Since suspensions  tend to show wall slip, the application of structured measuring surfaces in rheometers is widespread. Usually, for parallel-plate geometries, the tip-to-tip distance is used for calculation of absolute rheological values, which implies that there is no flow behind this distance. However, several studies show that this is not true. Therefore, the measuring gap needs to be corrected by adding the effective gap extension  Suspensions are nowadays used in numerous industrial areas . However, their absolute rheological properties are very difficult to determine, which, for example, results from colloidal forces and particle interactions ,2,3. A vThe low particle concentration in this near-wall region is caused by different phenomena, but one of the most important aspects is the geometric restriction between the particles and the wall itself. Since the particles cannot penetrate the wall, the minimum distance between a particle centre and a flat wall is the particle radius. Combined with shear induced particle migration away from the wall, the development of a particle-depleted area and, thus, wall slip can hardly be prevented . TherefoIn principle, wall slip can either be corrected or inhibited for absolute measurements. However, correction methods require a huge measuring effort and sometimes even more than one set of measuring equipment ,16,17. Taces see b is usuaNickerson et al.  conducted Joseph , of δ′~1s et al. , who anao et al. , who gluAs previous investigations show, the application of structured geometries leads to an increase in the effective gap size and, therefore, strongly influences the determination of viscosity values to an unknown extent. Nevertheless, structured geometries are widely used in commercial and especially in concrete rheometers to inhibit wall slip. Since there is a huge lack of information on how different geometries influence the rheological characterization of materials, we conducted research on three types of structured walls  in a parallel-plate rheometer. The materials investigated in this study were chosen with the aim to predict the behaviour of complex fluids from the results obtained for Newtonian fluids. Besides Newtonian fluids, the flow of fresh concrete should be imitated by a shear thinning fluid and different suspensions. In any case, the measured viscosities were corrected by comparing them to reference values and determining the effective gap size. For every system, the effective gap size could be determined, which is crucial for obtaining correct rheological properties for such geometries.Parallel-plate measuring systems consist of two parallel plates with radius  rotates . Due to tion (1) ,23:(1)γ˙Since the calculation of a single shear rate is necessary for further investigations, we chose the maximum shear rate occurring at the edge of the plate  for evaluating rrection :(4)τR=M2Following, the viscosity is calculated according to Equation (5):Measuring materials while wall slip is present leads to an apparent macroscopic shear rate rial see  and an aAccording to Yoshimura and Prud’homme, it is possible to correct the apparent shear rate measuring the same material at different gap heights ent of H :(6)γ˙R,aThe viscosity can then be expressed by the following equation:Besides the reference system  system, Anton Paar PP25-TG-1), further parallel-plate systems were developed to investigate the effective gap size and its dependency on various parameters like structure size, geometry, or nominal gap. These systems include a pyramidal and columnar structure  at In order to obtain reproducible and comparable results, all measurements conducted followed the same measurement preparation and procedure, which will be described in the following. Furthermore, every measurement has been repeated at least three times. The exact compliance with a protocol is essential while measuring complex fluids with roughened plates.After changing the measurement geometry, the zero gap was determined. In the case of smooth plates, the contact point of the plate surfaces was set as the zero gap. The approaching velocity was extremely low to reduce the influence of the zero gap error. For structured geometries, the zero gap was determined as the tip-to-tip distance.Following, the plates were lifted to a position that enabled the sample application. The Newtonian fluids and xanthan gum solutions were applied using disposable pipettes. The suspension was applied using a spatula to put as little strain as possible on the suspension, which should inhibit separation effects. Especially, the squeeze flow would cause a particle alignment that influenced the measurement. For structure depths of After applying the samples, the gap was set to the trimming gap The approaching velocity for setting the trimming and measuring gap was The measurement profile consisted of two sections. First, the samples were pre-sheared for Regarding the measurement values, the viscosity measured with structured plates is always lower than the true viscosity. This is due to the effective gap being bigger than the nominal gap, since the flow does not stop at the tip of the structures. Using the effective gap extension tems see  will be In this section the variations resulting from structured systems will be shown for all materials and similarities, and differences will be pointed out. The results presented are only exemplary. All obtained results are given in the The investigation of Newtonian fluids is meant to show the general influence of the structured geometries and can be used to calibrate the systems even for complex fluids, as it will be shown later. As simple Newtonian fluids, two different silicon oils Wacker AK 5000 , all geometries showed the same relationship between Since In contrast to the previous assumption that terature .A possible explanation for the increase of In principle, it is desirable to find a gap-independent correction function. As seen before, in investigating Newtonian fluids, ent gaps , which aAlthough the gap-dependency of However, the results presented at the beginning of this section showed that the relative influence of In order to develop a material-independent correction function, the two correction functions of both silicon oils were averaged for each system and will be called mean correction function from here on. The results are illustrated in Comparing both correction functions with regard to For the development of a general correction function, it is crucial to identify the influence of structural characteristics. Based upon the results presented earlier, the structure depth for pyramids, columns, and bars was varied at The emerging trend was similar for all structures, and Furthermore, the slope of the pyramidal structure values was higher. A possible reason for that could be the decrease in the number of elements with increasing structure height in case of the pyramids see . TherefoTo strengthen this finding, the column structure (systems 6–9) and the bars (systems 15–18) have been investigated in more detail, see It is obvious for both structures that, with decreasing structural elements, A possible explanation for the observed dependency of The relative influence of different modified geometries is viscosity-independent for the investigated range;A reduction of the measuring gap decreases the normalised viscosity due to the increasing influence of The values for Since the dependency of An increasing structure depth as well as a decreasing number of structure elements lead to increasing values of A summary of the following results for Newtonian fluids could be obtained:Comparing these results with previous studies, some aspects are important to be pointed out. In the study on a cleated geometry conducted by Nickerson et al. , they fos et al.  used a tIn order to analyse the influence of modified geometries on the measurement of shear thinning fluids, a solution of Since the flow behaviour of shear thinning fluids differs completely from that of Newtonian fluids, the influence of modified geometries might behave differently as well. Therefore, the normalised viscosity of a K 12,500 .By comparing the normalised viscosities for both flow behaviours, it becomes clear that the influence of the structured geometries in the case of xanthan gum was not as strong as for the Newtonian oils. While the values for the silicon oils deviated in a broad range, the xanthan gum solution was quite close to the true viscosity. Nevertheless, the increase of the effective gap can be observed for xanthan gum as well.A possible explanation for the narrow range close to the true viscosity can be found in the shear thinning behaviour of xanthan gum. In the case of structured geometries, the velocity of the upper plate was induced at the structure tips and over the complete structure depth. Therefore, the flow field entered the structure in the upper regions (tips), but the lower parts of the structure  remained unsheared. This leads to an overall lower “mean” shear rate compared to the reference system and, therefore, due to the shear thinning properties of xanthan gum, to a higher viscosity. Since this seems to counteract the increasing effective gap size, its influence decreases, leading to smaller deviations from the true viscosity.The dependency of measuring gap and the influence of shear thinning properties were analysed by measuring the normalised viscosity for three xanthan gum concentrations, d 1.4 mm .The shear thinning properties had no influence while investigating with structured geometries. Additionally, Analogous to the investigation on silicon oils, the influence of shear rate on the apparent viscosity was determined. To this end, the viscosity of a Comparing the apparent viscosity derived by the structured geometry with the reference viscosity, it becomes obvious that the apparent viscosity deviated especially for low shear rates. With increasing shear rate, this deviation became smaller, leading to nearly the same values for apparent and reference viscosity. The decreasing deviation between apparent and reference viscosity leads to the assumption that Since the xanthan gum solution showed a dependency on As The behaviour of Since shear thinning fluids behave completely different compared to Newtonian fluids, the correction function cannot be transferred directly. Therefore, new correction functions have been developed following the same procedure as described before. The uncorrected values measured with columns (system 6) at two different gap heights, The comparison between corrected viscosities and reference viscosity shows that the application of a gap-independent correction function In contrast to the Newtonian results, there seems to be no clear relationship between portance . For colSince an increase in the number of structural elements showed a decrease of solution .The relative influence of structured geometries is far smaller than that observed for Newtonian fluids and is also independent of the strength of the shear thinning effect in the range of the investigated fluids;In contrast to Newtonian fluids, no exact relationship between measuring gap and normalised viscosity was observed, which is the result of two counteracting effects—gap height and shear thinning—combined with the overall small influence of structured geometries;The values for The development of a gap-independent correction function Because of the relatively small changes of Concerning pyramids, an increase of structure depth, leading to a decrease of number of elements, leads to an increase of An increase of bars leads to an increase of The previous investigations on shear thinning fluids lead to the following results:Concerning all shear thinning fluids, it is important to mention that the development of a general material-independent correction function is not possible since these fluids differ from each other regarding the strength and onset of shear thinning properties.Comparing the results obtained for shear thinning xanthan gum solutions with the results observed for Newtonian fluids, it is obvious that n et al. , who staThe suspensions investigated in this study consisted of the silicon oil AK 5000 as matrix fluid and poly(methyl methacrylate) (PMMA) particles (In the first step, we compared the normalised viscosity and the effective gap extension It is worthwhile to mention that the reference values obtained for the The influence of adding PMMA particles to Newtonian oils on the normalised viscosity was investigated by measuring the p widths . The visIt is obvious that In order to analyse how these values behave for different concentrations, the These results show that no visible dependency between the relative influence of structured geometries and particle concentration existed, since all values deviated only slightly from each other without any apparent relation. The reason for this is surely the only weakly shear thinning flow behaviour of the suspensions with particle contents of up to t. % see . For claThe investigation of PMMA suspensions with different concentrations and the comparison to Newtonian fluids shows that the relative influence of structured geometries seemed to be the same for suspensions as well as for the Newtonian fluids used in this study. Since the flow behaviour of the suspensions was only weakly shear thinning, the assumption can be made that the relative influence mainly depends on the characteristics of the flow properties.Concerning the influence of parameters such as shear rate on the results obtained for suspensions, it is worthwhile to mention that we observed the same dependency as for Newtonian fluids. Therefore, a closer analysis of these parameters will not follow as it already happened in Since the investigated suspensions and silicon oils showed a comparable flow behaviour and reaction regarding the use of structured geometries, the same correction procedure as for Newtonian oils should be applicable to suspensions. Therefore, a gap-independent correction function was calculated and applied to the suspension viscosity. However, far more important than the gap independency is the material-independent correction. Since the derivation of a material-independent correction function was possible in the case of Newtonian oils, and the suspension flow behaviour only showed a slight tendency for shear thinning, the same material-independent correction function obtained for Newtonian fluids might be applicable to the suspensions. Therefore, the suspension viscosity was not only corrected using the gap-independent correction function derived from the suspension values but also using the material-independent function derived for Newtonian oils (see As expected, both methods yielded nearly the same results . BesidesThe normalised viscosities for the investigated concentrations, The normalised viscosity of suspensions shows the same gap dependency as silicon oils;For the investigated suspensions, no significant deviation in the effective gap extension Since the suspensions behaved only in weakly shear thinning manner, the assumption can be made that the relative influence of structured geometries mainly depends on the flow properties of the investigated materials;The viscosity can be corrected using a gap-independent correction function;Since the flow behaviour of the suspensions was only weakly shear thinning, the material-independent correction function derived for Newtonian oils could be applied to correct the suspension viscosity values, showing that slight deviations from Newtonian flow behaviour do not pose a problem.The results for the investigated suspensions can be summed up in the following statements:As described before, the results obtained for silicon oils and suspensions show approximately the same results concerning the effective gap extension o et al. , who obsThe use of structured geometries is a common way to prevent apparent wall slip while measuring wall slip-prone materials such as suspensions. The huge advantage of those geometries is the significantly lower measuring effort, since wall slip correction methods require measurements at two or more gaps. In this paper, we have demonstrated that the application of these geometries results in a higher effective gap size, which leads to the determination of incorrect viscosity values. However, the correction of viscosity is possible by adding the effective gap extension


Wind tunnel tests have become one of the most effective ways to evaluate aerodynamics and aeroelasticity in bluff bodies. This paper has firstly overviewed the development of conventional wind tunnel test techniques, including high frequency base balance technique, static synchronous multi-pressure sensing system test technique and aeroelastic test, and summarized their advantages and shortcomings. Subsequently, two advanced test approaches, a forced vibration test technique and hybrid aeroelastic- force balance wind tunnel test technique have been comprehensively reviewed. Then the characteristics and calculation procedure of the conventional and advanced wind tunnel test techniques were discussed and summarized. The results indicated that the conventional wind tunnel test techniques ignored the effect of structural oscillation on the measured aerodynamics as the test model is rigid. A forced vibration test can include that effect. Unfortunately, a test model in a forced vibration test cannot respond like a structure in the real world; it only includes the effect of structural oscillation on the surrounding flow and cannot consider the feedback from the surrounding flow to the oscillation test model. A hybrid aeroelastic-pressure/force balance test technique that can observe unsteady aerodynamics of a test model during its aeroelastic oscillation completely takes the effect of structural oscillation into consideration and is, therefore, effective in evaluation of aerodynamics and aeroelasticity in bluff bodies. This paper has not only advanced our understanding for aerodynamics and aeroelasticity in bluff bodies, but also provided a new perspective for advanced wind tunnel test techniques that can be used for fundamental studies and engineering applications. With the development of society and economy, numerous high-rise structures have been built all over the world. For example, the heights of Burj Khalifa tower, Shanghai Tower, etc., have exceeded 600 m and the aspect ratio of the 432 Park Avenue building has achieved 15:1. Bluff bodies may experience excessive levels of vibration under the action of wind, and the effect of wind on these bluff bodies becomes more significant than the effect of seismic activities. Therefore, the prediction of wind load and response of bluff bodies is a significant consideration for a structural design engineer. Despite significant advances in computational fluid dynamics (CFDs), the wind tunnel test is still regarded as an important way to evaluate the action of wind on structures.A high frequency base balance (HFBB) technique or a static synchronous multi-pressure sensing system (SMPSS) test technique is often carried out to obtain wind loads acting on a structure. These test techniques have been widely applied to bluff bodies to evaluate their performance. However, both the HFBB technique and the SMPSS technique are static measurements, in which wind loads are obtained from rigid test models and the effect of structural vibration (unsteady effect) is, therefore, excluded. The effect is conventionally small, but when it is in-phase with oscillating velocity the effect acts as aerodynamic damping and cannot be neglected. It has been affirmed that differences in wind loads measured from a static and a dynamic test are mainly ascribed to neglecting the unsteady effect . To inveThe aforementioned wind tunnel test techniques have their advantages and disadvantages in evaluating aerodynamics and aeroelasticity in bluff bodies. Few studies have comprehensively overviewed the characteristics and developments of those wind tunnel test techniques in recent years and reviews for advanced wind tunnel tests like hybrid aeroelastic-pressure/force balance wind tunnel test techniques are very limited. In this paper, the conventional wind tunnel test techniques, including the HFBB test, SMPSS test and aeroelastic wind tunnel test, are reviewed in detail. Then two advanced wind tunnel test techniques, a forced vibration test technique and a hybrid aeroelastic- force balance (HAFB) test technique, are comprehensively overviewed in light of the limitation of conventional wind tunnel test techniques. The characteristics of unsteady forces measured from the conventional and advanced wind tunnel tests are presented. Finally, new perspectives for advanced wind tunnel test techniques are summarized.The high frequency base balance (HFBB) test technique was proposed in the early 1980s ,5 and haThe technique involves the base force measurements of a nearly rigid scale model, which models only the external geometry of a structure. If the model is not rigid enough and responds dynamically to the wind force, the interaction between the dynamic response and the aerodynamic force would contaminate the base force measurements . TherefoIn brief, the HFBB test technique is an effective and expeditious way to obtain exact overall wind loads . Furthermore, only one test is needed to determine wind-induced responses for a series of structures with the same geometry. However, this technique has some limitations: The ideal linear translational mode shape and constant torsional mode shape of a HFBB are assumed or a correction factor is needed; only the fundamental modes of the two translational and one torsional mode shapes are concerned in the response analysis, and the higher mode shapes are neglected; the test model should be rigid enough and no significant motions during the test; the local wind force characteristics cannot be obtained.There have been lots of investigations on conventional buildings through the HFBB test and the technique has been developed for several decades ,14,15. IA SMPSS wind tunnel test technique is used to observe the wind pressure on the surface of the test model synchronously through the synchronous multi-pressure sensing system. Different from the HFBB test technique, pressure taps are installed at different levels of the test model and the distributed pressure is therefore recorded using a high-speed scanning pressure equipment. The base shear force and moment of the test model can be evaluated by using the high frequency pressure integration (HFPI) method ,20. As pA test model in a SMPSS test should be rigid enough to avoid evident wind-induced vibration so that the pressure measured in the test is precise . FurtherThe SMPSS technique has been widely used for estimating wind loads and wind-induced vibrations of structures due to the above illustrated advantages. However, there are some technical drawbacks inherent to it. For instance, a SMPSS test may make use of a large number of pressure taps to accurately predict wind forces acting on a test model. Furthermore, it is difficult and prohibitive to install the pressure taps especially in the presence of complicated geometric forms . For bluff bodies, the static measurements are prone to be affected by model motion. Lastly, the SMPSS test cannot consider the unsteady effect and the influence of structural vibration is excluded.An aeroelastic test technique makes use of an elaborate test model to directly obtain the aeroelastic process. The model is designed to not only have the exterior geometry of the prototype structure, but also the relevant dynamic parameters such as stiffness, mass and effective damping , so thatTypical schematic aeroelastic models are presented in Through an aeroelastic test, (1) wind-induced oscillations of a structure can be directly measured; (2) fluid-structure interactions can be well included; (3) high mode shapes of a test model can be well considered. Therefore, this test technique is widely accepted and used for evaluating wind-induced vibrations of structures. Meanwhile, the disadvantages of an aeroelastic test should also be highlighted: (1) The design and construction of an aeroelastic model is time-consuming; (2) during the test, dynamic parameters may vary with oscillating amplitude; (3) the wind loading information, which is required by engineers, cannot be obtained.In summary, by using a HFBB or a SMPSS wind tunnel test technique, static base forces or pressures of a test model are observed, which can be used for response predictions. The main problem of the test techniques is that the “unsteady” effect that has been proved to have a significant effect on response predictions is neglected. By using an aeroelastic test technique, wind-induced vibrations of a test model, which contains the “unsteady” effect, can be directly observed. However, wind forces of the test model which are required by the engineers cannot be obtained. Additionally, the aeroelastic test is complicated and time-consuming. To overcome the shortcomings in the conventional wind tunnel test techniques, advanced wind tunnel test techniques  that can include the unsteady effect are required.Due to the different limitations of static measurement tests (HFBB and SMPSS) and aeroelastic test, a forced vibration test is often carried out to evaluate unsteady wind forces of structures. A forced vibration test system mainly includes a test model, an actuator device, a signal generator, a power amplifier, a force measurement system and a response measurement system. An example of forced vibration test systems is given in As mentioned before, unlike distributed pressures, base forces do not include local wind force characteristics. Bearman and Currie  and BearThe above illustration indicates that a forced vibration test model with pressure measurements includes much more information than that with respect to base force measurements. Furthermore, by integrating unsteady distributed pressures along the height of a prism, observed from a forced vibration wind tunnel test, unsteady wind forces of the prism can be estimated. Due to the advantages of a forced vibration technique, it has been widely used for flutter derivative identification ,35, aeroHowever, the forced vibration test technique only takes parts of unsteady effect into account, the results of the test were not the same as the results of the real structure. The hybrid aeroelastic-force balance (HAFB) test technique can reflect the real results including a fully unsteady effect. In a HAFB technique, a test model oscillates freely under the action of wind and meanwhile unsteady wind forces of the test model are sampled. As the test model responds as a real structure and the fluid-structure interaction is well simulated, the observed unsteady wind forces are more accurate than those observed from a forced vibration test. The HAFB test technique was composed of two parts, including free vibration measurement and unsteady pressure measurement. In this section, the mechanism of the test technique was clarified clearly and the development of the HAFB test technique was introduced. An example of HAFB test system is given in Besides, the pressure measurement was synchronous with the response measurement, and the synchronization was realized by inputting a random and periodical signal generated by a signal generator. The pressure was sampled by using a synchronous multi-pressure sensing system (SMPSS). The sampling frequency and the duration time of unsteady pressure measurement are the same as those of the free vibration measurement.Very few studies have focused on the HAFB technique. Chiara Pozzuoli  has deveThe test model in Most recently, Gao and Zhu ,42 devisTo explain the effectiveness of the advanced wind tunnel test techniques, unsteady aerodynamics  and aeroelasticity  measured from the techniques are reviewed.The distributed pressure as well as the overall wind load of a prism can be evaluated from a SMPSS test. Wind pressures on the windward face of a structure are positive as the approaching flow directly acts on the face whereas they are negative on the other three faces (leeward face and side faces) due to suction. Wind forces in the along wind direction are defined as drag force and those in the crosswind direction are defined as lift force .For three-dimensional prisms, the local and overall mean, root mean square (RMS) lift and drag force coefficient are defined as follows. It should be noted in the crosswind direction the mean lift force coefficient is close to zero and is therefore neglected. The base moment coefficient can also be evaluated from the observed local pressure coefficients.z above the ground are defined asThe local wind force coefficients of a test model at height Integrating the obtained local wind force coefficients at each level, the generalized force coefficients are expressed asThe base force coefficient is expressed asBased on observed wind pressures and the above definitions, static wind force characteristics of structures have been comprehensively analyzed ,21,45,46Many studies have focused on unsteady wind forces on bluff bodies. Bearman and Obasaju  have invIn steady flow, bluff bodies can undergo vortex-induced-vibrations (VIVs) because of the von Karman vortices generated in wake flow which will create unbalanced forces on the structure. VIVs of prisms occur at the lock-in range where the exciting frequency may close to the natural frequency of the body. In addition, downstream structures behind the bluff bodies generating vortices may also undergo VIV because of the impingement of induced flow and the low pressure core region of the vortices . BecauseCooper et al.  and KataBased on observed unsteady pressures, spanwise correlations of cylinders have been analyzed ,58. It iThe above studies have not only advanced our understanding of characteristics of unsteady wind forces of structures, but also provide a foundation for further analysis. The following section will introduce the identified aerodynamic damping based on the unsteady wind forces obtained from a forced vibration test.The observed unsteady wind force is composed of a random component due to unsteady wake effects and turbulence, and a motion-induced force component. Based on a previous study , it is eThe corresponding base force is written asA complex aerodynamic impedance, Then, a dimensionless form of impedance As mentioned, the motion-induced force can be divided into an aerodynamic stiffness term and an aerodynamic damping term  into (15), the response predictions can be improved.For the purpose of convenient use, nonlinear mathematical models of the identified aerodynamic damping have been developed. Watanabe et al.  have proChen  has reviSelf-excited vibration  occurs when the total damping of a system is negative. It is in a steady state when the total damping Solving Equation (23), yieldsBy using the developed polynomial, the aerodynamic damping of a test model was evaluated and compared with that estimated by Equation (18) and the quasi-steady theory . The ideApart from the forced vibration technique, the aerodynamic damping can also be evaluated based on spectral and time series approaches  or auto-regressive and moving-averages (ARMA) techniques, half power bandwidth techniques and random decrement techniques (RDT) ,63,64). For forward inclined prisms, the generalized aerodynamic damping coefficients are shown in Chen and Kareem  pointed Vr = 28, 36 and 42, unsteady aerodynamic forces on the prism are shown in Self-excited galloping force is commonly estimated by a quasi-static method. However, the quasi-static force excludes the effect of structural oscillation and is therefore not applicable to predict galloping responses of structures with respect to unsteady effect. Bouclin and Geoola  have proThe identified unsteady galloping force includes three components: an aerodynamic damping component, an aerodynamic stiffness component and a residual force (buffeting force) component. Among them, only the aerodynamic damping or stiffness component inputs or dissipates energy to an oscillating system. Additionally, it has been validated that the effect of the aerodynamic stiffness is slight. On these considerations, a nonlinear mathematical model that is simplified as a fifth order polynomial was established to model the unsteady galloping force. It is expressed as Equation (25).It was found that the classical quasi-steady theory fails to predict the galloping response of a slender prism, as presented in As mentioned before, conventional static measurement tests fail to consider the unsteady effect, and the effect of structural vibration is not included. Particularly speaking, the test models used in the HFBB test technique and the SMPSS test technique are static, so the unsteady effect is completely excluded. For bluff bodies undergoing galloping, the quasi-steady theory fails because the unsteady effect on this occasion is very obvious. In this situation, the HFBB test technique and the SMPSS test technique should not be used. For the aeroelastic wind tunnel test, it can experience the fluid–structure interaction like a real building. However, only the wind-induced response of the prism can be observed, and the distributed pressure on the structure cannot be obtained. So aeroelastic characteristics of bluff bodies can be analyzed according to the data obtained from aeroelastic wind tunnel test. Aerodynamic characteristics cannot be received. To overcome the above shortcomings, the forced vibration test technique and HAFB test technique are proposed. They can be considered as the combination of SMPSS and aeroelastic test. The forced vibration test technique is widely performed to obtain local pressure and response of the test model. The model can oscillate at a certain frequency in the crosswind direction due to the motor fixed under the plate. But the technique only considers the one-way coupling effect and the structural motion is not like the real one. To some extent, forced vibration test can be used to obtain aerodynamic force acting on bluff bodies including aerodynamic damping force and aerodynamic stiffness force. But response of prisms in the real world cannot be predicted according to this technique. Due to the disadvantages of conventional wind tunnel test techniques and forced vibration test, HAFB test is developed with the purpose of improving the accuracy of prediction and obtaining more information. The detailed comparison of each test technique is shown in HFBB is widely applied to predict the wind-induced base shear and moment of bluff bodies. In a HFBB wind tunnel test, generalized wind force and base force can be obtained, which can be used for calculating wind force spectra and wind-induced response  accordinDespite the effectiveness of the HFBB test technique in analyzing aerodynamics in bluff bodies, it cannot obtain the point wind pressure, cross-correlation coefficient and coherence coefficient of wind pressure on the surface of a test model. SMPSS wind tunnel test can observe wind pressures on a static model synchronously. Integrating the observed wind pressure along the height of a test model, the local and generalized aerodynamic forces can be obtained. Wind-induced responses of the test model are, therefore, calculated according to the analytical scheme  using a Wind-induced response of a test model cannot be directly obtained from a HFBB or SMPSS wind tunnel test. Aeroelastic test is a straightforward way to measure wind-induced displacement response, torsion angle and acceleration. However, it cannot synchronously observe wind pressures or wind forces acting on a test model.In a forced vibration wind tunnel test, the unsteady effect of wind acting on a prism can be considered. A certain amplitude and frequency were given to determine the motion of the model. Pressure on the surface of the model was obtained through pressure taps. Then the generalized force and force coefficient, force spectra and power spectra were calculated. Besides, aerodynamic force, aerodynamic damping force and aerodynamic stiffness force can be identified. The calculation procedure for results obtained from a forced vibration wind tunnel test was shown in In a hybrid aeroelastic-pressure/force balance test, the unsteady effect was fully considered. Unsteady aerodynamic force was divided into three parts including aerodynamic damping component, aerodynamic stiffness component and residual component. According to the first two parts of the three components, a nonlinear mathematical model of the unsteady self-excited force can be deduced, while the order of the nonlinear model was obtained from order of unsteady self-excited force spectrum. As a result, the galloping response of a prism can be predicted. In general, the scheme to model the unsteady self-excited force on a bluff body is summarized in (1)The HFBB and SMPSS techniques neglect the unsteady effect, which has been proved to have a significant influence on the predicted wind-induced response. The response of the bluff bodies cannot be obtained directly from these two techniques.(2)The aeroelastic test can only obtain the wind-induced response of a bluff body, while the wind force acting on the model cannot be obtained.(3)Unsteady aerodynamic forces obtained from a forced vibration wind tunnel test takes part of the unsteady effect into consideration, because a test model is forced to oscillate and it only considers the effect from an oscillating model to its surrounding flow and cannot consider the feedback. As a result, unsteady aerodynamic forces on a forced oscillation model may have considerable differences with those on an aeroelastic model or a structure in the real world.(4)The hybrid aeroelastic-pressure/force balance test techniques are effective in measuring unsteady aerodynamic forces on bluff bodies during aeroelastic oscillations. The obtained forces can well include the ‘unsteady’ effect and can be used to address the shortcomings of classical theories  in predicting wind-induced responses of bluff bodies.(5)The HAPB/HAFB test techniques are highly recommended to investigate the unsteady aerodynamics and aeroelasticity of bluff bodies.In this paper, conventional wind tunnel test techniques  and advanced wind tunnel test techniques  were comprehensively overviewed. The main conclusions and recommendations are summarized as follows.


Background: Although many children with diseases of the kidneys and the urinary tract may not tolerate long journeys, the number of facilities that provide specialized care for these patients is limited. Therefore, the geographical accessibility of the required health services is critical especially in this patient group. We have analyzed the geographical accessibility of pediatric inpatient and nephro-urology services in Germany, Ireland, and the United Kingdom (UK).Methods: This study introduces a model to compare countries or regions regarding the geographical accessibility of their health services. We calculated the geodesic distances, travel distances, and travel time by car from evenly distributed random points to the nearest facilities that provide pediatric inpatient or nephro-urology outpatient services . The results were weighted by population density. We compared the three countries with regard to the accessibility of the named services.Results: Weighted median travel times from the random points to the nearest pediatric inpatient ward are < 30 min in all countries. Weighted travel times to the nearest point of pediatric service are shortest in the UK (median <50 min) and longest in Ireland (median <90 min), regardless of the type of service (p < 0.0001). Non-weighted travel times to the nearest pediatric inpatient ward and hemodialysis unit, however, are shorter in Germany than in the UK (p < 0.0001).Conclusions: There is a surprising disparity between the travel times to the nearest facility with pediatric nephro-urology service in these three industrialized European countries. Reasons may be differences in the geographical distribution of the population, the focus of the health care system, and a different degree of clinical networking. The geographical accessibility of health care facilities is a main factor of both the availability of services and their affordability . There hChildren with diseases involving the kidneys or the urinary tract comprise a marginal patient group that requires highly specialized medical care. The limited number of facilities within a health care system providing such specialized care also limits their geographical accessibility. The specific characteristics of this patient group, however, suggest that these patients do not tolerate long journeys to the outpatient clinic. Examples are newborns or young infants with congenital anomalies of the kidneys and the urinary tract (CAKUT), or patients visiting a hemodialysis unit several times a week, who cannot tolerate further travel-related absence from school or further restrictions in their social life. There is little data published on the impact of the geographical accessibility of specialized nephro-urology services on the outcome of pediatric patients. Children living further away from transplant centers are less likely to receive a kidney transplant compared to those living closer , 9. MoreEspecially in industrialized countries with sufficient resources, health care systems thus are expected to provide specialist treatment in pediatric nephrology and urology within short distance from home. There have been studies on the geographical accessibility of pediatric inpatient services in the regions of Japan , 14. To One aim of this study was to keep all calculations reproducible and transferable. Therefore, we utilized only publicly accessible data and applications.https://www.koordinaten-umrechner.de/ (accessed 09/2018 to 05/2019). Information about the Nomenclature of Territorial Units for Statistics (NUTS) 2016 regions  was obtained from the Eurostat online database. The data sets generated and analyzed during the current study are available from the corresponding author on reasonable request.Data about hospital services was mainly collected from public webpages. This included hospital websites, websites of physicians' associations and collaborations, press releases, job descriptions, and data published by health care providers and related institutes for statistics . Where the data about hospital services from public sources was not sufficient, the hospitals were contacted by phone or e-mail. Acquisition of data on local services in Germany was undertaken from 09/2018 to 11/2018, in Ireland from 12/2018 to 01/2019, and in the UK from 03/2019 to 05/2019. 2 geographical area for all three countries . There is a slight difference between the exact number of random points and the related country area (in km2) due to the method of first generating random points, then keeping only the points within the countries' boundaries. The distribution of the points was checked visually by scatter plotting them. We identified the NUTS (classification of 2016) level 3 polygon that each random point belongs to, thereby assigning the corresponding population density to each random point. We then calculated the geodesic (shortest path between two points on the surface of the earth) distance from every random point to every point of pediatric service  according to Karney, 2013 (U test (for pairwise comparison of the countries' data sets for each type of service) to test for statistical significance of the observed results.Data was analyzed using Python v3.7. The European Terrestrial Reference System 1989 Lambert Azimuthal Equal Area Europe (EPSG:3035) was employed as the coordinate reference system for all calculations, as it is recommended for purposes where true area representation is required . In ordeThe geographical distribution of the random points generated for each country as well as the sample of these random points that was used for the travel distance and travel time analyses shows no clusters or gaps , 2.p < 0.0001 according to both Kruskal-Wallis test and Mann-Whitney U-test).Median and mean geodesic distance, travel distance by car, and travel time by car from the random points to the nearest pediatric inpatient ward and to the nearest points of pediatric nephro-urology service  in each country, weighted by the population density of the NUTS 3 region that each random point belongs to, are shown in To assess the disparity in weighted travel times among these countries, we compared the numbers of institutions that offer the related pediatric  services and correlated them with the respective country‘s social indicators population, land area, number of doctors working in the health care system, number of live births, and total health care expenditure . In Germp < 0.0001 according to both Kruskal-Wallis test and Mann-Whitney U-test).Weighting by population density favors urban regions with a high population density. If the aim of a health care system is to provide specialized services also in less inhabited regions rather than to focus on urban areas, such weighting may therefore be inappropriate. To address this, we utilized non-weighted results . In contThe results of this study show a disparity between the geographical accessibility of institutions that provide specialized pediatric services in three industrialized European countries. Travel times to the nearest pediatric inpatient wards as well as to the nearest pediatric nephro-urology services weighted by population density are shortest in the UK, longer in Germany, and longest in Ireland.2) compared to the UK (272.4/km2) or Germany (234.0/km2) probably is a main reason for longer travel times. Secondly, the countries exhibit different types of population distribution. The fraction of the population living in urban areas is highest in the UK (83%), lower in Germany (77%), and lowest in Ireland (63%), according to the World Bank. The population density of NUTS 3 regions is widely distributed in the UK, whereas more narrowly scattered in Germany (This disparity is not in line with the inter-country differences in macroeconomic indicators (gross domestic product (GDP) per capita, total health expenditure per capita, or its amount of the GDP; data not shown). The results match better with other country-related characteristics. Firstly, Ireland's considerably lower population density  is lowest in the UK . This seConsidering our clinical experience that patients living far from a tertiary hospital are more likely to miss an appointment or discontinue the treatment than those living closer, the clinical relevance of the observed difference in travel times to specialized pediatric nephro-urology care between the three countries seems obvious. However, as mentioned above, the data published to support this thesis is very limited and more research into this field is required.This study introduces a model to compare countries or regions regarding travel distance and time from patients' homes to health service providers. The model uses random geographical points within one country  as starting points, and geographical coordinates of the real service providers  as destinations. To address the uneven distribution of the population within the countries, the results are weighted by the population density of the NUTS 3 region that the assumptive patients' homes belong to.As this model is based on publicly available data  and utilizes only publicly accessible applications, it can be easily and rapidly applied to any health service in any country by anyone who is capable of programming with Python, the most popular programming language nowadays according to PYPL index. In addition to comparing travel time and distance between countries or regions, the Python script we have established can be easily adapted to highlight gaps in the distribution of health services within a country or region. This can help health administrators to adjust networks of health services. The Python script we have used in this study is available from the corresponding author on request.To keep calculations feasible and easily transferable, the model is based merely on geographical location of health institutions and regional population density. Thus, it does not include the supply and demand aspect of service provision . In this study, we used NUTS 3 regions as the smallest available unit that (by its population density) determines the weighting of the results. However, the population density within such regions may be non-homogenous, possibly leading to results slightly differing from reality. As the relation of geodesic distances from the virtual patients' homes to the nearest service providers among the three countries is in line with the corresponding travel times, the disparities between the travel times among Germany, Ireland, and the UK we have shown here are not explained directly by the road infrastructure. Traffic congestion, however, may influence travel times. According to INRIX and TomTom traffic indices, Dublin is the most congested among the cities that hold a pediatric hemodialysis unit . Of the Our data shows a disparity between the travel times from assumptive patients' homes to the nearest pediatric inpatient ward and the nearest points of pediatric nephro-urology service in the three industrialized European countries Germany, Ireland, and the UK. Reasons for this may be differences in the overall population density, in the distribution of the population across the country, and in the focus of the health care systems. Furthermore, clinical networking within the field of pediatric nephrology in the UK seems to account for travel times in the UK being the shortest among the three countries. The model that is introduced in this study involves only publicly accessible data and applications and may easily be employed to compare travel times and distances in further countries or regions.The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.NT conceptualized and designed the study, collected data, designed instruments for the data analyses, carried out the data analyses, drafted the initial manuscript, and reviewed and revised the manuscript. DL provided the initial instruments for the data analyses, supervised the design of further instruments for the data analyses, supervised the data analyses, and reviewed and revised the manuscript. AK, MK, and MT supervised the data analyses, and reviewed and revised the manuscript. MM helped to prepare the analyses for publication, and critically reviewed and revised the manuscript. KD conceptualized the study, supervised the data analyses, and critically reviewed and revised the manuscript. All authors contributed to the article and approved the submitted version.KD was employed by the non-profit organization Deutsche Stiftung Organtransplantation. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


T depends on prior knowledge. (2) The accuracy of signal denoising is affected by filter length L. To solve the limitations, an improved MOMEDA (IMOMEDA) method is proposed in this paper. Firstly, the envelope harmonic-to-noise ratio (EHNR) spectrum is adopted to estimate the fault period of MOMEDA. Then, the improved grid search method with EHNR spectral entropy as the objective function is constructed to calculate the optimal filter length used in the MOMEDA. Finally, a feature extraction method based on the improved MOMEDA (IMOMEDA) and Teager-Kaiser energy operator (TKEO) is applied in the field of rolling bearing fault diagnosis. The effectiveness and generalization performance of the proposed method is verified through comparison experiment with three data sets.As a vital component widely used in the industrial production field, rolling bearings work under complicated working conditions and are prone to failure, which will affect the normal operation of the whole mechanical system. Therefore, it is essential to conduct a health assessment of the rolling bearing. In recent years, Multipoint Optimal Minimum Entropy Deconvolution Adjusted (MOMEDA) is applied to the fault feature extraction for rolling bearings. However, the algorithm still has the following problems: (1) The selection of fault period  Rolling bearings are vital components of mechanical systems and have been widely used in rotating machinery. However, rolling bearings often work in more demanding environments for a long time, such as corrosive, high temperature, large impact and high load conditions, which increases the uncertainty of the mechanical system, resulting in various fault defects of the bearings . In ordeThe fault diagnosis technique based on vibration signal has been widely used in practice because of its intuitive operation and reliable diagnosis results, where the vibration signal of the bearing is monitored by the acceleration sensors installed in the appropriate direction of the bearing seat or the box, and the signal is analyzed and processed to determine the bearing condition. Furthermore, some advanced measurement systems are applied in rolling bearings. Wang  designedFault feature extraction based on signal analysis is critical after the reliable vibration data source is available. In industrial production, when the bearing fails to work, the collected vibration signal will generate periodic transient pulses at a certain characteristic frequency . HoweverTo tackle the limitation, the deconvolution principle has been introduced for bearing fault periodic composition extraction, which achieved satisfactory results. In this line, Wiggins  proposedHowever, the MED method aims at maximizing the kurtosis value, and thus it is easily affected by strong noise. In the iterative process, only the pulse component in the vibration signal can be enhanced and this defect limits the development of MED method . To addrT. It is necessary to identify the periodic impulses signal through the multipoint kurtosis spectrum , the Equation (8) can be simplified as:The target vector traction . When t We then substitute y: The Finally, the multipoint kurtosis (MKurt) is defined as follows: t needs to take T as a prior knowledge. In practical engineering, it is usually impossible to meet this condition due to the complex working environments, which in turn limits the application of the MOMEDA method. In addition, the periodic pulse energy is determined by the filter length L. If the parameter setting is unreasonable, it will weaken or enhance the energy of the periodic impulse signal which will lead to misdiagnosis. Therefore, how to calculate the fault period T and filter length L for the MOMEDA method quickly is one of the major issues, aiming at improving its performance.However, the construction of target vector L2/L1 norm, Gini index and other norm indexes 30], we acovx(t) is demodulated by the TKEO and the corresponding spectrum is shown in The (C) FK methodFast kurtogram (FK)  is a timT search interval of traditional MOMEDA. Then, covx(t) and its Teager-Kaiser energy spectrum are shown in Compared with the MOMEDA-TKEO method and FK method, the amplitude obtained by the IMOMEDA-TKEO is clearer. Test 3 is carried out through the filter length optimized by the proposed method, without changing the fault period T directly affects the denoising effect of MOMEDA. However, the interval of T depends on prior knowledge and has trial and error. When the original signal is affected by complex background noise, the Mkurt spectrum may not provide correct fault period information according to the location of maximum multipoint kurtosis. For example, we get the pseudo fault period as T = 97.2. Furthermore, the optimal periodic pulse signal covx(t) cannot be obtained, as shown in It can be seen from the Mkurt spectrum that the search interval of fault period fd is the fault characteristic frequency, M is the multiple of the fault characteristic frequency; p(f) is the Teager-Kaiser energy spectrum [fdSNR index of the presented method is the highest, which indicates that the proposed method has better ability in weak feature representation.In order to further confirm the effectiveness of the proposed method quantitatively, a new signal-to-noise ratio (SNR)  is used spectrum . It can The specific experimental process of IMOMEDA-TKEO, MOMEDA-TKEO and the FK method is the same as The proposed method can achieve comparable effects as the MOMEDA-TKEO method as shown in T is invariant, filter length L is obtained by the proposed method and presented in L directly affects the deconvolution performance of the MOMEDA method. In addition, it can be seen from T directly affects the performance of the method.Under the condition that the fault period FK analysis results are shown in fdSNR for the IMOMEDA method is higher, which further implies the IMOMEDA has the qualitative and quantitative advantage over the original MOMEDA algorithm and FK method.From In order to verify the good generalization ability of the IMOMEDA-TKEO method, the further experiments were carried out with NASA bearing datasets.The test-bed shown in N is 4096 points.From the bearing parameters shown in Experimental comparative analysis is complete by utilizing the vibration signals of outer race and inner race. Furthermore, the vibration signal time and frequency analysis is presented in The experimental results of FK based method can extract fault features of outer race and inner race from the original vibration signals. Unfortunately, it can be seen from fdSNR index is calculated and shown in In order to intuitively compare the effectiveness of the method, the In order to further prove the validity of the proposed method, the practical test data from the self-made KUST-SY experiment platform is selected for subsequent analysis. The platform is able to simulate different failure conditions of rolling bearings. The structure of the test rig is shown in The failure behaviors of bearing system are diverse, including stripping, wear, fracture, crack, electrical erosion, indentation for the inner race and outer race. As displayed in N is 32768). We cannot get valuable information related to fault characteristic frequency from The time and frequency analysis is shown in T will lead to erroneous results in the MOMEDA method.It can be seen from fdSNR presented in The fault characteristic frequency for the inner race and outer race extracted by FK is shown in Through the comparative experimental analysis of IMOMEDA-TKEO, MOMEDA-TKEO and FK, the following can be summarized:(a) Combining with three case studies, the experimental results show that the IMOMEDA-TKEO method is better than the other method described in T of MOMEDA quickly with little time. Compared with the traditional MOMEDA method in which the parameters are set to the default value, the grid search method based on EHNR spectral entropy for selecting optimal filter length L does consume some time. However, optimization algorithm itself is not the main reason of time-consuming. The main reason is that as the iterations increases, the MOMEDA filter time keeps accumulating, leading to more training time of filter length. The purpose of this paper is to accurately calculate the fault period T of MOMEDA and select the optimal filtering length L of MOMEDA. The rationality and necessity of MOMEDA parameter selection are demonstrated on three different datasets. Although the calculation needs a certain time, it is within the acceptable range.(b) In this experiment, we select execution time and iterations to evaluate the complexity of the optimal parameter selection method and all the experiments are programmed in MATLAB 2018a on an Intel(R) Core(TM) i5-7300HQ CPU @ 2.50 GHz Laptop with 16.00 GB RAM. The parameter optimization time and iterations of IMOMEDA algorithm and the main program execution time of the comparative methods are shown in T of vibration signal, and set the filter length L reasonably by using the improved grid search method. The execution time is within the acceptable range.(c) It can be seen from T has to be set by a trial and error method. Furthermore, the range of T selected in the above experiments is the best result obtained by multiple attempts. If the selected range changes, the pseudo period will cause misdiagnosis. The proposed method introduces the EHNR spectrum to calculate the signal period T quickly and optimizes the filter length L by the improved grid search method. It transforms the iterative optimization problem into a single dimension optimization problem. Hence, the proposed method increases the operation efficiency and improves the deconvolution performance of MOMEDA method.(a) For the traditional MOMEDA method, the range of L is set as [L is given, while there is no upper limit, which makes the search range larger and the algorithm run for a long time. According to the specific characteristics of the signal, optimizing the search range can improve the efficiency of the algorithm. In addition, if we can obtain the corresponding relationship and change rule between the MOMEDA filter length L and period T on the basis of (a), it will further improve the operation efficiency and the practical application value of the proposed method.(b) In the proposed method, the search range of the filter length s set as . Only th(c) Compared with other types of signal characteristics, the practical test vibration signals for inner race from the rolling bearing fault simulation experiment platform are not only disturbed by noise, but also by harmonic signal of the outer race and rolling element. Under strong background noise, the IMOMEDA-TKEO method has been demonstrated to significantly outperform traditional signal processing methods in terms of noise reduction performance and fault feature representation robustness, which boosts the generalization ability.In this paper, a rolling bearing fault feature extraction method based on the IMOMEDA-TKEO is proposed. Some conclusions are obtained as follows:EHNR method is introduced to obtain the EHNR spectrum of the original signal and solves the problem that the fault period T of MOMEDA depends on prior knowledge.(1) The EHNR spectral entropy is proposed as the objective function, and the improved grid search method is introduced to solve the filter length L selection in MOMEDA.(2) The (3) Comparative experimental analysis of the proposed method with the MOMEDA-TKEO method and the FK method are completed. The proposed method can achieve the best filtering performance, which verifies the effectiveness and feasibility of the IMOMEDA-TKEO.L is not given, which makes the execution time and iterations of the algorithm take a certain time (it is within the acceptable range). In future studies, we will focus on improving the calculation accuracy of fault period T for complex signals and optimizing the search range of filter length L. We will also explore the change rule between the filter length L and fault period T to improve the performance of MOMEDA.In the proposed method, the upper limit of the filter length


In a broader perspective, considering that polymicrobial communities are ubiquitous in nature, the understanding of these mechanisms is the groundwork to control and modulate bacterial response to any environmental condition. The reduction of the complex nature of communities of microorganisms to a single bacterial strain could not suffice to recapitulate the in vivo situation observed in mammals. Furthermore, some bacteria can adapt to various physiological or arduous environments embedding themselves in three-dimensional matrices, secluding from the external environment. Considering the increasing awareness that dynamic complex and dynamic population of microorganisms (microbiota), inhabiting different apparatuses, regulate different health states and protect against pathogen infections in a fragile and dynamic equilibrium, we underline the need to produce models to mimic the three-dimensional niches in which bacteria, and microorganisms in general, self-organize within a microbial consortium, strive and compete. This review mainly focuses, as a case study, to lung pathology-related dysbiosis and life-threatening diseases such as cystic fibrosis and bronchiectasis, where the co-presence of different bacteria and the altered 3D-environment, can be considered as worst-cases for chronic polymicrobial infections. We illustrate the state-of-art strategies used to study biofilms and bacterial niches in chronic infections, and multispecies ecological competition. Although far from the rendering of the 3D-environments and the polymicrobial nature of the infections, they represent the starting point to face their complexity. The increase of knowledge respect to the above aspects could positively affect the actual healthcare scenario. Indeed, infections are becoming a serious threat, due to the increasing bacterial resistance and the slow release of novel antibiotics on the market.The comprehension of the underlying mechanisms of the interactions within microbial communities represents a major challenge to be faced to control their outcome. Joint efforts of  So much so that when a certain antibiotic is screened to be potentially effective to treat a patient, the needed antibiotic concentration to be delivered to the infection site must be 100 to 1000 times higher than the antibiotic concentration assessed in the susceptibility test  and Gram-positive as Streptococcus pneumoniae and Staphylococcus aureus detected though standard microbiology  and local (lung) microbiomes to aid the host in combating a lung infection are proposed . One stractivity . This isactivity .Lactobacillus genus, toward the well-being of their host is not restrained to a distal body district  . Other fThe Center for Disease Control Biofilm reactor was named after their inventors  and alloFinally, more recently, microfluidic devices have been developed. They can be, to some extent, designed and tailored to each experimental need. These systems are physiologically closer to the conditions that microbial communities may encounter, thus providing a more reliable experimental set up . BesidesIn Situ Hybridization (FISH) or with species-specific primers with quantitative qPCR . However, the realization of fully functional and effective models is still challenging due to both the complexity of the Mathematical models have characterized single-strain bacterial genetic evolution  and anti–competition for space and resources;–amensalism: one organism is inhibited or destroyed while the other organism remains unaffected;–exploitation: one organism takes advantage of the other for resources scavenging and metabolism;–commensalism: neither of the bacteria benefits from the other or provokes any harm;–and cooperation: both the organisms benefit from the relation.Mathematical models aim to relate the physical characteristics of the studied system and its dynamic evolution starting from the initial conditions until the system reaches an equilibrium. In case there are two bacterial strains and they coexist at the equilibrium, a bistability condition is reached . ConsideConsidering a relevant case of polymicrobial infections as Cystic Fibrosis, clinical data has been gathered regarding the microbial populations present under different conditions  . In the in vivo models have been developed to test antibiotics efficacy: an essential requirement for such a model is the progression from a spontaneous bacterial infection to a chronic stage associated with biofilm formation  is one notable example of how the absence of well-established laboratory protocols for the realization of bacterial co-cultures drives researchers to choose and optimize different methods and culture conditions that are case-dependent systems, which challenges the comparison of data among different laboratories. One of the most pernicious aspects of this disease resides in the high number of species composing the infectious microbiome , resultiormation . In vitrS. aureus is the main colonizer of lung mucosa from the childhood of the cystic fibrosis patients, until the establishment of P. aeruginosa colonies, which tend to prevail in S. aureus. Despite the majority of in vitro models assuming P. aeruginosa to carry on complete eradication of S. aureus, it is uncommon to isolate both bacterial species from Cystic Fibrosis-infected lungs. In an in vitro model, it has been demonstrated that the culture in dual-species biofilm led to a consistent decrease of S. aureus relative abundance without achieving its complete eradication , another research studied the reactions triggered at a genetic level during a short period (hours) of co-culture. In particular, various genomic analyses revealed how the competition for resources drives the first responses. The encounter of the two pathogens stimulated the up-regulation of genes related to the optimization of all those factors functional to the adaptation of metabolism, to excel in the competition for resources  (Two different co-culture systems were able to provide a deeper and more specific model for the early interaction of the two strains. It was supposed that esources . Pure cuR (qPCR) . In thisR (qPCR) . Despitein vitro co-cultures recently also played a key role in studying the causes of several evolutive changes observed in P. aeruginosa adaptation to the CF lung environment, like downregulation of lipopolysaccharide (LPS) and quorum sensing (QS) factors. P. aeruginosa might undergo two highly represented mutations  was supposed to be caused by the culture condition (growth in a low oxygen environment) since it was present in both mono-cultured and co-cultured bacteria. The second mutation  appeared instead only in co-cultured P. aeruginosa, thus it has been thought to be an adaptive response to the presence of S. aureus. Such an experiment has been performed through the establishment of a peculiar protocol, in which P. aeruginosa has been evolved for at least 150 generations in liquid close multiwell cultures, both in the presence and absence of S. aureus, since interactions between P. aeruginosa and S. aureus are thought to be crucial in this process of adaptation. Serial dilutions of cellular suspensions were performed to mimic the in vivo situation in which a small amount of P. aeruginosa encounters pre-established large colonies of S. aureus.The realization of utations . The firP. aeruginosa might take place in CF patients’ lungs, characterized by a switch from the native phenotype toward the so-called “mucoid” phenotype, which owes its name to the overproduction of alginate  efficiency against S. aureus , co-cultures were prepared in a peculiar way called “biofilm disruption assay”  assay for planktonic cultures and as an alternative to the other methods to assess antibiotic resistance in biofilms.Another field taking advantage of . aureus . This phn assay” . This prP. aeruginosa to form biofilm was studied. This ability was investigated through scanning electron microscopy and confocal microscopy of different P. aeruginosa strains on an airway epithelial cell   . Moreovein vitro testing methods provide the state-of-the-art although they were not developed to study the crosstalk among different bacteria and the effect of the three-dimensional environment. They represent important but limited examples available up-to-now: two-species cultures and biofilms as unique examples of polymicrobial cultures and 3D-environments, respectively. New developments in co-cultures and the study of in vitro bacterial three-dimensional substrates are needed to overcome the frontier for the production of realistic tools to be employed in the study of bacterial interactions and drug discovery and to switch to the reproduction of 3D environments and polymicrobial cultures. Within this scenario, we envision the need for in vitro methods that could impact on diverse applicative sectors.The study of the biological features of polymicrobial infections needs to be considered to develop effective antimicrobial strategies useful to treat complex and chronic diseases like CF and bronchiectasis. The available MO and LZ worked on the draft of the document and revised the manuscript with the help of GG. LZ organized the bibliography. FB and SVU wrote the first draft of some sections. PP organized and focussed the contributions and coordinated the revision and the answers to the reviewers. PP and LV designed the content and the structure of the review and critically revised the manuscript in all stages. All authors contributed to the final manuscript revision, read and approved the submitted version.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


This article studies quantum interference effects and their influence on the electronic transport through a parallel triple quantum-dot system coupled to normal and superconducting leads in the linear response and non-equilibrium regime. We model the system by a triple impurity Anderson Hamiltonian including the Coulomb intra-dot correlations in all quantum-dots. Using the non-equilibrium Green’s function formalism, we calculate the Andreev conductance and the transmittance for energies within the superconductor gap. Our results show that the Andreev reflection spectra, both in the presence and absence of Coulomb interaction, reveal Fano and Dicke-like resonances in analogy to the Fano and Dicke effects in atomic physics. As one of the main results, we obtain that the charge shows abrupt changes due to the Dicke effect. The interference phenomenon, which resembles the well-known Dicke resonance in atomic physics, appears to be of particular importance13. It manifests itself by a narrow (subradiant) and a broad (superradiant) line-shape, spontaneously emitted by closely linked atoms, separated by a distance smaller than the wavelength of the emitted light14. In the electronic case, the decay rates (level broadening) are produced by the couplings between localized levels and the conduction channel, and the proximity and effective couplings give rise to fast (super-tunneling) and slow (sub-tunneling) modes15.Electron transport through multiple quantum dots systems exhibits exciting interference effects such as Fano24. On the other hand, Fano effect is another quantum interference phenomena that has been studied in quantum transport for some time. This effect is due to the quantum interference from localized states and continuum states. It yields characteristic asymmetric Fano lineshape, characterized by the Fano factor q, which is a measure of the coupling strength between the continuum and the localized state.The presence of the Dicke effect has been predicted theoretically and experimentally in different nanoscopic systems30. The Cooper pairs31 transport, along with interference effects among electrons and holes, gives rise to novel and interesting phenomena33. Within this context, several features of the Dicke effect have been considered under the presence of superconductor correlations. In particular, it has been found that the Dicke effect occurs in the Andreev conductance spectrum by modulating the interdot coupling and the side-dot levels35. A description of the relationship between the induced electron pairing and the Dicke effect has been studied36 by focusing specifically on how the electron pairing and correlation effects are affected by the side-attached quantum dots, ranging from the interferometric to the molecular limits. However, a detailed description of the interplay between Dicke effects and the charging effect induced by the Coulomb intra-dot correlations, is still missing.The electronic transport through quantum dots (QDs), double quantum dots (DQDs) and triple quantum dots (TQDs) coupled to normal/ferromagnetic and superconductor leads, has been studied recentlyIn this paper, we present an investigation of the influence of the Dicke and Fano effects on electronic transport through a coupled triple quantum dot system coupled to normal and superconducting leads in a linear and non-linear regime.37 to obtain the relevant Green’s functions from the equations of motion. Our results show that the Andreev reflection spectra, both in the presence and absence of Coulomb interaction, reveal Dicke and Fano-like resonances in analogy to their counterpart effects in atomic physics. As one of the main effects, we obtain that the charge shows abrupt changes due to the Dicke effect, which has not been shown in previous reports.In particular, we study the interplay between the proximity effect due to the superconducting lead and the conjunction of two phenomena: Dicke and Fano-Andreev reflections. We shall focus on the electronic properties of TQDs within the regime of low temperatures and sub-gap energies The paper is organized as follows. “Results” introduces the model and describes the general background of the transport properties in the TQD system. Next, “Summary” presents the corresponding numerical results in the equilibrium and non-equilibrium regime for both non-interacting and interacting cases. Finally, “Appendix” closes with a summary and general conclusions.The system under consideration consists of a single-level central quantum dot (QD2) attached to one normal metallic and one superconducting lead and two side quantum dots QD1 and QD3) as shown schematically in Fig.  and QD3 BCS Hamiltonian31 of the superconducting (right) lead and reads:The second term stands for the QDs, given bym-th quantum dot with m-th quantum dot while The third term is the Hamiltonian of three coupled N, S) and the central quantum dot (QD2) :QD2 and the electrode The last term in Eq. (1) describes tunneling of electrons between the leads , we can obtainV is the bias voltage, and In order to obtain the lesser Green’s function As one may notice by inspecting Eqs. , 24b) a a24b) anThe LDOS of the quantum dots come from of matrix elements of the retarded Green’s function matrix (electron components in Nambu space). The LDOS for dots 1, 2 and 3 are, respectively: Then, the total DOS of the triple quantum-dot is given by the addition of LDOS of each QD:r as the ratio of leads coupling t is very small in respect to t is very near to the value of V is the applied voltage. In addition, we assume in all our calculations that We now discuss the transport properties within the Andreev regime. Within this regime, the range for the Fermi energy and QD levels is restricted in the range of the superconductor gap, Let us first consider the limit where the Coulomb interaction is neglected, t (interferometric regime) and an even smaller value of r does not exceed a certain critical value, the differential conductance exhibits one very narrow central peak and two broader and smaller side peaks symmetrically located with respect to the zero energy. However, for higher values of r, we observe that dI/dV presents six peaks in this interval of energy. In other words, these peaks split as r increases from a specific critical value , we have studied the impact on the sub-gap transport properties by varying the coupling ratio with the leads, r, when the level spacing between two side-QDs and the central QD is small In summary, we have investigated the electronic transport through a triple quantum-dot device coupled to normal and superconductor leads. The main focus was to study the Andreev features appearing in both the differential conductance and QD occupations under Coulomb correlations in the QDs. Within the interferometric regime (small Additionally, we found that when we consider the intradot interaction, the Dicke spectrum split into two symmetric sets equidistant to the electron-hole symmetry point. On the other hand, dramatic changes in the charge are produced when a subradiant state falls below the Fermi energy. This property could be used to store charge in quantum dots.