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A non-pneumatic tire, in which the tire pressure is replaced by an elastic support body, is called an airless tire with elastic support (ALTES). In this study, the differences in the aerodynamic characteristics between ALTES and pneumatic tire (PT) were comparatively investigated using URANS simulations based on the DrivAer model. It was found that the vehicle body drag coefficient of the vehicle equipped with ALTES was reduced by 27% compared to that of the vehicle equipped with PT and had a significant drag-reduction effect. The drag coefficient and moment coefficient of the ALTES wheels are 2 and 3.5 times those of the PT wheels, respectively, which offset the drag-reduction benefits. The drag coefficient and aerodynamic power loss of vehicles equipped with ALTES were 97–102% and 108.5%–114% of those of vehicles equipped with PT. It is reasonable and necessary to evaluate the aerodynamic performance of ALTES based on the aerodynamic power loss. By suppressing the wheel ventilation flow and reducing the wheel drag, the aerodynamic wheel cover can reduce the aerodynamic power loss of a vehicle equipped with ALTES by 12%, and the vehicle aerodynamic performance is comparable to that of a PT. • The aerodynamic performance of a non-pneumatic tire should take into account both drag and ventilation moment. • Non-pneumatic tires have a significant drag-reduction effect on the vehicle body. • Both of the drag and ventilation moment of non-pneumatic tires are higher than those of pneumatic tires. • The bluff-body shape and open-ventilated characteristics of non-pneumatic tires result in a higher level of aerodynamic loss. • Non-pneumatic tires equipped with an aerodynamic wheel cover can overcome their aerodynamic performance disadvantages. [ABSTRACT FROM AUTHOR]

Combining the improved delayed detached eddy simulation (IDDES) model and Ffowcs Williams-Hawkings (FW–H) equation, the potential of panhead leeward-side jet in pantograph aerodynamic noise control is numerically investigated. A typical double-strip pantograph is used as the baseline model. Two identical planar jets are arranged on the leeward side of the two strips to achieve active flow control. The results show that the planar jet can reduce the aerodynamic drag of the upstream strip and suppress its lift fluctuation, while the jet does not produce equivalent level of positive effects on the downstream strip. Benefiting from the suppression of lift fluctuation of the upstream strip, the far-field noise of the model equipped with jet achieves a 7 dB reduction in the main direction of panhead lift fluctuation, and the noise on the pantograph side is also reduced by 1–2 dB. The flow-field analysis reveals that the planar jet pushes the coherent flow structures in the panhead's wake downstream, which makes the high-intensity flow-field fluctuation away from the upstream strip. The absence of the positive effect of the jet on wake flow modification of the downstream strip can be attributed to the distinct inflow conditions of the two strips. • The panhead leeward jet can suppress the lift fluctuation of the upstream strip but has few effect on the downstream one. • The panhead leeward jet can reduce the pantograph aerodynamic noise. • The panhead leeward jet elongates the shear layer of the upstream strip and pushes the coherent structures downstream. • The different effects of the jet on the two strips' wake are due to their different inflow conditions. [ABSTRACT FROM AUTHOR]

The key issue for developing energy-saving high-speed trains is to optimize aerodynamic performance. Therefore, the strategy of installing shields beneath bogies (SBBs) is proposed to improve the aerodynamic performance of the train further. A high-speed train Case0 without SBB and two high-speed trains Case1 and Case2 with different SBBs are established. Numerical simulations are carried out using the IDDES method to analyze the differences in the aerodynamic characteristics of the three trains. According to the findings, the largest error in surface pressure between the wind tunnel test and numerical simulation is roughly 3.6%. The aerodynamic resistance of the head, middle, and tail car of Case1 is reduced by 29.7%, 7.3%, and −7.2% in comparison to Case0, respectively, and that of Case2 train is reduced by 56.7%, 32.1%, and 17.1%. The aerodynamic resistance of the whole Case1 and Case2 is reduced by 12.2% and 37.7%, respectively, with Case2 having a more significant drag reduction effect. The vortex shedding range of Case0 is approximately 1.35 times that of Case2. Moreover, the presence and type of the SBB will affect the flow field around and behind Bogie6, resulting in a 9° difference in the wake downwash angle between Case2 and Case0. A comprehensive analysis of the surface pressure and aerodynamic forces of the train, the pressure, velocity, turbulence kinetic energy, and vortex structure of the flow field reveals that Case 2 exhibits lower aerodynamic resistance and better characteristics in the flow field. The study's findings can serve as data support and reference for the research and development of next-generation high-speed trains and installing equipment on railways. • The shields beneath bogies (SBBs) are installed to improve the aerodynamic performance of the high-speed train further. • The drag force reduction effects of different forms of SBB on the carriages and the whole train are studied. • A comprehensive comparison is carried out on the train with different SBBs from the aerodynamic force and flow field. • The findings can support the development of next-generation high-speed trains and installing equipment on railways. [ABSTRACT FROM AUTHOR]

The recent popularity of electric vehicles and upcoming governmental regulations on emissions are forcing automotive manufacturers to build more efficient vehicles. An approach to improve the efficiency of a road vehicle is by reducing its aerodynamic resistance. One way of achieving this is through platooning, vehicles drive in close-proximity. Despite of the extensive literature available on platooning of multiple vehicle types, most studies have focused on the drag and flow effects around the truck only. This work aims to increase the understanding and recognize the usefulness of a truck-SUV platoon system as a whole using wind tunnel measurements on two scale models and CFD simulations. From the results of this study, it was found that the effects on the SUV are large and contribute to a significant portion of the drag reduction of the system, especially at short inter-vehicle distances where the drag of the car can become negative. The drag reduction for the truck is generally limited as the truck is a much larger vehicle. For the system, the results show that for very short distances it is beneficial for the truck to lead the platoon, while at longer separations there is no preferable order. • The combined drag of the platoon improves as the inter-vehicle distance decreases. • Very large drag reductions are possible for the SUV. • Significant difference in observed effects for the two vehicles due to their size. • Worse performance for truck when distances are very small. • The system is insensitive to yaw and vehicle order at longer separation distances. [ABSTRACT FROM AUTHOR]

An aerodynamic assessment is presented of two elite skaters, each in two different skating postures, at the ice-rink Thialf in Heerenveen, the Netherlands, via on-site Ring of Fire (RoF) measurements. This experimental approach adopts stereoscopic Particle Image Velocimetry (Stereo-PIV) to measure the flow upstream and downstream of the skaters. Both skaters transit through the RoF 20 times, 10 in each skating configurations. Athlete A skates with two hands on the back and with one arm on the back and one loose. Athlete B skates with one arm loose in a normal deep sit and in an extreme deep sit. All tests are performed at a nominal skating speed of 11 m/s. Firstly, the wake velocity fields of skater A with two hands on the back are presented throughout five different phases of the skate stroke. Significant variations in the distribution of the velocity deficit downstream of the athlete are observed, which suggest corresponding variations in the skater's aerodynamic drag. These velocity fields are also compared to literature and the similarities and differences are discussed between the flow around a static skater and that in the natural skating motion. Secondly, average streamwise velocity and vorticity fields for all 4 different postures are presented and compared. It is observed that for all cases the maximum velocity deficit in the wake is in the range of 0.45 ≤ u x * ≤ 0.55 and is located behind the lower back and upper legs. Furthermore, a characteristic vortex pair is observed downstream of the skater's hips for all four skating configurations, indicating it is independent of the athlete, the posture, and skating phase. The ensemble average aerodynamic drag is evaluated via a control volume approach along the wake behind the skater, accounting for the non-uniform flow conditions prior to the skater's passage. The uncertainty of the average drag measurements from the present RoF is about 5%. The results show that the optimization of the deep sit, e.g. the trunk and knee angle, yields a reduction by 7.5% of the skater's aerodynamic drag. Conversely, the difference in drag between two arms loose and one arm loose is not statistically significant. • Aerodynamic drag of ice-skaters measured on-site via the Ring of Fire approach. • The wake velocity fields for different skating phases have been shown. • Skating in a low-trunk posture yields a 7.5% reduction of drag area. • Skating with two arms on the back does not lead to a statistically significant drag reduction. [ABSTRACT FROM AUTHOR]

To reduce the aerodynamic drag of high-speed trains, this work proposes an air blowing configuration on the head and tail cars of high-speed trains. The variation in the aerodynamic drag and slipstream velocity is analyzed under different blowing velocities, and the flow mechanism for train aerodynamic performance alteration is explained. The results show that under the blowing speeds of U b = 0.05 U t , 0.10 U t , 0.15 U t , and 0.20 U t , where U t is the train speed, the total drag coefficient (C d) decreases by 5.81%, 10.78%, 13.70%, and 15.43% compared to the without-blowing case, respectively. However, with the increase in the blowing speed, the reduction trend of C d tends to be smoother; namely, the decrement ratio compared to the previous blowing speed for the head car is 9.08%, 0.11%, 0.60%, and 1.14% for U b = 0.05 U t , 0.10 U t , 0.15 U t , and 0.20 U t , respectively. The blowing measure generates an air gap between the coming flow and train surface, consequently causing a reduction in the viscous and pressure drag. In addition, the structure size and strength of the wake flow under different blowing cases show a decreasing trend from U b = 0.00 U t to 0.10 U t and then an increasing trend from U b = 0.10 U t to 0.20 U t. Thus, considering the blowing cost, efficiency, and flow structure evolution comprehensively, the case of U b = 0.10 U t is recommended. Under this blowing speed, the reduction ratio of the aerodynamic drag is 9.18%, 12.77%, 10.90%, and 10.78% for the head, middle, tail car, and total train, respectively. • A unique active air blowing design to reduce the aerodynamic drag of high-speed trains. • Significant reduction of the pressure drag on head middle, and tail cars by the proposed approach. • The obtained optimal blowing speed is 0.10 U t (U t is the train speed) with consideration of the blowing cost and efficiency. • Under the optimal blowing speed, the train's overall aerodynamic drag coefficient is reduced by 10.78%. [ABSTRACT FROM AUTHOR]

This work details the assessment of the performance of the drag-porosity model implemented in ARPS (Advanced Regional Prediction System) atmospheric Large-Eddy Simulation (LES) solver for simulating the atmospheric boundary layer developing over the urban canopy with comparison with literature. The flow within and above an idealized urban canopy consisting of a staggered array of cubes with various packing densities modeled with the drag-porosity approach immersed into a neutral, Coriolis-free atmospheric boundary layer at high Reynolds is investigated. Besides one-points statistics, particular interest was given to the ability of the model to reproduce the turbulent coherent structures and their characteristic scales. A detailed analysis of one-point statistics, one- and two-dimensional spectra and two-point correlation functions revealed the presence of typical structures and features found in wall-bounded turbulent flows (two-scale behavior in the roughness sublayer, ejections, sweeps, self-similar wall-attached large scale streaky motions, canopy-independent very large scale motions). Further investigation to identify the interaction mechanisms between large and small scales based on spectral filtering highlighted an interaction mechanism that resembles an amplitude modulation process, as observed in literature on wall-bounded flows. These findings therefore show that the proposed approach is able to reproduce all the key features of the flow developing over urban terrain. • Evaluation of a drag-porosity model for LES of flows over urban canopies. • Drag alone drives turbulent boundary layer processes at high Re above canopy-top. • Canopy density influence on the scale content in and above the roughness sublayer. [ABSTRACT FROM AUTHOR]

The experimental errors of the buffeting force caused by the scale effect become more significant when the reproduced turbulence scale is not very large in comparison with the characteristic dimension of the test model. From a fluid mechanics perspective, the scale effect can be divided into the three-dimensional (3D) effect in the spanwise direction and the distortion effect in the streamwise direction. A theoretical model of the 3D drag admittance is proposed to study the influence mechanism of the scale effect. Wind tunnel tests were conducted to clarify the effects of turbulence intensities (I u) and scales (L u / D) on the spatial distribution characteristics of drag force on a stationary train under crosswinds. The results indicate that the I u mainly influences the drag coefficient. In comparison, the L u / D is the key parameter in determining the scale effect on the drag force, which becomes more significant when the turbulence scale decreases. Based on the wind tunnel tests, an improved and double-exponential coherence model of drag force is proposed, resulting in a generalized 3D AAF model to rapidly modify the scale effect on the drag force of a stationary train. 2D AAF provides us with new insight into the effect of distortion. The results indicate that L u / D plays a dominant role in determining the 2D AAF in the high-frequency domain. A decrease of L u / D results in a more rapid drop-off of 2D AAF at a higher frequency caused by the distortion of small-scale vortices in the turbulence approaching a two-dimensional train. The proposed 2D AAF model is of high accuracy in modifying the distortion effect. [Display omitted] • The scale effect on the drag of a stationary train under crosswinds is studied. • A three-dimensional theoretical framework is proposed to study the scale effect. • An improved coherence model for the drag force of a stationary train is proposed. • A closed-form formula of 3D AAF of a stationary train is proposed. • Empirical models are proposed to modify the 3D and distortion effects. [ABSTRACT FROM AUTHOR]

Wind loading on lattice structures is studied locally by considering the applied forces on individual angle members. This work represents a comprehensive Computational Fluid Dynamics (CFD) analysis of wind loading on angle members, which includes a quantitative evaluation of the aerodynamic drag and lift loading and also a qualitative investigation of the wake characteristics behind the angle member. The focus is on the flow around the angle member cross-section which has two equal length sides at a right angle to each other. CFD has been used to investigate the flow around a single angle member characterized by its length D , thickness T , and angle of incidence to the oncoming flow. Both Reynolds-Averaged Navier–Stokes (RANS) and Large-Eddy Simulation (LES) are evaluated for a Reynolds number of 37,700, where the k- ϵ RANS and Wall-Adapting Local Eddy-viscosity (WALE) LES models are respectively used. The results are compared with the wind tunnel experiments reported in the literature (Wardlaw, 1967; Slater, 1969; Prud'homme et al., 2014) and the National Building Code of Canada (Canadian Commission on Building and Fire Codes and National Building Code of Canada, 2005), where the drag and lift coefficients at a range of incidence angles were reported. Quantitatively, the RANS performs well when compared to experiments for the aerodynamic coefficients at a range of incidence angles and less well for angles where a cavity style flow is formed and the separation becomes influential. This is in contrast to other bluff body cases where RANS does not perform so well. The LES is thus run at selected incidence angles for comparison and to better understand the flow where the RANS performs well and less well. Details of the turbulence in the wake are difficult to interpret when using RANS modeling, but this behavior is well illustrated with the LES model and details are reported here. • Evaluation of the aerodynamic drag and lift loading on angle members. • Investigation of the wake characteristics behind the angle member. • Both RANS and LES are evaluated for a Reynolds number of 37,700. • RANS performs well againt litterature at a range of incidence angles. • LES better understand the flow where the RANS performs less well. • This study constitutes a basis for evaluating the mask effect over multiple members. [ABSTRACT FROM AUTHOR]

The effect of drafting on the aerodynamic drag of a long-track speed skater is investigated in-field, at the 400m ice-rink of Thialf, Heerenveen. The Ring of Fire system is used to measure the flow downstream of an elite, isolated skater at approximately 11 m/s, transiting repeatedly through a tunnel filled with Helium-filled soap bubble flow tracers. Large-scale stereoscopic particle image velocimetry is used at an acquisition frequency of 500 Hz to obtain the near to far wake up to 11 m distance behind the skater. Over these 11 m, the center of gravity of the wake can shift up to 10 cm laterally, depending on the phase of the skating motion, and it moves about 15 cm to the floor. The former suggests that a trailing skater should slightly adapt its trajectory to achieve the lowest aerodynamic drag by drafting. The drag reduction of a trailing skater is estimated from the measurements on the isolated rider, assuming that the trailing rider's drag reduction only stems from the loss in total pressure in the wake of the first rider. The drag reduction is obtained with varying lateral and longitudinal distance between the leading and hypothetical trailing rider. It is observed that the peak reduction (∼40%) steeply decays with increasing lateral offset: at an offset of 50 cm the reduction is negligible. Instead, with increasing longitudinal offset, the decay is more gradual: at a distance of 11 m the reduction of 17% remains significant. The in-field estimations of the drag reduction are supported by wind tunnel measurements conducted on scaled skater models. Finally, the results obtained on the ice-rink indicate that a trailing skater should follow a slightly wider trajectory of about 20 cm, in comparison to the leading skater, to achieve the peak drag reduction during the entire skating stroke. • The air flow in the wake of a transiting skater is measured on a 400-m ice rink using the Ring-of-Fire. • The wake's center of gravity can shift 10 cm laterally and 15 cm to the ice floor 11 m downstream of skater. • The aerodynamic drag reduction of a hypothetical trailing skater is estimated from the in-field wake velocity measurements. • The in-field estimated drag reductions are compared to wind tunnel experiments on scaled skater models. • For minimum aerodynamic drag, a trailing skater should follow a slightly wider trajectory, in comparison to the leading one. [ABSTRACT FROM AUTHOR]

The significant increase in aerodynamic drag is still a key problem restricting the further speed increase of maglev trains. The Improved Detached Eddy Simulation (IDDES) turbulence model based on the Shear Stress Transport (SST) k - ω model was employed to explore the potential and effect of blowing/suction control on the aerodynamic drag reduction of the maglev train. In this study, the aerodynamic characteristics of the train's wake region, and the interference mechanism of blowing/suction on the flow field were studied. The results indicate that the best drag reduction rate ζ d of 8.69% of the train and the lift reduction rate δ l of 30.4% of the tail car are achieved when blowing along the normal direction of the surface at 0.5 times of the train speed at the separation region near the tail nose. In this case, the ζ d and δ l grow with the increase of blowing speed; however, the growth rate of ζ d decreases when the blowing speed reaches 0.2 times to 0.5 times of the train speed. Studies in this paper propose an effective active flow control method to reduce the train aerodynamic drag, and provide theoretical guidance for the development of drag reduction technology for maglev trains. • The flow field characteristics of the maglev train near the wake area were obtained. • The blowing/suction control method was proposed. • The interference mechanism and drag reduction effect of the blowing/suction control method on the maglev train were examined. [ABSTRACT FROM AUTHOR]

The safety of high-speed trains is considerably compromised by increasing speed trends. Thus, braking plate technology has been applied to high-speed trains. The purpose of this study is to clarify the evolution law of the flow field of the vehicle during the opening of the braking plate and analyze the influence of the plate movement on the aerodynamic performance of the train. In this study, the flow field was computed using incompressible Navier–Stokes equations and the shear-stress transport (SST) k –ω turbulence model, and the unsteady flow over the opening brake plates was simulated using moving overlapping grids and dual time-stepping. The numerical method was verified through comparison with wind tunnel data (error <8%). The results reveal that the upstream braking plate significantly decreases the aerodynamic forces of the downstream plate during opening of the plates and causes the aerodynamic drag of the downstream braking plate to fluctuate significantly when it increases. The operation of the braking plate produces a small increase in the drag force of the train body (2.6%), but it significantly decreases the lift force of the train body (by up to 94%), especially during the opening of the braking plates. The flow field in the upper part of the train is significantly changed by the opening of the braking plate. In particular, the pressure in the cavity of the braking device changes sharply, and the surface is subjected to a large pulse pressure. • The maximum aerodynamic forces of the braking plates appeared during opening. • Effect of opening braking plate on aerodynamic drag and lift of train body. • The evolution of the flow in the upper part of train during the opening of plate. • Compared with upstream plate, the flow field of downstream plate are more complex. [ABSTRACT FROM AUTHOR]

Societies rising environmental awareness in the context of advancing climate change is continuously shifting the focus of automotive development to vehicle efficiency and thus also to aerodynamic resistance. However, customers favor the familiar vehicle aesthetics, so research and development must find new solutions to reduce drag based on a given body shape. Dimple patterns extruded outward from the vehicle skin are considered in this study with the aim of reducing flow separation and thus lowering drag. They are applied to the realistic and already optimized notchback DrivAer vehicle model. Delayed detached-eddy simulations validated by experiments are used to assess the potential of this concept. In order to see the dependence of the concept on essential parameters, the location, size as well as height of the dimple pattern are varied in this study. All applied patterns on the rear window of the DrivAer model reduce drag and lift by up to 1.9% and 19.2%, respectively, compared to the reference geometry without dimple pattern. A large-scale dimple pattern is less sensitive with respect to the dimple height, whereas a smaller pattern exhibits more significant variations with respect to drag and downforce as the dimple height is varied. Compared to conventional vortex generators, the dimple pattern approach shows less potential to increase downforce, but allows a significant reduction in drag compared to the classical approach, with the consequence of increasing efficiency and reducing emissions. • A dimple pattern does not increase the friction component of drag and is capable of significantly increasing vorticity. • Every applied dimple pattern reduced lift but only patterns on the rear window were able to reduce drag. • The use of dimple patterns on the DrivAer body results in almost complete elimination of detachment on the rear window. • Dimple patterns on the roof increase the pressure on it, but have a strong negative influence on the pressure distribution downstream. [ABSTRACT FROM AUTHOR]

A full-scale 48-foot shipping container was instrumented with surface pressure taps and loaded onto a number of single- and double-stacked container freight trains. Surface pressure data enabled the container pressure drag coefficient to be evaluated for a range of different train loading configurations, container positions along the train, and atmospheric wind conditions. Field-based measurements show that for low and high crosswind conditions surface pressure distributions measured on the front face of the instrumented container are in good agreement with those reported in past studies. However, the magnitude of the pressure drag coefficient was found to be typically 50% lower for all loading cases analysed compared with previous seemingly analogous wind-tunnel and numerical investigations. For high crosswind conditions, the drag coefficient was found to increase and correlated well with the level of asymmetry observed in the measured pressure distributions. This was true for all cases regardless of the position along the length of the train. In the absence of direct information of the incident free-stream wind conditions, the level of asymmetry in the pressure distributions was found to provide a viable indirect method for assessing the impact that crosswinds have on the aerodynamic drag of freight trains. • A full-scale shipping container was instrumented with surface pressure taps. • Pressure drag was evaluated for different loading configurations and container locations. • Drag coefficient was 50% lower compared with previous investigations. • An indirect method for assessing the effect of wind conditions on drag was established. • Estimates for aerodynamic efficiency and loading configuration considerations are made. [ABSTRACT FROM AUTHOR]

This study investigates the effect of a deflector on Ahmed model drag and the flow on the slant during steady crosswind conditions by experimental methods at a based-height Reynolds number of 2.45 × 105. The length deflector is 0.09 times the length of the slant. The deflector was fixed at the leading edge of the slant with an upward angle of 5° to the horizontal axis. Force, pressure, and global skin-friction measurements were conducted to understand the relation between drag, pressure distribution, and flow fields on the slant surface. The results showed that the deflector reduces aerodynamic drag at low yaw angles. This result is due to the breakdown structure of longitudinal vortexes and the separation bubble, which leads to fully separated flow on the leading edge of the slant. However, at yaw angles above 8°, a large reversed flow region forms on the slant and the drag of the model increases. A similar flow structure on the slant is observed for yaw angles of 12° and 15°. Conversely, the model lift is reduced with the deflector at all yaw angles tested. Detailed pressure distribution and complex flow structure on the slant provide insight in this study. • Flow on slant for models with and without deflectors during cross-wind conditions is studied. • Adding deflector leads to break down the separation bubble and longitudinal vortexes on slant at low yaw angles. • Separation bubble and longitudinal vortexes occur again at yaw angles above 8° and drag of the model increases. • Relation between complex flow fields, pressure distribution on the slant and drag of the model is investigated. [ABSTRACT FROM AUTHOR]

This study investigated the variations in aerodynamic drag and the pressure wave effects caused by more than two pods operating in a Hyperloop tube. Three speeds, i.e., 100, 200, and 300 m/s, and two distances between two separate pods, i.e., 43 and 86 m, were simulated using SST k-ω turbulence model. The results indicate that drag of single- and multi-pod systems have similar tendencies that increase with increasing pod speed. For pod speed exceeding the critical Mach number, irrespective of the connectedness or separation of the pods, the last pod always experiences the highest drag. Otherwise, at lower speeds, the differences between the drag acting on each of the pods is negligible. The distance between two pods has a minor influence of less than 4% on the drag. The effects of two different pod shapes, i.e. rectangular and semicircular, on the drag of three connected pods are also small. The average drag per pod was calculated, and it was observed that when more pods were operated, the average drag reduced significantly. The operation of the connected-pod system creates a slightly lower drag than that of a separate-pod system. • Multipod operations were simulated for three different pod speeds. • The last pod experiences the highest drag for both connected and separated pods. • The distance between two separated pods has less than 4% influence on drag. • The average drag per pod decreases with increasing number of pods. • The connected pod system has lower average drag than the separated pod system. [ABSTRACT FROM AUTHOR]

For a passenger vehicle, approximately 25% of the total aerodynamic drag originates from the wheels, making the aerodynamics of wheels a significant factor for the overall performance of a vehicle. To understand the complex flow field created by the rotational condition and geometry of these bluff-bodies, numerical simulations are often used. However, computations are frequently performed in domains that replicate open road conditions, differing from the conditions of wind tunnels. Therefore, to properly validate a CFD procedure and to correlate physical tests to numerical results, interference effects of the wind tunnel need to be investigated and their impact on the aerodynamics of wheels analysed and compared to that of open road calculations. In this study, numerical simulations on the DrivAer model were performed using different tyres and rims in both open road conditions and with the inclusion of a detailed model of a slotted walls wind tunnel. The results of the simulations are compared to experimental data, consisting of forces and flow field measurements. It was found that the inclusion of the wind tunnel in the computations improves the prediction of the flow fields, resulting in better prediction of both the absolute drag values and the drag deltas between configurations. • The drag prediction improves with the wind tunnel geometry in the simulations. • The largest discrepancies between domains occur for tyres with lateral grooves. • Open road simulations overpredict the drag difference between open and closed rims. • The wind tunnel blockage alters separations at the tyre and rim. • The ground simulation changes the interaction between the front and the rear wheels. [ABSTRACT FROM AUTHOR]

Adverse weather conditions in the northern regions during winter might lead to accumulation of ice or snow on bridge cables and consequently trigger ice shedding. The falling ice poses a risk for the traffic and pedestrians below, which often results in bridge closures and/or insurance claims. A passive surface-modifying device in the form of a steel wire mesh tautened on the surface of a cable sheath was previously found to substantially reduce the risk associated with falling ice through improved ice retention. In this paper, the cross-flow aerodynamic behaviour of bridge cables fitted with steel wire meshes is investigated in detail. The wire mesh generally resulted in higher levels of aerodynamic drag when compared with a plain cable sheath, although the Reynolds number dependence was highly reduced. Several angles of attack were investigated and no instability issues with respect to the across-wind vibrations were observed for any of the tested wire mesh configurations based on the quasi-steady theory. Even though vortex shedding was still present in all cases, the intensity of the fluctuating lift forces was much lower than that of the plain cable. To investigate basic aspects of the flow structure produced by the wire mesh, two flow visualisation techniques, combined with streamwise wind speed measurements in the near wake, were further used and are discussed. • Innovative surface modifications of bridge cables are investigated in a wind tunnel. • Three wire mesh configurations with a different number of wire ropes are compared. • The wire mesh was designed to reduce the risk of ice shedding from bridge cables. • The test methods include static tests, wake measurements and flow visualisation. • Aerodynamic damping is evaluated using quasi-steady theory. [ABSTRACT FROM AUTHOR]

The improved delayed detached-eddy simulation (IDDES) method is used to investigate the aerodynamic performance of a high-speed train with cowcatchers having different ground clearances. The numerical results were verified by wind tunnel tests, and validation on the Reynolds number and geometry generality were also conducted. It was proved that the ground clearance under the cowcatcher has a significant effect on the train's time-averaged and transient aerodynamic forces. A smaller ground clearance reduces the aerodynamic drag of the front part of the train body, strength the negative lift while increases the drag of the middle and rear parts as well as the positive lift. Smaller ground clearance also significantly reduces the aerodynamic of its neighboring downstream components. A lower ground clearance makes its power spectral (PS) value stronger and increases the dominant frequency of the neighboring downstream components. The flow pattern on the train body and its surroundings will also be locally affected by the ground clearance, such as slipstream and vorticity distribution. • Ground clearance of cowcatcher is a significant factor to train aerodynamics. • Smaller ground clearance reduces the aerodynamic drag of head car while increase others. • Flow patterns are locally affected by the ground clearance. • The conclusions about aerodynamic drag are generical to other high-speed trains. [ABSTRACT FROM AUTHOR]

The aerodynamic drag of a trailing cyclists in a tandem formation is investigated at different lateral and longitudinal separations. A Ring of Fire (RoF) experiment is conducted at the Tom Dumoulin bike park of Sittard-Geleen in the Netherlands. The method is based on stereoscopic Particle Image Velocimetry (Stereo-PIV) measurements followed by invoking the conservation of momentum expressed in a control volume to evaluate the drag force of the cyclists. Three cyclists perform a series of individual tests as well as four different drafting tests, varying their order in the group. All tests were performed at a nominal riding speed of 45 km/h; the longitudinal displacement of the drafters varied between 0.32 m and 0.85 m and the lateral displacement varied between ±0.20 m among different runs. The results from the RoF measurements show the flow field interactions between the two drafting cyclists as well as the wake of the second cyclist. It is observed that the amount of drag reduction for the trailing rider is mainly caused by the change in inflow conditions. The drag reductions of the trailing cyclist are in the range from 27% to 66% depending on the longitudinal and lateral separation from the leading rider. The aerodynamic advantage of the drafting rider decreases with increasing lateral and longitudinal separation between riders, with the lateral separation found to be more relevant. Furthermore, based on the analysis of the individual wakes, the drag reductions found by the RoF, and the drag reduction measured by Barry et al. (2014), a model is introduced that predicts the aerodynamic gain of the trailing rider based on his or her position with respect to the leading rider. Validation of the model with data from literature shows that in the near wake the model prediction is in line with literature, with an overestimation of the drag reduction when the longitudinal distance is between 0.1 m and 0.3 m. • The drag reduction felt by a trailing cyclist in a tandem configuration is measured by the Ring of Fire. • The drag reduction decreases as the lateral and longitudinal separation increases, where the former one is the most relevant. • Flow visualizations show that the drag reduction of the trailing rider can be ascribed to the changing inflow conditions. • A mathematical expression is proposed that describes a model to predict the drag reduction under drafting conditions. [ABSTRACT FROM AUTHOR]

This paper describes an investigation of the aerodynamic performance of a Hyperloop pod equipped with an axial compressor using CFD simulation. The compressor is expected to reduce the drag if the operational speed of the pod exceeds the Kantrowitz Limit (KL). To evaluate the effectiveness of the compressor, two types of pod have been designed: those with and those without a compressor. A validation study was undertaken for supersonic flow around a cylindrical body in a wind tunnel. Several combinations of pod Mach number and blockage ratios were examined. The axial compressor was modeled with the fan interface model and our original method, which we call the source terms model. Results are similar, but our model has the advantage that it can be applied to higher pressure ratios without inducing numerical instabilities. The power consumption was reduced owing to the compressor by decreasing the pressure accumulated in front of the pod. The effectiveness of the compressor was shown to be greater for conditions well above the KL, with a maximum power reduction of 47.5%. Simulations performed half-scale, and at a different system pressure, showed that the effectiveness of the compressor was not greatly influenced by these parameters. [Display omitted] • Pods with and without an axial compressor are compared by CFD. • The compressor is modeled by the source term in NS equations. • The pressure upstream of the pod is reduced by the axial compressor. • Maximum power reduction is estimated to be 48%. [ABSTRACT FROM AUTHOR]

Double-unit trains composed of the same type of high-speed trains (HSTs) are relatively common. The connection of different types of trains, however, has been rarely studied. Therefore, we adopted an improved delayed detached-eddy simulation (IDDES) method to investigate the aerodynamic characteristics on double-unit trains formed by different types of HSTs. Two standardized HSTs—Train A and Train B at present in China—are used to form four different cases (A-A, A-B, B-A, and B-B). We conducted algorithm validation and grid independence to ensure the reliability of numerical results. The results showed the drag of case B-B was smaller when the same HST units were connected, whereas the drag of case A-B was smaller when the different HST units were connected. Downstream the coupling region, the structures of vortex shedding presented obvious differences for each case, which further led to different fluctuation intensities in wake vortices. The order of the height of the wake vortices was as follows: h 1(A-A) > h 3(B-A) > h 2(A-B) > h 4(B-B). The tail of the upstream vehicle had a dominant influence on the slipstream peak in the coupling region. • The minimum drag of double-unit train formations with different combinations is obtained. • The discontinuous coupling region has an obvious influence on the downstream vortices. • The tail of the upstream vehicle has a dominant influence on the slipstream peak in the coupling region. [ABSTRACT FROM AUTHOR]

A wind-tunnel study has been conducted to evaluate changes in the aerodynamic behaviour of stay cables of a bridge due to ice accretion from freezing rain and, due to different cable surface geometries. Numerical simulations were performed to predict the three-dimensional ice shapes resulting from freezing rain for a variation of environmental conditions: wind speed, wind direction, air temperature, velocity of precipitation and duration of precipitation. Stay-cable models were fabricated using laser selective sintering for three different surface geometries (double helical fillet, concentric rings, and double helical strakes) with ice formed under the same environmental conditions. Additional models with the helical fillet geometry were fabricated to evaluate the effect of the air temperature at which ice was formed, the effect of wind direction during the precipitation event and the effect of the duration of the precipitation. The aerodynamic drag and lift forces acting on the models were measured for different wind speeds and cable-wind angles. The overall study evaluated the mean aerodynamic force coefficients of ten models in smooth flow, and two of the models were also tested in turbulent flow. The critical Reynolds number regime for the models of stay cable with ice accretion shapes occurred at a lower Reynolds number (below 100,000) than for the un-iced cables and the drag coefficient was less sensitive to increases in Reynolds numbers than the equivalent un-iced cables. These effects were attributed to the combined role of the ice-shape in determining the separation point combined with the higher surface roughness of the ice. An important observation was that for the three surface geometries and the range of test conditions that were considered, the maximum drag coefficients for ice-accreted models ranged from approximately 1.1 to 1.3, and were up to 50% greater than those observed without ice. The model with helical strakes and ice accretion exhibited the highest drag coefficient and exhibited more variation in lift coefficient with changes in Reynolds number than the other two cable surface geometries. The aerodynamic forces of an ice-accreted model followed similar trends in smooth and turbulent flow, but the drag coefficients were greater in turbulent flow. The presence and shape of the ice appeared to have a more dominant effect on the separation behaviour of the models than the presence of turbulence (for an ice-accreted model). The aerodynamic coefficients did not change greatly between the cases where ice was accreted at − 0. 5 ° C and − 1. 5 ° C due to the similar ice shape and thickness near the separation locations. The ice shape formed at − 5 ° C was thicker but more localized and resulted in drag and lift coefficients that had greater variation with changes in the cable-wind plane. The effect of ice duration from 10 h to 20 h resulted in a increased variation in mean lift coefficient behaviour due to the asymmetric shape that resulted from the ice accretion parameters. • Stay cable aerodynamics were evaluated with and without freezing rain precipitation • Conditions included many geometries, temperatures, precipitation angles, and time • The critical Reynolds number regime occurred below Re = 100,000 • Drag coefficients were greater in turbulent flow than in smooth flow • Drag coefficients increased by up to 50% with ice accretion • The impingement point varied based on the precipitation conditions • Site-specic considerations on icing and aerodynamics should be considered [ABSTRACT FROM AUTHOR]

The current research attempts to describe the association between the aerodynamic drag and flow structures induced by train running in time-averaged and time-dependant views. A ICE2 train model is used as the research object, with the flow field solved by Large Eddy Simulation (LES). The validation is carried out through comparison with existing research data, and grid independence study in terms of time-averaged and second-order statistics. Numerical results are firstly presented by description of the time-averaged near wake flow topology based on surface flow visualization. Then, the wake integration method is used both time-averaged and instantaneously to link the aerodynamic force with integrated flow quantities on different wake planes: from the time-averaged perspective, the contribution of different flow terms to total drag is presented and compared; from the instantaneous perspective, the overall wake variation tendency is linked to the fluctuating component of aerodynamic force, based on the wake convection velocity and downstream distance of wake planes. The dynamic features of flow quantities in different locations are presented, with cross-correlation method used to describe the correlation between fluctuating aerodynamic drag and wake motion. Approaches and findings presented in the current study could serve as fundamentals which support future study. • Large eddy simulation employed to solve flow field around a generic ICE2 high-speed train model. • Surface flow visualization techniques utilized to illustrate the time-averaged near wake flow topology. • Wake integration method applied to link the aerodynamic force with integrated flow quantities on different wake planes. • Dynamic features of flow quantities presented, with cross-correlation method used to describe correlation between aerodynamic drag and wake motion. [ABSTRACT FROM AUTHOR]

Vacuum Tube Transportation (VTT) system utilizes the low-pressure environment to achieve aerodynamic drag reduction and reduce the energy consumption of high-speed trains. Within this system, even for trains operating at subsonic speed, the flow acceleration around the streamlined train may leads to local supersonic flow. The appearance of the supersonic flow will dramatically alter the flow field structure and aerodynamic load of the high-speed train. Therefore, this study investigates two types of representative flow fields under different blockage ratio through the improved delayed detached eddy simulation (IDDES) method. The initialization, propagation, reflection and intersection process of shock waves in transonic case are captured and analyzed. It is shown that, the boundary layer thickness and width of vortex group in wake has experienced a slight increase with the increase of vacuum level. Compared with the subsonic case, the occurrence of choked flow and shock wave in transonic flow field lead to a decrease of wake region's vortex group width. As a result, the total drag coefficient of the train is increased by approximately one order of magnitude, and the dominant St number of tail car's drag coefficient has shifted from 0.16 to 0.11. [ABSTRACT FROM AUTHOR]