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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]

In this study, the numerical solution of a high-speed train with several simplified bogies is investigated. The time-averaged flow field around the train, the surface pressure, and the aerodynamic forces on the train are discussed. The results reveal that a simpler bogie structure can achieve a higher underbody flow velocity and change fluctuations beneath the train owing to the resulting turbulence level. The simplification of bogies has a smaller effect on the side slipstream velocity and pressure compare to which in underbody, and at 3 m away from the centre of the track, the simplified bogie with wheels and a simple side frame used in this study obtains similar results to cases wherein more complex bogies are used. The surface pressure under the train is affected by bogie simplification, especially in the bogie cabin end area, resulting in aerodynamic drag and lift variations. If underbody flow or aerodynamic drag and lift forces are the focus of study, then the geometry of the centre region of the bogie, i.e. its main structures features, should be maintained in simplified models. • The unsteady aerodynamic performance of high-speed train was investigated with various bogie geometries. • The time-averaged flow fields and aerodynamic forces were analysed. • With simpler bogie structure, higher underbody flow velocity can be achieved. • At 3 m away from the centre of track, the effect of bogie structure is small. • The time-averaged aerodynamic force coefficients changed with simplifying bogie geometry. Abstract. [ABSTRACT FROM AUTHOR]

The drag reduction of commercial vehicles is the principal challenge to improving fuel saving and decreasing air pollution. Accordingly, several flow control devices, including gap fairing, cab roof fairing, boat tail, and side skirt were introduced to reduce the aerodynamic drag exerted on heavy vehicles. Although such devices have exhibited good drag reduction performance, their aerodynamic performance can be improved further. Moreover, a comprehensive and systematic research is required, including drag force measurement, flow field analysis and real-scale proving ground test, to evaluate the fuel consumption of heavy vehicles. In this study, the drag reduction effect of the aero full package (AFP) of a tractor–trailer, which integrated gap fairing, flap-type side skirt and lower inclined air deflector boat tail, was investigated through wind tunnel experiments and proving ground test. On the basis of the results of drag and PIV measurement, the integrated AFP was found to significantly modify the flow structure around the vehicle, thereby leading to a 26.5% reduction in drag coefficient compared with the reference tractor-trailer model (C D = 0.693). Furthermore, the proving ground test using real tractor–trailer verified that the integrated AFP provides 13.4% fuel saving. The present results will provide practical information for improving the aerodynamic performance and fuel efficiency of heavy vehicles. • The aero full package for tractor-trailer consist of gap fairing, flap-type side skirt and LIAD boat tail was newly proposed. • The aero full package reduces drag coefficient up to 26.5% compared with reference tractor-trailer. • The flow structures around tractor-trailer model is revealed through PIV experiments. • Real-scale proving ground test confirmed 13.4% fuel saving with the aero full package. [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]

An experimental study was conducted to investigate the dynamic ice accretion processes on bridge cables with different surface modifications (i.e., 1. Standard plain, 2. Pattern-indented surface, and 3. Helical fillets). An Icing Research Tunnel available at Iowa State University (i.e., ISU-IRT) was used to generate two typical icing conditions (i.e., dry rime icing vs. wet glaze icing). During the experiments, while a high-speed imaging system was used to capture the transient behaviors of surface water transport and dynamic ice accretion processes over the surfaces of the cable models, a high-accuracy force measurement system was utilized to quantify the dynamic aerodynamic loadings acting on the cable models under different icing conditions. It was found that the addition of surface features (i.e., pattern-indented surface and helical fillets) could dramatically influence the dynamic ice accretion process and the final topology of the ice structures accreted on the cable surfaces. While the ice accretion on the standard plain cable and the cable with helical fillets was found to reduce the drag forces by about 25%, the ice accretion on the pattern-indented cable model would induce about 30% greater aerodynamic drag force acting on the cable model. • Transient details of the icing processes on different bridge cable models were revealed. • Effects of different surface modifications on the ice accretion process were evaluated. • The drag force evolution of bridge cables during icing process were examined. [ABSTRACT FROM AUTHOR]

The use of passive ducting from the vehicle nose out of the front wheel opening was studied as a method of modifying the wake profile of a NASCAR Xfinity Series race vehicle to influence its effect on the drag of a trailing vehicle. Utilizing a fixed inlet area due to geometric constraints, exit angle, height and exit/inlet area ratio were explored using multi-vehicle Computational Fluid Dynamics (CFD) simulations and wind tunnel validation. Results indicate that appropriately positioned ducts reduce trailing car drag throughout the range of vehicle spacing studied; most significantly, it reduces the drag peak for the trailing car at approximately 1/4 to 1/2 car length spacing. Increased exit area is shown to enhance drag reduction for the trailing car at all vehicle spacings. Increasing exit angle closer to perpendicular to the direction of travel was shown to be effective as well, even being capable of eliminating the drag peak for the trailing car. Aerodynamic efficiency was increased over the non-ducted baseline configuration with the duct exit angle nearer to the direction of travel, while it was negatively impacted with the duct exit angle perpendicular to the direction of travel. Wind tunnel testing confirmed that aerodynamic efficiency improvement was due to an increase in negative lift under the car, while Kiel probe measurements validated the shape and positioning of the wake changes caused by the ducts. [ABSTRACT FROM AUTHOR]

Numerical simulations have been conducted to investigate the flow structures and the force generation of two-dimensional regular and isotoxal-star polygonal cylinders in low Reynolds number (≈150) cross-flow. The isotoxal-star polygons are essentially N -sided regular polygons with the addition of N V-grooves. Generally, transforming the regular polygon into the equivalent isotoxal-star polygon increases the normalized circulation, resulting in an increase in the magnitude of lift fluctuation and mean drag generation; this effect is sensitive to cylinder orientation. The influence of each V-groove on the overall flow structure and force generation of the isotoxal-star polygon is dependent on its placement and orientation, and the V-grooves fall into three main categories. Type I V-grooves are windward facing V-grooves containing the stagnation point; they are generally ineffective at enhancing force generation. Type II V-grooves are those upstream of the separation point but do not contain the stagnation point. They may enhance the magnitude of lift fluctuation if windward oriented, or yield negligible lift enhancement if oriented towards the transverse direction. Type III V-grooves are those downstream of the separation point, and they enhance the magnitude of lift fluctuation if they are oriented towards the vertical direction. • Numerical study of 2D regular and isotoxal-star polygonal cylinders in low Reynolds number (≈150) cross-flow. • The isotoxal-star polygons are essentially N -sided regular polygons with the addition of N V-grooves. • The influence of the V-grooves falls into three main categories. • Type I V-grooves are windward facing and contain the stagnation point. Generally, they do not enhance force generation. • Type II V-grooves are upstream of the separation point. They increase lift fluctuation if windward oriented. • Type III V-grooves are downstream of the separation point. They increase lift fluctuation if vertically oriented. [ABSTRACT FROM AUTHOR]

Predictions from embedded-LES are presented of a model of a section of a double-stacked freight wagon subjected to different local loading configurations. In total, 15 different upstream ( G f r o n t ) and downstream ( G b a s e ) gap spacings were simulated to characterise the change in the flow topology. The mean flow fields indicate that the inter-wagon flow undergoes a significant topology change over the range of G f r o n t = 1.77 W – 3.23 W (W = wagon width). For G f r o n t ≤ 1.77 W , the mean recirculating flow in the gap covers its entire length. In contrast, for G f r o n t ≳ 3.23 W , the complete wake closure for the upstream wagon occurs enabling the upstream shear layers to impinge on the entire downstream surface, in turn, increasing the rate of change of drag force as G f r o n t is increased. The change in the wake shedding frequency, directly affecting the base pressure and consequently the drag, due to the variation in G f r o n t and G b a s e , is presented. [ABSTRACT FROM AUTHOR]

Abstract As train speed increases, the aerodynamic drag reduction becomes increasingly significant. The aim of this paper is to find out appropriate bogie positions to reduce the drag of a high-speed train. In this paper, based on the three-dimensional steady incompressible Reynolds-averaged N-S equations and the Realizable k - ε two-equation turbulence model, the effects of bogie positions on the aerodynamic performance and near wake of the three-carriage high-speed train are presented. The mesh resolution and methodology are validated against wind tunnel test. The results show that the front bogie position of the head car has a significant impact on the aerodynamic performance of the head car, leading to different aerodynamic drag forces. When the bogie moves towards the rear by 1 m and 2 m, the aerodynamic drag forces of the head car reduce by 7.75% and 10.56%, and the total drag decreases by 5.57% and 6.58%, respectively. [ABSTRACT FROM AUTHOR]

Abstract Three minivan cars tested in real flow conditions are investigated through base pressure distribution, force balance and velocity measurements. Discontinuous transitions of permanent wake reversals and bistable dynamics between two well-defined states are observed varying ground clearance, pitch and yaw, with open and closed air-intake. These transitions prove that the cars undergo the same global z -instability as the square-back Ahmed body (M. Grandemange, M. Gohlke and O. Cadot, Physics of Fluids, 25 , 2013). The contribution of the global instability to the lift coefficient is estimated to 0.012 and 0.024 depending on the wake state. Eventually, the potential of direct passive control of the z -instability is demonstrated by improving from 4.6 % to 8.3 % the drag reduction obtained by closing the front air-intake of the car. [ABSTRACT FROM AUTHOR]

Abstract In the present study, flow behind a square cylinder is investigated using control rods. The control rods are placed in the shear layer and studied using particle image velocimetry, hotwire anemometry and flow visualization techniques. Experiments are conducted in a low-speed wind tunnel for three different Reynolds numbers 485, 805 and 1290. However, detailed analysis of flow is only presented at a Reynolds number of 485. The aim is to modify the wake behind a square cylinder for possible suppression of vortex Street and drag reduction. The control rod diameter is kept constant as 0.2 times of the cylinder diameter. Depending upon the gap between control rods and the cylinder, the flow structure, vortex dynamics, the distance of the shear layer roll up behind the cylinder is modified. An attempt is made to correlate the different flow structures with corresponding drag forces. Several parameters such as time-averaged drag coefficient, Strouhal number, recirculation length, fluctuating velocity, turbulence intensity, and Reynolds stress are measured. To understand the flow phenomena and its intricate structure, flow visualization images are also captured in the same setup with smoke visualization. Maximum up to 22% drag reduction is observed at a particular control rod position. Highlights • Wake modification behind a square cylinder using control rods. • Particle image velocimetry (PIV), hotwire anemometer and flow visualization techniques are used to characterize the flow. • Effect of control rod position on the flow structure, vortex shedding and the drag coefficient. • The maximum reduction in drag coefficient (up to 22%) is observed for particular control rod position in the shear layer. • The flow visualization images give a clear picture of flow structure modification and suppression of vortex shedding. [ABSTRACT FROM AUTHOR]

Abstract This study investigates the yaw angle effect on the aerodynamic performance of a hatchback model fitted with a roof spoiler. Although the aerodynamic performance of road vehicles is highly dependent on its yaw angle, it has rarely been considered in studies pertaining to roof spoiler applications. Two common types of spoilers are studied, namely, strip and wing types. The hatchback model is based on the Ahmed body at 35 ° slant angle. The yaw angle affects the aerodynamic performance of the hatchback model negatively regardless of the presence of a spoiler. However, the impact is more pronounced with the absence of a spoiler, particularly at higher yaw angles. Most importantly, the use of a spoiler prevents the bi-stability behaviour of flow which occurred in the model without the spoiler. Furthermore, the strip spoiler reduces both the drag and lift coefficients (C d and C l), whereas the wing type results in a greater reduction in C l while penalising C d. The results of the surface pressures indicate that the changes in the flow around the rear slanted body are the main factors affecting the aerodynamic performance of the hatchback model using the spoiler. Highlights • The non-symmetric bi-stable flow are detected for the Ahmed body at critical yaw angle. • Use of spoilers prevented bi-stable flow. • Strip spoiler reduced both the drag and lift coefficients. • Rear wing produced greater reduction in lift coefficient while penalising drag coefficient. [ABSTRACT FROM AUTHOR]

Abstract In a team time trial (TTT), the main strategy is based on drafting, where team members alternately take the lead while others ride behind the leading cyclist. TTTs can contain up to 9 riders of the same team. To the best of our knowledge, systematic aerodynamic studies of drafting groups from 2 up to 9 riders have not yet been published. Therefore, this paper presents such an analysis for up to 9 drafting cyclists in a single paceline, with wheel-to-wheel spacings d = 0.05, 0.15, 0.5, 1 and 5 m. A total of 47 Computational Fluid Dynamics (CFD) simulations are performed with the 3D RANS equations, standard k-ε model and scalable wall functions and validated with wind-tunnel measurements. In groups of up to 5 identical riders with d up to 1 m, the last rider has the lowest drag but this is not the case for larger groups. A closely drafting group of 7, 8 or 9 riders has an average drag that is about half that of an isolated rider. However, for much longer theoretical single pacelines, a staggered peloton configuration can yet be about two times more drag efficient. Graphical abstract Image Highlights • CFD simulations of aerodynamic drag of 2 up to 9 in-line drafting cyclists. • Different wheel-to-wheel spacings: d = 0.05 m, 0.15 m, 0.5 m, 1 m and 5 m. • CFD simulations are validated with wind tunnel measurements. • Traveling in paceline of 7, 8 or 9 riders is about twice more efficient than alone. • Average paceline drag is compared with other cycling configurations such as pelotons. [ABSTRACT FROM AUTHOR]

A steady slot suction near the free-end leading edge of a finite-length square cylinder was used to control its aerodynamic forces. The aspect ratio ( H / d , where H and d are the height and width of the cylinder, respectively) of the tested cylinder was 5. The tested suction ratio Q , which was defined as the ratio of the suction velocity at the slot ( U s ) to U ∞ , ranged from 0 to 4. It was found that the overall aerodynamic forces reduce quickly with the increase of Q from 0 to 1, then recovers slightly with Q from 1 to 2, and keep approximately constant with Q  ≥ 2. The maximum reduction of the overall mean drag, fluctuating drag and fluctuating lift occurs at Q  = 1, which reaches 3.6%, 17.8% and 45.5%, respectively. The steady slot suction reduces the aerodynamic forces not only near the free end, but also over the entire cylinder span. At Q  = 1, the shear flow emanating from the free-end leading edge reattaches on the free end forming a recirculation bubble. The enhanced momentum transport between the free-end shear flow and the wake suppresses the spanwise vortex shedding and aerodynamic forces the most effectively. [ABSTRACT FROM AUTHOR]

A cycling peloton is the main group of cyclists riding closely together to reduce aerodynamic drag and energy expenditure. Previous studies on small groups of in-line drafting cyclists showed reductions down to 70 to 50% the drag of an isolated rider at same speed and these values have also been used for pelotons. However, inside a tightly packed peloton with multiple rows of riders providing shelter, larger drag reductions can be expected. This paper systematically investigates the drag reductions in two pelotons of 121 cyclists. High-resolution CFD simulations are performed with the RANS equations and the Transition SST-k-ω model. The cyclist wall-adjacent cell size is 20 μm and the total cell count per peloton is nearly 3 billion. The simulations are validated by four wind-tunnel tests, including one with a peloton of 121 models. The results show that the drag of all cyclists in the peloton decreases compared to that of an isolated rider. In the mid rear of the peloton it reduces down to 5%–10% that of an isolated rider. This corresponds to an “equivalent cycling speed” that is 4.5 to 3.2 times less than the peloton speed. These results can be used to improve cycling strategies. [ABSTRACT FROM AUTHOR]

An athlete's riding posture is a key element for aerodynamic drag in cycling. Tandem cycling has the complication of having two athletes in close proximity to each other on a single tandem bicycle. The complex flow-field between the pilot and stoker in tandem cycling presents new challenges for aerodynamic optimisation. Aerodynamic drag acting on two tandem road race setups and two track time-trial setups were analysed with computational fluid dynamics (CFD) simulations. For validation purposes, wind tunnel measurements were designed providing drag measurements from both tandem athletes simultaneously using a quarter-scale model. A max drag force deviation of 4.9% was found between the wind tunnel experiments and CFD simulations of the quarter-scale geometry. Full-scale CFD simulations of upright, crouched, time-trial and frame-clench tandem setups were performed. The drag force experienced by individual athletes in all investigated tandem setups was compared to that of solo riders to enhance understanding of the aerodynamic interaction between both tandem athletes. The most aerodynamic tandem setup was found to be the frame-clench setup which is unique to tandem cycling and had a C D A of 0.286 m 2 , and could provide an advantage of 8.1 s over a standard time-trial setup for a 10 km time-trial event. [ABSTRACT FROM AUTHOR]

A consistent mesh refinement study, relating to the prediction of aerodynamic forces about an experimentally validated reference train geometry, is presented in this paper. The flow about a high-speed train has a multi-scale character which poses challenges for the design of computationally effective meshes. The purpose of this study is to assist in the development of guidelines for effective drag prediction of high-speed trains using numerical simulation. These guidelines should assist CFD practitioners by identifying the regions of the mesh that are critical for the correct estimation of drag as well as providing information on appropriate mesh characteristics, such as volume and surface element length scales. Numerical assessments are validated against an experimental drag measurement program and the extent to which RANS is sufficiently predictive for industrial design is discussed. The results obtained in the work suggest that the mesh about the train nose is essential for the proper assessment of the aerodynamic drag acting on the vehicle. [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 paper investigates the response to crosswind of a high-speed freight train for intermodal transportation, through wind tunnel tests. A 1:20 model of a freight train composed by an engine and two flat-car vehicles was instrumented with force balances to measure the aerodynamic coefficients of the flat-car plus container assembly and of the container alone. Aerodynamic coefficients strongly affects the train stability and the anchorage limits of the container itself. During the wind tunnel tests, eight different loading configurations were considered and aerodynamic coefficients were determined for yaw angles (i.e. angle between train and wind) ranging from 0° to 90°. Experimental results show the benefits in terms of drag reduction due to the presence of a laden vehicle upstream. As far as the rollover risk is concerned, the less critical condition is found for a vehicle preceded by a fully laden wagon and followed by an empty one. More in particular, at low yaw angles, the worst condition for a vehicle occurs when the wagon ahead is empty while, for yaw angles between 45° and 55°, which are the most critical for ‘low speed’ trains, the differences with respect to the other configurations reduce significantly. [ABSTRACT FROM AUTHOR]

The aerodynamic drag of a high-speed train can contribute significantly to its energy consumption. Hence, the purpose of this paper is to find out a new compound mode of bogie cut outs to achieve drag reduction for a Chinese high-speed train. In this paper, a Detached Eddy Simulation method based on the Realizable k - ε turbulence model was used to investigate the underbody flow features of high-speed trains with different compound modes in the angles of bogie cut outs at Re  = 1.85 × 10 6 . The time-averaged aerodynamic drag was compared with experimental data from wind tunnel tests. The results show that the DES simulations present high accuracy in predicting this kind of flow underneath the train body, and those numerical results closely agree with the experimental data. The variations of bogie cut outs' angles only cause the changes of flow structures around the bogies and in the wake. As a result, obtain different aerodynamic drag forces. Most of drag of the train is experienced by the streamlined head and all bogie regions. A new compound mode of bogie cut outs is proposed in the present paper, achieving 2.92% drag reduction for a three-car model. [ABSTRACT FROM AUTHOR]

Due to the infeasibility of using a wind tunnel to measure the aerodynamic drag coefficient of a high-speed train entering and passing a tunnel, a new moving model test method is proposed. A photoelectric sensor is fixed at the bottom of the train to scan the printed zebra stripes which are pasted along the track, from which the train displacement is obtained and the speed and acceleration can be calculated. The train aerodynamic drag coefficient before entering the tunnel and the drag coefficient of a train running inside the tunnel can be obtained based on Newton's Second Law. The results show that the aerodynamic drag coefficient of the train before entering the tunnel barely changes at different positions which can be considered as constant, but the value decreases as the train passes through the tunnel. This phenomenon is caused primarily by the transient airflow inside the tunnel. Comparisons with the verified numerical simulations are performed and good agreement with difference less than 7% is reached, which implies that the moving model method proposed in this paper is feasible and reliable. [ABSTRACT FROM AUTHOR]

Aerodynamic drag reduction of heavy vehicles is one of the main issues in fuel saving and environmental gas emission. In general, 50% of the total aerodynamic drag is induced from the region in the front surface of the vehicle and the gap between the tractor and trailer. Various forebody drag-reducing devices for tractor–trailer vehicles were introduced, however, conventional gap fairings or side extenders have technical limitations in reducing the aerodynamic drag exerting on the cab-roof space and gap between the tractor and trailer effectively. In this study, wind tunnel tests were conducted to investigate the drag reduction effects of the newly proposed gap fairings. The proposed gap fairing reduced drag by 16.4% at maximum as well as changed the gap length and deflecting angle. The aero cab fairing (ACF), which is a combination of cab-roof and gap fairings, and the extended aero cab fairing (EACF) considerably reduced the drag coefficient by approximately 11.1% and 17.5%, respectively. A particle image velocimetry was used to investigate flow characteristics, such as spatial distributions of mean velocity, vorticity, and turbulent kinetic energy around the scaled-down tractor–trailer model (1:17) with and without gap fairings to analyze the drag-reduction mechanism of the proposed gap fairings. The proposed gap fairings including ACF and EACF significantly reduced the mean velocity, vorticity in the gap region between the tractor and trailer, and the turbulent kinetic energy on the trailer's front surface. Results can provide useful information for improving the design of new additive flow control devices in reducing the aerodynamic drag of heavy vehicles. [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]

The bobsleigh bumper shape was optimized to reduce aerodynamic drag using three-dimensional Reynolds-averaged Navier–Stokes analysis and surrogate modeling. The k – ω shear stress transport turbulence model was used as turbulence closure. The numerical results for the drag coefficient of the bobsleigh showed reasonable agreements with the experimental data. On the basis of the results of a parametric study using six parameters related to the shape of the front and rear bumpers, three parameters, i.e., the distances between the bumpers and the ground and the leading angle of the front bumper, were selected as design variables, and the drag coefficient was employed as an objective function for optimization. Latin hypercube sampling was used to select twenty-five design points in the design space. A surrogate model of the objective function was constructed using response surface approximation. The aerodynamic drag of the bobsleigh was reduced by 3.08% with the optimization compared with the reference design. [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]

To analyze the difference of drag force coefficients of bridge sections obtained by the global force and pressure distribution methods, three-dimensional numerical simulations on the drag force coefficients of three typical sections are carried out. Firstly, after the numerical simulation method is validated, the percentage contribution of the friction drag force coefficient of the three sections under varying wind attack angles are investigated. Furthermore, the effect of the Reynolds number and handrails on the percentage contribution of the friction drag force coefficient is analyzed, and the results show that while the Reynolds number has an insignificant effect, the handrails’ effect is significant. Due to the importance of the contribution of the friction drag force for streamlined sections, more cases are simulated to investigate the percentage contribution of the friction drag force coefficient by varying the aspect ratio and fairing angle using the Sutong Bridge’s cross section as an example. The correction coefficients of the drag force coefficients obtained by the pressure distribution method for streamlined sections are obtained by the least square principle. The research conclusions can provide a reference for the differences of the drag force coefficient measured by the global force and pressure distribution methods for engineering practice. [ABSTRACT FROM AUTHOR]

The distribution variation and correlation of envelope interference factor ( EIF ) of the base torsion responses for principal building were studied by applying the rigid model wind tunnel test. The influence factor including different breadth ratio ( B r =0.4, 0.6, 0.8, 1.0, 1.2 and 1.4), height ratio ( H r =0.8, 1.0, 1.2 and 1.4), reduced velocity and approaching turbulence intensity were considered. Furthermore, the mechanism and occurrence condition of wake vortex-excited resonance were also studied. Results showed that the maximum value of EIF was 1.9 when vortex-excited resonance failed to happen. The correlation coefficient of EIF between B r =0.8 and B r =1.0 was 90.2%. Vortex-excited torsion resonance which induced from wake of interfering building occurred when B r =0.4, and the maximum value of EIF reached up to 2.98. With increased H r , the value of EIF increased. Good correlation was shown among different height ratios. For breadth ratios range from 0.3 to 0.5, it was suggested the reduced velocity should be out of the range of 3.33–5.56 to avoid occurring vortex-excited torsion resonance. [ABSTRACT FROM AUTHOR]

Investigating the aerodynamic reduction of drag in heavy vehicles, such as trucks or tractor-trailers, has considerable significance given the strong influence on related industries. The underbody flow that passes through the underside of heavy vehicles induces considerable drag while interacting with rolling wheels and other structures. Nonetheless, the reduction of drag caused by underbody flow has received less attention than that attributed to upper and forebody flows. Side skirts are common underbody drag-reduction devices that consist of straight panels curtaining the underspace between the front and rear wheels to control the underbody flow in the ground clearance. In this study, we propose two different types of side skirts with flaps or additional inclined inner panels to maximize drag reduction. Effects of these devices are quantitatively evaluated by wind tunnel tests and computational fluid dynamics analysis. In wind tunnel tests with 1/8 scaled-down vehicle models, drag coefficient is reduced by more than 5% for both side skirts. Effects of various physical dimensions or angle variations on drag reduction are determined. Large-eddy simulation (LES) estimated similar drag reduction with reduced vortical activities, loss of streamwise momentum, strength of turbulent kinetic energy and global pressure difference, compared to the case without side skirts. [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]

As a vehicle travels through a corner, the flowfield observed from the vehicle׳s frame of reference becomes curved. This condition results in the relative flow angle and freestream velocity changing both across the width and along the length of the body. Wall-resolved Large Eddy Simulations were used to simulate a simple vehicle shape through three different radii corners. The variable flow angle and acceleration affected the pressure distribution along either side of the body and caused an increase in the size of the outboard C-pillar vortex, and an inboard decrease. Furthermore, an outboard extension of the separation bubble at the bluff trailing face resulted in a gentler downwash angle off the backlight surface, with the opposite occurring inboard. At a Reynolds number of 1.7×10 6 , a 19.2% increase in aerodynamic drag occurred for a five car-length radius corner when compared to the straight-line condition. In addition, a yawing moment acted against the rotation of the body through the corner, and a side force acted towards the centre of the corner. An exponential trend related the curvature of a vehicle׳s path to the increase in aerodynamic drag, with a linearity exhibited for the increase in yawing moment and side force. [ABSTRACT FROM AUTHOR]

The effects of deflectors on the aerodynamic drag and near wake of an Ahmed model with a 25° slant angle were experimentally investigated. Experiments were conducted in a closed circuit low speed wind tunnel at a Reynolds number of 8.7×10 5 , based on the free stream oncoming velocity ( U ∞ ) and model length ( l ). The deflectors were placed at the side edges and leading edge of the slant. The height or width of the deflectors was 1%, 2% and 3% of l . Pressure distribution on the rear slanted surface and the vertical base was measured using pressure scanners. The flow velocities and static pressure were measured by Cobra probe at 0.5 l and l downstream in the near wake. Oil film flow visualization technique was deployed to capture the flow structure on the slant. In the absence of control, shear layer separation from the side edges and leading edge of the slant results in remarkable local low pressure regions on the slant, which contribute significantly to the aerodynamic drag. There is a D-shape separation bubble on the slant and the near wake is dominated by one pair of counter rotating trailing vortices. When the height of the deflector placed at the side edges of the slant is 1% of l , its effects on the near wake are negligibly small and the corresponding drag is increased slightly, compared to the uncontrolled case. With the deflector height increased to 2% and 3% of l , the D-shape separation bubble and the trailing vortices were noticeably suppressed, with the drag reduction reaching 3.5% and 7.2%, respectively. The deflector at the leading edge of slant changes the near wake of the model similar to that behind an Ahmed model of larger slant angle (e.g. 30° or 35°), eliminating the D-shape separation bubble on the slant. The corresponding drag reduction reaches 9.3%, 10.7% and 10.9% for the deflector width of 1%, 2% and 3% of l , respectively. For the 25° slant Ahmed model, the deflector placed at the leading edge of the slant is more efficient in reducing drag and suppressing the trailing vortices than those at the side edges of slant. [ABSTRACT FROM AUTHOR]

The flow field around a rigid finite square cylinder inclined away from the vertical direction was numerically investigated via large eddy simulations. The cylinder under investigation had a height-to-width ratio of 18 and the inclination angle, defined as the angle between the cylinder orientation and the vertical direction, varied from forward inclination to backward inclination. Pressure measurements and flow visualizations were taken in a wind tunnel to validate the numerical simulations. The forward inclination is found to enhance the downwash, which is initially observed behind the free end of the vertical cylinder, and amplify it to a downward axial flow. Conversely, the backward inclination promotes upwash, which originates behind the base of the vertical cylinder, to be an upward axial flow. On the other hand, the vertical cylinder produces two pairs of counter-rotating streamwise vortices (quadrupole wake) in its wake, but only one pair of vortices (dipole wake) is observed to form behind both the forward and backward inclined cylinders. Moreover, only a free-end vortex pair is exhibited behind the forward inclined cylinder whereas a base vortex pair exists behind the backward inclined cylinder. They are considered to generate the downward and upward axial flow respectively. It is anticipated that the presence of the axial flow significantly influence both the pedestrian-level wind conditions and the aerodynamic characteristics of the cylinder. [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]

Abstract: The optimization of the nose shape of a high-speed train entering a tunnel has been performed using genetic algorithms (GA). This optimization method requires the parameterization of each optimal candidate as a design vector. The geometrical parameterization of the nose has been defined using three design variables that include the most characteristic geometrical factors affecting the compression wave generated at the entry of the train and the aerodynamic drag of the train. A large set of three-dimensional, turbulent, compressible, unsteady simulations of realistic train models have been done, and this information has been used to fit a metamodel. The metamodel is used by the GA to evaluate each optimal candidate in a more efficient way. The optimal designs that minimize the maximum pressure gradient and the aerodynamic drag are in good agreement with the literature. To complete this single-objective optimization, a multi-objective optimization has been developed, and a Pareto front has been obtained. The use of metamodels has permitted to analyze the influence of each design variable. [Copyright &y& Elsevier]

Abstract: Multi-bundled conductors of large cross-section are becoming popular due to the increasing application of 1000kV ultra-high voltage (UHV) transmission lines. However, the aerodynamic characteristics of these conductors have not been investigated systematically, especially the global drag coefficient which is of a vital importance to determine the equivalent static wind load on the conductors. Therefore, a series of wind tunnel experiments were carried out to measure the global drag coefficients of multi-bundled conductors with the conductor diameter, bundle number, wind turbulence intensity and wind attack angles varied. Necessary measures were taken to ensure the accuracy of the test results. The results show that the global drag coefficients vary significantly with the wind attack angles and they decrease with the increase of the conductor diameter and bundle number. In any attack angle, the global drag coefficients of multi-bundled conductors are much smaller than those of a single conductor in the same wind velocity. The biggest global drag coefficients corresponding to different conditions investigated in this study are less than those recommended in the design codes worldwide. This suggests that the global drag coefficient of multi-bundled conductors specified in the current codes can probably be reduced by taking proper consideration of the shielding effect. This will lead to economic benefit in the construction of the UHV transmission lines. [Copyright &y& Elsevier]

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]

Abstract: Drag coefficient (C D ) in the urban roughness sublayer over industrialized and urban equatorial areas under low wind conditions are reported in this paper. An observation tower was set up in (1) the field of the Institusi Teknologi Tunku Abdul Rahman in the middle of the Prai Industrial Park (5°22′N, 100°23′E), employing a Gill UVW propeller anemometer and a temperature sensor placed at a height of 10m above the ground level and (2) on the top of a faculty building in Universiti Sains Malaysia (USM) (5°21′N, 100°18′E), employing a sonic anemometer at 18m above the ground level. Meteorological data were collected for three months in the years 2006 and 2010. Monin–Obukhov similarity theory using local scales was first tested at the sites studied and was found to be applicable. The relationships between C D and mean wind speed, V, local friction velocity, u ⁎l , and atmospheric stability, ζ l , are also discussed (subscript “l” denotes local). Generally, C D is strongly dependent and increased with u *l for both sites. C D was also confirmed to be influenced by atmospheric stability where it is at its maximum when u *l (and V) is large, which generally occurs in neutral atmospheric conditions. This relationship was seen at both sites, suggesting its generality. Lastly, the measured C DN value obtained was also used to calculate C GN , geostrophic drag coefficient (=1.9×10−3), which is similar to the reported values in the literature. [Copyright &y& Elsevier]