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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]

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]

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 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]

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 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]

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]

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 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]

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]

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]

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]

Abstract: The passage of high speed trains causes aerodynamic effects in tunnels, and considerable pressure transients are generated because of the restricted airspace within the tunnel. This leads to passenger discomfort, noise surrounding the tunnel, resistance to train movement, and possible damage to the train body and tunnel facilities. For the real assessment of the aerodynamic pressures in the tunnel induced by the passing trains of the high speed rail in Taiwan, a series of field measurements were performed near the portal and the shaft or the adit of the tunnel during normal operation. The measurements were conducted for several train speeds, including the maximum operation speed of the high speed rail in Taiwan, which is 300km/h. Pressure sensors were deployed along the tunnel to investigate the propagation of pressure waves. The results show that the train nose entry/exit generated a compression wave propagating throughout the tunnel, resulting in a sharp increase in pressure. Conversely, the train tail entry/exit generated an expansion wave causing a pressure drop. The successive reflections of these pressure waves between both ends of the tunnel were observed. The pass-by of a train inside the tunnel also induced an immediate local pressure drop due to aerodynamic drag. Based on the measurement results, the spatial variation of the train-induced pressure peaks inside the tunnel is discussed. Furthermore, the relationship between the pressure peaks and the train speed is established, and the influence of the cross-sectional area of the tunnel is also presented. [Copyright &y& Elsevier]

The present study examines the possibilities of transferring drag measurement results on reduced-scale train models to the respective full-scale vehicle. A comprehensive experimental and numerical study of the boundary layer and skin friction along trains is performed, focusing on Reynolds number effects. The data are supplemented by an extensive literature study and compared with different approaches from flat plate theory. Good agreement can be found when using the appropriate empiric coefficients and boundary conditions. Simultaneously, the difficulties in determining the skin friction drag of trains due to three-dimensional effects and surface roughness become apparent. The ground simulation analysis, including the effects of ground roughness due to ballast and sleepers, reveals a significant effect of the ground conditions on the vehicle's aerodynamic drag. Additionally, the effects of elements mounted on the train roof are investigated for different upstream flow conditions. Finally, a scaling method is proposed to transfer drag results from model-scale to full-scale trains based on the findings. • Trackbed roughnesses strongly influence the underbody friction of trains. • Boundary layer profiles of trains can be approximated by the flat plate theory. • Reynolds number effects in train aerodynamics occur both at model and full-scale. • Roof elements can strongly alter the drag and wake of the train. [ABSTRACT FROM AUTHOR]

Based on 3- D, unsteady compressible Navier-Stokes equations and SST k - ω two-equation IDDES turbulence model, the effect of cross passage on aerodynamic characteristics of evacuated tube transportation is simulated by overset mesh technology. The flow structure, aerodynamic force and aerothermal environment in three train/tube configurations are studied. The results show that there is a linear relationship between the length of the disturbed region of normal shock wave and running time. Cross passage can reduce both aerodynamic drag and aerothermal environment in the case of a single train passing. The transport channel and diffusion channel are formed to transport the air from tube A (B) to tube B (A). However, the lateral force fluctuates greatly. After the train leaves the cross passage, the lateral force coefficient periodically decays and oscillates. While, in the case of two train crossing, the existence of the cross passage could aggravate pressure field and aerothermal environment in the tube. Besides, the aerodynamic drag coefficient fluctuates during the intersection of two trains and the lateral force coefficient of the train fluctuates greatly and the maximum lateral force coefficient of two trains crossing is 2.86 times that of a single train passing. • The overset mesh technology could simulate the train running in the tube. • There is a linear relationship between the length of the disturbed region and time. • Cross passage reduces both aerodynamic drag and heating in single train case. • Transport and diffusion channels are formed to transport air in single train case. • Cross passage aggravates aerodynamic performance in two trains crossing case. [ABSTRACT FROM AUTHOR]

Quadcopters are attracting a growing interest in many applications such as cargo delivery or surfaces inspection. These applications are often subjected to flights in the proximity of walls or ground that generate external forces and torques on the vehicle due to aerodynamic effects, because of what strong safety guarantees are required. In this research a methodology based on dynamic meshes was developed and applied to computational simulations to reproduce the flight of the drone over an obstacle. Thus, the effect of the ground proximity on the drone performance was assessed, and also its combination with the flow around the body at different translational velocities. The results shown a decrease of the drag force, and an increase of the lift and forward pitch moment due to the presence of the ground. These effects are magnified by the translational velocity, which also deviates the flow generated by the propellers and delays the interaction with the obstacle. Both in the approaching and leaving to the obstacle, increases of up to 60% in the pitch moment are observed. This sudden variations must be properly counteracted to guarantee the stability and safety of the drone operation. • The flight of a quadcopter over an obstacle was reproduced by CFD methodology. • The effect of ground proximity was assessed for single rotor and quadrotor UAV. • Ground effect reduces drag force and increases lift force and pitch moment. • Significant flow oscillations are noted close to ground at low translational speeds. • Strong changes of pitch moment are seen at the reaching and leaving of the obstacle. [ABSTRACT FROM AUTHOR]

The importance of aerodynamics in cycling is not a recent discovery. Already in the late 1800s it was recognized as a main source of resistance in cycling. This knowledge was only rediscovered in the late 1970s and 1980s, when aerodynamic concepts were applied to bicycle equipment and cyclist positions, leading to new world hour records and Olympic medals. The renewed interest for cycling aerodynamics is significantly growing with the production of a vast literature, focused on increasing the comprehension of cycling aerodynamics and on improving the aerodynamics of bicycle equipment. Finding the connection between the different subfields of cycling aerodynamics and linking new research with past discoveries is crucial to efficiently drive future studies. Therefore, the present paper provides a comprehensive review of the history and the state-of-the-art in cycling aerodynamics, focusing on one of its main aspects: the bicycle. First, a short history of the bicycle is presented. Next, some cycling power models are outlined and assessment methods for aerodynamic drag are discussed, along with their main advantages and disadvantages. The core of this review paper addresses the components constituting the bicycle: frame and tubes, wheels, handlebar and other equipment. Finally, some future perspectives on bicycle aerodynamics are provided. Image 1 • Aerodynamic development of bicycle and its main components throughout history. • Linked to historical evolution of the world hour record. • Specific focus on frame, wheels and handlebar. • Providing future perspectives for research and development. [ABSTRACT FROM AUTHOR]

The geometries of bogies have a significant influence on the aerodynamic performances of high-speed trains (HSTs) in crosswinds, especially for wind tunnel tests. The delayed detached-eddy simulation (DDES) method was used to calculate the aerodynamic characteristics of 1/8th scaled HST models with differently simplified bogies under crosswinds. The numerical results were verified by wind tunnel tests using a same model. It was proven that the complexity of bogies has a significant influence on the HST models' aerodynamic drag and rolling moment at various yaw angles. The flow field of the HST models under a 30° crosswind was then employed to study the effects of the bogies. With the simplification of the bogie structure, the positive and negative pressure in bogie cabins increased while that on the top surface seldom changed. Various flow patterns were found on the windward side and top surface of bogie cabins. The differences of the flow fields underneath the train models were quantitatively analyzed using the vertical velocity profiles near each bogie. • Aerodynamic drag and rolling moment caused by various bogies under crosswinds. • Pressure distribution on bottom and top central line of train bodies. • Differences on flow patterns on boards of bogie cabins. • The underbody flow in term of velocity profiles vary due to bogie complexity. [ABSTRACT FROM AUTHOR]