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

The 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]

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]

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]

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: 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]

Abstract: The dispersion of windborne debris that is caused by the object''s aerodynamics and shape is investigated. It is shown that the flight may be divided into two phases. The initial phase, where the relative velocity is high and the rate of rotation low, is characterised by high transverse accelerations. The second phase, which has much lower accelerations, can be modelled as that of a compact object, where only the drag force is significant. The initial conditions for the second phase are determined by the initial phase. Approximate solutions to the equations of motion are proposed for rectangular plate and rod type debris. The results are is good agreement with wind tunnel observations. [Copyright &y& Elsevier]

Abstract: This paper presents the results of an investigation on the effect of wind turbulence for the reduction of drag for a speed skater. A speed skater competing in an indoor oval is subjected to turbulent flow conditions. Measurements of flow characteristics in a speed skating oval were made and a simulation of those conditions was developed in the NRC 2m×3m Wind Tunnel. Wind tunnel tests on mannequins of a speed skater with force measurement using an external balance have indicated that the reduction of drag coefficient with Reynolds number was strongly affected by the flow conditions. The effects were not consistent for three different positions of a speed skater. [Copyright &y& Elsevier]

The aerodynamic characteristics of a hyperloop vehicle with or without channels operating at subsonic speeds in a low-pressure tube were simulated using a scale-adaptive simulation (SAS) method based on the shear-stress transport (SST) κ-ω turbulence model. The aerodynamic behavior of the hyperloop pod, fluid flow, and heat transfer phenomena in the tube were analyzed and evaluated. The grid resolution was analyzed, and the numerical algorithm was validated using a wind tunnel test. The results show that the effects of using channels on the pressure drag of the pod are mainly noticeable at the end of the deceleration stage, and using channels has a great influence on the friction drag relative to the pressure drag, especially during deceleration. The friction drag of such a pod increased with the inclusion of an axial channel in both the acceleration and deceleration stages, whereas it was decreased using a radial gap inside the pod in conjunction with an axial channel during deceleration. The flow field around the pod tail is greatly affected when channels are used relative to other areas around the pod. Using both radial gap and axial channel can effectively reduce the abnormally high temperature in the wake of the pod when the pod surpasses the critical speed. The effects of the radial gap on the flow field in the radial gap are different during different stages of operation, especially in terms of the pressure field. The inclusion of a radial gap is not conducive to improving the surrounding flow field, especially during deceleration. It was also found that the aerodynamic profile design of the channel exit is very important, and the pod speed must be considered. • Effects of the radial gap on the flow field differ between different stages • Flow fields around the pods were affected by the channels, especially the radial gap • Use of channels has a small effect on pressure drag except during deceleration • High temperatures from shockwaves can be reduced using radial and axial channels [ABSTRACT FROM AUTHOR]

Wind tunnel experiments on flow interference of adjacent (spacing ratio P / B ​= ​1.75) twin square cylinders in various attack angles α were conducted at a Reynolds number of 8.0 ​× ​104 considering both horizontal and diagonal arrangements of models. Investigations on the mean and fluctuating aerodynamic force coefficients, mean and fluctuating wind pressure coefficients, spanwise correlations, power spectrum densities (PSD) of lift, and Strouhal numbers of two cylinders, as well as the inter-cylinder correlations between them, are presented. The results show that the aerodynamic characteristics of two diagonal cylinders differ significantly from those of horizontal ones. Compared with the single-cylinder, the pressure distributions on adjacent cylinders are dramatically changed, leading to the negative drag, attractive lift, or repulsive lift on the downstream cylinder. At small α , diagonal cylinders present stronger mean drag and higher fluctuating forces than horizontal cylinders do. Besides, a stronger spanwise correlation is found for the diagonal ones. The vortex shedding pattern can be divided into four stages for the horizontal arrangement and six for the diagonal case. PSD curves and Strouhal numbers are similar between two cylinders for both arrangements, suggesting that the two closely spaced cylinders act like one bluff body in the vortex shedding. • Horizontal and diagonal cylinders have quite different aerodynamic properties. • Two closely spaced square cylinders act like one bluff body in vortex shedding. • Negative drag and attractive lift on the downstream cylinder exist at α ​≤ ​30°. [ABSTRACT FROM AUTHOR]

This study deals with the effects of wind turbulence on the mean aerodynamic drag, lift and moment coefficients of a stationary ice-accreted section model of a bridge cable for different wind angles of attack. The experiments were conducted in the Reynolds number range between 2.5·104 and 1.36·105. While the yawed/inclined cable model was experimentally iced in a climatic wind tunnel, the development of the ice shape was registered using photogrammetry. Later on, the ice-accreted model was reproduced on a smaller scale via three-dimensional printing. For the model in cross-flow, a series of wind tunnel tests were performed in low and moderately turbulent flow conditions with turbulence intensities of 3% and 12%, respectively. The obtained results were compared with those obtained experimentally for a circular smooth cylinder. The principal finding is that the flow turbulence significantly reduces the drag coefficient. • The experimental process was conducted to create an ice accretion on a cable model. • The ice-accreted model was reproduced in smaller scale by 3D printing method. • The aerodynamic coefficients were tested in low and moderately turbulent wind. • The effect of wind turbulence on aerodynamic coefficients was examined. [ABSTRACT FROM AUTHOR]

This study numerically investigates the effects of Jacobs bogies on the aerodynamic behaviors of a high-speed train using improved delayed detached eddy simulation (IDDES) at R e ​= ​3.3 ​× ​105. The results of the numerical simulations have been validated against the experimental data obtained from a previous reduced-scale moving model test and a wind tunnel test. The slipstream velocity, wake flow, underbody flow and aerodynamic drag of the HST are compared between the conventional bogie case and Jacobs bogie case. The results show that the use of Jacobs bogies can reduce the TSI values of the slipstream velocity at trackside and platform positions by 11.07% and 22.40%, respectively, which thereby shows a positive effect on improving the safety level of trackside workers and passengers standing on the platform. The Jacobs bogies are found to decrease the maximum values of the slipstream velocity and turbulence kinetic energy occurring at the intermediate bogie regions beneath the HST by 30.08% and 41.32%, respectively, which is beneficial for weakening the ballast flight phenomenon. The Jacobs bogies significantly narrow the scale of the longitudinal vortex structure in the wake propagation region. Additionally, the application of Jacobs bogies lowers the aerodynamic drag values of the vehicles and contributes to a 10% total drag reduction. • IDDES is validated to be a good engineering tool for the prediction of unsteady flow field around HST. • The aerodynamic behaviours of conventional HST and Jacobs HST are comparatively analyzed. • The use of Jacobs bogies is beneficial to alleviate the ballast flight phenomenon. • Jacobs bogies can improve the safety level of passengers and trackside workers. [ABSTRACT FROM AUTHOR]

The Reynolds number effects on body limbs of a cyclist model, namely leg and arm, are investigated via robotic volumetric Particle Image Velocimetry measurements in the velocity range from 5 ​m/s to 25 ​m/s. The near wakes of such body limbs feature recirculation regions whose width and length are governed not only by the taper of the body parts, but also by the presence of coherent streamwise vortical structures. Moreover, the interaction with the wakes of the upstream body parts plays a role in the local wake properties. While reductions of the wake width are observed on both lower leg and arm with increasing free-stream velocity, the wake of the upper leg follows an opposite trend increasing in size at higher velocity. Such variations of wake width with the Reynolds number are related to the behaviour of the local drag coefficient, indicating a drag crisis behaviour on both leg and arm. The distribution of the so-called critical velocity upon these body segments is discussed, as it determines the freestream speed where a minimum value for the drag can occur. • Cyclist mannequin near-wake measured by robotic volumetric particle image velocimetry in a wide range of velocities. • Reynolds number effects governed bycoherent streamwise vortical structures and wake interactions, in addition to limb taper. • Variations of the local limb wake width with Reynolds number are related to the behaviour of the local drag coefficient. • Distribution of the so-called critical velocity , corresponding to minimum drag coefficient, upon the body limbs is presented. • Present findings serve as a reference for the design of low-drag skinsuits. [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]

In this paper, the two-dimensional lift, drag and pitching moment aerodynamic admittances of three typical bridge deck sections, including a streamlined box section, an edge section and a truss section are identified by wind tunnel tests using a statistical identification approach and high-frequency dynamic balance measurement technique. An empirical formula for determining the two-dimensional aerodynamic admittance is proposed and compared with the commonly used Sears function and Davenport function. The results show that the two-dimensional lift and moment aerodynamic admittances for the three types of bridge decks are larger than Sears' function in the concerned wavenumber range of the buffeting analysis. This indicates that the Sears function may underestimate the vertical and torsional buffeting response of prototype long-span bridges. The two-dimensional drag aerodynamic admittance of the truss section corresponds strongly with Davenport's function, which verifies the reasonableness of Davenport's function that was originally developed for the two-dimensional drag aerodynamic admittance of open lattice structures. Furthermore, it was found that the two-dimensional aerodynamic admittance is insensitive to the mean wind velocity. The effect of wind attack angle on the two-dimensional aerodynamic admittance of bridge decks cannot be neglected. • The 2D AAFs of a streamlined box section, an edge section and a truss section are identified. • The empirical formulas of the 2D lift, drag and pitching moment AAFs are given. • The 2D AAFs are compared with the commonly used Sears function and Davenport function. • The effects of the mean wind velocity and angle of attack on the 2D AAFs are analyzed. [ABSTRACT FROM AUTHOR]