Search

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 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]

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]

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]

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]

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]

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]

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]

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]

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]

A simulation methodology is introduced to provide improved estimates of the true efficiency gains that can be achieved using aerodynamic drag-reduction technologies for ground vehicles, with a particular application to heavy-duty-vehicle operations. The approach is based on a stochastic simulation methodology that applies route-specific wind conditions to predefined duty cycles, using wind-climate data calculated from publicly-accessible data sets. The simulation tool accounts for vehicle duty-cycle, seasonal wind conditions, the direction of travel, and diurnal effects to generate resultant-velocity and yaw-angle predictions along a specified route. Aerodynamic drag and associated fuel-use predictions are then made based on vehicle aerodynamic performance models generated from sources such as wind-tunnel measurements, road/track measurements, or computational fluid dynamics (CFD) simulations. The simulation software was used to evaluate the benefits of drag-reduction technologies associated with heavy-duty vehicles (HDV) travelling at low speeds on two major routes in Canada. The fuel-savings rate was determined for several drag-reduction technologies and tractor-trailer-height-matching approaches on two highway routes, and at vehicle ground speeds of 50 ​km/h, 80 ​km/h, and 100 ​km/h. This approach provides a representative range of fuel savings that considers the effect of different wind climates on the performance of the drag-reduction strategies. The results suggest that drag-reduction technologies, and particularly height-matching approaches, can have important contributions to reducing fuel use and the associated greenhouse gas emissions at speeds as low as 50 ​km/h. Vehicle weights ranging from 13,600 ​kg (30,000 lb) to 54,500 ​kg (120,000 lb) were evaluated to identify the relative contributions of rolling resistance and aerodynamic drag on a vehicle's total fuel consumption. Although the proportion of a vehicle's total fuel consumption from aerodynamics is reduced at low speeds, the fuel savings that can be achieved with standard drag reduction approaches can provide a meaningful reduction in fuel use, a reduction in fuel costs, and a reduction in greenhouse gas emissions. • Drag-reduction technologies and proper height-matching can result in meaningful fuel savings at 50 to 60 km/h. • Route-specific software provided higher-fidelity fuel savings predictions than conventional methods • Drag due to rolling resistance was the dominant contributor to fuel consumption for a vehicle weight of 54,500 kg. • The proportion of fuel consumption due to aerodynamic effects was greater above 80-90 km/h. [ABSTRACT FROM AUTHOR]

The effect of varying the aspect ratio of a 1:8 scaled Ground Transportation System (GTS) was numerically investigated using the steady formulation of the RANS k − ω SST model. Cases of different aspect ratios (defined as the ratio of the varying width to the baseline width (Eq. (1)), with and without a boat-tail, were considered. For the GTS without boat-tail, a quasi-linear decline in the drag coefficient was found with increasing aspect ratio, resulting from a variation in the strength of the side shear layers, and the consequent change in the vertical symmetry of the near-wake. The average length of the wake was found to increase with increasing aspect ratios, resulting from an increase in the size of high Reynolds shear stress regions in the wake. For the GTS with boat-tail, variation of aspect ratio was found to have no effect on the critical boat-tail angle , albeit the magnitude of reduction in the drag coefficient differed between low and high aspect ratio configurations. Variation of the ground clearance (G ∗) of the GTS (without boat-tail) concluded a G ∗ value corresponding to minimum drag coefficient (G ∗ = 0.083) , while retaining of vertical quasi-symmetry was observed for G ∗ ≥ 0.29. • The effect of Aspect Ratio was numerically investigated on the 1:8 scale GTS. • Reduction in the drag coefficient was found with increasing Aspect Ratio. • Length of the wake was found to increase with increasing Aspect Ratio. • Aspect ratio had no effect on the angle resulting in minimum drag reduction. • Ground clearance effect on GTS drag coefficient was investigated. [ABSTRACT FROM AUTHOR]

An experimental investigation was conducted to examine the effects of helical fillets on dynamic ice accretion process over the surfaces of bridge stay cables and evaluate its effects on the aerodynamic characteristics of the stay cables under both dry rime and wet glaze icing conditions. The experimental study was performed in the Icing Research Tunnel of Iowa State University (i.e., ISU-IRT). Four bridge stay cable models, including a standard plain cable model and three helical filleted cable models of different helical pitch lengths, were used for the experimental investigation. During the experiment, in addition to using a high-speed imaging system to record the dynamic ice accretion process over the cable surfaces, a Digital Image Projection (DIP) based 3D scanning system was also utilized to quantify the 3D shapes of the ice structures accreted on the test models. While a high-resolution digital Particle Image Velocimetry (PIV) system was used to characterize the wake flows behind the cable models during the ice accreting process, the time variations of the aerodynamic drag forces acting on the test models were also measured by using a pair of force/moment transducers mounted at two ends of the cable models. It was found that, under the rime icing condition, the helical filleted cable models accreted more ice structures than the standard plain cable model. However, the helical filleted cable models were found to have less ice accretion under the wet glaze icing condition. The pitch length of the helical fillets was also found to affect the ice accretion process substantially. Under the rime icing condition, while the aerodynamic drag forces acting on the cable models were found to decrease continuously with more rime ice accreting over the cable surfaces, the drag reduction due to the rime ice accretion was found to be less obvious for the helical filleted cable models, in comparison with that obtained for the standard plain cable model. Under the glaze icing condition, the aerodynamic drag forces acting on the cable models were found to decrease quickly at the initial stage of the glaze icing process, and then increase gradually with the increasing ice accretion time at the later stage of the ice accreting process. PIV flow field measurements were correlated with the force measurement data to elucidate the underlying physics for a better understanding of the variation characteristics of the aerodynamic forces acting on the cable models under different icing conditions. • Transient details of the icing processes on helical fillet cables were revealed. •Effects of additional helical fillets on the ice accretion process were evaluated. •The drag coefficient evolution of bridge cables during icing process were examined. •The flow field around the iced stay cables was tested by PIV technology. [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 impact of flow-normal ribs and small-scale surface roughness on the drag and vortex shedding of a circular cylinder was investigated. Three rib heights, four relative rib spacings and three different forms of micro-roughness were combined to produce 28 unique surface coatings for the cylinder. The drag was measured in a wind tunnel for Reynolds numbers in the range 20,000 < R e < 160,000 , representing nearly a decade change centred about the drag crisis. The drag measurements were complemented by hot-wire measurements in the wake to investigate the vortex shedding frequency. The results show significant average drag reduction, up to 23%, for most of the ribbed geometries compared to a smooth cylinder for R e < 160,000. Increasing the rib height was found to reduce the critical Reynolds number and increase the minimum drag coefficient. Varying the rib spacing resulted in an "optimal" spacing, approximately five times the rib height, that caused the lowest critical Reynolds number. Increasing the micro-roughness resulted in a reduction in the critical Reynolds number and an increase in the minimum drag coefficient. • Extensive parametric study of micro roughness superimposed on ribs on a cylinder. • Identification of a passive reduction of 23% in average drag over the test Re range. • Optimal rib spacing is 5 ​h. [ABSTRACT FROM AUTHOR]

The use of rear cavities at the base of a square-back Ahmed body has been experimentally evaluated as a passive control device under cross-wind conditions with yaw angles β ≤ 10 ° , by means of pressure, force and velocity measurements. A comparative study has been performed at a Reynolds number R e = 10 5 , considering the reference square-back body (i.e. the body without any passive control device), and the same body implementing both straight and curved cavities as add-on devices. It is shown that the performance of a straight cavity, which is widely acknowledged as a robust drag reduction device for car models, is hindered under moderate cross-wind conditions, and does not constitute an efficient control strategy, especially when compared with a curved cavity. In particular, when the freestream is aligned with the body, the curved cavity provides a stronger attenuation of the fluctuating nature and the bi-stable dynamics of the wake (characteristic of the wake behind a square-back Ahmed body) than the straight one. Besides, the reduced size of the near wake, which is provoked by flow re-orientation and the reduced span between the rear edges of the curved cavity, leads to an important base pressure recovery, that translates into relative reductions of the drag of 9.1 % in comparison with the reference case (i.e. 2.6 % , with respect to the straight cavity). The results are considerably improved under cross-wind conditions, since the increase with the yaw angle of the force is particularly intense for the body with the straight cavity and attenuated for the model with the curved cavity. Thus, the relative reduction of the drag coefficient with respect to the reference body becomes negligible for the straight device at a yaw angle of 10°, while it still represents approximately a 10 % for the curved cavity. Furthermore, flow visualizations show that the wake is deflected as the incident flow is increasingly yawed, leading to the formation of a single leeward vortex core that approaches progressively the body, decreasing the base pressure. This phenomenon is minored when a curved cavity is implemented, increasing the low pressure induced at the body base. • We analyze the turbulent wake behind a square-back Ahmed's body and the effect of rear cavities under cross-wind conditions. • A curved cavity, obtained through shape optimization, is a robust control device for wake symmetrization and drag reduction. • The performance of a straight cavity as drag reducer is highly hindered under moderate cross-wind conditions. [ABSTRACT FROM AUTHOR]