Flow Structures In Wake Research Articles

A study on the wake structure of an ascending submersible with silk flexible appendages using continuous wavelet transform and dynamic mode decomposition

This study proposed a new method for installing silk flexible appendages on the surface of the submersible to modify the wake structure of ascending submersibles, and explored a method of drag reduction of ascending submersible. A comparative analysis of the flow structure of submersibles with varying appendage lengths was conducted to understand the disturbance characteristics of the wake flow structure of submersible, and the high-speed particle image velocimetry (PIV) measurement experiment was conducted at a Reynolds number of 6456. Analysis of the time-averaged flow field showed that the flexible appendages could disrupt the two large-scale vortices in the wake of the submersibles during the sailing process, but this ability diminished with increasing appendage length. Furthermore, continuous wavelet transform (CWT) and dynamic mode decomposition (DMD) were used to analyze the mechanism behind this phenomenon. The analysis results showed that flexible appendages based on CWT had an inhibitory effect on large-scale flow, and this effect gradually decreased with increasing appendage length. The results indicate that the flexible appendages can disrupt the wake vortex structure, reduce vortex energy, and facilitate the transition from large-scale vortex to small-scale vortex. Additionally, excessive disturbance is generated in the wake region when the flexible appendage is too long, hindering the shedding of small-scale vortices and resulting in an increase.

Aeroacoustics of a Square Finite-Wall-Mounted Cylinder in Pressure-Gradient Flows

An experimental investigation of the wake flow structures, surface pressure fluctuations, and noise production of a square finite-wall-mounted cylinder with an aspect ratio of 2.4 is presented. The cylinder was immersed in flows with favorable, near-zero, and adverse pressure gradients at a Reynolds number of 48,000, based on cylinder width. Far-field noise, unsteady surface pressure, and particle image velocimetry measurements were taken simultaneously using the open-jet pressure-gradient test rig in the UNSW Anechoic Wind Tunnel. Favorable and adverse pressure gradients were found to enhance and suppress the cylinder tonal noise, respectively. A favorable pressure gradient reduced the size of the recirculation region in the wake, which intensified the free-end downwash and suppressed the junction upwash. The intensities of the cylinder surface pressure fluctuations were slightly increased at the primary and secondary shedding Strouhal numbers (based on cylinder width) of approximately 0.1 and 0.2. Conversely, the recirculation region expanded under an adverse pressure gradient, with the downwash weakened and the upwash enhanced. The peaks in the surface pressure spectra were noticeably attenuated at the primary shedding Strouhal number and completely suppressed at the secondary Strouhal number. Wake flow structures correlated with the surface pressure fluctuations and far-field noise were identified to understand the enhancement or suppression mechanisms of the surface pressure fluctuations and the associated shedding regimes.

Analysis of unsteady wake flow in centrifugal pump volute by using mode decomposition method

To analyze the coherent structure of wake flow in volute and its corresponding frequency information, dynamic modal decomposition (DMD), classic and spectral proper orthogonal decomposition (SPOD), were employed to decompose the transient flow field of the centrifugal pump volute. The snapshot set was constructed by means of the velocity field data at different moments in volute based on the large eddy simulation (LES) approach. The basic principles of the DMD, POD, and SPOD methods were compared in detail, and the decomposition results of the three methods on the wake flow structure in volute were compared and analyzed. The analysis results show that DMD can decompose the wake flow into coherent structures with different frequencies, including the basic steady-state structure, the dynamic modal flow field structure characterizing rotor–stator interaction (the first three modes with frequencies of 145.81, 291.61, and 437.43 Hz, respectively), and dissipative modal flow field structure characterizing fragmentized vortex (the fourth mode with a frequency of 486.03 Hz) in the volute. The POD can decompose the wake flow into flow structures with different energy levels. The first four modal energies account for more than 66% of total energy, which represents the large-scale structure with higher energy, and its dominant frequencies correspond to the blade passing frequency (145 Hz) and its frequency multiplication (290 Hz). The SPOD can not only decompose the complex wake flow into structural features of different energy levels but also has single-frequency characteristics of its modal structures. Compared with DMD and POD methods, the SPOD has the advantages of both, and can reflect the evolution characteristics of the wake flow in the volute.

Numerical study of a generic ship's airwake for understanding bi‐stability mechanism

A distinguishing feature of the bi-stable wake is that the wake persists in either of two preferred states for a sufficiently long time. Aiming to understand the persistence mechanism, this paper numerically investigates the airwake characteristics of the Chalmers ship model (CSM) using large eddy simulation with a wall-adapting local-eddy viscosity model and is complemented by experimental testings for validations. There are two cases of interest: (i) the baseline CSM with a sharp-edged superstructure front that induces massive boundary layer separation; (ii) the front-rounded (FR) CSM with suppressed flow separation. During a characteristic time ( $t^*$ ) of 1142 (26.5 s), the baseline case has a frequently switching wake, whereas the FR wake maintains a stable asymmetric structure with only one switch attempt. To understand the different wake behaviours, the study starts by analysing wake flow structures, vortex cores and the wake dynamics, followed by investigating the instantaneous flow physics. Results suggest that the baseline wake has a weak bi-stable pattern, whereas the FR wake behaves similarly to a reflectional symmetry breaking state of a potential bi-stable wake. The wake switching is found to be driven by the tilting of (vertical-oriented) $z$ -vorticity sheets from either side of the base toward the centre. This tilting behaviour is subjected to the high-magnitude vorticity that sheds from the upstream flow separation at the front sharp edges. With the sharp edges rounded in the FR case, the upstream vorticity is mitigated and the tilting effect is significantly reduced, leading to a more stable wake structure. The reasoning provided in the paper potentially explains the persistence mechanism of the bi-stable wake.

Reconstruction Of Time-Averaged 3D Pressure Fields In The Wake Of An Ahmed Body With Pressure From PIV

In this contribution, we present and compare time-averaged pressure fields reconstructed from scanned stereoscopic 2D3C-PIV data and volumetric 3D-PTV data (Shake-The-Box) with classic pointwise surface pressure tap measurements. Measurements were conducted in the near-wake region of an one-quarter scaled Ahmed body (Ahmed 1984) with 25° slant angle at height-based Reynolds numbers of Re = 5.06 · 10^4 and 1.13 · 10^5, respectively, i.e. free-stream velocities of 10.5 m/s and 23.5 m/s. The measurements are a continuation of previous measurements (Gericke et al. 2023, Ladwig et al. 2023) with a focus on the more complex flow topology concerning the wake flow structure. Measurements were conducted in a low-speed wind tunnel of Göttinger design with a semi-closed test section at Bochum University of Applied Sciences. The investigated flow field in the near wake region is dominantly characterized by a strongly three-dimensional flow field based on a mutually influenced vortex system in the wake, which challenges the utilized pressure reconstruction algorithm based on the Reynolds-averaged Navier-Stokes equations. The present study addresses challenges in pressure reconstruction due to the presence of highly transient rotational and shearing flow in the context of a practical application. The results show good agreement between reconstructed PIV/PTV-based pressure fields and reference pressure measurements using classic wall pressure measurements. The global characteristics of the pressure distributions are correctly reproduced. The agreement is particularly good in areas of unidirectional flow and local smaller velocity gradients. In shear zones as well as separation and, e.g., recirculation areas, larger deviations are clearly visible, as the velocity gradients present near the surface are not sufficiently captured at all.

The groove effect on wake characteristics of rotating cylinders

In the present study, active and passive flow control methods have been implemented to investigate their effects on the wake flow structures of a circular cylinder. Grooves having circular, rectangular, and triangular cross sections have been applied to the cylinders exposed to the rotation rates, α, from 0 to 0.79. The experiments have been conducted by particle image velocimetry at a Reynolds number of Re = 5 × 103. The contour graphics of time-averaged results have been presented. Moreover, the variations in velocity profiles have also been depicted. The experimental results revealed significant variations for flow patterns, wake structures, and turbulence parameters due to the effects of both groove geometries and rotational motion. In the stationary cases, for turbulence intensity, the circular grooved cylinder exhibited a 15% increase, while the triangular grooved cylinder showed a slightly higher increase of around 20% compared to that of the bare cylinder (BC). Conversely, in non-stationary cases, the rectangular grooved cylinder displayed the most prominent reduction in turbulence intensity, decreasing by approximately 10% compared to that of the BC. The groove type has considerably affected the flow structures of the wake regions, especially for the lower rotation rates.

Aerodynamic noise reduction of high-speed pantograph by introducing planar jet on leeward surface of panhead

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

Scale-adaptive simulation of the separated flow past a 90°-inclined prolate spheroid

The separated flow past a 6:1:1 spheroid is numerically investigated by means of the scale-adaptive simulation technique. The Reynolds number based on the free-stream velocity and the diameter at middle-section of the spheroid is located in the subcritical regime, i.e., Re = 3900. In comparison with the circular cylinder at the same Reynolds number, about 35% drag reduction is acquired by the spheroid, and the fluctuations of lift and drag are suppressed effectively. According to the detailed comparison, the satisfactory drag reduction and suppression of fluctuating force obtained by the spheroid are closely associated with the higher base-pressure and lower turbulent fluctuations in the near wake. Abundant contrasts of the different spanwise sections are presented to reveal the mechanism of constrained flow and apex effect of the spheroid. In addition, in order to provide reliable data for testing and developing turbulence models, a large number of turbulence statistics are computed and compared with previous data of the circular cylinder and sphere at comparable Reynolds numbers. Lower Reynolds stress peaks and less vigorous coherent structures indicate that the three-dimensional force and constrained flow caused by the spheroid can lead to the formation of steady shear layer and vortex separation. Furthermore, proper orthogonal decomposition and dynamic mode decomposition are employed to understand the large-scale wake flow structures behind the spheroid. The modal analysis results confirm that the wake of the spheroid is more stable than the circular cylinder, reconfirming the effective flow control.

Influence of inflow conditions on simplified heavy vehicle wake

In the current study, the impact of various inflow conditions, including turbulent wind profiles and turbulent intensity, on the wake flow topology of a simplified ground transportation system (GTS) model was investigated using the improved delayed detached eddy simulation. The reliability and accuracy of the numerical method adopted in this paper were verified against the results comprising the aerodynamic drag and the wake flow structure of the GTS model obtained from the large eddy simulation and the experimental data. The research results indicate that turbulent winds characterized by logarithmic and uniform velocity profiles resulted in significantly different wake flow topologies yet exhibit the same dominant frequency. The turbulent intensity also plays a crucial role in the wake of the GTS model. It is observed that an increase in turbulence intensity corresponds with a rise in the aerodynamic drag. Specifically, when the turbulence intensity is set at 15%, there is a 3.68% increase in the aerodynamic drag of the GTS model compared to a case where the turbulence intensity was only 5%. In addition, the turbulent intensity is critical to the dominant frequency characteristics of the wake region of the GTS model. These results demonstrate that both the velocity profiles and the turbulence intensities significantly influence the wake flow topology and aerodynamic drag of the GTS model, providing a valuable reference for establishing appropriate inflow conditions and exploring the formation mechanism of flow topology in the wake of the GTS model.

Effect of the tip speed ratio on the wake characteristics of wind turbines using LBM‐LES

AbstractIn this paper, the wake characteristics of Zell 2000 wind turbine under different tip velocity ratios are studied by using the lattice Boltzmann method and large eddy simulation. The adaptive mesh refinement method is performed to capture the fine flow structure and wake characteristics development. In this paper, we mainly focus on the effect of the tip speed ratio on the flow structure and unsteady characteristics of wind turbine wake. The three‐dimensional flow vorticity structures, the section vorticity diagram, the pressure fluctuation of wake and the lift coefficient of wind turbine wake are utilized to explore the effect of the tip speed ratio on the unsteady physics mechanism of wind turbine wake. With the increase of the tip speed ratio, the distance between two adjacent vortex rings along the axial direction gradually decreases, as the position of the broken vortex circles gradually approaches the center of the blade, separated vortexes are rapidly generated, and the coherent structure appears closer to the wind turbine. A relationship is established between the tip speed ratios and the positions of the broken vortex circles. It is further found that the dominant frequency amplitude gradually increases with the increase of tip speed ratio and the pressure amplitude spectra of vortex increases with the decrease of the distance between the wake and the center of the blade axis. The above series of studies can provide significant physical insight into deep understanding the influence of the tip speed ratio on the wake characteristics of wind turbines.

Numerical investigation on the effect of bionic fish swimming on the vortex-induced vibration of a tandemly arranged circular cylinder

The effect of bionic fish swimming on the vortex-induced vibration (VIV) of a circular cylinder arranged in tandem at a low Reynolds number of 150 is numerically investigated in this work. The bionic fish placed upstream of the cylinder with gap ratios of 1, 3, and 5 and that located downstream of the cylinder with gap ratios of 3 and 5 are examined in the simulations that were carried out in the reduced velocity range of Ur = 2–15. It is found that both the gap ratio and the reduced velocity have a significant influence on the VIV response and wake flow structure. When the bionic fish is placed upstream, the maximum response amplitude of the downstream cylinder is much greater than that of an isolated one. Two flow regimes are identified in terms of the shear layer reattachment, i.e., the continuous reattachment and the alternate reattachment. Comparing the vortex shedding frequencies of the cylinder and the swimming fish, it is found that the frequency of the cylinder is always locked in the fish swimming frequency, and multiple frequencies occur at Ur = 5. When the bionic fish is arranged downstream, four flow regimes are observed, including the extended-body, continuous reattachment, alternate attachment, and co-shedding regimes. Furthermore, the time-mean energy transfer coefficient of the cylinder is considerably higher at Ur = 5 than that when the fish is placed upstream of the cylinder.

A numerical study on the hydrodynamics of a swimming crocodile model

Aiming to uncover the propulsion mechanisms underlying a cruising crocodile, we conduct computational fluid dynamics (CFD) simulations on the flow around a simplified three-dimensional model of the Crocodylus siamensis. The locomotion of the crocodile model is realized through undulating its body and tail, mimicking a crocodile-like swimming pattern. At a cruising speed of U∞ = 0.5 m/s (corresponding to a Reynolds number Re = 9.95 × 105 based on U∞ and the body length L), the hydrodynamics of the crocodile model are investigated, taking into account effects of the undulation parameters (i.e., amplitude A and frequency f). The normalized undulation parameters cover broad ranges of 0.6 ≤ A* = A/W ≤ 1.0 and 0.25 ≤ f * = fW/U∞ ≤ 0.625, where W is the body width. The CFD simulations are conducted in ANSYS Fluent, with the SST k–ω turbulence model and user-defined functions for dynamic mesh being used. Numerical results reveal that A* and f * render profound effects on the hydrodynamic performance of the crocodile model. The time-mean axial force coefficient (CA¯) and power coefficient (C¯Power) exhibit rapid growth with increasing A* and/or f *, while the root mean square lateral force coefficient (Cy,rms) is more dependent on f * than on A*. It is further found that, irrespective of A*, CA¯ and C¯Power can be well scaled with Strouhal number St (= 2fA/U∞) or St2(1 − U∞/c). Furthermore, distinct flow patterns are observed in the wake of the crocodile model undulating at different St, corresponding to the drag, transition (or cruising), and thrust type swimming, respectively. Discussion is made on the wake flow structures and their connections to the generation of the hydrodynamic forces. The findings from this work contribute to the understanding of the propulsion mechanisms of the swimming crocodile, meaningful for the design of efficient biomimetic amphibious robots.

Revisiting the surface-mounted cube: An updated perspective of the near wake and near-wall flow field

The flow around a surface-mounted cube has been investigated for decades, yet the fundamentals of the mean flow have not been updated to reflect the latest advances regarding the flow around surface-mounted finite-height square prisms in general. One of the main gaps is the flow field very near the cube walls, especially the sides, and its relationship to the near wake. To investigate these features, large-eddy simulations of the flow around a surface-mounted cube were carried out at a Reynolds number Re =1×104. Two boundary layers were considered: a thin and laminar boundary layer and a thick and turbulent one, to provide an overview of the flow around the cube for contrasting boundary layers. The major flow structures in the mean wake were the horseshoe vortex, the arch vortex and the dipole structures, with other regions of vorticity also present. The time-averaged flow fields presented similar flow features for both boundary layers, but the thicker and turbulent one caused the horseshoe vortex to be located closer to the cube, the wake to become narrower and shorter, the coherent structures and downwash to weaken, the pressure to decrease around the sides and top of the cube, the drag force coefficient to decrease, the normal force coefficient to increase, and the trailing edge vortices to weaken. Intermittent flow reattachment on the top and side faces of the cube, as well as corner vortices, were found for both boundary layers, while the side and free end vortices were found exclusively with the thicker boundary layer, yielding a headband vortex. The three-dimensional flow structures and features were related to the near-wall flow field on the cube and ground plane surfaces, giving a complete and updated description of the mean flow characteristics.

FLOW STRUCTURES IN THE WAKE REGION OF TWO INCLINED FLAT PLATES IN A SYMMETRICAL TANDEM ARRANGEMENT

Flow properties and wake structures behind bluff bodies have fascinated many researchers for years. Such interest in this phenomenon is due to various applications of bluff bodies in engineering and industry. In this study, flow structures and the downstream wake behind two inclined flat plates in a symmetrical tandem arrangement have been studied experimentally at Reynolds number of 23,000. Constant-temperature hot wire anemometers (CTAs) were employed to quantitatively measure the near wake properties in terms of shedding frequency, velocity components, and turbulent kinetic energy (TKE). The effects of gap ratio (<i>g/D</i>) between the plates and the angle of attack (α) on the wake flow structure have been probed in detail quantitatively. It was found that for a given gap ratio <i>g/D</i>, the Strouhal number decreases as the angle of attack increases. Moreover, for a given angle of attack a, the Strouhal number increases as the gap ratio <i>g/D</i> increases. Interestingly, it was observed that for a given gap ratio <i>g/D</i>, the coherent TKE peak production decreases as the angle of attack increases.

Wake flow structure and hydrodynamic characteristics of flow around a C-shaped cylinder with variable attack angle at low Reynolds numbers

A numerical investigation is conducted on the flow over a C-shaped cylinder in the low Reynolds number range of Re = 40–160. The effect of attack angle (α) ranging from 0° to 180° is examined simultaneously. Wake evolution and vortex structure as well as the hydrodynamic characteristics are analyzed. Seven flow patterns are identified based on the location of boundary layer separation points and the evolution of near-wall vortices. The boundary layer separation points lock on the two ends of the C-shaped cylinder, resulting in the typical Karman vortex street (Pattern I). A separation point shifts to the curved surface in Pattern II-1 and Pattern II-2, and a quasi-stagnation vortex (QS) is formed within the groove in Pattern II-2. In Pattern III-1 and Pattern III-2, the QS fills the groove. The subordinate vortex is observed in the groove close to the lower end (Pattern IV). The complicated vortex merging occurs around the lower end in Pattern V. The separation points lock on the two ends, exhibiting a pair of counter-rotating vortex shedding downstream of the two ends (Pattern VI). No vortex shedding is found in Pattern VII. Additionally, the characteristic parameters and the hydrodynamic coefficients are related, and they are associated with the flow pattern partition. Four types of vortex street are identified in the wake of the C-shaped cylinder, including no vortex street, 2S vortex mode and decayed vortex street, 2S vortex mode and secondary vortex street (2S-SVS), and P + S vortex mode and secondary vortex street in vortex evolution (P + S-SVS).

Bi-stability of the Wake Flow of a Hatchback Car under Zero Yaw Angle Condition

<div>The aerodynamic performance of automobile especially drag and lift was largely determined by the wake flow, which is three-dimensional, unsteady, and turbulent. The styling of the rear back of the vehicle body has much influence on the wake flow structure, typically including squareback, notchback, and hatchback. Bi-stability of the wake flow of vehicle body makes the aerodynamic force oscillating, which affects the energy consumption and driving stability. This article investigates the bi-stability of wake flow of a hatchback SUV in full-scale automotive wind tunnel. Both aerodynamic force and surface pressure on the rear back of the vehicle were measured. Time series of aerodynamic force and pressure footprint are used to confirm the existence of bi-stability. The effects of some sensitive factors on the bi-stability have been analyzed. The results show that for the given condition with bi-stability phenomenon existing, the change of drag and lift can be 6.36% and 111%, respectively. The bi-stability of wake of the tested hatchback SUV is related to the change of flow structure in the wake of the vehicle, which can be explained by the drag crisis of hatchback vehicle under critical slant angle.</div>

Vertical and Spanwise Wake Flow Structures of a Single Spire over Smooth Wall Surface in a Wind Tunnel

The aerodynamic interaction between the wake flow structure behind a single spire with a smooth wall boundary layer at a long streamwise location was observed in a wind tunnel experiment. The application of a single spire is intended to generate a wake flow similar to the one generated behind a skyscraper. A quarter elliptic wedge spire was used and a long streamwise distance of up to 26 times the spire’s height was adopted to ensure the development of the boundary layer and the wake recovery. To grasp how the smooth wall boundary layer interacts with the wake as well as how the wake recovers downstream, vertical and lateral velocity profiles were examined. Despite only one spire being utilized, it was found that the role of the spire as a vortex generator was confirmed the boundary layer height in the with-spire case increased compared to that of the without-spire case. Moreover, the velocity deficit recovery process was observed vertically and streamwise. However, within the boundary layer, the recovery rate in the streamwise direction was lower compared to the above it. This finding indicates that within the boundary, the turbulence generated can sustain the wake caused by the spire, reducing the recovery rate. Based on the current lateral velocity analysis, the final streamwise distance required by the wake to fully recover could not be predicted due to the large velocity deviation of 2.15% at the end of the streamwise distance.

Fountain Flow Visualization in Quadrotor Wake Decreasing Rotor Thrust In-Ground Effect

Wake visualizations and thrust measurements of a quadrotor model were conducted to clarify the relationship between thrust and wake flow structure in-ground effects. The adjacent rotor distance, rotor height above ground, and the presence of a plate representing a generic quadrotor centerbody were varied. Thrust decreased when lowering the quadrotor to the ground when the adjacent rotor distance was over 2.2 times the rotor radius. Laser light sheet flow visualization showed quadrotor wake flow structures. Furthermore, visualization using particle image velocimetry showed an averaged vector map of wakes and fountain flow at the center of the quadrotor wake, which caused circulation with rotor passing flow and decreased rotor thrust. The center plate increased the sum of the rotor thrust and plate lift; however, circulation flow formed under the plate and caused the rotor thrust to decrease the in-ground effect. The results clarified that fountain flow development decreased the quadrotor thrust in-ground effect. Reducing the adjacent rotor distance weakened the development of fountain flow in the quadrotor wake and eliminated the decrease in rotor thrust near the ground.

Flow control for aerodynamic drag reduction of a high-speed train with diversion slots on bogie regions

A diversion slot is one of the potential mechanical devices to reduce high-speed train underbody aerodynamic resistance. This research aims to investigate the effectiveness of using diversion slots as a means of passive flow control to reduce the resistance of a high-speed train. Two different diversion slot designs, i.e., the big diversion slot (Bds) and the small diversion slot (Sds), placed at two installation locations near the bogie cabin end walls in six configurations are used. The results indicate that drag of the tail car is significantly reduced by 7.8%, 5.5%, 9.0%, and 9.4% using the configurations in cases 2 and 4–6, while an increase in 0.4% is experienced in case 3. Consequently, the total train aerodynamic resistance reduces by 1.9%, 0.2%, 3.0%, 4.2%, and 0.4% in cases 2–6, respectively, as compared to case 1. By evaluating the flow structure, we found that the diversion slots trigger flow separation, deflecting the airflow from entering the bogie regions, increasing flow turbulence and reducing the flow velocity. It efficiently improves the wake flow structure by reducing the wake strength, thereby increasing the tail nose surface pressure, thus reducing the tail car's aerodynamic drag. This study proposes a novel approach for reducing aerodynamic drag in high-speed trains, improving the underbody flow and wake characteristics.

A novel wake flow control method for drag reduction of a high-speed train with vortex generators installing on streamlined tail nose

The head/tail of a high-speed train has been designed in a streamlined way to achieve good aerodynamic performance, which leads to the flow separation point moving close to the tail nose tip. Therefore, the conventional way with some add-ons, i.e., the passive flow control, to suppress the flow separation in advance is not a good choice for this train wake flow improvement. Also, with the increase of the train speed, it is urgent to study new methods for the aerodynamic drag reduction of the train. The wake of the high-speed train is characterized by a pair of counter-rotating vortices, contributing to low surface pressure on the streamlined tail and posing a risk to the train operation. Thus, lowering the intensity of counter-rotating vortices and enhancing the surface pressure become a significantly potential drag reduction method. In the current study, a novel wake flow control method, named the vortex intensity reduction theory (VIRT), for the drag reduction of a high-speed train with vortex generators installing on the streamlined tail nose, was proposed to generate a pair of vortices with opposite rotating directions, expecting to weaken the wake vortices and have a higher-pressure distribution on the tail, as compared to the base case. The results show that with the installation of vortex generators (VGs), the train wake flow intensity is suppressed, and the influence region is reduced, resulting in the better train wake flow structures, as compared to the train without VGs. The VGs have significant impact on the aerodynamic performance of the tail car, while this effect is not evidently observed on the head and middle cars. The VGs contribute to the surface pressure increase on the streamlined tail, resulting into a reduction of pressure difference between the head and tail cars. As a result, a reduction of 5.11% in the aerodynamic drag and a reduction of 14.93% in the aerodynamic lift of the tail car are obtained, while for a three-car grouping train model, the reductions are about 2.23% and 72.66%, respectively. Therefore, the VIRT based on VGs proposed in this paper can effectively reduce the aerodynamic force of the tail car and alleviate the intensity of wake flow of the high-speed train, which will provide a newly potential drag reduction method of the next generation high-speed train.