Time-averaged Drag Coefficient Research Articles

Near-Field Aeroacoustic Shape Optimization at Low Reynolds Numbers

We investigate the feasibility of gradient-free aeroacoustic shape optimization using the flux reconstruction (FR) approach to study two-dimensional flow at low Reynolds numbers. The overall sound pressure level (OASPL) is computed via the direct acoustic approach, and optimization is performed using the gradient-free mesh adaptive direct search (MADS) algorithm. The proposed framework is assessed across three problems. First, flow over an open cavity is investigated at a Reynolds number of Re=1500 and freestream Mach number of M∞=0.15, resulting in a 7.9 dB noise reduction. The second case considers tandem cylinders at Re=200 and M∞=0.2, achieving a 16.5 dB noise reduction by optimizing the distance between the cylinders and their diameter ratio. Finally, a NACA0012 airfoil is optimized at Re=10,000 and M∞=0.2 to reduce trailing edge noise. The airfoil’s shape is optimized to generate a new four-digit NACA airfoil at an appropriate angle of attack to reduce OASPL while maintaining the baseline time-averaged lift coefficient and preventing an increase in the baseline time-averaged drag coefficient. The optimized airfoil is silent at 0 dB and the drag coefficient is decreased by 24.95%. These results demonstrate the feasibility of shape optimization using MADS and FR for aeroacoustic design.

Numerical investigation on flow characteristics and thermal performance of phase change material in spherical capsules with smooth or dimpled surfaces

The utilization of phase change spherical capsules with dimpled surfaces in a packed-bed latent thermal energy storage (PLTES) system provides a novel approach to addressing the intermittency and fluctuation in solar thermal utilization processes. In the context of accounting for wall effects, a comparative analysis of the flow characteristics and thermal performance of smooth and dimpled spherical phase-change capsules is conducted using the Large Eddy Simulation (LES) and an enthalpy-porosity method. Results show that the dimpled spherical capsule exhibits a substantial improvement in thermal performance compared to the smooth spherical capsule, with a maximum increase of 28.3 % in time-averaged Nusselt number when the difference in time-averaged drag coefficient is within 2 %. Compared to the smooth spherical capsules, the transient characteristics of dimpled spherical capsules exhibit significant changes, with the fluctuation amplitude of the drag coefficient and the fluctuation frequency of the lift coefficient both increasing by approximately twice. The local heat transfer performance also exhibits significant differences. The increase in surface temperature enhances the melting rate of PCM inside spherical capsules, but with the temperature rise, the enhancement efficiency decreases. The dimpled structure has a promoting effect on the melting process of PCM inside spherical capsules, and with the increase in surface temperature, this promoting effect becomes more pronounced. The time for PCM to achieve complete melting can be reduced by up to 19.07 %. The work contributes a novel idea for the application of non-smooth structures in PLTES systems.

Aerodynamic drag improvements on a circular cylinder using passive Venturi actuators

Scale-adaptive simulations (SAS) of three-dimensional flow around a circular cylinder fitted with a passive version of a novel flow control method (passive Venturi actuator, PVA) are performed at a diameter-based Reynolds number of Re = 28 000. The PVA consists of one or more narrow slits located at the top and/or bottom sides of the cylinder that connected to the throat of the axial Venturi slit in this cylinder. The main purpose of the study is to investigate the influence of divergence angle and narrow slit location relative to the axial Venturi slit on the aerodynamic performance of the cylinder. To this end, four models were designed with various PVAs. Additional models, a plain cylinder (unmodified model) and a cylinder fitted with an axial Venturi slit (model without a narrow slit), were also used for quantitative comparison. SAS predicts that an additional 5% reduction in the time-averaged drag coefficient, ⟨CD⟩, was observed when two narrow slits located on the surface at an angle of ±80° from the front stagnation line were fitted to the cylinder with an axial Venturi slit. Reducing the divergence angle of the PVA leads to improvements in ⟨CD⟩ and root mean square of fluctuating force coefficients, CD−rms and CL−rms. It is found that a cylinder with a PVA that has two narrow slits and a divergence angle of 6° can produce a 28.6% reduction in ⟨CD⟩, a 58.5% reduction in CD−rms, and an 81.2% reduction in CL−rms, when compared to the plain cylinder.

Wake suppression of a cylinder immersed in turbulence using rotating rods

In this study, we conducted three-dimensional direct numerical simulations to investigate the control performance of the free-stream turbulence past a circular cylinder with two small rotating rods at Re = 3900. In the case A and case B, two control rods with different rotating ratio α are positioned symmetrically at the circumferential angle θ=±45° and θ=±120° from the forward stagnation point of the main cylinder, respectively. Here, α is the ratio of the tangential velocity of the control rod surface ur to the inflow velocity U∞. Compared to the bare cylinder immersed in turbulence, both installation positions of the rotating rods at α = 2 result in a significant suppression on the wake flow. Specifically, the time-averaged drag coefficient Cd¯ was reduced by 25%, and the root mean square value of the lift coefficient measured to reduce by up to 30%. Statistical analysis is then performed, in terms of the Reynolds stresses, mean field, and the turbulent wake visualization to show variations in the flow dynamics. As expected, the front-mounted control rods inject kinetic energy into the boundary layer, effectively suppressing the turbulence fluctuations on both sides of the main cylinder. The work done by viscous forces around the rotating control rods and the suppression of turbulent fluctuations contribute to the pressure recovery observed in the rear-mounted control rod case. Proper orthogonal decomposition method is further employed to analyze the key features of the controlled wake of two cases with α = 2.

Flow past rotating cylinders using deterministic vortex method

This study examines the performance of an in-house code based on a deterministic vortex method on circular and square cylinders rotating with constant velocity. Subjecting a bluff body to rotary about its axis effectively reduces drag forces, suppresses the fluctuating forces, and increases the lift forces (known as the Magnus effect). Rotating cylinders are well known for their use in rotor ships, rotor aircrafts, and wind turbines. This study aims to establish the accuracy of the in-house code for the rotating circle cylinders based on the results from the literature and examine the behavior of the rotating square cylinders (a less-studied case). A square section, as a many-sided shape, can be considered when the behavior of more complex geometries is of interest, such as trajectories of the sediments, or replacing circular sections when autorotation is of importance. Both cylinders are exposed to a uniform flow of Re = 200 and an imposed rotation rate range of 0 ≤ α ≤ 5.5 for the circular, and 0 ≤ α ≤ 5 for the square cylinder. The present numerical tool is able to predict the vortex shedding suppression as a result of forced rotation. For both cases, a systematic increase in the time-averaged lift coefficient (≈25.5 for the circular and ≈26 for the square case when α = 5) and a general descending trend in the time-averaged drag coefficient (≈0.03 for the circular and ≈0 for the square case when α = 5) are detected. The second mode of vortex shedding for the circular cylinder is captured within a narrow range of rotation rates. A noticeable feature of the flow over rotating square cylinders is the emergence of near-field wakes in the near-body region, which develop independently from the wake behind the body. To the best of the authors’ knowledge, this is the first comprehensive study of a rotating body carried out using a deterministic vortex method-based numerical tool.

A Solution Verification Study for URANS Simulations of Flow Over a 5:1 Rectangular Cylinder Using Grid Convergence Index and Least Squares Procedures

Abstract A verification study was conducted for URANS (Unsteady Reynolds-Averaged Navier–Stokes) simulations of flow around a 5:1 rectangular cylinder at a Reynolds number of 56,700 (based on the cylinder depth) using the k–ω SST (shear stress transport) turbulence model and the γ−Reθ transition model for three types of grids (a fully structured grid and two hybrid grids generated using Delaunay and advancing front techniques). The grid convergence index (GCI) and least squares (LS) procedures were employed to estimate discretization error and associated uncertainties. The result indicates that the LS procedure provides the most reliable estimates of discretization error uncertainties for solution variables in the structured grid from the k–ω SST model. From the six solution variables of interest, the highest relative uncertainty was typically observed in the root-mean-square (rms) of lift coefficient, followed by time-averaged reattachment length and peak rms of pressure coefficient on the top and bottom surfaces of the cylinder. The solution variable with the lowest uncertainty was Strouhal number, followed by time-averaged drag coefficient. It is also noted that the GCI and LS procedures produce noticeably different uncertainty estimates, primarily due to inconsistences in their estimated observed orders of accuracy and safety factors. To successfully apply the procedures to practical problems, further research is required to reliably estimate uncertainties in solutions with “noisy” grid convergence behaviors and observed orders of accuracy.

The time-averaged aerodynamic performance of three circular columns arranged inline at a subcritical Reynolds number

This study conducted a series of wind tunnel tests to present the time-averaged aerodynamic performance of three equidistant circular columns arranged inline at various incidence angles and center-to-center pitch ratios under a subcritical Reynolds number of 6.4 × 104. The incidence angle α and pitch ratio L/D change in the range of 0°–90° and 1.2–4.0, respectively. The results show that the time-averaged aerodynamic performance of the three columns varies significantly with α as well as L/D. Based on the pitch ratio, the time-averaged aerodynamic coefficients can be approximately classified into four types: minimum pitch ratios (1.2≤L/D < 1.4), small pitch ratios (1.4≤L/D < 2.5), medium pitch ratios (2.5≤L/D < 3.5), and large pitch ratios (3.5≤L/D ≤ 4.0). The aerodynamic interference of the time-averaged aerodynamic coefficients under these four types is discussed by comparing the results obtained for the three columns with those for a single one. The time-averaged drag coefficient mainly experiences a reducing effect at the minimum and small pitch ratios and a certain extent of amplifying effect at the medium pitch ratios. The time-averaged lift coefficient appears non-zero values at some incidence angles, the range of which depends on the pitch ratio. The mechanism of the aerodynamic interference is also discussed by analyzing the time-averaged wind pressure distribution.

On the distributed blowing control of flow around a square cylinder at a low Reynolds number

This paper numerically investigates the distributed blowing control of the flow around a square cylinder at a low Reynolds number of Re=200. The air holes are placed at the windward or leeward stagnation points. The control effect of the unsteady aerodynamic forces is notably different with different spanwise distributions of air holes, which is represented by the dimensionless air hole area ratio G. The simulation results indicate that the windward blowing control (WBC) shows better control effect than the leeward blowing control (LBC) in reducing the time-averaged drag coefficient Cd¯ of the square cylinder while the fluctuating lift coefficient Cl′ can be significantly suppressed by the LBC. Besides, the surface pressure distribution, the turbulence kinetic energy (TKE) and Reynolds shear stress (RSS) distributions and the shedding vortices in the wake are analysed and compared in detail under different control conditions. Finally, through the instantaneous 3D vortex structures and the spanwise fluctuating velocity w′ distribution in the flow field, the suppression of the unsteady aerodynamic forces is mainly related to the 3D vortex pairs induced by the blowing jet.

Aerodynamic Drag Improvements on a Simplified Heavy Vehicle using Three-sided Plain and Notched Base Flaps

An experimental investigation has been undertaken to determine the effects of plain and notched base flaps on the drag performance of simplified tractor-trailer combination without any intermediate gap, Generalized European Transport System (GETS). Both plain and notched base flaps are rigid, made up of three identical flaps whose length is equal to the width of the GETS model, and not angled inward or outward. The experiments examined three-sided flap configurations corresponding to various combinations of seven heights of the plain part (from 10 to 40 mm in steps of 5 mm), four notch amplitudes (from 2.5 to 10 mm in steps of 2.5 mm), and five notch wavelengths (from 10 to 50 mm in steps of 10 mm). It is shown that the drag performance of the plain flap at zero yaw highly depends on the height of the plain flap. The maximum drag reduction occurs for e/w=0.1 yielding a drag reduction of 1.9% when compared to the GETS model without flap (baseline GETS). It was shown that the time-averaged drag coefficient increased slightly until a maximum was reached at e/w=0.3 but then decreased slightly with increasing e/w. Under zero yaw angle conditions, GETS model with a notched base flap, e10-a05.0-λ20, gives the lowest drag. The addition of this base flap to the GETS model resulted in a 2.8% drag reduction. This notched base flap was shown to be more effective not only at reducing &lt;CD&gt; under yawed flow conditions tested but also at reducing time-averaged side coefficient under yawed flow conditions tested, compared to the e10-a00.0-λ00 flap.

The effect of transverse inclination angle on hydrodynamic characteristics of two tandem circular cylinders in an X arrangement

A numerical investigation of the flow around two stationary circular cylinders in an X arrangement, with a Reynolds number of Re = 3900 and a spacing ratio of L/D = 4.0, is conducted using Large-Eddy Simulation. Eight different transverse inclination angles (α = 0°–30°) are considered to investigate their effects on several aspects, including the time-averaged drag coefficient (Cd-ave), root-mean-square lift coefficient (Cl-rms), Strouhal number (St), instantaneous flow structure, and time-averaged pressure distribution. The numerical results demonstrate that when the inclination angle exceeds a critical value of αc = 12.5°–15°, the flow pattern in the gap between two cylinders transitions from the reattachment regime to the co-shedding regime. This flow transition indicates that the wake vortex periodically detaches from the upstream cylinder and impinges on the downstream cylinder, resulting in a sharp increase in force coefficients, especially for the downstream one. When compared with α = 15°–30°, the force coefficients for both cylinders at α = 0°–12.5° are reduced, which is favorable to the structural stability. Furthermore, an increase in α leads to a monotonic rise in Cd-ave for the downstream cylinder. This can be primarily attributed to the amplified pressure coefficient on the front face of the downstream cylinder, which is caused by the reduced shielding effect from the upstream one.

Effects of annulation on low Reynolds number flows over an orthocone

This study numerically examines the influences of transverse annulation around a cone surface on the characteristics of a flow over an orthocone. This work is inspired by Spyroceras, a fossilized genus of nautiloid cephalopods from the Paleozoic era, whose method of locomotion is understudied. As a baseline case, a flow over a smooth orthoconic model with a blunt cone end is investigated numerically at Reynolds numbers from 500 to 1500. As Reynolds increases, two different shedding mechanisms—hairpin-vortex wake and spiral-vortex wake—are captured. We notice that an introduction of annulation over the cone surface changes the critical Reynolds number for the transition of the shedding mechanism. The dominant shedding frequency increases with the Reynolds number for the smooth and annulated cone flows. Moreover, the annulation reduces the dominant frequency for the same Reynolds number and increases the time-averaged drag coefficient. Modal decompositions—Proper Orthogonal Decomposition (POD) and Spectral Proper Orthogonal Decomposition (SPOD)—are used to capture the coherent structures and their oscillating frequencies. We have captured modes corresponding to the hairpin-vortex wake and spiral-vortex wake shedding mechanisms. Comparing the leading POD modes for the smooth and the annulated cone flows, we find that the annulation can reduce the twisting effects of the coherent structures in the wake. Additionally, we find that the SPOD analysis can identify modes presenting both hairpin-vortex wake and spiral-vortex wake in one flow condition as leading modes, while the POD leading modes only reveal one shedding mechanism in each flow.

Analysis of Flow Characteristics around a Square Cylinder with Boundary Constraint

Based on the two-dimensional hydrodynamic model of the finite volume method and structured multigrid, the flow characteristics around a square cylinder with boundary constraint are analysed. The gap ratio G/D (G is the distance from the cylinder to the channel boundary, and D is the side length of the square cylinder) does not change the four flow patterns. Under the laminar vortex street phase, the boundary constraint only reduces the scale of the vortices. The vortex centres are pressed toward the boundaries of the channel, and a low velocity zone is formed near the boundary, but the law of vorticity attenuation along the flow direction is not changed. The flow pattern classification map shows that the boundary constraint increases the Reynolds number required to generate the turbulence flow pattern, and the range of the Reynolds number in the flow pattern of the laminar vortex street has a maximum increase range. The correlation between the time-averaged drag coefficient or the vortex shedding frequency and Reynolds number under different gap ratios indicates that the resistance of the square cylinder and the vortex shedding frequency increase accordingly with the strengthening of the boundary constraint. When G/D &lt; 3.5, the increase is particularly obvious. Meanwhile, the correlation characteristics between the resistance or the vortex shedding frequency of the square cylinder and the Reynolds number are unrelated to the boundary constraint strength.

Dairesel Silindir Çevresindeki Pasif Akış Kontrolünün Sayısal İncelenmesi

The effects of a perforated cylinder on the passive flow control around a circular cylinder mounted on a wall were investigated. The perforated cylinder was placed outside of the single circular cylinder concentrically. The large-eddy simulation was used to resolve the flow field. The study was aimed at both reducing the drag coefficient of the single cylinder and controlling the fluctuating forces acting on the single cylinder caused by vortex shedding in the downstream wake. The results showed that the structure of the downstream wake flow of the single cylinder changed significantly after placing the perforated cylinder. For example, von Karman vortices disappeared, and the maximum magnitude of turbulent kinetic energy, TKE, in the downstream wake was reduced. The time-averaged drag coefficient of the single cylinder was decreased by 69%. In addition, the maximum value of the lift coefficient of the single cylinder was reduced by eight times when the perforated cylinder was placed outside the single cylinder.

Computational fluid dynamics prediction of the aerodynamic difference between stationary and moving trains

Moving model simulations have been a key method of predicting the aerodynamic performances of High-Speed Trains (HSTs). Ideally, the aerodynamic characteristics of a train moving or being blown by the wind are the same with appropriate ground configurations. In a numerical simulation, there are differences due to interpolation errors and errors caused by model movement. The impact of the error caused by the movement on the result is not known. Therefore, in this study, stationary and moving cases were used to assess the magnitude of the movement’s effect using the Improved Detached Eddy Simulation (IDDES) method. A wind tunnel test validated the numerical algorithm at 60 m/s and a common yaw angle of 0°. Moreover, the spatial and time discretization satisfied the high accuracy requirements, as determined through a mesh independence study and convective Courant number testing. The time-averaged drag coefficients predicted by the moving case were similar to those of the stationary case, especially the total drag coefficients. In contrast, differences were determined in the stationary and moving cases in terms of the flow structure and slipstream. The motion encouraged the streamwise vortices around the tail car and the wake vortices to expand along the spanwise direction and the wall-normal direction, and the vortex cores shifted away from the outer surface of the vehicle. As a consequence, the average value and the standard deviation of the slipstream increased. Therefore, moving model simulations require more caution. These findings can help researchers make directional corrections in the numerical simulation of train-tunnel systems.

Streamwise vortices, cellular shedding and force coefficients on finite wing at low Reynolds number

Numerical simulations for flow past a finite rectangular wing with a NACA 0012 section at $Re=1000$ for various semi-aspect ratios ( $0.25\le sAR \le 7.5$ ) over a range of angles of attack ( $0^{\circ }\le \alpha \le 14^{\circ }$ ) reveal streamwise vortices, which increase in strength and number to occupy an increasing spanwise extent with increase in $\alpha$ . They result in non-monotonic spanwise variation of local force coefficients and increased strength of wing-tip vortex for $\alpha &gt;8^{\circ }$ . Viscous and pressure drag dominate for low and high sAR, respectively. The time-averaged drag coefficient first decreases and then increases with increase in $sAR$ . Vortex shedding for $\alpha =14^{\circ }$ is single cell and parallel for $sAR&lt;3$ . Shedding is in two cells with an oblique angle that varies with time, leading to large spanwise variation in the root mean square of local force coefficients for higher $sAR$ . Various types of dislocations, reported earlier in wakes of bluff bodies, are seen for different $\alpha$ and $sAR$ . Dislocations for $\alpha =14^{\circ }$ appear at the same spanwise location for $sAR=3$ and at different spanwise locations for $sAR\ge 4$ . Vortex shedding for $\alpha =12^{\circ }$ and $sAR=5$ exhibits one cell structure in the near wake and two cells in the far wake due to splitting and reconnection of vortices near the mid-span in the moderate wake. Linkages form between counter-rotating spanwise vortices for $sAR\ge 1$ . Additional linkages between shed- and wing-tip vortices are observed for lower $sAR$ . At each $\alpha$ , the strength of the wing-tip vortex and radius of its core, estimated using Rankine and Lamb–Oseen models, increases up to a certain $sAR$ beyond which it is approximately constant.

Numerical Investigation of Vortex Shedding from a 5:1 Rectangular Cylinder at Different Angles of Attack

Although flow around a 5:1 rectangular cylinder at small angles of attack (AoA) has been extensively studied, when the AoA becomes larger, the research is rare. Therefore, this study performs Unsteady Reynolds-Averaged Navier-Stokes simulations (URANS) using the k-ω SST turbulence model for unsteady flow around a two-dimensional 5:1 rectangular cylinder at different AoAs up to 45°. A strong dependence of the flow characteristics on AoA is observed through the analysis of the time-averaged lift coefficient, drag coefficient, and Strouhal number. The peak of lift and drag coefficient is observed to be correlated, respectively, to the leading- and trailing-edge vortex based on the analysis of the flow. The x′-directional length of the main recirculation bubble on the top side and the distance from the bubble center to the leading edge of the cylinder both reach the maximum when α = 15°. In addition, the standard deviation σx′ of the time-averaged velocity U¯x′ along the cylinder shows a trend of increasing at first and then decreasing, and that σy′ also shows the same trend at α = 20°~45°; it fluctuates within a range of 0.05~0.2 at α = 0°~20°. Finally, two principal modes of vortex shedding are observed with α = 15° being their turning point, (i) “1 + 1” mode: in one vortex shedding period, two major vortices shed off from the top and bottom sides of the cylinder at α ≤ 15°; (ii) “2 + 2” mode, four vortices shed off from the top and bottom sides of the cylinder at α &gt; 15° in one vortex shedding period.

Experimental and numerical investigation of flow structure of grooved cylinder

Making modifications to a cylinder is one of the most effective ways to change the flow in the cylinder wake region. This study aimed to investigate experimentally and numerically the flow structure of the wake region of a smooth cylinder and rectangular grooved cylinders (roughness coefficients are ϵ/D = 0.02, 0.04, and 0.06, ϵ is the roughness height, and D is the diameter of the cylinder) by using the Particle Image Velocimetry (PIV) and the Computational Fluid Dynamics (CFD). Numerical studies were conducted with the Reynolds-averaged Navier-Stokes (RANS) equations. The experimental studies were performed at Re = 5000 in an open water channel, while the numerical studies were performed at Reynolds numbers in the range of 5000 ≤ Re ≤ 20000. A good agreement was observed between the experimental and the numerical results in terms of the time-averaged patterns, time-averaged velocity profiles, and Strouhal numbers. The saddle points of the rectangular grooved cylinders were closer to the surface compared to those of the smooth cylinder. The results revealed that the time-averaged drag coefficient reduced by 30% for the largest groove roughness (ε/D = 0.06) at Re = 20000. In conclusion, the present study shows that rectangular grooved cylinders are effective in reducing the time-averaged drag coefficient.

Direct numerical simulation of different finite cylinders rolling on a horizontal surface: The thickness effects on the aerodynamics

The flow around a cylinder rolling along a horizontal ground plane is investigated using direct numerical simulation for a Reynolds number based on the cylinder diameter, D, of ReD=3×104. Three cylinder thicknesses, T/D=0.040, 0.126, and 0.400, are considered. The time-averaged drag coefficients are found to be 0.87, 0.69, and 0.97 for the three cylinders, respectively. The non-monotonic variation in drag coefficient with thickness suggests a transition in the proportion of contributions of friction and pressure drag to the total drag which changes about 30% with thickness increase. Indeed, the ratio of friction drag to pressure drag varies from 0.333 to 0.085 and finally to 0.013 for the three cylinders. The pressure coefficient become more negative in the aft of the thicker cylinders, because the cylinder become more bluff body which suggests more pronounced pressure drag. Temporal fluctuations in the drag coefficient associated with vortex shedding events increase monotonically with thickness, though the root-mean-square of the drag coefficient follows the same trend as the mean drag coefficient. The lift coefficients are −0.057, 0.066, and 0.64 for the three respective cylinders. The negative value for the thinnest cylinder indicates down force. The transition from negative lift on the thin cylinder to positive lift on the thick cylinder is associated with elevated surface pressure just upstream of the ground contact point as thickness increases. As the flow is more detached from the cylinder sides, the friction lift is less significant with thickness increase. Furthermore, the pressure coefficient is negative spatially larger in the top of the thicker cylinders which the expansion offers a more significant upward pressure lift.

High Drag States in Tidally Modulated Stratified Wakes

Abstract Large-eddy simulations (LES) are employed to investigate the role of time-varying currents on the form drag and vortex dynamics of submerged 3D topography in a stratified rotating environment. The current is of the form Uc + Utsin(2πftt), where Uc is the mean, Ut is the tidal component, and ft is its frequency. A conical obstacle is considered in the regime of low Froude number. When tides are absent, eddies are shed at the natural shedding frequency fs,c. The relative frequency is varied in a parametric study, which reveals states of high time-averaged form drag coefficient. There is a twofold amplification of the form drag coefficient relative to the no-tide (Ut = 0) case when lies between 0.5 and 1. The spatial organization of the near-wake vortices in the high drag states is different from a Kármán vortex street. For instance, the vortex shedding from the obstacle is symmetric when and strongly asymmetric when . The increase in form drag with increasing stems from bottom intensification of the pressure in the obstacle lee which we link to changes in flow separation and near-wake vortices.

Effect of the angle of attack on flow around an elliptic cylinder near a moving wall

The present work investigates the combined effect of the angle of attack (AOA) (the angle between the semi-major axis of the cylinder and the vertical axis), and the gap ratio G/D (where G represents the distance between the cylinder center and the moving wall and D denotes the semi-major axis of the elliptic cylinder) on the flow around an elliptic cylinder near a moving wall. Here, AOA covers ±15°,±30°, and ±45° with G/D ranging from 0.6 to 2.5. The Reynolds number (based on the free-stream velocity and the semi-major axis) is fixed to 150. The resulting Kármán vortex street, the two-layered wake, and the secondary vortex street have been investigated and visualized. The resulting patterns have been classified and mapped out in the (G/D, AOA)-space. At small gap ratios, a clockwise rotation of the cylinder (negative AOA) leads to a stronger suppression effect between the moving wall and the backside of the cylinder. This results in more transitions between the different wake patterns than for the counterclockwise rotated cylinder (positive AOA). As the cylinder is rotated more, either clockwise or counterclockwise (i.e., as |AOA| increases), the crest value of the drag coefficient decreases while the crest value of the lift coefficient first increases and then decreases for G/D≥1.0. For a given G/D, the time-averaged drag coefficient decreases with the increased rotation of the cylinder, except for an increase observed as AOA increases from 0° to 15° for G/D≤0.9. Moreover, the lift force is directed upward and downward for positive and negative AOA, respectively, and its magnitude decreases with the increase in G/D when the cylinder is counterclockwise rotated, while it increases when the cylinder is clockwise rotated.