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Research Papers: Flows in Complex Systems

Large Eddy Simulation Exploration of Passive Flow Control Around an Ahmed Body

[+] Author and Article Information
Siniša Krajnović

Division of Fluid Dynamics,
Department of Applied Mechanics,
Chalmers University of Technology,
Gothenburg SE-412 96, Sweden
e-mail: sinisa@chalmers.se

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received August 20, 2013; final manuscript received March 9, 2014; published online September 10, 2014. Assoc. Editor: Meng Wang.

J. Fluids Eng 136(12), 121103 (Sep 10, 2014) (10 pages) Paper No: FE-13-1506; doi: 10.1115/1.4027221 History: Received August 20, 2013; Revised March 09, 2014

Large eddy simulations (LES) are used to study passive flow control for drag reduction in a simplified ground vehicle. Add-on devices in the form of short cylinders are used for the formation of streaks in the streamwise direction that lead to the separation delay. The results of the present numerical simulations are compared with the experimental data and show good agreement. The two-stage flow control mechanism is analyzed from the LES results. It was found to be in agreement with the previous experimental observations that the counter-rotating vortices behind the impinging devices influence the separation only indirectly through the longitudinal vortices further downstream.

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References

Figures

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Fig. 1

(a) Shape of the Ahmed body, (b) Zoom of the rear of the geometry, and (c) Ahmed body seen from behind

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Fig. 2

The relevant parameters of the cylindrical roughness elements

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Fig. 3

Time-averaged flow around the cylinders. Vortex cores are shown in black and streamlines projected onto the symmetry plane are shown in white. Flow is from left to right.

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Fig. 4

Planes of the instantaneous velocity at: (a) x/λz = -2.5, (b) x/λz = -1.7, and (c) x/λz = -0.9, (d) x/λz = -0.05

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Fig. 5

Planes of the instantaneous streamwise vorticity ωx at: (a) x/λz = -4.2, (b) x/λz = -3.8, (c) x/λz = -3.4, (d) x/λz = -3.0, (e) x/λz = -2.5, (f) x/λz = -2.1, (g) x/λz = -1.7, (h) x/λz = -1.3, (i) x/λz = -0.9, (j) x/λz = -0.5, (k) x/λz = -0.05, (l) x/λz = 0.4, and (m) x/λz = 0.8

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Fig. 6

Planes of the instantaneous streamwise velocity u¯/Ue at: (a) x/λz = -4.2, (b) x/λz = -3.8, (c) x/λz = -3.4, (d) x/λz = -3.0, (e) x/λz = -2.5, (f) x/λz = -2.1, (g) x/λz = -1.7, (h) x/λz = -1.3, (i) x/λz = -0.9, (j) x/λz = -0.5, (k) x/λz = -0.05, (l) x/λz = 0.4, and (m) x/λz = 0.8

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Fig. 7

Planes of the time-averaged streamwise vorticity ωx at: (a) x/λz = -4.2, (b) x/λz = -3.8, (c) x/λz = -3.4, (d) x/λz = -3.0, (e) x/λz = -2.5, (f) x/λz = -2.1, (g) x/λz = -1.7, (h) x/λz = -1.3, (i) x/λz = -0.9, (j) x/λz = -0.5, (k) x/λz = -0.05, (l) x/λz = 0.4, and (m) x/λz = 0.8

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Fig. 8

Planes of the time-averaged streamwise velocity u¯/Ue at: (a) x/λz = -4.2, (b) x/λz = -3.8, (c) x/λz = -3.4, (d) x/λz = -3.0, (e) x/λz = -2.5, (f) x/λz = -2.1, (g) x/λz = -1.7, (h) x/λz = -1.3, (i) x/λz = -0.9, (j) x/λz = -0.5, (k) x/λz = -0.05, (l) x/λz = 0.4, and (m) x/λz = 0.8

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Fig. 9

Planes of the resolved Reynolds stress in m2/s2 in the streamwise direction u¯2 at: (a) x/λz = -4.2, (b) x/λz = -3.8, (c) x/λz = -3.4, (d) x/λz = -3.0, (e) x/λz = -2.5, (f) x/λz = -2.1, (g) x/λz = -1.7, (h) x/λz = -1.3, (i) x/λz = -0.9, (j) x/λz = -0.5, (k) x/λz = -0.05, (l) x/λz = 0.4, and (m) x/λz = 0.8

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Fig. 10

Planes of the resolved Reynolds stress in m2/s2 in the spanwise direction w¯2 at: (a) x/λz = -4.2, (b) x/λz = -3.8, (c) x/λz = -3.4, (d) x/λz = -3.0, (e) x/λz = -2.5, (f) x/λz = -2.1, (g) x/λz = -1.7, (h) x/λz = -1.3, (i) x/λz = -0.9, (j) x/λz = -0.5, (k) x/λz = -0.05, (l) x/λz = 0.4, and (m) x/λz = 0.8

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Fig. 11

Planes of the resolved Reynolds stress in m2/s2 in the wall normal direction v¯2 at: (a) x/λz = -4.2, (b) x/λz = -3.8, (c) x/λz = -3.4, (d) x/λz = -3.0, (e) x/λz = -2.5, (f) x/λz = -2.1, (g) x/λz = -1.7, (h) x/λz = -1.3, (i) x/λz = -0.9, (j) x/λz = -0.5, (k) x/λz = -0.05, (l) x/λz = 0.4, and (m) x/λz = 0.8

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Fig. 12

Counter-rotating vortices visualized using isosurfaces of the time-averaged streamwise vorticity component ωx = ±300. The streaks are shown using an isosurface of u¯/Ue = 0.95.

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Fig. 13

(a) Velocity streaks on the roof of the model at y/k = 0.5 from the wall. The flow is from left to right. (b) The amplitude of the streaks A∧st(x,y/k = 0.5) as a function of the distance from the cylinders array scaled with the spanwise spacing x˜/λz at spanwise positions z/λz = ±0 (triangles), z/λz = ±1 (circles), z/λz = ±2 (+),z/λz = ±3 (.),z/λz = ±4 (*),z/λz = ±5 (dash-dotted line), z/λz = ±6 (diamonds), z/λz = ±7 (dashed line), and time-averaged line (solid line).

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Fig. 14

Time-averaged flow structures around the rear of the body for (a) natural flow and (b) controlled flow. Vortex cores are shown in red (see online version). Particle traces are shown in white on the vehicle. The streamlines are projected on two planes parallel with the symmetry plane of the body and one plane parallel with the vertical base of the body.

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Fig. 15

Particle traces on the rear slanted surface from the present LES: (a) natural flow and (b) controlled flow

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Fig. 16

Streamlines projected on the symmetry plane: (a) natural flow, (b) controlled flow (medium grid), and (c) controlled flow (fine grid)

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Fig. 17

Time-averaged vorticity component ωx in planes (a) and (b) 0.35 H, (c) and (d) 0.52 H, (e) and (f) 0.69 H, and (g) and (h) 0.87 H

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Fig. 18

Time-averaged streamline velocity in the plane parallel with the slanted surface at the constant distance Y/k = 0.08 above it. The flow is from top to bottom. The surface is colored with U/U∞. (a) Natural flow (LES) and (b) controlled flow (LES).

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Fig. 19

Comparison of the surface pressure coefficient Cp on the slanted surface of the model. (a) Natural flow (LES) and (b) controlled flow (LES).

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Fig. 20

Comparison of the surface pressure coefficient Cp between the natural flow (dashed line) and the controlled flow (solid line) from fine grid LES on (a) the slanted surface and (b) the rear face of the body. The dash-dotted line is LES of the controlled flow using the medium grid. Experimental data for the natural and the controlled flows are plotted with circles and triangles, respectively. The profiles are shown for y = 0.

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