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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

Ahmed, S. R., Ramm, G., and Faltin, G., 1984, “Some Salient Features of the Time Averaged Ground Vehicle Wake,” SAE Paper No. 840300.
Lienhart, H., and Becker, S., 2003, “Flow and Turbulent Structure in the Wake of a Simplified Car Model,” SAE Paper No. 2003-01-0656.
Krajnović, S., and Davidson, L., 2005, “Flow Around a Simplified Car, Part 1: Large Eddy Simulation,” ASME J. Fluids Eng., 127(5), pp. 907–918. [CrossRef]
Krajnović, S., and Davidson, L., 2005, “Flow Around a Simplified Car, Part 2: Understanding the Flow,” ASME J. Fluids Eng., 127(5), pp. 919–928. [CrossRef]
Basara, B., Krajnović, S., and Girimaji, S., 2008, “PANS vs. LES for Computing of the Flow Around a 3D Bluff Body,” 7th International ERCOFTAC Symposium on Engineering Turbulence Modeling and Measurements, Limassol, Cyprus, June 4–6.
Pujals, G., Depardon, S., and Cossu, C., 2010, “Drag Reduction of a 3D Bluff Body Using Coherent Streamwise Streaks,” Exp. Fluids, 49, pp. 1085–1094. [CrossRef]
Krajnović, S., and Han, X., 2012, “LES and PANS of Passive and Active Control of Flows Around Generic Vehicle Bodies,” Seventh International Conference on Computational Fluid Dynamics (ICCFD7), Big Island, HI.
Smagorinsky, J., 1963, “General Circulation Experiments With the Primitive Equations,” Monthly Weather Rev., 91(3), pp. 99–165. [CrossRef]
Krajnović, S., and Davidson, L., 2002, “Large Eddy Simulation of the Flow Around a Bluff Body,” AIAA J., 40(5), pp. 927–936. [CrossRef]
Krajnović, S., 2009, “LES of Flows Around Ground Vehicles and Other Bluff Bodies,” Philos. Trans. R. Soc. A, 367(1899), pp. 2917–2930. [CrossRef]
Krajnović, S., and Davidson, L., 2003, “Numerical Study of the Flow Around the Bus-Shaped Body,” ASME J. Fluids Eng., 125(3), pp. 500–509. [CrossRef]
Hemida, H., and Krajnović, S., 2008, “LES Study of the Influence of Train Nose Shape on the Flow Structures Under Cross-Wind Conditions,” ASME J. Fluids Eng., 130(9), p. 091101. [CrossRef]
Krajnović, S., Ringqvist, P., Nakade, K., and Basara, B., 2012, “Large Eddy Simulation of the Flow Around a Simplified Train Moving Through a Crosswind Flow,” J. Wind Eng. Ind. Aerodyn., 110, pp. 86–99.
Krajnović, S., Bengtsson, A., and Basara, B., 2011, “Large Eddy Simulation Investigation of the Hysteresis Effects in the Flow Around an Oscillating Ground Vehicle,” ASME J. Fluids Eng., 133(12), p. 121103. [CrossRef]
Krajnović, S., Sarmast, S., and Basara, B., 2011, “Numerical Investigation of the Flow Around a Simplified Wheel in a Wheelhouse,” ASME J. Fluids Eng., 133(11), p. 111001. [CrossRef]
Krajnović, S., Osth, J., and Basara, B., 2011, “LES Study of Breakdown Control of A-Pillar Vortex,” Int. J. Flow Control, 2(4), pp. 237–257. [CrossRef]
Osth, J., and Krajnović, S., 2012, “The Flow Around a Simplified Tractor–Trailer Model Studied by Large Eddy Simulation,” J. Wind Eng. Ind. Aerodyn., 102, pp. 36–47. [CrossRef]
Krajnović, S., 2011, “Flow Around a Tall Finite Cylinder Explored by Large Eddy Simulation,” J. Fluid Mech., 676, pp. 294–317. [CrossRef]
Andersson, P., Brandt, L., Bottaro, A., and Henningson, D., 2001, “On the Breakdown of Boundary Layers Streaks,” J. Fluid Mech., 428, pp. 29–60. [CrossRef]

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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