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Flows in Complex Systems

Unsteady Aerodynamic Flow Investigation Around a Simplified Square-Back Road Vehicle With Drag Reduction Devices

[+] Author and Article Information
Bahram Khalighi

General Motors Global R & D,MC 480-106-256, 30500 Mound Rd., Warren, MI, 48090bahram.khalighi@gm.com

Kuo-Huey Chen

General Motors Global R & D, MC 480-106-256, 30500 Mound Rd., Warren, MI, 48090kuo-huey.chen@gm.com

G. Iaccarino

Mechanical Engineering Department,  Stanford University, Stanford, CA, 94305jops@stanford.edu

J. Fluids Eng 134(6), 061101 (May 29, 2012) (16 pages) doi:10.1115/1.4006643 History: Received October 26, 2011; Revised April 16, 2012; Published May 29, 2012; Online May 29, 2012

The unsteady flow around a simplified road vehicle model with and without drag reduction devices is investigated. The simulations are carried out using the unsteady RANS in conjunction with the v2 -f turbulence model. The corresponding experiments are performed in a small wind tunnel which includes pressure and velocity fields measurements. The devices are add-on geometry parts (a box with a cavity and, boat-tail without a cavity) which are attached to the back of the square-back model to improve the pressure recovery and reduce the flow unsteadiness. The results show that the recirculation regions at the base are shortened and weakened and the base pressure is significantly increased by the devices which lead to lower drag coefficients (up to 30% reduction in drag). Also, the results indicate a reduction of the turbulence intensities in the wake as well as a rapid upward deflection of the underbody flow with the devices in place. A reduction of the unsteadiness is the common element of the devices studied. The baseline configuration (square-back) exhibits strong three-dimensional flapping of the wake. The main shedding frequency captured agrees well with the available experimental data. Comparisons with the measurements show that the simulations agree reasonably well with the experiments in terms of drag and the flow structures. Finally, a blowing system (Coanda jet) is investigated numerically. In this case a drag reduction of up to 50% is realized.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Geometry of the square-back model: (a) square-back model, (b) view of the base, (c) the cavity device, and (d) the boat-tail device

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Figure 2

Schematics and dimensions for the models: boat-tail (α = 9 deg) and the cavity models

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Figure 3

Mesh refinement strategy

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Figure 4

Time-averaged velocity profiles at three locations in the wake in the symmetry plane for all three meshes (simulations and measurements)

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Figure 5

Time-averaged turbulent kinetic energy profiles at three locations in the wake in the symmetry plane for all three meshes

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Figure 6

Fore-body surface pressure distribution in the symmetry plane over the SB1 model

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Figure 7

Instantaneous near wake velocity (streamlines) in the symmetry plane: (left) CFD simulation for SB1; (right) CFD simulation for SB2

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Figure 8

Mean velocity field (streamlines) in the symmetry plane (left) CFD simulation, (right) PIV measurements: (top) SB1, (middle) SB2, (bottom) SB3

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Figure 9

Mean velocity field (streamlines) in the horizontal center plane for the SB1 model: (left) CFD simulation, (right) PIV measurements

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Figure 10

Instantaneous near wake pressure distributions in the symmetry plane: (left) CFD simulation for SB1; (right) CFD simulation for SB2

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Figure 11

Visualization (simulation) of the stream-wise velocity region inside the vortex core in the Wake: (left) SB1, (right) SB2

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Figure 12

Visualization (simulation) of the vortex core pressure iso-surface (lower pressure values) in the wake of the SB1 model

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Figure 13

Measured and simulated profiles of the normalized k/U2 component of the normal Reynolds stress at x/Ly  = 1.5 downstream of the trailing edge for SB1 and SB3 models (time-averaged)

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Figure 14

Simulated skin friction over the plates (SB2 model) in the symmetry plane: (left) lower plate, (right) upper plate.

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Figure 15

Mean pressure profiles in the plane of symmetry for SB1 and SB3 models

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Figure 16

Drag coefficients comparison between CFD and measurements for SB1, SB2, and SB3 models. The values are normalized by the measured drag for the SB1 case.

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Figure 17

Locations of the numerical probes

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Figure 18

Time history of the pressure (left column) and velocity (right column) in the wake for the square back (SB1): (top) probes 1, 2, 3; (middle) probes 7, 8; (bottom) probes 3, 5, 6

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Figure 19

Time history of the pressure (left column) and velocity (right column) in the wake for the boat-tail (SB3): (top) probes 1, 2, 3; (middle) probes 7, 8; (bottom) probes 3, 5, 6

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Figure 20

Spectra of the pressure (left column) and velocity (right column) signals in the wake for the square back (SB1): (top) probes 1, 2, 3; (middle) probes 7, 8; (bottom) probes 3, 5, 6

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Figure 21

Spectra of the pressure (left column) and velocity (right column) signals in the wake for the boat-tail (SB3): (top) probes 1, 2, 3; (middle) probes 7,8; (bottom) probes 3, 5, 6

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Figure 22

Schematic of the Coanda jet geometry (modifications are made in the rear of the SB1 model for the Coanda jet case). This figure represents one quarter of the geometry.

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Figure 23

Time-averaged stream-wise velocity (left) and the turbulent kinetic energy profiles (right) in the symmetry plane of the Coanda jet wake (vj is the conda jet velocity)

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Figure 24

Spectra of the pressure (left column) and velocity (right column) signals in the wake for the Coanda jet: (top) probes 1, 2, 3; (middle) 7, 8; (bottom) 3, 5, 6

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Figure 25

Simulated drag coefficients for the Coanda jet cases (U is the free-stream velocity). The values are normalized by the measured drag for the SB1 case.

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