A STUDY OF VORTICAL STRUCTURES PAST THE LOWER PORTION OF THE AHMED CAR MODEL

A squareback simplified car model is exposed to a forced oscillating yaw and results are compared to static measurements. Tests are conducted at Reynolds number Re = 3.7×105 and a maximum Strouhal number St=fLrefU0=0.053. Phase average force and moment measurements exhibit a phase shift between static and dynamic tests that increases with oscillating frequency. To gain better understanding of the origin of the phase shift phenomenon, this paper proposes to characterize the flow evolution around the model using PIV measurements. Phase-shift seems to originate from the pressure recovery area where velocity fields exhibit a time delay in their response to dynamic yawing. Moreover, lateral vortical structures appearing on lee side from β = 15° increase this phase-shift and consequently appear to be favourable to the lateral stability of the vehicle.

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Forces and Flow Structures on a Simplified Car Model Exposed to an Unsteady Harmonic Crosswind

Journal of Fluids Engineering ◽

10.1115/1.4025466 ◽

2013 ◽

Vol 136 (1) ◽

Cited By ~ 7

Author(s):

Valérie Ferrand

Keyword(s):

Phase Shift ◽

Transient Flow ◽

Measurement Techniques ◽

Flow Topology ◽

Average Force ◽

Ground Vehicle ◽

Vortical Structures ◽

Car Model ◽

Image Velocimetry ◽

Evolution Phase

A ground vehicle traveling along a road is subject to unsteady crosswinds in a number of situations. In windy conditions, for example, the natural atmospheric wind can exhibit strong lateral gusts. Other situations, such as tunnel exits or overtaking induce sudden changes in crosswinds, as well. The interaction of this unsteady oncoming flow with the vehicle and the resulting aerodynamic forces and moments affect the vehicle stability and comfort. The objectives of the current study are to improve the understanding of flow physics of such transient flow and ultimately to develop measurement techniques to quantify the vehicle’s sensitivity to unsteady crosswind. A square back simplified car model is exposed to a forced oscillating yaw and results are compared to static measurements. Tests are conducted at Reynolds number Re = 3.7 × 105 and reduced frequencies ranging from 0.265 × 10−2 to 5.3 × 10−2. Unsteady side force and yawing moment measurements are associated with particle image velocimetry flow fields to interpret dynamic loads in link with flow topology evolution. Phase average force and moment measurements are found to exhibit a phase shift between static and dynamic tests that increases with oscillating frequency. Velocity fields reveal that the phase-shift seems to originate from the rear part of the car model. Moreover, lateral vortical structures appearing on the lee side from β = 15 deg increase this phase-shift and consequently appear to be favorable to the lateral stability of the vehicle.

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Numerical Simulations of Ground Simulation for a Realistic Generic Car Model

Volume 1C, Symposia: Fundamental Issues and Perspectives in Fluid Mechanics; Industrial and Environmental Applications of Fluid Mechanics; Issues and Perspectives in Automotive Flows; Gas-Solid Flows: Dedicated to the Memory of Professor Clayton T. Crowe; Numerical Methods for Multiphase Flow; Transport Phenomena in Energy Conversion From Clean and Sustainable Resources; Transport Phenomena in Materials Processing and Manufacturing Processes ◽

10.1115/fedsm2014-21164 ◽

2014 ◽

Cited By ~ 1

Author(s):

E. Guilmineau

Keyword(s):

Experimental Data ◽

Major Change ◽

Front Wheel ◽

Flow Solver ◽

Vortical Structures ◽

Car Model ◽

Computational Fluid Dynamics Cfd ◽

Drag And Lift ◽

Volume Method ◽

Good Agreement

Investigations of the aerodynamic influence of rotating wheels and moving ground on a realistic generic car model are presented. For this study, Computational Fluid Dynamics (CFD) are carried out with the flow solver ISIS-CFD, which is based on a finite-volume method. This paper presents the effects of ground simulation (GS) for the realistic car model DrivAer. Comparisons of the pressure between experimental data and numerical results show a good agreement. A moving ground and rotating wheels reduces the drag and the lift. Though the forces decreases at the front wheel due to the wheel rotation locally, the major change in drag and lift happens directly on the car body itself. The main vortical structures that develop around the wheels are also presented.

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