Research Papers: Flows in Complex Systems

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

[+] Author and Article Information
Valérie Ferrand

Université de Toulouse,
Institut Supérieur de l'Aéronautique et
de l'Espace (ISAE),
10 Avenue Edouard Belin,
31400 Toulouse, France
e-mail: valerie.ferrand@isae.fr

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received July 19, 2012; final manuscript received August 28, 2013; published online October 7, 2013. Assoc. Editor: D. Keith Walters.

J. Fluids Eng 136(1), 011101 (Oct 07, 2013) (8 pages) Paper No: FE-12-1336; doi: 10.1115/1.4025466 History: Received July 19, 2012; Revised August 28, 2013

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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Fig. 1

Experimental test bench: elevated floor and turntable dispositive

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Fig. 2

One period of the dynamic yaw movement for f* = 1.325 × 10−2 and -10 deg≤β≤10 deg

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Fig. 3

Top, side, and front views of the “Willy” body

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Fig. 4(a)

The two-component force balance and (b) front and rear load cell contribution to a point side force “Fcalibration

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Fig. 5

Schematic view of force balance dynamic calibration equipment

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Fig. 6

PIV configurations

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Fig. 7

Static evolution of Cy with β

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Fig. 8

Static evolution of CN with β

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Fig. 9

Static evolution of the center of pressure XCP/Lref with β

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Fig. 10

Static mean velocity field at X/Lref = 0.5, colored by the normalized mean vorticity Ωx.Lref/U0. Data extracted from PIV measurements.

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Fig. 11

Static mean velocity field at Z/Lref = 0.45, colored by the normalized mean length (U2+V2)1/2/U0. Data extracted from PIV measurements.

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Fig. 12

(U2+V2)1/2/U0 distribution along the curve C. Data extracted from PIV measurements.

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Fig. 13

Cy and CN measured for the reduced frequencies f* = 0.265 × 10−2, 1.325 × 10 − 2, 2.65 × 10−2, 5.3 × 10−2, and -10 deg≤β≤10 deg compared to the corresponding static curve

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Fig. 14

Cy and CN measured for the reduced frequencies f* = 0.265 × 10−2, 1.325 × 10−2, 2.65 × 10−2, 5.3 × 10−2, and 10 deg≤β≤30 deg compared to the corresponding static curve

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Fig. 15

Dephasing Δϕ (in deg) of the dynamic case (f* = 5.3 × 10−2) compared to the static case for Cy and CN. The two yaw ranges -10 deg≤β≤10 deg and 10 deg≤β≤30 are presented.

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Fig. 16

(U2+V2)1/2/U0 distribution along the curve C. Comparison of static and dynamic cases.

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Fig. 17

Comparison of static and dynamic (f* = 5.3 × 10−2) mean velocity fields at X/Lref = 0.5 for β = 28 deg, colored by the normalized mean vorticity Ωx.Lref/U0. Data extracted from PIV measurements.

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Fig. 18

Side force coefficients associated with the front and rear load cell measurements. Static and dynamic cases (f* = 5.3 × 10−2) for the two yaw angle ranges -10 deg≤β≤10 deg and10 deg≤β≤30 deg




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