"The Use of a Bluff Body Wake Generator for Wind Tunnel Studies of NASCAR Drafting Aerodynamics"

Authors: Robert Dominy*, Michael Wilson*, Geoff Le Good**, David Sims-Williams*

Affiliation: * Durham University, School of Engineering, UK
**GL Aerodynamics, UK

Corresponding Author
Dr R.G. Dominy
Durham University, School of Engineering, South Road, Durham, DH1 3LE, UK
Email: ku.ca.mahrud|ynimod.g.r#ku.ca.mahrud|ynimod.g.r
Telephone: +44 191 334 2513

It is well documented that the aerodynamic performance of a NASCAR race car is compromised when closely following another car. The consequences of operating in the wake of the upwind vehicle include handling imbalance and reduced overall aerodynamic downforce and drag.

Although previous wind tunnel studies have been performed to investigate this race situation the model scale and Reynolds numbers of those tests have generally been compromised in order to accommodate two or more test vehicles within the confines of a typical automotive wind tunnel. In particular it has been found to be the length of the wind tunnel’s moving ground plane and pressure gradients within the test section that have caused the greatest restriction on the useful wind tunnel section test length. Full scale testing has also been performed but mostly with compromised ground simulation ( fixed ground ) and at a financial cost that makes such testing inaccessible to many teams.

This study addresses that issue by the development and evaluation of a very short, bluff body that creates an accurate representation of the wake flow from the leading car thus adding only very slightly to the tunnel working section length that is required for conventional, single car testing and overcoming the need to reduce the scale of the test model. As a consequence teams are able to use a single, standard scale model for both isolated and drafting studies in conventional wind tunnels. The concept is built on that of Wilson et al [1] who initially evaluated the technique for open-wheel race cars. The wake structure that is required to be generated by the bluff body was initially determined by performing CFD simulations of the flow about an asymmetric NASCAR ‘Car of Yesterday’ although the technique is equally useful for current car geometries.

The simulation was based on a wind tunnel model at a test speed of 50 m/s giving a test Reynolds number of 6.7 x 10^6; comfortably into the fully turbulent regime. The moving ground was simulated for these studies although wheel rotation was not.

The wake structure demonstrates clearly an axymmetric characteristic for this vehicle geometry. Using a bluff body of the type proposed by Wilson [1] this wake structure has been recreated accurately using a bluff body wind tunnel model of approximately 1/5th of the length of the complete model. The model can be located upstream of the moving ground plane, if present, in order to provide maximum worling section length to accommodate the test model.

The paper presents the initial evaluation of the wake of an isolated car together with details of the bluff body design and data on pressures, velocities and turbulence structures in the wake.

[1] Wilson, MR., Dominy, RG., Sims-Williams, DB., The Aerodynamic Characteristics of a Race Car Wing Operating in a Wake, SAE 2008-01-0658

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