Real world drag coefficient - is it wind averaged drag

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Real world drag coefficient - is it wind averaged drag

Windsor, S.

Drag reduction of road vehicles continues to be the holy grail of aerodynamicists with renewed importance of aerodynamics as OEMs strive to reduce CO₂ and improve fuel economy. New legislation such as Worldwide harmonized Light vehicles Test Procedure (WLTP) is increasing the emphasis on real world boundary conditions (with moving ground wind tunnel testing) and vehicle configurations, in an effort to obtain more realistic fuel consumption figures. However, WLTP in common with all other drive cycles used in the automotive sector is based on zeroyaw aerodynamic measurements. This implicitly assumes conditions vehicles rarely see: the onset wind vector is at zero yaw, the atmosphere is still and undisturbed by other road users. This paper will show that many modern cars (particularly low drag saloon cars) have noticeable drag coefficient (C_D) minima at zero yaw, with some having up to 10% increase in C_D for relatively small yaw angle changes around zero. The concept of wind averaged drag (C_DW) will be discussed as a means of assessing real world aerodynamic performance. Three methods of calculating C_DW are investigated and the MIRA method is preferred for application to passenger cars. This demonstrates that including yaw angle effects via C_DW provides a significantly different perspective on the aerodynamic contribution to fuel economy.