The aerodynamic drag of a road vehicle is defined by the measurement obtained at zero yaw in the idealised simulation environment found in the wind tunnel, or when using CFD. This beneficial environment has allowed a continuous improvement in the aerodynamics of passenger cars over many years but it is recognised that it may be unrepresentative of the conditions in which the car usually operates. While the simulated ambient conditions are characterised by very low turbulence and are steady state, the car in the real world experiences an unsteady environment, caused by the effects of the natural wind and by traffic, which can include high levels of turbulence. The potentially inappropriate simulated aerodynamic drag value is then used to determine fuel economy. In the past a wind-averaged drag measurement has been proposed to account for varying yaw conditions experienced by a vehicle on the road, but this is still an essentially steady state solution. In this paper the additional unsteady drag component arising from the vehicle operating in a turbulent flow field is assessed using a quasi steady analysis. The increase in drag at a given yaw angle is shown to be a function of the drag and the gradient of drag with yaw. This modified data is then used to recalculate an unsteady wind-averaged drag coefficient. The standard measure of aerodynamic drag; the drag coefficient at zero yaw, is not representative of the drag obtained in the real world, which includes steady and unsteady wind effects. At moderate car speeds the unsteady wind averaged drag can be nearly 10% higher than the zero yaw drag. The unsteady drag, derived in this paper, is based on perturbations in the onset flow, affecting velocity magnitude and direction. It will be an underestimate for many vehicles as the effects of free stream turbulence on body features are not included.


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    Title :

    Aerodynamic drag in a windy environment


    Contributors:


    Publication date :

    2014


    Size :

    12 Seiten, Bilder, Tabellen, 16 Quellen




    Type of media :

    Conference paper


    Type of material :

    Print


    Language :

    English




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