In the first part of the paper blockage correction in closed test sections was revisited with a "classical approach" of the author. Introducing a newly derived buoyancy term into the correction formula matching corrected values with small scatter was yielded for the drag coefficient at zero yaw and for blockage ratios of up 15%. In the absence of experimental data beyond this blockage ratio the limit of the procedure could not yet be defined. The buoyancy term physically originates from wake distortion, which was neglected in earlier considerations. In order to establish the interference effect over the solid part of the vehicle a ring-vortex model was deployed, similar to the use of nozzle interference corrections in open jet wind tunnels. The result was verified by numerical and experimental data generated by a fast-back and square-back type of a passenger car. The second part of the paper deals with the determination of the interference effects due to blockage in climatic wind tunnels with small test section sizes. For this purpose the different blockage terms were derived as a function of the axial coordinate. This was achieved by employing the ring-vortex model. It turned out that for small nozzles, as they are common practice in climatic wind tunnels, the solid blockage term dominates the other interference effects. In order to counteract the varying interference effects over the length of the vehicle, it is suggested to readjust the tunnel speed with the interference velocity calculated at the front bumper and keep the speed of the chassis dynamometer at the desired test speed in order to maintain the correct thermo-management of the motor. Also for the remaining part of the vehicle, the interference effects are largely increased and thus, the flow about the vehicle is falsely represented. It is further shown that the vehicle position in front of the nozzle is of influence. The closer the vehicle is placed towards the nozzle the smaller are the negative interference velocities at the bumper. This result also depends on the method how the tunnel speed is determined. Finally, it is demonstrated that with a limited amount of physically based equations and a simple iteration-process the variable interference effect can be calculated over the length of the vehicle with basic computer spread sheets. For a selected climatic wind tunnel merely the main geometrical vehicle dimensions and the distance between front bumper and nozzle are variable parameters, which need to be set up into the scheme.


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    Title :

    On buoyancy and wake distortion in test sections of automotive wind tunnels


    Contributors:


    Publication date :

    2013


    Size :

    23 Seiten, Bilder, 20 Quellen



    Type of media :

    Conference paper


    Type of material :

    Print


    Language :

    English






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