Computational Fluid Dynamics (CFD) is an important and extensively used tool for aerodynamic development in the vehicle industry today. As manufacturers wish to substitute physical tests on prototype vehicles with virtual simulations, validation of the virtual methods by comparison to wind tunnel experiments is a must. A proper validation can only be performed if the wind tunnel geometry with representative boundary conditions is included in the numerical simulation and if the flow is well predicted for the empty wind tunnel. One of the important flow parameters to predict is the longitudinal pressure distribution in the test section, which is dependent on both the wind tunnel geometry and the settings of the boundary layer control systems. This work investigates the effects of inlet angularity and different boundary layer control systems, namely basic scoop suction, distributed suction and moving belts, on the longitudinal pressure distribution in the Volvo Cars full scale aerodynamic wind tunnel using CFD and a systematic design of experiments approach. The study shows that the different suction systems used to reduce boundary layer thickness upstream of the vehicle have statistically significant effects on the longitudinal pressure distribution in the test section. However, the estimated drag difference induced on a typical vehicle by the difference in horizontal buoyancy between the tested settings is within the test-to-test accuracy of the physical wind tunnel, leading to the conclusion that force calculations in simulations are fairly insensitive to the tested parameters on the intervals investigated.
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AbstractThe aerodynamic drag, fuel consumption and hence CO 2 emissions, of a road vehicle depend strongly on its flow structures and the pressure drag generated. The rear end flow which is an area of complex three-dimensional flow structures, contributes to the wake development and the overall aerodynamic performance of the vehicle.This paper seeks to provide improved insight into this flow region to better inform future drag reduction strategies. Using experimental and numerical techniques, two vehicle shapes have been studied; a 30% scale model of a Volvo S60 representing a 2003MY vehicle and a full scale 2010MY S60.First the surface topology of the rear end (rear window and trunk deck) of both configurations is analysed, using paint to visualise the skin friction pattern. By means of critical points, the pattern is characterized and changes are identified studying the location and type of the occurring singularities. The flow field away from the surface is then analysed using PIV measurements and CFD for the scale model and CFD simulations for the full scale vehicle. The flow field is investigated regarding its singular points in cross-planes and the correlation between the patterns for the two geometries is analysed.Furthermore, it is discussed how the occurring structures can be described in more generalized terms to be able to compare different vehicle geometries regarding their flow field properties.The results show the extent to which detailed flow structures on similar but distinct vehicles are comparable; as well as providing insight into the complex 3D wake flow.
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