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.
The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. 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.
The Volvo Cars aerodynamic wind tunnel has had a vortical flow angularity pattern in the test section since its original commissioning in 1986. The vortical flow nature persisted after an upgrade in 2006, when the fan was replaced and a moving ground system was introduced. It has been hypothesized that the cause for this flow angularity pattern was leakages around the heat exchanger installed in the settling chamber. The present paper tests this hypothesis by measuring the flow angularity in the test section before and after sealing the leakages. The findings show that the leakage path around the heat exchanger does not influence the flow angularity, and that the current pattern is different compared to the commissioning after the upgrade. This prompted an investigation of the influence from the turbulence screens, which were changed after the upgrade commissioning. These investigations indicate that the probable cause of the vortical flow angularity pattern is residual swirl from the fan. Force measurements on a reference car with and without extra induced flow angularity show that the flow angles measured in the tunnel for regular operation are most likely small enough to not have a significant effect on the measured aerodynamic forces.
Many aerodynamic wind tunnels used for testing of ground vehicles have advanced ground simulation systems to account for the relative motion between the ground and the vehicle. One commonly used approach for ground simulation is a five belt system, where moving belts are used, often in conjunction with distributed suction and tangential blowing that reduces the displacement thickness of the boundary layer along the wind tunnel floor. This paper investigates the effects from aft-belt tangential blowing in the Volvo Cars Aerodynamic wind tunnel. First the uniformity of the boundary layer thickness downstream of the blowing slots is examined in the empty tunnel. This is followed by investigations of how the measured performance of different vehicle types in several configurations, typically tested in routine aerodynamic development work, depends on whether the tangential blowing system is active or not. Numerical simulations are also used to explain the flow field origin of the force differences measured in the wind tunnel. Results show that even though the displacement thickness behind the blowers varies along the width of the blowing slots, it is significantly reduced compared to the case of no blowing; furthermore, it is also shown that deactivating the blowing altogether has an effect not only on the absolute forces but also on the deltas measured between different configurations, and that this phenomenon is more prominent if the vehicle has a larger base area.
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