Analysis of the Unsteady Flow Field of a Passenger Vehicle

Doktorsavhandling, 2015

Political legislation and consumer requirements for a sustainable society require that vehicle manufacturers focus on reduced fuel consumption and lower emissions. A reduction in the overall energy consumption can be directly linked to a decrease of the driving resistance, where the aerodynamic drag significantly contributes at velocities above 70km/h. On passenger vehicles, and this is particularly true for Sports Utility Vehicles (SUVs), the total aerodynamic drag mainly consists of the so called pressure drag, which is the difference between the stagnation pressure at the front and a pressure deficit in the base wake. This thesis experimentally and numerically investigates the wake shape behind a Volvo XC60 in order to further improve the understanding of the main flow physics, both for time-averaged flow and in unsteady conditions. To increase the base pressure, and hence, to reduce the aerodynamic drag, tapered extensions are implemented. Their effect on the surrounding near-wake and their impact on the global forces under different yaw angles are studied and discussed. All experimental measurements are conducted in the Volvo Cars Aerodynamic Wind-Tunnel, where in addition to the traditional time-averaged measurements, a procedure for the determination of the fluctuating pressure is used. Numerically, a methodology based on Spalart-Allmaras Delayed-Detached Eddy Simulation, SA-DDES, suitable for complex geometries at high Reynolds numbers, is developed. The results confirm that the extensions reduce drag and act as a truncated boat-tailing device smoothing out the pressure recovery zone. It is found that key to the design of the extensions is the guidance of the flow such that the flow enters the separation zone in a longitudinal direction. This is accomplished through a guiding vane or "kicker". It is also shown that this device enables a better yaw response of the vehicle that is less sensitive to changes in yaw conditions, while rear-lift values are kept within acceptable limits. With respect to the numerical results, an unsteady methodology that can handle the complex geometries typical for automotive flows has been developed. The methodology is tuned to fit the requirements in lead time that are acceptable and feasible for industrial applications. The results confirm an improved representation of the flow field that is comparable to the wind tunnel measurements.