Bimodal Wake Cycling with Turbulent Flow

The vehicle wake region is of high importance when analyzing the aerodynamic performance of a vehicle. It is characterized by turbulent separated flow and large low pressure regions that contribute significantly to drag. In some cases the wake region can oscillate between different modes which can provide engineering challenges during vehicle development. Vehicles that exhibit bimodal wake behavior need to have their drag values recorded over a sufficient time period. When simulating such a vehicle in CFD this can be a problem as the period of oscillation can be very long, on the order of 10’s of seconds or minutes. Running such simulations with sufficient time to capture the average drag would be prohibitively expensive in terms of CPU hour cost during the vehicle design process. As an alternative to running a long simulation it is possible to influence the wake behavior through the use of turbulence. By adding turbulence to the upstream flow the wake can be prevented from settling into particular mode and will instead rapidly cycle between modes, allowing for an average value to be calculated in a much shorter simulation time. This average value can then be used to compare different vehicle designs and make informed decisions during the development phase.