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Control of streamwise vortices


Many technical applications in the field of automobile and building aerodynamics are affected by the presence of streamwise vortices. For example, these vortices form at the C-pillar of a notchback car which can cause more than 20% of the vehicle’s total drag. Also, trains exposed to strong cross winds generate large and strong vortices on the lee side which can tip the train over. This effect has to be considered during the train’s development because it is part of the certification process. Buildings in cross wind situations form streamwise vortices as well. They arise in proximity to the roof where these vortices generate strong suction forces.

Streamwise vortices originate from shear layers which roll up into a helical shape. During this process, vorticity is transported from the outer region to the vortex core, thereby increasing the circulation along the vortex axis. This causes a low pressure region which may not be desirable. The vortex strength increases until vortex breakdown occurs, a very well-known phenomenon on delta wings.

A sweptback semi-span step is chosen as a generic model to investigate these streamwise vortices. The step simulates the flow structures at the C-pillar of the generic car model "Ahmed-Body". As expected, the separating shear layer rolls up into a streamwise vortex. An elimination of this vortex may significantly reduce the vehicle’s drag. A means for controlling the shear layer and the corresponding vortex is provided by active flow control. The velocity profile of a shear layer possesses an inflection point which renders it unstable. This characteristic leads to the well-known Kelvin-Helmholtz instability which is sensitive to certain frequencies even at low amplitude. Thus, the shear layer may be manipulated with little effort. If controlled at its source, the vorticity transported to the vortex core of the rolling up shear layer can be contained as well. This provides substantial control authority of the vortex strength and the resulting low pressure.

In the future, an effective and efficient control of streamwise vortices may result in reduced fuel consumption of road vehicles, improved cross wind stability of trains, and decreased structural loads on buildings.


Christoph Strangfeld

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