Sunday, July 28, 2013

Front-wing vortex breakdown

With the assistance of Mercedes GP, Jacques Heyder-Bruckner completed a PhD thesis in 2011 which analysed the front wing-wheel interaction on a racing car. Of particular interest in this work is the fact that Detached Eddy Simulation (DES) was used to represent the phenomenon of vortex breakdown, which occurs when the front-wing approaches low ride-heights under conditions of roll and pitch.

Whilst Reynolds Averaged Navier-Stokes (RANS) simulations are the CFD workhorse of modern motorsport, RANS is known to be inadequate for representing separated flows, and in particular for representing large-scale vortical phenomena, such as vortex evolution and breakdown. In contrast, Detached Eddy Simulations use RANS to represent the attached flow, but directly solve the Navier-Stokes equations in the regions of separated flow.

Q-criterion isosurfaces of the vortex breakdown phenomenon, taken from the instantaneous DES flowfield, are depicted here. Bruckner points out that “the vortex breakdown moves forward as the wing is moved closer to the ground…The large vortex expansion…is composed of a recirculation region enclosed by the spiralling tail shed from the vortex breakdown.This causes high pressure fluctuations on the endplate and flap, resulting in a more unstable wing with variations in downforce and drag three times larger than at higher ride-heights.” (p116).