Coupling of bi-stable modes for squareback automotive geometries in close-proximity

This paper presents a study identifying the link between the wakes of the lead and trail body in a two-body platoon with varying longitudinal separation. Base pressure measurements and planar velocity fields in the wake of the lead body have been captured separately using pressure scanners and particle image velocimetry (PIV) in a wind tunnel facility. These results are supplemented by computational fluid dynamics (CFD) simulations for the key configurations, allowing simultaneous analysis of the base pressures and wakes of both models. Three distinct flow regimes are identified, with the first of these occurring at the closest tested separation of 0.1 body lengths ( L ). There is little interaction between the inter-body volume and the surrounding flow field at this separation. The wake of the trail body is like that of an isolated body. A second regime, in which there is more fluctuation in the inter-body gap but weak correlation between the wakes of the two bodies, is explored at separations between 0 . 2 L and 0 . 5 L . A third regime is identified at separations at and above 0 . 75 L . The defining behaviour is that both bodies exhibit bi-stable behaviour and that the correlation of the lateral wake bias on the bodies is negative showing they are ‘locked’ into opposing states at each temporal instance. • Bi-stability is investigated for two Windsor bodies with varying separation. • Three regimes are identified across the range of tested separations. • At close spacings the bodies act as one with little dynamic behaviour in the gap. • At intermediate spacings both wakes exhibit a high frequency lateral oscillation. • At far spacings anti-correlated bi-stable behaviour is seen with linked switching.