  The aim of this research work is to analyse the large scale response of a
vehicle wake to on-road perturbations by using an instrumented vehicle and a
combination of scale one wind tunnel tests, track trials and on road
experiments. More precisely, in all these tests, we focus on the analysis of
the asymmetry of the pressure distribution at the base. Proper Orthogonal
Decomposition (POD) is used. For all cases considered, POD analysis reveals two
dominant modes, respectively associated with vertical and horizontal wake large
scale reorganisation. More than 50\% of the total energy is carried by these
two modes and this value increases significantly for on-road tests. Noteworthy,
the low-frequency energy content of the temporal coefficients of these modes is
significantly higher on-road. Low frequencies (even very low ones) then play a
major role, corresponding to a quasi-static perturbation domain of the velocity
seen by the vehicle. We show that a quasi-steady exploration of the on-road yaw
angle statistical distribution during a wind tunnel test captures phenomena
similar to those observed on the road and is therefore interesting to evaluate
on-road aerodynamic performances. This also opens perspectives for developing
closed loop control strategies aiming to maintain a prescribed wake balance in
order to reduce drag experienced on the road.
