Due to the increased interest in the aerodynamics of truck /trailer combinations as a means to reduce their fuel consumption, experiments have been conducted on a model of a trailer to study possible changes in its near wake arising form attached aft-flaps. The model was fitted with vertical flaps on both sides of the back plate, which were set to oscillate at given frequencies and amplitudes. Experiments were carried out using the PIV system in a Water Channel at Monash University. The oscillation of the flaps perturbed the flow in such a way that the shear layer was significantly changed and the roll up of vortices was moved further upstream, causing a rise in the rate of entrainment and a shorter reattachment length. It also had the effect of a reduced wake size and higher velocities within the wake. These are all effects that are consistent with an increase in the base pressure at the back plate of the trailer and therefore also a reduced drag coefficient, since they are strongly connected