Drag reduction mechanisms of a car model at moderate yaw by bi-frequency forcing

A bi-frequency open-loop control strategy aiming to combine both high- and low-frequency forcing effects is used to experimentally reduce the drag of a simplified car model at a slight yaw angle of 5 degrees. The unforced mean wake features a lateral asymmetry which induces a low base pressure footprint close to the leeward side and increases drag compared to the aligned model. Forcing is performed with pulsed jets along the windward trailing edge. High-frequency forcing acts as a time-invariant flap. The fluidic flap effect deviates the windward shear layer towards the leeward side and reduces the wake bluffness, but the lateral asymmetry of the near wake is still observed. The drag reduction related to this high-frequency forcing is about 6% with a high actuation efficiency. A modulation of the high-frequency forcing with a low-frequency component is then introduced in order to modify the mass and momentum exchange in the separating shear layer at the windward trailing edge. We find that the modulated forcing provides the ability to manipulate the mean wake orientation while maintaining the fluidic flap effect. Among all wake orientations, those reducing drag are the ones having a mean symmetric wake. The bi-frequency control strategy leads to a maximum drag reduction of 7% for the best choice of frequencies. Importantly, the bi-frequency control is more efficient than the single high-frequency forcing, the actuator requiring only half the actuation energy and presenting an actuation efficiency multiplied by 3. Finally, the physical mechanisms related to drag reduction are carefully analyzed. In particular, we show that the wake symmetrization reduces the global production of turbulent kinetic energy in the shear layers. These results open up opportunities for closed-loop control of wake asymmetries.