Julie A. Vernet

Turbulent Boundary Layer Upstream, Over and Downstream a Cylindrical 2D Bump

Most flows of relevance in technical applications are exposed to the combined effect of surface curvature and pressure gradient. The applicability of knowledge from canonical wall-bounded flows is hence limited when it comes to these complex flows and geometries. The present investigation is an attempt to contribute to this topic, by experimentally investigating the turbulent boundary layer developing upstream, over, and downstream a wall-mounted cylinder section. Hot-wire anemometry is used and although such measurements are impaired in the separated region, we show here how to exploit the readings to get information on the separated region. Single-point streamwise velocity statistics are presented and discussed with focus on the effect of back flow (separation) on the statistics and the emergence of an internal boundary layer.

Plasma Streamwise Vortex Generators for Flow Separation Control on Trucks

An experimental study of the effect of Dielectric Barrier Discharge plasma actuators on the flow separation on the A-pillar of a modern truck under cross-wind conditions has been carried out. The experiments were done in a wind tunnel with a 1:6 scale model of a tractor-trailer combination. The actuators were used as vortex generators positioned on the A-pillar on the leeward side of the tractor and the drag force was measured with a wind-tunnel balance. The results show that the effect at the largest yaw angle (9 degrees) can give a drag reduction of about 20% and that it results in a net power reduction. At lower yaw angles the reduction was smaller. The present results were obtained at a lower Reynolds number and a lower speed than for real driving conditions so it is still not yet confirmed if a similar positive result can be obtained in full scale.

Flow separation control by dielectric barrier discharge plasma actuation via pulsed momentum injection

Control of a turbulent boundary layer separating on a half-cylinder mounted on a flat plate has been investigated using a Dielectric Barrier Discharge (DBD) plasma actuator placed along the apex of a cylinder. The main focus of the study has been to evaluate if the control ability of the actuator can be improved through pulsed actuation compared to its steady counterpart. Investigations of the electric wind induced by the DBD plasma actuator in still air, when mounted on the flat plate, revealed that while the steady actuation produces an electric wind similar to a wall jet, the pulsed actuation creates a train of co-rotating vortices. The vortices are the result of a starting vortex produced by the actuator at each actuation pulse. A parametric study showed a dependence of the size, shape and propagation velocity of the vortices on the pulse frequency and duty cycle. With the actuator mounted along the apex of the cylinder, Particle Image Velocimetry measurements of the uncontrolled and controlled flow with a free-stream velocity of 5 m/s showed a clear reduction of the recirculation region downstream the cylinder when using plasma actuation. An even higher control effect could be achieved with pulsed actuation compared to the steady actuation. Phase-locked measurements of the unsteady actuation showed that pulsed actuation periodically shifted the flow separation location resulting in the propagation of vortical structures in the recirculation region. The size of the vortical structures showed a dependence on the pulsed actuation timing parameters.Control of a turbulent boundary layer separating on a half-cylinder mounted on a flat plate has been investigated using a Dielectric Barrier Discharge (DBD) plasma actuator placed along the apex of a cylinder. The main focus of the study has been to evaluate if the control ability of the actuator can be improved through pulsed actuation compared to its steady counterpart. Investigations of the electric wind induced by the DBD plasma actuator in still air, when mounted on the flat plate, revealed that while the steady actuation produces an electric wind similar to a wall jet, the pulsed actuation creates a train of co-rotating vortices. The vortices are the result of a starting vortex produced by the actuator at each actuation pulse. A parametric study showed a dependence of the size, shape and propagation velocity of the vortices on the pulse frequency and duty cycle. With the actuator mounted along the apex of the cylinder, Particle Image Velocimetry measurements of the uncontrolled and controlled flow w...