

	Mechanics På svenska

Doctoral defense

Plasma actuators for separation control on bluff bodies


Defendant	Main Advisor	Extra Advisor	Date
Julie Vernet	Henrik Alfredsson	Gunilla Efraimsson	2017-03-17

Opponent
Kwing-So Choi, University of Nottingham

Evaluation committee
Michel Cervantes, LTH, Luleå, Sweden Jonathan Morrison, Imperial College London Simone Sebben, Chalmers

Abstract

This thesis deals with the experimental realisation of an active flow control technique, that utilises dielectric barrier discharge (DBD) plasma actuators with the ultimate goal to delay flow separation occurring on the A-pillar of truck tractors. The first part of the thesis deals with the development of in-house built DBD plasma actuators and evaluates their performance when placed on a curved surface. The behaviour and parameter dependence of the electric wind in quiescent air were investigated by means of Laser Doppler velocimetry (LDV) and were found to agree with the literature. Furthermore, during the two halfperiods (strokes) of the alternating current, the electric wind was investigated through phase-resolved LDV data, which revealed that while the velocity during both strokes remains positive, it di?ers in magnitude with nearly a factor of two between the strokes. The turbulent boundary layer around a generic geometry, which shares the main flow features with the flow around an A-pillar, i.e. a half cylinder mounted on a flat plat which is approached by a turbulent boundary layer, was as well characterised by means of hot-wire anemometry (HWA) in order to obtain the characteristic length and time scales of the separating shear layer. The second part of the thesis constitutes the heart of the project, viz. a detailed investigation of flow separation control by means of di?erent types of DBD plasma actuators spanning a wide range of operating parameters performed on the generic geometry. In particular, the possibility of reducing the separation length behind the blu? body by means of a spanwise oriented plasma actuator was assessed via pressure measurements along the wall and in the wake of the cylinder and showed that a double arrangement of the spanwise actuator was able to reduce the reattachment length downstream the cylindrical bump which resulted in significant drag reduction. For the spanwise oriented actuators, both steady and pulsed actuations with various frequencies and duty cycles have been studied by means of planar particle image velocimetry (PIV) with respect to the starting vortex and its e?ect on the global flow field. When the steady actuation helps in reducing the recirculation bubble by injecting momentum close to the flow separation, the pulsed actuation directly a?ects the vortical structures in the shear layer with a clear dependency on the pulse frequency and duty cycle. Although using spanwise oriented DBD plasma actuators proved to be successful in delaying turbulent boundary layer separation and moving further upstream the reattachment, it is sensitive to the actual location of the actuator and looses its efficiency with increasing velocity. Therefore, vortex generator (DBD-VG) plasma actuators, in which the electrodes of the actuators are oriented in the streamwise direction so that the array of actuators produces counter-rotating (CtR) streamwise vortices, have been employed as well. A detailed investigation of the flow field by means of planar and stereoscopic PIV has been performed and the appearance of bi-modality in the actuated flow could be revealed by means of statistical and structural analysis. Large scale streamwise structures, thought to be reminiscent of the secondary instability, in the nominal two-dimensional wake flow, are energised by the actuation until a phenomenon of lock-on of these vortices occurs at high driving voltages. In the third part, as a final proof-of-concept study, an array of DBD-VGs has been mounted on an actual truck model and direct balance measurements in an aerodynamic wind tunnel have been performed to assess the drag under no-control and control conditions for various velocities and yaw angles. A clear reduction of the drag has been demonstrated resulting in a power gain higher than the power consumption of the actuators. Although this study was carried out for lower Reynolds numbers than for real driving conditions, the results are encouraging but further development of the plasma actuators is necessary before we will see them on the road.