Plasma actuators for separation control on bluff bodies

Sammanfattning: This thesis deals with the experimental realisation of an active flow controltechnique, that utilises dielectric barrier discharge (DBD) plasma actuatorswith the ultimate goal to delay flow separation occurring on the A-pillar oftruck tractors.The first part of the thesis deals with the development of in-house builtDBD plasma actuators and evaluates their performance when placed on acurved surface. The behaviour and parameter dependence of the electric windin 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 investigatedthrough phase-resolved LDV data, which revealed that while the velocity duringboth strokes remains positive, it differs in magnitude with nearly a factor oftwo 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. ahalf cylinder mounted on a flat plat which is approached by a turbulent boundarylayer, was as well characterised by means of hot-wire anemometry (HWA)in order to obtain the characteristic length and time scales of the separatingshear layer.The second part of the thesis constitutes the heart of the project, viz. adetailed investigation of flow separation control by means of di↵erent types ofDBD plasma actuators spanning a wide range of operating parameters performedon the generic geometry. In particular, the possibility of reducing theseparation length behind the bluff body by means of a spanwise oriented plasmaactuator was assessed via pressure measurements along the wall and in the wakeof the cylinder and showed that a double arrangement of the spanwise actuatorwas able to reduce the reattachment length downstream the cylindricalbump which resulted in significant drag reduction. For the spanwise orientedactuators, both steady and pulsed actuations with various frequencies and dutycycles have been studied by means of planar particle image velocimetry (PIV)with respect to the starting vortex and its effect on the global flow field. Whenthe steady actuation helps in reducing the recirculation bubble by injectingmomentum close to the flow separation, the pulsed actuation directly affectsthe vortical structures in the shear layer with a clear dependency on the pulsefrequency and duty cycle. Although using spanwise oriented DBD plasma actuatorsproved to be successful in delaying turbulent boundary layer separationand moving further upstream the reattachment, it is sensitive to the actual locationof 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 ofactuators produces counter-rotating (CtR) streamwise vortices, have been employedas well. A detailed investigation of the flow field by means of planarand stereoscopic PIV has been performed and the appearance of bi-modalityin the actuated flow could be revealed by means of statistical and structuralanalysis. Large scale streamwise structures, thought to be reminiscent of thesecondary instability, in the nominal two-dimensional wake flow, are energisedby the actuation until a phenomenon of lock-on of these vortices occurs at highdriving voltages.In the third part, as a final proof-of-concept study, an array of DBD-VGshas been mounted on an actual truck model and direct balance measurementsin an aerodynamic wind tunnel have been performed to assess the drag underno-control and control conditions for various velocities and yaw angles. A clearreduction of the drag has been demonstrated resulting in a power gain higherthan the power consumption of the actuators. Although this study was carriedout for lower Reynolds numbers than for real driving conditions, the resultsare encouraging but further development of the plasma actuators is necessarybefore we will see them on the road.