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Licentiate seminar

Plasma actuators for separation control - design and application


Defendant Main Advisor Extra Advisor Date
Julie Vernet Henrik Alfredsson Ramis Örlü 2014-12-19

Opponent
Marios Kotsonis, TU Delft, the Netherlands

Evaluation committee

Abstract

This thesis deals with the experimental realisation of an active flow control technique, that utilises dielectric barrier discharge (DBD) plasma actuators, to re-energise boundary layers subjected to adverse pressure gradients. The ultimate goal of the research is to delay flow separation occurring on the A-pillar of trucks tractors within the Flow Research on Advanced and Novel Control Efficiency (FRANCE) project. The first part of the project consists of the development of manufacturing techniques for in-house built DBD plasma actuators and to evaluate 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). The electric wind was found to develop similar to a wall jet and the results of the parametric study are in agreement with the literature, viz. increase of wind velocities with increasing driving voltage and frequency and with decrease of dielectric sheet thickness. The measured evolution of the induced wall jets served also as a benchmark data base for a companion numerical project. Furthermore, during the two half-periods (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 differs in magnitude with nearly a factor of two between the strokes. Since the phase dependence of the electric wind is observed only in the vicinity of the actuator, the assumption of a steady induced force in simulations seems to be justified. For the second part of the project, a double DBD plasma actuator was used to control flow separation occurring on a cylindrical bump approached by a turbulent boundary layer. The uncontrolled flow that develops on the flat plate upstream the bump, separating on the half-cylinder geometry and reattaching on the downstream flat plate was characterised using hot-wire anemometry. Finally, detailed pressure measurements at the wall and in the wake of the cylinder showed that the double actuator was able to reduce the reattachment length downstream the cylindrical bump which resulted in a drag reduction up to 30% when the actuator was placed a few millimetres upstream the separation point. However, drag reduction was observed for a wide range of actuator positions and it is hypothesised that the measured reduction could be due to different mechanisms, either as a re-energising of the shear-layer or as addition of momentum close to the wall thereby delaying separation and increasing the base pressure on the backside of the half-cylinder.
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