Christoph Hollenstein, Pénélope Leyland, Peter Ott, Philip Peschke
The applications of surface dielectric barrier discharge plasma actuators, (DBD), have known a considerable interest recently in the field of flow control. In general, such actuators consist of two electrodes separated by an insulation covering the lower electrode entirely, while the upper electrode remains exposed to the flow. This actuator is driven either by a high frequency AC-voltage or by fast rising voltage pulses. In the former case, a sinusoidal voltage of several kilovolts with frequencies of a few hundred to several thousand Hertz is applied to the electrodes. The actuator generates a body force tangentially to the surface which provides a variety of possibilities to manipulate or to create flows. Inside an existing mean flow, the actuator can be used to impart momentum into a boundary layer and to alter the velocity, the turbulence distribution or to control the transition from laminar to turbulent. The amplitude of the force, created by the plasma actuator can be altered by varying the driving voltage. In the case of the fast rising voltage pulse (FRP) driven DBD actuators, pulses with an amplitude of several kilovolts and with a rise/decay time (from 10%-90% max. voltage) of a few nanoseconds are applied. The wall jet’s velocities of these actuators are quite low, and the actuator acts as an equivalent additional energy source. A weak compression wave is generated at the edge of the flow/air-exposed electrode which prevails whatever the flow regime, and seems to be the predominant mechanism of these FRP actuators. A major part of the experimental work up to now using surface dielectric barrier discharge (DBD) actuators was carried out in flow speeds up to 30 m/s, mostly with actuators driven by AC-voltage. The main identified mechanism of successful control with such actuators at such low speeds is the induced ionic wind effect, which can attain velocities limited to a few meters per second, hence influencing mainly only very low speed flows [8]. With increasing flow speed, however, this influence becomes insufficient and the actuator’s control capacity generated by these AC-driven DBD has not yet achieved expectations [2]. Recently, DBD actuators driven by nanosecond pulsed FRP-DBD have been successfully used to reattach leading edge separation in high speed flows [1]. Even though this is not a phenomenon frequently encountered under realistic flight conditions these findings encouraged to investigate possible effects of FRP-DBD on shock buffet, which is a problem pertaining to transonic cruise flight conditions, and is of interest in compressor design. In order to evaluate whether FRP-DBD actuators have beneficial effects on the aerodynamic performance in high subsonic flows and in particular in mitigating shock buffeting phenomena, a series of experimental investigations has been carried out in the CIRA PT-1 transonic wind tunnel. The flow speed was varied between M=0.4 and M=0.85 at angles of attack from -2° to 8° and Reynolds numbers between Re=1.7∙106 and Re=2.5∙106. Tests were performed on a constant wing span model of a BAC 3-11 supercritical airfoil, with 11% of maximum thickness, able to reach shock buffet conditions at Mach numbers in the range (M=0.75-0.88), at low angles of attack (0-5°) and Reynolds number about 2 - 3 million, [3].[4]. During the tests, steady and unsteady pressure measurements were acquired to obtain quantitative measurements. This paper provides a short description of the experimental setup, presents the main results regarding the effect of the plasma presence on the aerodynamic performance and on the shock buffet phenomenon.
2013
