1. ACTIVE CONTROL OF NATURAL TOLLMIEN-SCHLICHTING WAVES USING
PLASMA ACTUATORS
Abstract
Understanding, predicting and controlling laminar to turbulent transition is of great interest due to
the wide range of practical applications where transition is significant. Generally, turbulent flow has
an order of magnitude higher skin friction. Fuel savings of up to 25 % would be possible for a large
commercial aircraft if laminar flow could be maintained over the wings. At low to moderate
Reynolds number several types of airfoils show the development of Tollmien-Schlichting (T-S)
instabilities. These T-S waves become the dominant cause of transition which is further amplified
by an adverse pressure gradient.
Natural T-S instabilities occur randomly as wave packets. Effective damping can only be achieved
by a closed loop feed-forward control algorithm. The current work presents an experimental study
on active control of natural T-S waves developing on the suction side of an NACA 0012 airfoil. A
closed-loop control system has been implemented using the filtered-x Least Mean Squares adaptive
algorithm based on Finite Impulse Response filters. Surface mounted microphones are used as
sensors. The controller drives a Dielectric Barrier Discharge plasma actuator placed along the span
of the airfoil. In contrast to the conventional sinusoidal signal, the actuator is powered using a
continuously adapted signal selected by the controller in order to damp the incoming wavetrain of
T-S waves. High speed 2-component Particle Image Velocimetry is used to characterize the flow in
the vicinity of the actuator. Several cases are tested using both open-loop and closed-loop actuation.
Tested freestream velocities range from 17 to 25 m/s at chord Reynolds of 2.2×105 to 3.3×105
respectively.
Results indicate the suppression of the tonal component of unstable T-S waves with closed-loop
actuation. In the case of closed loop control, spectral analysis of error microphones shows the
reduction of natural T-S waves up to 7.65 dB, 3.7 dB and 2 dB for freestream velocities 17, 20 and
25 m/s respectively. The RMS reduction of error microphone signals indicates a reduction of
approximately 50% for 17 m/s while a significant reduction of approximately 17% and 11 % is
maintained for freestream velocities 20 and 25 m/s. In the case of open-loop control, the actuation
at naturally stable actuation frequencies results in a dampening of instabilities due to the dominant
blowing effect of the actuator whereas amplification of instabilities occurs at unstable actuation
frequencies. In the open loop, the actuator is operated using non-adapted single-frequency
sinusoidal signal.