Control of leakage flows using periodic excitation

Abstract

Many flows can be characterized as leakage flows from high-pressure sides to lowpressure sides, resulting in increased inefficiency. This paper experimentally explores reducing leakage flows using periodic excitation. A synthetic jet actuator was used as the periodic excitation source in this experiment. The synthetic jet when excited periodically produces momentum but no net mass flux. Two speakers were used in these experiments as actuators: a high-frequency compression driver and a low-frequency woofer. The speaker exit cavity was covered with plates leaving only a small rectangular orifice exit for the synthetic jet. An airtight plexiglass box with a small gap was used to create a simulated leakage flow. The synthetic jet was then directed normally into the leakage flow to reduce the effective gap. The variables considered were as follows: the momentum flux ratio of the actuator over the leakage flow, the reduced frequency, the ratio of the height of the clearance gap over the thickness of the gap, and the ratio of the width of the orifice exit over the thickness of the gap. The ratio of discharge coefficient with actuation over the baseline discharge coefficient was used as the figure of merit. The results showed that the momentum flux ratio exerted a great influence on the leakage flow. A reduction of nearly 70 % in discharge coefficient ratio was achieved at a momentum flux ratio of 70. The reduced frequency was also found to have a significant effect, with the effectiveness of the actuator decreasing as the reduced frequency increased. The geometric parameters had little effect on the leakage flow. Thesis Supervisor: Kenneth S. Breuer Title: Visiting Associate Professor of Aeronautics and Astronautics