Abstract
A plasma actuator comprising a dielectric layer sandwiched between upper and lower electrodes can induce a flow from the upper to lower electrode by means of an externally-applied electric field. Our objective is to clarify the mechanism by which such actuators can control the cavity tone. Plasma actuators, with the electrodes elongated in the streamwise direction and aligned in the spanwise direction, were placed in the incoming boundary of a deep cavity with a depth-to-length ratio of 2.5. By using this experimental arrangement, the amount of sound reduction (“control effect”) produced by actuators of differing dimensions was measured. Direct aeroacoustic simulations were performed for controlling the cavity tone by using these actuators, where the distributions of the body forces applied by the actuators were determined from measurements of the plasma luminescence. The predicted control effects on the flow and sound fields were found to agree well with the experimental results. The simulations show that longitudinal streamwise vortices are introduced in the incoming boundary by the actuators, and the vortices form rib structures in the cavity flow. These vortices distort and weaken the two-dimensional vortices responsible for producing the cavity tone, causing the tonal sound to be reduced.
Highlights
Self-sustained oscillations in a flow over a cavity as shown in Figure 1—such as a sunroof of an automobile or various gaps between parts of a high-speed transportation vehicle—can radiate intense tonal noise as a cavity tone
The variation sound-reduction levelwas at the fundamental of 1500 of of the voltage of Epathe applied to the actuators measured, wherefrequency the spanwise pitchHz of as thea function lower the voltage
To clarify the mechanism by which streamwise plasma actuators aligned in the spanwise direction reduce the cavity tone with the acoustic resonances, direct aeroacoustic simulations were performed, as well as wind tunnel experiments
Summary
Self-sustained oscillations in a flow over a cavity as shown in Figure 1—such as a sunroof of an automobile or various gaps between parts of a high-speed transportation vehicle—can radiate intense tonal noise as a cavity tone. This high-intensity noise occurs at a single peak frequency, making it a very unpleasant for many people. Rossiter [1] described an oscillation mechanism similar to that presented for edge tones by Powell [2] In this mechanism, the interactions of vortices with the downstream edge of the cavity radiate acoustic waves, which cause the formation of new vortices at the upstream edge. An acoustic resonance sometimes occurs in the cavity—such as a one-quarter-wavelength-depth mode—making the cavity tone more intense [3]
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