# Experimental Tests and Aeroacoustic Simulations of the Control of Cavity Tone by Plasma Actuators
###### 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.  
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Control of the cavity tone by a plasma actuator (PA) has also been investigated [6; 7]. Figure 2 shows a typical configuration of a dielectric-barrier-discharge plasma actuator. It comprises an upper exposed and a lower embedded electrode, with a dielectric layer sandwiched between the electrodes. By applying a high AC voltage to the electrodes, momentum is transferred from the charged plasma particles to the neutral gas through collisions between the plasma components and neutral air molecules. As a result, a flow is induced from the upper electrode toward the lower one. Plasma actuators have many advantages: they are simple configurations without moving parts, have fast time response and low mass, with the ability to be attached easily onto objects, as compared with other flow-control actuators, such as a piezo-actuator, an impinging jet, or a synthetic jet.  
[PERSON] et al. [6] succeeded in reducing the cavity tone by using spanwise-aligned plasma actuators, with the electrodes elongated in the streamwise direction (\"streamwise plasma actuators\"), which introduced a spanwise variation of velocity in the cavity flow. However, the sound-reduction mechanism of this approach has not been clarified in detail.  
The objective of the present investigation is to clarify the mechanism by which streamwise plasma actuators reduce the cavity tone. In preliminary experiments, the effects of the spanwise width and pitch of the lower electrode on the control of the cavity tone were studied. In Section 2, both our experimental methodologies and the simulations performed to clarify the mechanism through which the plasma actuators control the intensity of the cavity tone are discussed. The comparison of predicted results with those measured is discussed in Section 3. The predicted and measured results are discussed in detail in Section 4. In Section 5, conclusions are summarized.