# Experimental Tests and Aeroacoustic Simulations of the Control of Cavity Tone by Plasma Actuators
## 5 Conclusions  
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. The predicted flow and acoustic fields, both with and without control, are in good agreement with those measured. To the authors' knowledge, this is the first time that direct aeroacoustic simulations of the control of aerodynamic noise by plasma actuators have been performed, including comparisons with experimental data.  
On the measured effects of lower electrode width on the control, the cavity tone was effectively reduced for a lower-electrode width of \(d_{\mathrm{l}}\) = 3, 5, and 7 mm, while the control effects on the cavity tone were negligible for \(d_{\mathrm{l}}\) = 1 mm. The predicted and measured flow fields show that streamwise vortices are induced by the actuators for \(d_{\mathrm{l}}\) = 3 mm in the absence of a freestream.  
The results show that aerodynamic sound can be controlled at the relatively high freestream velocity of 30 m/s by 31 dB by plasma actuators with the spanwise fine pitch of actuators of \(s\) = 4 mm and \(d_{\mathrm{l}}\) = 3 mm at the applied voltage of \(E_{\mathrm{pa}}\) = 5 kV\({}_{\mathrm{p}}\)-p, even though they can only induce flows with a maximum velocity of 2 m/s. In comparison with the control by blowing jets, the plasma actuators can more effectively reduce the cavity tone by introducing streamwise vortices with the fine displacement.  
The predicted phase-averaged flow fields show that the streamwise vortices by the control are introduced into the cavity flow as longitudinal vortices. Due to these vortices, large-scale vortices with spanwise axes that cause the cavity tone are distorted and weakened. Moreover, the coherence of the velocity fluctuations in the spanwise direction decreases, which also contributes to the reduction of the cavity tone.  
Although the exact estimation of the power consumption is a future problem, the effectiveness of the plasma actuators as the devices for noise reduction can be shown in this paper. The technological problems related with high-voltage safety or high-frequency effects are also remaining challenges for application to actual vehicles.  
This work was supported by JSPS KAKENHI grant number JP17K06153 and through the application development for Post K computer (FLAGSHIP 2020) by the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT). The authors appreciate Mr. [PERSON] for contributions throughout the manufacturing of the actuators.  
[PERSON] conceived and designed the experiments, conducted the simulations, and wrote the paper; [PERSON] performed the experiments; [PERSON] and [PERSON] analyzed the data; [PERSON] contributed reagents/materials/analysis tools; and [PERSON] and [PERSON] supervised the research.  
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.