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日本語

The research team of Associate Professor Hiroshi Yokoyama has developed a method for reducing aerodynamic noise via plasma generation in the air. The process known as cavity flow, the flow of air over a hole or concave shape, is capable of generating significant aerodynamic noise. The plasma actuator is a device that can induce various flows in air via plasma generation. Using a plasma actuator to suppress this noise, the team achieved a reduction of up to 35 dB of aerodynamic noise. Furthermore, the periodic switching off and on of the plasma actuator at an appropriate frequency led to a better performance in sound reduction when compared to the continuous operation of the plasma actuator under the same power consumption. The results of their research were published in Physics of Fluids on October 9, 2020.

Cavity flows, such as the flow of air between carriages on high-speed trains or around the landing gear configurations of aircraft, radiate aerodynamic noise at a level which can cause discomfort for passengers. This problem has recently been tackled by the use of a flow-induced device consisting of top and bottom electrodes and a dielectric layer between them, known as a plasma actuator, to achieve flow control.

The research team demonstrated that the plasma actuator successfully weakened vortices that can cause intense levels of sound. Thus, the aerodynamic noise from the cavity flow was reduced by as much as 35 dB. Furthermore, to reduce the power required for driving the plasma actuator, the plasma actuator was periodically switched off. This type of driving is termed as "intermittent control." Intermittent control at an appropriate frequency leads to higher sound reduction when compared with continuous control under the same power consumption. The simulations of flow and sound in a supercomputer clarified a weakening effect on the vortices, which cause intense sound, thanks to the control via a plasma actuator. Moreover, the cavity tone was continuously reduced even via intermittent control at an appropriate frequency.

The plasma actuator’s performance is currently still inhibited by the limitation in achievable induced flow speed and the treatment of ozone generation with plasma. However, the research team believes that this noise reduction method will contribute, directly or indirectly, to the design of more comfortable transport vehicles. Aerodynamic noise becomes increasingly problematic as the speed of transport vehicles is increased. Hence, the development of a reduction mechanism for aerodynamic noise can help us to realize faster transport vehicles, while keeping the noise levels down.

The mechanism of acoustic radiation from the cavity flow is similar to that of a whistle. If a finger is moved to or from the mouth during whistling, the whistle tone stops and starts. It is important to consider the effect of the speed at which the finger is moved. Furthermore, it is important to determine whether the tone can be stopped by sufficiently fast movement. By utilizing the fast time response of the plasma actuator, the results of this study addressed this question. The results indicated a continuous reduction in the tone via control at an appropriate frequency, which is dependent on the cavity flow configurations.

This study was supported by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan under JP17K06153.