Khan M., Seto K., Teramoto K.

Jet screech is the most remarkable aerodynamic noise which is caused by acoustic feedback loops and capable of causing tailplane structural failure. There were many efforts to suppress this screech component; however, most of them used a stuff that protruded into jet flow which reduced the thrust and finally decreased the overall efficiency of the aircraft. In the present experimental study, investigations have been carried out to evaluate the performance of the new control technique of jet screech with a spherical reflector. In order to reduce the jet screech the reflector was placed at the nozzle exit. The placement of the spherical reflector at the nozzle exit controlled the location of the sound image source as well as minimized the sound pressure at the nozzle exit. It has been indicated that the new technique suppressed not only the screech tones but also the broadband noise components. Furthermore the spherical reflector was very much effective in reducing overall sound pressure in the upstream region of the nozzle exit. Hence, proposed spherical reflector could protect the upstream noise propagation and may be promising countermeasure to protect the fuselage of an aircraft from acoustic fatigue by incorporating it into a streamline form. In the present paper the performance the proposed technique was examined acoustically as well as aerodynamically. Acoustic data was analyzed by using 8-channel data acquisition apparatus and the aerodynamic evaluation was done by Schlieren apparatus along with high-speed video camera. The thrust loss due to the new technique was also evaluated directly in the vertical wind tunnel.