Toward Noise Mitigation in High Speed and High Reynolds Number Jets Using Plasma Actuators

Abstract

5 . Eight actuators, distributed azimuthally inside the nozzle, near the nozzle exit, were used to excite various azimuthal modes of the jet over a large frequency range (StDF of 0.1 to 5.0). Dynamic pressure measurements were used to investigate the growth and decay of perturbations and instability waves in the flow, PIV measurements were used to evaluate the effects of control on turbulence field, and far field acoustic was measured to evaluate the control effect on the radiated noise field. The jet responded to the forcing over the entire range of excitation frequencies, but with varying degrees. The growth and decay of perturbations imparted to the flow by the actuators and the ensuing instability waves were found to be similar to the reported results in the literature using other techniques. Excitations with lower frequencies coupled to the flow, and the instability waves grew slowly, saturated farther downstream, stayed saturated for longer time, and then decayed gradually. The saturation and decay of instability waves moved farther upstream as the excitation frequency increased. Instability waves with higher azimuthal modes exhibited faster decay. Preliminary far field acoustic results show a significant noise increase (2 to 4 dB) when the jet is excited around the jet preferred mode instability frequency (StDF=0.2-0.5), which becomes larger from the shallower angle to the jet axis (30°) to the larger angle (90°), in agreement with the results in the literature. Noise reduction of 0.5 to over 1.0 dB is observed over a large excitation frequency range - this reduction seems to peak around StDF =1.5 to 2.0 at 30° angle, but around StDF =3.0 to 3.5 at 90° angle. While forcing the jet with higher azimuthal modes, with azimuthal modes limited to m=0 to 3 in the current experiments, is advantageous for noise mitigation at 30° angle and lower frequencies, the effect is not as clear in higher forcing frequencies and at 90° angle. These preliminary results are quite encouraging and show the potential of the technique. However, a great deal more work must be done to better understand the effects and to improve the technique.