Broadband Shock-Associated Noise in Screeching and Non-Screeching Underexpanded Supersonic Jets

The effect of screech tones on the broadband shock-associated noise of underexpanded jets is investigated experimentally. Screech is removed by means of a notched nozzle, and the properties of the broadband shock-associated noise in the screech-free configuration are compared to that in a screeching flow. It is first demonstrated that the suppressing technique used is nonintrusive in that it does not alter the shock-cell structure of the jet plume. It is then shown that screech has an effect on the aerodynamics of the jet, which induces changes in the broadband shock-associated noise. Indeed, screech accelerates the damping of the shock-cell pattern, leading to an attenuation of the broadband shock-associated noise and a shifting of this noise component to higher frequencies. Moreover, a tuning between the peak frequency of the broadband shock-associated noise and the screech frequency is observed. It is also deduced from the directivity of the broadband shock-associated noise in the far field that the convective velocity in the shear layer is modified in the presence of screech tones.