Parametric study of the Hartmann-Sprenger tube

Experiments were conducted with a Hartmann-Sprenger tube (H-S) to study the effect of different parameters on the frequency and amplitude of acoustic fluctuations excited when the H-S underexpanded jet impinges on an in-line cavity. Time averaged shadowgraphs were acquired to study the flow field between the underexpanded jet and the cavity for varying parameters of the H-S tube. It was observed that the H-S tube primarily excited two different modes. The first mode corresponds to the jet regurgitant mode (JRG) where the frequency of oscillations scales as a function of the cavity depth. The other mode is screech where an oscillating shock is formed in front of the cavity. The screech mode excites a higher acoustic frequency than the JRG and it is observed to be a strong function of the pressure ratio R, and distance between the jet and the cavity X. At a fixed cavity length, varying standoff distance X could excite either the JRG or screech. At very low standoff distances (X/D-j < 0.8), the current study indicates that there is a mode switch from screech to JRG. A cavity to jet diameter, D-c/D-j > 1 was found to sustain JRG over a wide range of X. Diameter ratios D-c/D-j < 1 sustained high frequency screech modes in a wide range of H-S tube parameters.