Effect of Lip-Thickness on the Acoustic Characteristics of Hartmann Resonator

Abstract

This paper studies the effect of lip-thickness on the various acoustic characteristics of a Hartmann resonator placed axisymmetrically in the flow field of a supersonic jet. Hartmann resonator is a device wherein supersonic jet impinges at the open end of a cylindrical cavity, which is closed at the other end. Such cavities have numerous applications such as flow control, atomization, ignition initiation etc. The various acoustic parameters such frequency, over all sound pressure level, acoustic slice power etc is determined for a cavity with constant lip-thickness and are compared without lip-thickness. The parameters being considered are the nozzle pressure ratio, stand-off distance (S) between nozzle exit and the cavity inlet, cavity length (L). The jet diameter and the cavity inlet diameter are kept constant (7 mm) for both the cavities. Acoustic pressure is measured in the far field region at emission angles varying from 37 to 135, measured from the jet flow direction. It is observed from spectra that the fundamental frequencies of cavities without and with lip are observed to decrease with increase in cavity length. The effect of lip is observed to have negligible effect on the resonance frequency. The maximum OASPL at a nozzle pressure ratio (NPR) of 4 is observed at an emission angle of 60 – 90 for a cavity without lip where as it is almost at 37 for a cavity with lip. The increase of OASPL to a higher value is more pronounced at a higher nozzle pressure ratio thus illustrating that the effect of lip-thickness. The slice power directivity also shifts to a higher value without lip.