A systematic study of supersonic jet noise

Abstract

The purpose of this work is to study the acoustic fields associated with two different nozzle configurations; a rectangular and a circular. Both nozzles are designed with the same exit Mach number and have an identical momentum and energy flux. By presenting a comparison of the two nozzles, it proposed to establish and identify the dominant noise generating mechanisms. A basic difference in shape changes the relative importance of different noise mechanisms. The other main aim of this study is to establish scaling laws of supersonic jet noise. A shock tube is a very versatile apparatus for such an analysis. By first changing the driver, driven pressure and molecular weights, a wide range of stagnation pressures and temperatures could be achieved. The case with which these conditions are simulated is, however, traded off with the short test time, of the order of milliseconds. A short test time allows the use of a heat sink nozzle and eliminates the use of an anechoic chamber.

So far tests have been made in the range of 1000-5000°R, for different levels of expansion and an exit Mach number of 2.7. In comparing the two nozzles, it is found that the rectangular nozzle is indeed quieter than the circular nozzle. The rectangular nozzle is more effective under overexpanded conditions, and a factor of 1.6 in acoustic efficiency at low temperature (1200°R) and about 3 at high temperature is related to a rapid deceleration of the jet through a system of strong shocks. The low acoustic efficiency of the overexpanded rectangular jet is related to a rapid deceleration of the jet through a system of strong shocks. At high temperature, this effect is not observed because an important density ratio exists across the shear layer which becomes very unstable due to the Taylor instability.

For both the circular and rectangular nozzle, the effect of temperature showed an increase in the directivity angle at high temperature which is correlated to an increase in eddy convective velocity, rather than refraction due to density gradients, which seems to play a secondary role. The low temperature overexpanded jet showed a difference of about 2.6 db in the OPWL between the two nozzles. However, at this condition, for the rectangular nozzle, a difference of 8 db between the maximum and minimum noise direction is observed. Hence, a suitable orientation of the nozzle could cause a considerable reduction in the noise level. The rectangular nozzle seems to be very effective under overexpanded conditions. The scaling laws, which are in the preliminary stages, were developed for the change in the OPWL as a function of stagnation pressure. For the circular nozzle, additional scaling was done for temperature and acoustic efficiency.

These scaling laws need to be verified for additional temperatures. Also, further work should be initiated in the potential use of the rectangular nozzle as a noise suppressor and as a model for better comprehension of noise generating mechanisms.