An experimental study of noise radiated from supersonic elliptic jets

Abstract

Results 6om Mach 1.5 elliptic jet experiments are presented. In order to simulate the velocity and density of a heated jet, the jet is operated with a gas mixture of helium and air. As a result, evidence of Mach wave emission is seen in the h e l i d a i r jet when compared to the pure air jet case. Using a glow discharge excitation technique, the jet can be excited to predominantly varicose or flapping modes. Hot-wire phase measurements downstream of the potential core show the structures to have a near sonic velocity relative to the ambient environment. Sound pressure level measurements show the noise to radiate at a higher angle to the jet axis and in a more directional manner for the helidairjet compared to the pure air jet. The elliptic jet is also seen to radiate less noise than a similar axisymmctric jet. Azimuthal modal decomposition measurements show that there is a significant increase in the flapping mode of the jet as the jet velocity increases. These results demonstrate that there are significant differences between unheated and heated elliptic jets in the Mach number 1.5 speed range that have been found previously for axisymmetric jets. u Nomenclature a = speed of sound = equivalent diameter = frequency = pressure P R = distance from jet exit SPL = sound pressure level St = Strouhal number, fDqAJj T = temperature U = velocity