Entropy noise from shock containing nozzles of military jets at afterburner

Abstract

This paper considers the noise of military jets operating at afterburner. Boosting by the afterburner, the jet velocity and temperature are much higher than those of laboratory jets. Under these conditions, do we anticipate the noise components of these jets the same as those of laboratory jets, i.e., consisting mainly turbulent mixing noise? Do we expect new noise sources in these jets, since they operate at much higher velocities and temperatures and that the combustion processes in the afterburner are highly unsteady. In this paper, we explore one plausible new noise mechanism; the generation of entropy noise inside the shock containing nozzles of military jets. It is well-known that hot and cold spots (entropy waves) from unsteady combustion when convecting through a non-uniform mean flow would create fluctuating pressure and hence acoustic radiation. We investigate this possibility by performing numerical simulation of the flow and noise inside a rectangular nozzle typical of that of the F-22 Raptor. These nozzles are imperfectly expanded. There are shocks inside. We find that because shocks have very steep velocity gradients, their presence leads to the generation and emission of strong entropy noise when entropy waves from the afterburner pass through them.