Effect of Nozzle Internal Contour on Jet Aeroacoustics

Abstract

A frequent problem that confronts designers of jet rigs and nozzles used for aeroacoustic testing is the unknown effect of the state of the flow upstream of the nozzle exit on aeroacoustic performance. Though the effect of the inflow conditions on flow development has been studied extensively, there is limited information on their effect on noise. Flow distortion, both in pressure and temperature, thickness and state of the boundary layer, quality of the measurement system and anechoic chamber, and analysis procedure could all play a role in the observed variations in data from different experiments. In order to understand the effect of the thickness of the boundary layer upstream of the nozzle convergent (entrance) section and in the nozzle itself, a computational and experimental investigation has been carried out. Three nozzles of identical exit diameter have been designed and tested in the Boeing Low Speed Aeroacoustic Facility (LSAF). These three nozzles have a shallow conic section, a short cubic contraction and an ASME flow path (contraction followed by a constant section), respectively. Simultaneous aerodynamic and acoustic measurements have been made with a six-component force balance and far field microphone arrays for a wide range of nozzle operating conditions. The numerical simulations and the measurements of nozzle coefficients indicate that the initial boundary layers are vastly different, with the cubic producing a thin boundary layer and the conic a much thicker one. Surprisingly, the noise levels from these two nozzles are virtually identical especially at the higher frequencies. An important practical conclusion drawn from this observation of insensitivity to nozzle exit conditions is that it should be possible to compare noise spectra from different facilities even if measurements of inflow conditions are not available. The ASME nozzle generates higher levels of noise, the magnitude of which is more pronounced at elevated jet temperatures. The characteristics and the stability of the boundary layers inside the nozzles are examined to glean some insights to the observed differences in noise.