How Small can a Nozzle be for Accurately Scaling Jet Noise to a Larger Nozzle?

Abstract

Most university researchers typically end up using small nozzles to study jet noise. The data acquired from many of such existing studies has been used to develop jet noise prediction schemes. However, it is not quite clear as to how small the nozzle can be before such data becomes unreliable. There are two-fold issues here: (1) it may be difficult to obtain similarity in upstream boundary layer characteristics as well as in the flow development in the jet plume for the smaller nozzles; and (2) for fixed jet exit velocities the Reynolds numbers for the smallscale nozzles may not provide the same results because of their possible laminar nature at the nozzle boundary layer compared to those obtained using larger nozzles with turbulent flows and hence larger Reynolds numbers (Re). Resolving these issues is the backbone of this paper. The microphone measurements of the noise produced by the jet and schlieren flow visualization of the jet were analyzed for jets for small and high Reynolds numbers (Re range=60400-1090000). The acoustic measurements show that at polar angles greater than 30° (with respect to the downstream direction) identical lossless spectra are obtained after suitable scaling corrections for the jet velocity and diameter. At the lower polar angles, the lower Reynolds number jets have lower jet-noise levels for Helmholtz numbers (normalized frequency) greater than 0.2. The overall sound pressure levels at 30° are found to be proportional to Log (Re).