Scaling of Fluctuating Pressures in Jet Impingement

Abstract

Scaling laws for the determination of fluctuating surface pressure loads on STOL aircraft due to jet impingement have been investigated using a quarter-scale model of a boilerplate test facility at NASA Langley, in which a JT-15D engine with a rectangular outer nozzle blows over a small curved airfoil representing the upper surface of a wing with flap extended. Whereas the full-scale engine has an internal hot core nozzle, there was no internal nozzle in the model, so that, not only was a condition of perfect mixing simulated, but also, this was done with a cold jet. It is known that the overall impingement pressures should be expressible in terms of five parameters, the pressure coefficient, the Strouhal number, the jet to ambient temperature ratio, the jet Mach number, and the jet Reynolds number. Comparing surface pressure spectra on the basis of the first two parameters, it is found that the model spectra peak at pressures which are 1.6 times the values indicated by the relevant scaling laws. This is tentatively attributed to the effect of the temperature ratio. Overall pressure coefficients are found to compare well, and to be independent of either temperature ratio, Mach number, or Reynolds number. It is concluded that a scaled cold jet run at low Reynolds and Mach numbers can be used to predict full-scale impingement pressures, although some uncertainty remains over the corrections for jet temperature effects, the corrections for different noncircular nozzle geometries, the effect of a separate core nozzle, and the effect of engine noise. These can only be resolved by suitable experimental programs.