Reynolds-Number Effects on the Acoustic Disturbance Environment in a Mach 6 Nozzle

Abstract

To understand the impact of the Reynolds number on the disturbance field inside a high-speed wind tunnel, we use direct numerical simulations to model turbulent boundary layers along the walls of a quasi-two-dimensional nozzle configuration. These simulations are performed at four different unit Reynolds numbers, ranging from [Formula: see text] to [Formula: see text]. The predominantly hydrodynamic fluctuations inside the boundary layer are nearly unaffected by the freestream forcing of the impinging acoustic radiation from the opposite wall. Within the nozzle-core region, the disturbance environment is found to be spatially uniform and solely acoustic in character. Comparisons with tunnel noise measurements are made using the numerical data. For the first time, comparisons of fluctuations in the same physical quantity (the streamwise mass flux) have been published, as opposed to earlier comparisons involving static-pressure fluctuations based on the simulations and pitot-pressure fluctuations recorded in the wind tunnel. The NASA 20 in. Mach 6 wind tunnel observations corroborate the predicted trend of decreased root-mean-square variations in pressure and mass flux as the Reynolds number rises. Additional details of the acoustic radiation field are quantified and should be useful toward a digital synthesis of the tunnel disturbance environment that would enable realistic simulations of the natural transition process.