Effects of Vibrational Nonequilibrium on the Acoustic Noise Radiated by a Compressible Boundary Layer

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Turbulence-generated acoustic noise is of critical concern in the nozzle flows of conventional high-speed wind tunnels, where the disturbance environment encountered by models in the freestream is substantially stronger than that experienced in atmospheric flight and leads to much reduced transition Reynolds numbers. To obtain more accurate comparisons of experimental, computational, and free-flight data, a new control mechanism is needed to reduce freestream disturbance levels. Therefore, the aim of the present work is to investigate the ability of vibrational nonequilibrium processes to attenuate acoustic radiation emitted by turbulent boundary layers in high-speed facilities.

Predicting the attenuation from vibrational nonequilibrium processes remains a challenge, and there exist limited experimental data for model validation, particularly at elevated temperatures. To better understand the absorption properties of various gas mixtures, a heated acoustic chamber is developed to measure the attenuation of CO2, N2O, and mixtures of CO2/He, CO2/N2,and N2O/He at temperatures up to 529 K. In mixtures of CO2/He at room temperature, an increase in helium is found to decrease the peak attenuation modestly, but increase the peak attenuation frequency. At higher temperatures, the peak attenuation increased substantially, but as the helium fraction increased, the rate of increase in peak attenuation drops and the values asymptote at lower temperatures. These results illustrate that varying the fraction of helium in mixtures of CO2/He can shift the attenuation to a desired frequency range, providing a method to control acoustic radiation.

The effects of vibrational nonequilibrium processes on turbulence-generated acoustic noise are investigated in a Mach-2.8 shock-tunnel facility at the University of Maryland. CO2, N2, He, and He/CO2 mixtures are injected into the lower boundary layer of the flow through a porous plate located in the upstream region of the test section. A four-point Focused Laser Differential Interferometer (FLDI) positioned above the turbulent boundary layer is used to obtain freestream fluctuation measurements assumed to be representative of entropic fluctuations propagating along streamlines and acoustic disturbances along Mach lines. Compared to a boundary layer of pure air, the injection of 30%, 35%, and 40% He/CO2 mixtures resulted in reduced fluctuation powers correlated along a Mach line in the frequency range of 200−800 kHz. Minimal reductions in fluctuation power were measured along corresponding streamlines; therefore, it could be concluded that the vibrationally active gas species in the boundary layer primarily affected acoustic radiation and not entropic disturbances.

As measurements are affected by noise radiated from the boundary layers on all four walls of the facility, a mathematical disturbance model is created to examine the sensitivity of the measured attenuation to acoustic disturbances propagating from the lower boundary layer only. Disturbances are modeled as Gaussian wave packets propagating along Mach lines from the four test section walls and along streamlines. Modeling the acoustic disturbances from the lower boundary layer with a 15−30% amplitude reduction resulted in amplitude spectral densities and cross power spectral densities that agreed well with the FLDI measurements. Thus, the injection of a vibrationally active gas into a turbulent boundary layer has the potential to significantly reduce acoustic-disturbance amplitudes in the freestream, greatly expanding the utility of conventional high-speed facilities to study flows in which transition plays an important role.