Fluctuations of density, pressure and temperature in a turbulent mixing region

Abstract

Observations of density fluctuations have been made in a turbulent mixing jet with variable Mach number and enthalpy. On the basis of mean velocity and enthalpy distributions the density fluctuations have been related to local pressure and temperature fluctuation intensities. It is found that the pressure fluctuations determined in this way are in agreement with previous microphone-based observations, although there is some uncertainty about the use of microphones in turbulent flows. The present observations show the maximum pressure fluctuations in the shear layer close to the nozzle to be in excess of the values estimated from turbulence intensity data by 26%, while closer agreement is found in the self-preserving region further away from the nozzle. The temperature fluctuations were found to be consistent with observations made with fine wire probes, and the pressure-temperature covariance showed a reversal of sign across the shear layer consistent with mixing in the presence of the mean property gradients. Increases of flow Mach number substantially reduced the relative intensity of both pressure and temperature fluctu­ations. For mixing conditions dominated by a large mean temperature gradient, turbulent scales and spectra were similar to those of velocity fluctuations. Under conditions where the mean temperature gradient was eliminated and pressure fluctuations dominated the density variations, the scales were significantly increased and the spectra showed a more rapid decrease with frequency.