Wavevector-Frequency Spectrum of Transition Zone Wall Pressure Fluctuations
The laminar-to-turbulent transition zone continues to be of interest from the flow noise point of view. Lauchle has developed a theoretical model to predict the radiated component (wavenumber less than or equal to the sonic wavenumber) of the pressure fluctuations that occur in this zone G. C. Lauchle, J. Acoust. Soc. Am., 67, 158-168, (1980) . It is also recognized that wavenumbers greater than the sonic wavenumber are also important because pressure fluctuations at these higher wavenumbers may couple with structural vibrations. In this work, the modeling of the wavenumber-frequency spectrum of the pressure fluctuations in a transitioning boundary layer is attempted. This analysis assumes that the pressure field within a turbulent burst is statistically similar to that under a fully-developed turbulent boundary layer, and therefore follows known turbulent boundary layer models. The statistics of the entire transition zone is modeled by introducing a burst generation function (an indicator function) which is independent of the statistics of a single turbulent patch. The statistical properties of this burst generation function have been measured in a subsonic wind tunnel at low Mach numbers. Extensive measurements of the intermittency factor, burst frequency, indicator function, and space-time correlations have been performed over the entire transition zone at three different flow velocities. An empirical model of these quantities has been inferred. By combination with Chase modeling valid for a turbulent boundary layer D. M. Chase, J. Sound. Vibr., 70, 29-47 (1980) , a model of the wall pressure fluctuation wavevector-frequency spectrum is deduced for the transition zone. This model, valid for wavenumbers greater than the sonic wavenumber, is compared in the low wavenumber region with its equivalent for a turbulent boundary layer. This model therefore allows the future prediction of the coupling of pressure fluctuations inside the transition zone with structural vibrations, and the calculation of the noise generated by the transition process.
- Pub Date:
- FLOW NOISE;
- Physics: Acoustics