Estimation of the Direct Acoustic Radiation from Turbulent Spots. Using BoundaryLayer Velocity Measurements

摘要

To provide an understanding of the acoustic source characteristics of theboundary layer transition region, unsteady velocity field measurements of an isolated, artificially generated turbulent spot were made in a zero-pressure gradient laminar boundary layer. These measurements were performed in a water channel, using a laser Doppler velocimeter. They provide quantitative information describing the large scale unsteady displacement thickness fluctuations due to the passage of a turbulent spot. Fluctuations of the displacement thickness are related to the radiated noise through the Liepmann acoustic analogy. The description of the large-scale velocity field is based on ensemble-averaged unsteady velocity data, from which the displacement thickness is calculated. These results are used to calculated the velocity normal to the plate, as well as the characteristic rise times, ti, of the displacement thickness. Comparison of the results at different steamwise stations shows that the rise time and the mass flux deficit peak amplitudes of the normal velocity increase early in the spot development, but level off. For all these parameters, the rate of spatial growth is greatest in the stations closest to the generation point.... Acoustic radiation, Turbulent spots, Boundary layer transition region, Velocity field, zero pressure gradient, displacement. thickness fluctuations.