ESTIMATION OF THE DIRECT ACOUSTIC RADIATION FROM A TRANSITIONAL BOUNDARY LAYER USING VELOCITY MEASUREMENTS

摘要

The acoustic source strength of turbulent spots in a laminar boundary layer is estimated using unsteady velocity field measurements of an isolated, artificially generated turbulent spot made in a zero pressure gradient laminar boundary layer. The measurements were performed in a water channel, using a laser Doppler velocimeter, in order to provide quantitative information describing the large scale fluctuations in the wall-normal mass flux which occur during the passage oi a turbulent spot. These quantities are related to the radiated noise through the Liepmann acoustic analogy. The unsteady mass flux is characterized by three scales: the peak value of the unsteady displacement thickness, the time for the displacement thickness to change from its undisturbed value to its maximum, and the convection velocity of the turbulent spot. Comparison of the results at different streamwise stations shows that the time and length scales describing the large scale unsteady mass flux increase with distance from the generation point, with the highest growth close to the generation point, The lime scale, normalized by the time for the spot to traverse a natural transition zone, ranged from 0.06 to 0.74 over the locations measured. In addition, the measurements showed that the streamwise variation in the normal mass flux consists of three peaks: a wallward peak at the spot leading edge, a positive peak directly upstream, and wallward peak at the trailing edge. This distribution can be described as a superposition of simple sources. A scaling analysis performed using these experimental results suggests that the sound radiated by the large scale motion due to an isolated turbulent spot has a dipole character, the dipole strength increasing as the spot grows. Extrapolation to a natural transition zone indicates that the acoustic source strength of the large scale intermittent motion is highest in the middle part of the transition zone. [References: 30]