TY - JOUR
T1 - Estimation of the direct acoustic radiation from a transitional boundary layer using velocity measurements
AU - Krane, M. H.
AU - Pauley, W. R.
N1 - Funding Information:
The authors would like to acknowledge the _nancial support of the O.ce of Naval Research "Code 0014 OA# and the Applied Research Laboratory E+F Program[ This paper was written during a visit by one of the authors "M[H[K[# to the Centre d|Acoustique of the Ecole Centrale de Lyon\ sponsored by the French Ministere de l|Enseignement Superieur et de la Recherche[
Copyright:
Copyright 2017 Elsevier B.V., All rights reserved.
PY - 1995/4/13
Y1 - 1995/4/13
N2 - 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 of 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 time 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.
AB - 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 of 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 time 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.
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U2 - 10.1006/jsvi.1995.0169
DO - 10.1006/jsvi.1995.0169
M3 - Article
AN - SCOPUS:33744702783
SN - 0022-460X
VL - 181
SP - 737
EP - 763
JO - Journal of Sound and Vibration
JF - Journal of Sound and Vibration
IS - 5
ER -