Hani G. Alexander
William C. Meecham
Mech. Aerosp. and Nucl. Eng. Dept., Univ. of California, Los Angeles, CA 90024
Noise generated by a jet impinging on a surface is of interest in itself for such applications as vertical takeoff. Here we consider the image argument [see A. Powell, J. Acoust. Soc. Am. 32, 982 (1960) and W. C. Meecham, ibid. 37, 516 (1965)]. It shows that there can be no dipole sound for a large flat surface, only the volume, quadrupole aerosound. The sound generated by the viscous layer at the surface is neglected, and is examined by us here and shown to be indeed negligible. The simulation was carried out in two separate steps. The first step was to generate the hydrodynamic field which was compared to well-documented experimental results of similar flows. Once an adequate hydrodynamic field was generated, the theoretical treatment of Curle and Lighthill relating surface generated sound and the nature of aeroacoustic sound, respectively, was employed to get order-of-magnitude values for the sound intensity and develop directivity patterns. The source region considered was the viscous region near the surface. The k(epsilon) turbulence model was employed and did a fair job of depicting the fluid field. Velocity profiles of the free-jet region were very successful, while the model broke down some in the ``wall jet'' region of the flow. Surface pressure and shear stress distribution was in good agreement with experimental values. The acoustic values were then obtained and the viscous layer was found to generate very low intensity levels, in the neighborhood of 25 dB, much less than the volume sound.