Influence of acoustic forcing on the near field development of a heated plane jet

Acoustic effects on the temperature spectra, spreading rate of heat, and fl ow structures of a two-dimensional hot-air jet are studied experimentally. The Reynolds number of the hot jet is approximate to 3500, and the ratio of the jet-exit density to the ambient is 0.78. Without any forcing, the jet exhibits oscillations with a fundamental frequeney f(0) (116 Hz), correspon ding to a Strouhal number of 0.31. Under an acoustic forcing, the hot jet i s investigated by local temperature measurements and flow visualization. It is found that the acoustic forcing can enhance, or suppress the natural f( 0)-oscillations, depending on the forcing frequency F-e. When the forcing f requency is in the band of f(0)+/-4.5 Hz, the natural jet oscillations are enhanced with sharper spectral peaks at a frequency interval of f(0)/2 in t he temperature spectra. Correspondingly, larger and more organized vortices are observed in the shear layers, which contribute to the more jet spread of heat. When the forcing frequency is out of the narrow band of f(0)+/-4.5 Hz and below 2f(0), the spectral components of the fundamental frequency a re suppressed, and the dominant frequency in the flow becomes F-e rather th an f(0). Under the forcing, the hot jet starts to develop the vortical stru ctures at a streamwise location closer to the nozzle, and hence spreads mor e heat compared with the unforced hot jet. (C) 2001 Elsevier Science Inc. A ll rights reserved.