Organized motions in a plane turbulent jet under controlled excitation



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The organized motions in the near field of a plane jet under controlled excitation has been investigated experimentally by employing hot-wire techniques. A plane jet flow facility suitable for this study has been built. The facility has some unique features and consists of separate formica-lined wooden modules joined to each other through flexible connections so that the nozzle is vibration-free. Air flows through a 44:1 two-dimensional contraction before issuing through a 1.25 in. x 55 in. slit into a large room. Sinusoidal excitation is induced in the- jet by a loudspeaker attached to the settling chamber. This study has been carried out at the cavity resonant frequency 70 Hz and at a fixed exit pulsation amplitude 1.4% of the exit velocity. The jet velocity is varied to cover a Strouhal number range 0.15 - 0.60 corresponding to the jet Reynolds number range 31,600 - 8,000. The effect of the controlled excitation on the mean velocity field and thus on the mass, momentum and kinetic energy fluxes, and the entrainment rate is found to be small. The effect on the turbulent field is more noticeable but, unlike in a circular jet, the wave fundamental amplitude is a small fraction of the rms turbulent velocity. The difference between the plane jet and the circular jet responses to excitation may be attributed to the difference in vortex pairing in the two flows. The amplitude and phase distributions of the wave fundamental show that the disturbance which is induced as a symmetric mode remains symmetric at downstream stations. The wave data in the near field free shear layer agree with the spatial stability theory of Michalke (1965). Assuming a single mode form for the wave fundamental, the spatial growth rate, wavenumber and phase velocity have been determined for the Strouhal number range 0.15 - 0.60. The growth rate is much higher and the wavelength much smaller in the shear layer than on the centerline of the jet. The phase velocity data show that in the lower Strouhal number range the plane jet is a nearly nondispersive waveguide. The wave fundamental reaches its maximum relative amplitude at the Strouhal number 0.18 (at a streamwise distance of- about 4 slit-widths from the jet exit). These new data in a plane jet provide an insight into the orderly structure in the near field of a plane jet and are likely to be important in the formulation of an appropriate theory for turbulent shear flow.