Experimental Investigations On Impinging Shear Flows

The experimental investigations were performed on the impinging shear flow that is comprised with a jet shear layer impinging a plate parallel to the jet. On the basis of the primary studies upon the nonlinear interactions between the unstable waves and the self-excited oscillations in the flow, experiments were focused upon the attenuation effects on the oscillations of tripping wires, side jets and deflected plates, and upon the mechanisms for the attenuations.By the method of double-frequency acoustic excitations, experiments were conducted on the influences of phase differences between the fundamental and subharmonic waves upon the "subharmonic resonance"- the nonlinear interaction between the waves. The results indicate that there is especially a narrow rang of phase difference in which the amplitude of subharmonic waves is notably diminished and that the diminishment is affected by initial amplitudes of the waves. Experiments of "detuned subharmonic resonance" or "tri-wave coupling" were also carried out with the technique of bicoherence analysis. All the results show that the fundamental waves play a role of "catalyzer" in these nonlinear interactions.It was found in preliminary tests that sound pressures of the oscillations were diminished with tripping wires or side jets when the wires or the jets were introduced into the impinging shear flow in certain ways. In order to find out the mechanisms for the diminishment of the oscillations, controlled experiments were performed on non-impinging shear flows. In these experiments acoustic excitations were employed to play the role of the feedback of the upstream-propagating disturbances of pressure in the impinging shear flow. Under different positions of a tripping wire or speeds of the side jet, the controlled experiments were made, including the receptivity of the shear layer near jet exit to acoustic excitations, the spatial development of fluctuating velocity and the profiles of mean velocity.Experimental studies on the influences of the deflected plates upon the oscillations were focused on the attenuations of sound pressures of the oscillations with the bent plates of various angles and lengths. The results show that the bent plates with angels of 30~60 and a length about an oscillating wavelength diminish the sound pressures notably. With the same angle of 45, the half-wavelength bent plates diminish the pressures, and the one-wavelength or longer bent plates do better.