EXPERIMENTS ON THE NONLINEAR STAGES OF EXCITED AND NATURAL PLANAR JETSHEAR-LAYER TRANSITION

An experimental investigation focusing on the nonlinear stages of plan ar jet shear layer transition is presented. Experimental results for t ransition under both ''natural'' and low level artificial forcing cond itions are presented and compared. The local spectral dynamics of the jet shear layer is modeled as a nonlinear system based upon a frequenc y domain, second-order Volterra functional series representation. The local linear and nonlinear wave coupling coefficients are estimated fr om time-series streamwise velocity fluctuation data. From the linear c oupling coefficient, the mean dispersion characteristics and spatial g rowth rates may be obtained. With the estimation of the nonlinear powe r transfer function, the total, linear and quadratic nonlinear spectra l energy transfer may be locally estimated. When these measures are us ed in conjunction with the local quadratic bicoherency and linear-quad ratic coupling bicoherency, the local system output power may be compl etely characterized and the effect of nonlinearity on local mean flow distortion assessed. Particular attention is focused upon quantifying the linear and nonlinear power transfer that characterizes the differe nt stages of the jet shear layer transition for both natural and excit ed flows. The quadratic power transfer that occurs with deviation from the perfect resonant wavenumber matching condition is clarified as is the dynamic mechanism of subharmonic resonance. The mechanism of spec tral broadening is described and contrasted for natural and artificial ly excited flows.