Amplitude-modulated excitation of a turbulent mixing layer

The evolution of a momentary, spanwise-uniform disturbance in a plane mixing layer was studied experimentally. Since interaction between the disturbance and the two-dimensional (primary) instability of the mean flow is unavoidable, excitation by a low-level, two-dimensional, time-harmonic carrier wave train provided a clear phase reference. The disturbance was generated by a lower frequency pulsed amplitude modulation of the excitation waveform. The duration of the modulating pulse was equal to the carrier wave period, and the degree of modulation was either 2 or 4. A demodulation technique was used to discriminate between the response to the modulating pulse and the harmonic excitation. This technique decomposes the response into a family of modal two-dimensional wave packets, allowing detailed study of the disturbance, and, in particular, the propagation, amplification, and some of the nonlinear aspects of the behavior of its leading modal components. The fundamental packet is advected with the mean velocity of the two streams. Its streamwise extent and dominant frequencies remain virtually unchanged with downstream distance. The passage of the disturbance is accompanied by a spatial and temporal change in the momentum thickness of the harmonically excited flow. Cross-stream distributions of the streamwise velocity perturbation within the disturbance are similar to those of the harmonically excited flow at streamwise stations having the same momentum thickness. High turbulence levels, not prevalent in the harmonically excited shear layer, are detected within the disturbance and suggest the possibility of transient mixing enhancement.