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This paper proposes a technique for developing parametric models of modally degenerate resonators from stimulus-response data. The technique complements traditional empirical frequency response estimates that are commonly used for testing MEMS resonators, however, the parametric models have distinct advantages when the modal frequency differences are close enough to frustrate estimates of quality factors, natural frequencies and mode orientations. The proposed technique also completely rejects parasitic coupling between the stimulus and pick-off electrodes. It is shown how a modification of the algorithm using demodulated measurement signals greatly reduces storage and computational requirements.

This experimental study examined the effects of controlled vortex generation and interactions created by axisymmetric excitation of a transverse jet, with a focus on the structural and mixing characteristics of the flow. The excitation consisted of a novel double-pulse forcing waveform applied to the jet, where two distinct temporal square wave pulses were prescribed during a single forcing period. The two distinct pulses produced vortex rings of different strength and celerity, the strategic selection of which promoted vortex ring interactions or collisions in the nearfield to varying degrees. Jet flow conditions corresponding to a transitionally convectively/absolutely unstable upstream shear layer (USL) in the absence of forcing, at a jet-to-crossflow momentum flux ratio of J = 10, and to an absolutely unstable USL at J = 7, were explored for a jet Reynolds number of 1800. Acetone planar laser-induced fluorescence (PLIF) imaging was utilized to quantify the influence of different prescribed temporal waveforms. All forcing conditions enhanced the spread, penetration, and molecular mixing of the jet as compared to the unforced jet, though to differing degrees. Interestingly, when the jet was convectively unstable, forcing which promoted vortex collisions provided the greatest enhancement in molecular mixing, whereas the absolutely unstable jet produced the greatest enhancement in mixing when the vortex rings did not interact, with important implications for optimized jet control.

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