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When a plane jet exhibits flapping motion, two distinctive peaks, f1 and f2, appear in the spectrum of fluctuating velocity. The mean frequency is half of the frequency, fv, of vortices shedding from a two-dimensional body that is installed in a jet nozzle contraction. Jet mixing has been enhanced by the flapping motion with a low frequency, f3=f1-f2, which is considered to be caused by an inharmonic excitation. Vortical structures in the flapping jet are investigated by using flow visualization, image correlation and laser Doppler velocimetry. Vortices are seen to roll up asymmetrically at the jet nozzle exit, at a double frequency of that of the vortex shedding from the cylinder. Two types of vortices are formed by coalescence between these vortices and the vortices shedding from the cylinder. The periodicities of these two types of vortices are found to be equal. It is noted that vortices with f1 or f2 do not exist in the flow. The f1 and f2 are caused by the modulation of amplitude of the coalesced vortical structures with half of the flapping frequency, f3.

nihon kikai gakkai ronbunshū. b

Vortical Structures in a Two-Dimensional Jet Under Flapping Motion Generated by a Wake of a Cylinder Installed in a Nozzle Contraction (2nd Report, Flow Visualization and Image Correlatioin Analysis)