An experimental investigation of vortex pairing in a circular jet under controlled excitation



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The large-scale coherent structures in the near field of circular free jets have been experimentally studied by organizing these structures through controlled acoustic excitation. Hot-wire and flow-visualization investigations have been carried out in three air jets (of diameters D = 2.54 cm, 5.27 cm, and 7.62 cm) with both initially laminar and turbulent boundary layers, over a wide range of the jet Reynolds number DUe/v. The roll-up of the initial axisymmetric shear layer into vortex rings, and their subsequent evolution and interactions like pairing, are found to strongly depend on the excitation Strouhal number. It is also found that under certain conditions of excitation, the jet turbulence can be decreased to values lower than those of the unexcited jet. The principal thrust of this research was the mechanics of the vortex pairing process in the circular jet. [...]