Flow Visualization of Excited Coaxial Water Jet With Phase Difference

Flow visualization and measurements of mean and fluctuating velocities were performed on a coaxial jet at a Reynolds number of 3000 in an open water tank using hot-film anemometry, particle image velocimetry (PIV) and laser-induced fluorescence (LIF). The coaxial jet, with a velocity ratio of 0.6, was excited at the forcing frequencies f_e = f_n, f_n/2, where f_n is the initial vortex frequency of the unexcited jet. The annular and circular jets were subjected to differently phased excitation of the vortical structure in the inner and outer shear-layers, and the effects were studied. For a coaxial jet excited at half of the initial vortex frequency, the vortex pattern and interaction dynamics in the near-field of the jet depend on the phase of the excitations. Vortex-pairing is promoted by forced excitation with specific phase differences, which produces rapid diffusion of the jet width. For the forcing frequency f_e = f_n, forced excitation with various phase differences has little effect on the vortex pattern in the shear-layers, but the center-line velocity increases more than that of a coaxial jet subjected to a forced excitation of f_e = f_n/2.