Effect of axisymmetric forcing on the structure of a swirling turbulent jet

With the aid of the Stereo PIV technique, the local structure of swirling free turbulent jets is investigated under the following flow conditions: the Reynolds number was equal to 8900, and the swirl number varied from 0 to 1.0. The effect on the jet flow structure of external periodical forcing, applied with an axisymmetric mode to the inlet velocity, was studied. Also investigated were the flow response to the forcing at two Strouhal numbers, St = 0.52 and 1.2, and with various forcing amplitudes. Additional measurements with application of the conditional sampling approach were performed in order to analyze spatio-temporal dynamics of the large-scale ring-like vortices, generated in the forced jet at a low swirl rate. The greatest effect of the forcing on the swirling jet structure was observed in the case of the high swirl rate (S = 1.0), which was previously considered to be largely insensitive to external forcing. The forcing at St = 1.2 with relatively high amplitude resulted in an abrupt change in the turbulent structure of the flow: an increase of total turbulent kinetic energy and strong anisotropy of its components took place.