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 axi-symmetric 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. (C) 2008 Elsevier Inc. All rights reserved.