Large eddy simulation of turbulent jet controlled by two pulsed jets: Effect of forcing frequency

A turbulent jet with Reynolds number of 71000, forced by a pair of pulsed jets operating 180 degrees out of phase, is investigated using large eddy simulation. The effect of the pulsing frequency on mixing efficiency and coherent structures is considered in detail through six cases: an unforced case and five cases with different frequencies (St(D) = 0.05, 0.1, 0.15, 0.2 and 0.3, Strouhal number based on jet diameter). The mixing efficiency is increased when the pulsing frequency is low and conversely decreases at high frequency. In the present work, the optimal frequency is found to correspond to a Strouhal number of 0.15. The coherent structures observed in the forced case are complex and are found to be significantly different with the unforced case. These structures alternate in the forced plane with flapping motion and are symmetrical in the unforced plane. The spanwise structures form at the same frequency as the pulsed jets. After formation, they grow in azimuthal direction and become tilted when moving downstream. Vortex pairing occurs when the leading part of a structure catches up to the trailing part of its predecessor. The forcing frequency does not influence the vorticity magnitude of the coherent structures but does impact their space to develop. In lower frequency case, fewer structures that are larger size are formed. In higher frequency case, more structures are generated but they are limited in the radial direction and the size of them is smaller. Based on the results, it is clear that the optimal mixing enhancement case corresponds to an effective balance between the number and size of vortex structures. (C) 2019 Elsevier Masson SAS. All rights reserved.