Asymmetric forcing of convectively unstable transverse jets

The present paper explores the effect of asymmetric and helical excitation of the flow about the exit plane of a jet injected perpendicularly into crossflow. Both acetone planar laser-induced fluorescence and stereo particle image velocimetry were used to quantify transverse jet response at relatively high jet-to-crossflow momentum flux ratios (J = 61 and 41), which in the absence of external excitation produced a highly penetrating jet with a naturally convectively unstable upstream shear layer (USL) and asymmetric cross-sectional shape. For various excitation conditions, in some cases involving complete clockwise or clockwise perturbations and, in other cases, localized perturbations, alterations in the spectral character of the USL were observed, including lock-in to the applied frequency and quasiperiodicity involving applied and natural frequencies. For forcing frequencies and amplitudes producing lock-in, asymmetric excitation was found to accelerate USL vorticity roll-up and improved mean symmetry in the jet cross section. With these alterations in jet structure, molecular mixing between jet and crossflow fluid was improved in both center plane and cross-sectional planes of the transverse jet. Proper orthogonal decomposition analysis applied to the images revealed 2D and 3D mode coefficient plots with interesting topological features. In many cases, these phase diagrams revealed attractor like shapes, especially when excitation frequencies and amplitudes produced lock-in of the USL, suggesting that with further study, such topologies could be used as characteristic signatures for mixing optimization and low-order model development.