Optimization of Controlled Jets in Crossflow

The controlled acoustical excitation of a round gas jet injected transversely into crossflow, is studied. Use of an open-loop, or feedforward, controller in the experiments allowed for straightforward comparisons to be made among jet responses to different conditions of acoustic excitation of jet fluid. It was found that, for a variety of excitation frequencies, optimal temporal pulse widths during square wave excitation produced clear and distinctly rolled-up vortical structures as well as increased jet penetration into the crossflow forjet-to-crossflow velocity ratios of 2.6 and 4.0. In some cases, application of forcing frequencies corresponding to subharmonics of the upstream shear layer mode for the unforced transverse jet also produced increased jet penetration, coincident with deeply penetrating, distinct vortical structures in the jet. In other instances, merely exciting at the optimal pulse width and at a low excitation frequency (in comparison to the unforced transverse jet shear layer frequency) yielded the best jet penetration and spread. These results on optimization may be interpreted in terms of a universal timescale associated with vortex ring formation and propagation.