UCLA

This work describes the experimental characterization of the instabilities forming in the near-field of the variable density transverse jet shear layer, and mechanisms by which jet behavior and mixing may be controlled. Jets comprised of mixtures of gaseous nitrogen and helium are injected from a nozzle mounted flush with an injection wall, issuing into air crossflows at a constant jet Reynolds number of 1800. Jet-to-crossflow density ratio S is

varied between 1.00, representing the pure nitrogen jet, and 0.14, the pure helium jet, by changing the proportions of nitrogen and helium composing the jet fluid. Jet-to-crossflow momentum flux ratio J is varied at incremental values between infinity>J>2 at each value of S. The results of single-component hotwire measurements in the jet shear layer indicate that a transition to global instability likely occurs as J is altered below approximately 10 and/or as S is brought below approximately 0.45-0.40. This transition is characterized by several clear spectral features, including sharp spectral peaks and resistance to low level acoustic forcing for the globally unstable (self-excited) case, and broadband oscillations with high receptivity to applied forcing for the convectively unstable case. Evidence of a Hopf bifurcation is found under examination of the growth of the shear layer oscillation magnitude under variation of the control parameter J, as well in the response to pure-tone forcing near the shear layer's natural mode frequency. These observations appear to link many previous independent studies of both equidensity transverse jets and low density free jets, which may become globally unstable under alteration of J and S, respectively. However, the dynamical character of the transition to global instability in the low density transverse jet displays differences under independent variation of J and S, which may indicate the predominance of different modes. Particle image velocimetry is also used to expand upon these hotwire measurements using a less intrusive and planar technique, confirming the quantitative findings related to the shear layer behavior.

Planar laser-induced fluorescence measurements of jet fluid concentration in various cross-sections of the jet indicate that at large values of J and S, the Re = 1800 transverse jet is highly asymmetric about its (often) assumed plane of symmetry. Mixing quantification shows that mixing may scale according to a normalized spatial coordinate x=sqrt(JD), and that lower density jets are less well mixed than their higher density counterparts at equivalent scaled downstream distances from the point of injection. Acoustic excitation of a globally unstable transverse jet under a variety of dierent square waveforms yields optimal mixing enhancement at a particular square wave duty cycle associated with highest penetration of individual vortex rings into the crossflow. Knowledge of the transverse jet shear layer's stability characteristics remains an important factor in optimizing jet control.