Experiments conducted in axisymmetric low-density jets reveal that the transition to global instability and the frequency selection of the global mode depend on the operating parameters of density ratio, momentum thickness, and Reynolds number. The onset of the global mode was mapped in the Reynolds number–momentum thickness operating domain for density ratios $S \,{=}\, \rho_{j}/\rho_{\infty}$ ranging from 0.14 to 0.5. The results provide convincing evidence of the universality of global oscillations in low-density jets and indicate that conditions in the jet exit plane are responsible for driving the global instability.

Experiments conducted in helium axisymmetric jets with an annular coflowing air stream yield critical values of the velocity ratio U2∕U1 needed to suppress global instability inherent in these low-density flows. Global mode suppression was achieved for coflowing velocities less than approximately 20% of the jet centerline velocity, though the critical velocity ratio displayed a nonmonotonic relationship with the initial shear layer momentum thickness. The experiments are supported by spatio-temporal inviscid stability theory, where the convective-absolute transition was tracked in an operating domain including U2∕U1 and D∕θ. For initially thick shear layers, the experimental observations are in good agreement with linear theory, but deviate considerably as the separating shear layer thickness is reduced.

Viscous linear stability calculations are presented for model low-density axisymmetric jet flows. Absolute growth transitions for the jet column mode are mapped out in a parametric space including velocity ratio, density ratio, Reynolds number, momentum thickness, and subtle differences between velocity and density profiles. Strictly speaking, the profiles used in most jet stability studies to date are only applicable to unity Prandtl numbers and zero pressure gradient flows—the present work relaxes this requirement. Results reveal how subtle differences between the velocity and density profiles generally used in jet stability theory can dramatically alter the absolute growth rate of the jet column mode in these low-density flows. The results suggest heating/cooling or mass diffusion at the outer nozzle surface can suppress absolute instability and potentially global instability in low-density jets.

Experiments conducted using a low-density axisymmetric jet reveal that the self-excited nature can be altered by modulating the external pressure field using an acoustic driver. A region exists in the forcing frequency-amplitude space where the self-sustaining frequency is entirely absent and solely replaced by the forcing frequency and harmonics thereof. Inside this region, the centerline streamwise velocity disturbance level can be significantly increased or decreased. Visual evidence suggests these alterations coincide with dramatic changes in jet spreading and demonstrates that forcing can provide an effective means of controlling mixing in self-excited jets. Results support theoretical predictions made by Pier [B. Pier, Proc. R. Soc. London, Ser. A 459, 1105 (2003)].

We describe an automated, volumetric particle image velocimetry (PIV) and tracking method that measures time-resolved, 3D zooplankton trajectories and surrounding volumetric fluid velocity fields simultaneously and non-intrusively. The method is demonstrated for groups of copepods flowing past a wall-mounted cylinder. We show that copepods execute escape responses when subjected to a strain rate threshold upstream of a cylinder, but the same threshold range elicits no escape responses in the turbulent wake downstream. The method was also used to document the instantaneous slip velocity of zooplankton and the resulting differences in trajectory between zooplankton and non-inertial fluid particles in the unsteady wake flow, showing the method's capability to quantify drift for both passive and motile organisms in turbulent environments. Applications of the method extend to any group of organisms interacting with the surrounding fluid environment, where organism location, larger-scale eddies and smaller-scale fluid deformation rates can all be tracked and analyzed.