The modal distributions of coherent structures evolving near the nozzle of a circular jet are considered. The effects produced on the instability modes by transverse curvature, flow divergence, inhomogeneous inflow conditions, and the detailed shape of the mean velocity profile, are investigated both theoretically and experimentally. Linear stability analysis applied to a thin shear layer surrounding a Iarge-diameter jet (i.e. a jet whose diameter is large in comparison with a typical width of the shear layer) indicates that many azimuthal modes are equally unstable. An increase in the relative thickness of the shear layer limits the number of unstable modes, and only one helical mode remains unstable at the end of the potential core. The linear model used as a transfer function is capable of predicting the spectral distribution of the velocity perturbations in a jet. This provides a ratiodl explanation for the stepwise behaviour of the predominant frequency resulting from a continuous increase in the jet velocity.

Experimental results from a systematic exploration of the dynamics and control of large-scale, organized structures in an isolated, circular jet issuing into a uniform cross ow with a jet-to-cross ow velocity (or blowing) ratio of 6 are presented. Surveys of the unsteady ow eld were conducted using a single sensor hot-lm probe, and the instantaneous and time-averaged scalar elds were explored using Mie scattering-based ow visualization. A spinning mechanical valve was used to modulate the jet ow with a single frequency. The forced ow response illustrates the ow receptivity to high frequencies near the jet exit (associated with the jet instabilities) and to low frequencies farther downstream (where the counter-rotating vortex pair dynamics are dominant). Open-loop forcing in a jet Strouhal number around Sr D = 0.1 was recognized and demonstrated to be effective in organizing unsteadiness and enhancing mixing and entrainment in the ow eld. Measurements of the time-averaged velocity and scalar elds are used to demonstrate increased mass entrainment and mixing as a result of the unsteady forcing. Nomenclature A = cross-sectional area of channel, m 2 D j = diameter of jet nozzle, m f = frequency, Hz h = channel height, m l = ow structure length scale, m Re = Reynolds number, (U D j =º) r = velocity ratio, (U j =U 1 ) Sr D = Strouhal number, identical to f D j =U U = local mean velocity, m/s U j = jet exit velocity, m/s U 1 = cross ow velocity, m/s u 0= local turbulence intensity, m/s x = axial distance, 0 at jet centerline y = vertical distance, 0 at jet injection wall z = spanwise distance in channel, 0 at center ± = boundary-layer thickness on channel horizontal wall, m º = kinematic viscosity for air, m 2 /s ½ = density of air at room temperature, kg/m 3

Leading nonlinear interactions generated by waves externally superimposed on an axisymmetric jet are considered theoretically, and the results verified experimentally. The mean flow in the jet loses its axial symmetry whenever the jet is excited simultaneously by two different azimuthal modes of the same frequency. Subharmonic resonance occurs in this flow whenever the conditions warrant such an occurrence, generating azimuthal modes which may not have been present otherwise in this flow. Some of these resonance conditions are explored.