DYNAMICS OF COHERENT STRUCTURES AND TRANSITION TO TURBULENCE IN FREE SQUARE JETS

We report results of time-dependent numerical simulation of spatially developing free square jets initialized with a thin square vortex-shee t with slightly rounded corner-regions: The studies focus on the near field of jets with Mach number 0.3-0.6 and moderately high Reynolds nu mbers. A monotonically-integrated large-eddy-simulation approach is us ed, based on the solution of the unfiltered inviscid equations and app ropriate inflow/outflow open boundary conditions. The simulations show that the initial development of the square jet is characterized by th e dynamics of vortex rings and braid vortices. Farther downstream, str ong vortex interactions lead to the breakdown of the vortices, and to a more disorganized flow regime characterized by smaller scale elongat ed vortices and spectral content consistent with that of the Kolmogoro v (K41) inertial subrange. Entrainment rates significantly larger than those for round jets are directly related to the enhanced fluid and m omentum transport between jet and surroundings determined by the vorte x dynamics underlying the axis-rotation of the jet cross-section. The first axis-rotation of the jet cross-section can be directly correlate d with self-induced vortex-ring deformation. However, subsequent jet a xis-rotations are the result of strong interactions between ring and b raid vortices, rather than being correlated with successive self-induc ed vortex-ring deformations, as previously conjectured based on labora tory observations. The interaction between braid and ring vortices has the effect of inhibiting the periodic self-induced axis-rotations obs erved in the case of isolated square vortex rings. (C) 1996 American I nstitute of Physics.