Large eddy simulation of the near-field vortex dynamics in starting square jet transitioning into steady state

Large eddy simulation (LES) is carried out to study the vortex dynamics in the near-field of a starting turbulent square jet as well as its evolution into a developed steady jet. Simulations are conducted at Reynolds numbers (Re = UjD/nu) of 8000 and 45 000 based on the nozzle hydraulic diameter (D) and jet velocity (U-j). A Reynolds stress model was used to simulate the internal flow in the nozzle which provided the inlet conditions for the LES of the jet. To validate the simulations, turbulence statistics are compared with experimental results available for a steady square jet. Evaluation of the probability density function, skewness, and flatness of the centerline streamwise velocity (U-c) shows deviation from the Gaussian distribution in the near-field. Evolution of the self-induced deformation of the leading vortex ring is investigated to further clarify the role of axis-switching. The axis-switching is initiated earlier at low Reynolds number while the completion of the axis-switching process occurred at the same dimensionless time for both Reynolds numbers. The role of pressure distribution on vortex ring deformation is investigated. It is shown that the influence of pressure-induced azimuthal instability tends to deform a two-dimensional vortex ring topology into a three-dimensional one and revert back to a two-dimensional character again. The break-down and diffusion of the tip of the vortex are also studied. Evolution of the shear layer from a starting jet to a developed jet is studied in terms of the vorticity field. For a starting jet, entrainment is shown to occur in the presence of corner hairpin vortices. Published by AIP Publishing.