Direct Numerical Simulation of Axisymmetric Jets

摘要

We present results from numerical simulations of the evolution of the Kelvin-Helmholtz instability for an unforced, subsonic, compressible axisymmetric, spatially-evolving shear layer. In addition, we study the effect of small, random pressure fluctuations at the nozzle orifice on the growth of the mixing layers. These fluctuations model inflow perturbations in experimental flows arising from turbulence and boundary layers in the nozzle. The finite-difference numerical model used to perform the simulations solves the two-dimensional time-dependent conservation equations for an ideal fluid using the Flux-Corrected Transport algorithm and timestep-splitting techniques. No subgrid turbulence model has been included. In the absence of perturbations, the calculations indicate that the large scale development of the unforced jet shear layer has an underlying degree of organization. This is the result of a feedback mechanism in which the shear layer ahead of nozzle edge is modulated by the far field induced gy the mergings on downstream, near the end of the potential core of the jet. The studies with random high frequency perturbations on the inflow velocity show that they effectively tend to break the temporal correlations between the structures. Keywords: Coherent structures; Free Shear flows.