Coherent structures in the near-field of rectangular jets

The unsteady, 3-D, incompressible Navier-Stokes equations are solved numerically to study the near-field characteristics of an unforced AR=2 rectangular jet at Re=750. Simulations are performed for two different inflow conditions: i) a developing thin boundary layer; and ii) a fully-developed velocity profile. For the former case, the jet remains symmetric with respect to its minor and major axes for the entire simulated time interval. For the fully-developed case, however, azimuthal asymmetric modes begin to grow during the very early stages of the jet evolution. The asymmetries, which originate near the centers of the longer sides of the jet, lead to a very complex flow characterized by finer-scale coherent structures and enhanced lateral spreading rates and entrainment. The computed results underscore the sensitivity of asymmetric jet flows to inflow conditions, clarify the characteristics of the near-field coherent structures, and demonstrate the strong coupling and non-linear interaction between the ω_θ- and ω_x-dynamics.