Confined swirling jet impingement on a flat plate at moderate Reynolds numbers

The behavior of a swirling jet issuing from a pipe and impinging on a flat smooth wall is analyzed numerically by means of axisymmetric simulations. The axial velocity profile at the pipe outlet is assumed flat while the azimuthal velocity profile is a Burger's vortex characterized by two non-dimensional parameters; a swirl number S and a vortex core length delta. We concentrate on the effects of these two parameters on the mechanical characteristics of the flow at moderate Reynolds numbers. Our results for S=0 are in agreement with Phares et al. [J. Fluid Mech. 418, 351 (2000)], who provide a theoretical determination of the wall shear stress under nonswirling impinging jets at high Reynolds numbers. In addition, we show that the swirl number has an important effect on the jet impact process. For a fixed nozzle-to-plate separation, we found that depending on the value of delta and the Reynolds number Re, there is a critical swirl number, S=S*(delta, Re), above which recirculating vortex breakdown bubbles are observed in the near axis region. For S>S*, the presence of these bubbles enhances the transition from a steady to a periodic regime. For S<S*, the flow remains steady and the results show that the introduction of swirl reduces the maximum pressure and radial skin-friction coefficients over the wall. (C) 2009 American Institute of Physics. [DOI:10.1063/1.3063111]