Development of boundary conditions for direct numerical simulations of three-dimensional vortex breakdown phenomena in semi-infinite domains

The suitability of various open boundary conditions is evaluated for direct numerical simulations of three-dimensional, incompressible, spatially and temporally evolving, swirling laminar jets in domains that extend to infinity in the downstream and radial direction. From the point of view of specifying conditions at the open boundaries, this class of flows is particularly challenging due to its ability to support traveling waves. Towards this end, several radial boundary conditions are implemented and tested with respect to their ability to conserve local and global mass, to handle low and high entrainment flow, and to avoid the introduction of artificial waves propagating from the boundaries into the interior: a free-slip condition, two types of homogeneous Neumann conditions, and a radiation condition in spirit of the outflow boundary condition. Global mass is conserved automatically within machine accuracy in the free-slip and simple radiation case, while the Neumann conditions require some iterative modification to conserve mass. This yields a computationally less efficient scheme which additionally exhibits poorer conservation properties due to the limited number of iterations. The free-slip condition typically requires the largest radial extent of the computational domain due to its impermeable character which is particularly problematic for the high entrainment flow. Hence, the radiation condition has been found as the most suitable lateral boundary condition for both high and low entrainment jets. (C) 2004 Elsevier Ltd. All rights reserved.