Application of direct and large-eddy simulation methods to late wakes of submerged bodies

Late wakes of submerged bodies are simulated in support of the ONR mechanics and energy conversion turbulence program. Direct numerical simulations (DNS) is used to examine the dynamics, energetics, and structure of wakes evolving to late times. Particular emphasis is placed on quantifying the structural differences between wakes of towed and self-propelled bodies and on quantifying the influences of environmental shear and stratification. The DNS results clearly show that wakes of towed bodies lead to larger-scale coherent structures at late times. Stratification tends to confine the wake in the vertical direction and provides a mechanism for the radiation of internal waves. Shear is found to interrupt the upscale cascade of vorticity. In addition to DNS, large-eddy simulation (LES) is considered as a means of accessing higher Reynolds numbers and of enabling broader parameter studies. As a first step in this direction, the LES approach is validated by repeating prior DNS on much coarser meshes. A parameter-free dynamic subgrid-scale model is used for this purpose and good results for low-order statistics are obtained on a mesh that is a factor of 4 coarser in each direction as compared with the DNS. When the effect of the increased time step is taken into account, this represents a factor of 4/sup 4/ = 256 reduction in computational expense. Thus LES can greatly reduce the computer time needed for wake studies and can allow significant increases in Reynolds number.