Mass conservation limitations in overset CFD with unsteady moving grids

Overset grid techniques (also known as Chimera grids, composite grids, or overlapping grids) were introduced in 1966. Overset methods were originally applied to compressible flow simulations for multi-body external aerodynamics. In the early 2010's, overset methods were extended to incompressible underwater and naval surface ship simulations. The use of moving overset grids in incompressible flow simulations poses a unique challenge--how to account for the instantaneous mass conservation imbalance when the grid volume changes. If the body mesh moves by a small amount, relative to the size of the overlapping grid cells, then the fringe-donor interpolation stencils can simply be updated based on the new cell positions. However, if the mesh motion requires current hole cells to be reactivated, or new hole cells removed from the simulation, the total fluid volume contained within all active grid cells changes instantaneously. In an incompressible flow simulations, the instantaneous volume change results in a significant pressure oscillation, due to the pressure-mass coupling inherent in PSIO or SIMPLE based algorithms. To further complicate the issue, reducing the simulation time-step actually increases the amplitude of these pressure oscillations. This presentation will explore this time-step phenomina and show some recent work attempting to overcome this limitation.