The effects of grid resolution on hypersonic viscous flow simulations are investigated in detail. Previous studies have shown that grid plays an important role on the aerothermodynamic predictions for a hypersonic vehicle. We investigate flows over simplified geometry to analyze the behavior of grid resolution when the flow is solved using the Navier-Stokes equations. It shows that grid 

independent solutions can be achieved with fine meshes where the surface-based cell Reynolds number is in the order of 10. The reason for fine meshes is analyzed using wall functions derived using the boundary layer approximation. It is found that the truncated error of numerical schemes is increased dramatically when the flow approaches the surface, which explains the fact that near-surface cells should be much smaller than faraway cells to maintain the numerical error at a low level.