Does anyone have any useful knowledge for a perfect meshing in Flow3D? I do not understand cell size ratio and aspect ratio. thanks.

Well, a perfect mesh is cubic. This gives best accuracy and convergence rate. The aspect ratio is one and the adjacent cell size ratio is one too. In practice, of course, it may be too expensive to have such a mesh. For a non-uniform grid, the mesh generator attempt to smooth out the variation in cell sizes in a given coordinate direction, dx(i)/dx(i+1), but it cannot control the aspect ration (dx/dy, dx/dz and dy/dz), so the preprocessor reports these number for the user information. For example, an aspect ratio of > 3 may become problematic for convergence and/or accuracy, especially for flows with surface tension.