r is ball radius, h is box Length, width and height, 2r<h. How to calculate this kind of field. How to set BC. . Any suggestion are helpful. Thx in advance.

nado

I guess it depends on what you want of the analysis.

If its just the steady state flow field around the ball, you can make the system steady and let the flow past a stationary ball.

At each time step, shall i generate a new grid?

If you are doing a steady state analysis there is no need to generate the grid every time.