by based on which formula we calculate rotational offset value for frozen rotor type boundry conditin? i can't find this formula in help document. pls guide me.

The "offset" command lets you rotate the impeller (or a portion of it if you're doing a periodic simulation), without physically rotating the impeller and its mesh. The code takes into account the rotation even if the mesh is always in the same position. This command has much more sense for periodic analysis. Hope this helps!