I am relatively new to CFD. I want to use 3D mesh for some computational work on Wind turbines. If I have to sacrifice some grid points to reduce the run-time, which direction should I choose to coarsen the grid without affecting much of the final output? Usually which directions are more important to be fine-meshed than the other? Thanks a lot for your help.

Removing points along the blade's span would be the place to start. You need points off the blade to resolve the boundary layer and points in the chordwise direction to resolve transition or other phenomena.

Dear John Chawner, Thanks a lot for your delicate response. I appreciate your suggestions. Would you please explain the grid directional importance a little more or give me links to some reference books, papers or articles. Waiting for your kind response and thanking you in anticipation.