Hello,

Consider 2 concentric cylinders rotating with w1 and w2 in the same direction. Between does 2 cylinders there is an incompressible viscid fluid. Why is the pressure gradient a function on the radius, and not the angle (the flow is induced by the rotation and in the rotation (teta) direction)?

Tanx.

If the flow is steady, then think about what you are asking - a pressure gradient in the circumferential direction would imply that the pressure at zero degrees around the cylinder would be different than the pressure at 360 degrees around the cylinder. But that would make no sense, unless you have some mechanism to arbitrarily insert discontinuities in the flow.