Question

Why do we consider specified shear stress as zero at cylindrical domain boundary in ansys

Answer 1

If your boundaries represent actual geometry and not part of a larger domain, you must consider NO SLIP condition for the walls as well as the circle. This would allow for boundary layer development before, and around, the circle (rotating cylinder). These two boundary layers would interact in the actual flow and you would expect to see recirculation regions in the flow. With a slip boundary, or zero shear condition, these elements would not be captured.

(PLZ MARK MY ANSWER AS BRAINLIST)