Question

Which of the following is true?

(a) Flow is rotational inside the boundary layer and irrotational outside

(b) Flow is irrotational inside the boundary layer and rotational outside

(c) Flow is rotational both inside and outside of the boundary layer

(d) Flow is irrotational both inside and outside of the boundary layer

I have been asked this question during an online exam.

I would like to ask this question from Types of Fluid Flow topic in division Kinematics of Flow and Ideal Flow of Fluid Mechanics

Answer 1

Correct choice is (a) Flow is rotational inside the boundary layer and irrotational outside

The best explanation: When a torque is applied to a fluid particle, it undergoes a rotation. Thus, the rotation of a fluid particle will alwayds be associated with shear stress. Shear stress is in turn dependent on the viscosity. Hence, rotational flow occurs where the viscosity effects are predominant. Since, viscosity effects are predominant inside the blundary layer, the flow will be rotational in this region. However, outside the boundary layer, the viscosity effects are negligible. Hence, flow can be treated as irrotational outside the boundary layer.