Question

Which of the following is true?

(a) Flow is laminar inside the boundary layer and turbulent outside

(b) Flow is turbulent inside the boundary layer and laminar outside

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

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

This question was addressed to me in class test.

This intriguing question comes from Types of Fluid Flow in section Kinematics of Flow and Ideal Flow of Fluid Mechanics

Answer 1

The correct answer is (a) Flow is laminar inside the boundary layer and turbulent outside

For explanation I would say: Flows can be characterized as laminar or turbulent on the basis of Reynold’s number Re = ρvd / μ, where ρ is the density, d is the pipe diameter and μ is the viscosity. For Re < 2000, the flow will be laminar and Re > 4000, the ow will be turbulent. For laminar flow, the viscosity effects must be high (μ should be high) as inside the boundary layer. Outside the boundary layer, the viscosity effects are negligible. Hence, the flow will be turbulent.