Question

How is the boundary layer thickness defined? (ue is the outer edge velocity)

(a) u = 0.99ue

(b) u = 0.89ue

(c) u = 0.90ue

(d) u = 0.50ue

This question was addressed to me in class test.

Query is from Boundary Layer Properties in portion Boundary Layers, Laminar & Turbulent Boundary Layers, Navier Stokes Solutions of Aerodynamics

Answer 1

Right choice is (a) u = 0.99ue

Best explanation: Inside the boundary layer, the velocity increases along the y – direction until it becomes equal to the freestream velocity. The thickness of boundary layer δ is defined as the point from the surface where the velocity is 0.99 times the free stream velocity.