Question

In which case are the velocity and thermal boundary layer equal?

(a) Pr = 0

(b) Pr = 1

(c) Pr > 1

(d) Pr < 1

I got this question during an internship interview.

This question is from Boundary Layer Properties topic in section Boundary Layers, Laminar & Turbulent Boundary Layers, Navier Stokes Solutions of Aerodynamics

Answer 1

Right answer is (b) Pr = 1

For explanation I would say: In most of the cases the two thermal and velocity boundary layers are not same except in one exceptional case when the Prandtl number  =  1, in which case δt = δ. When Prandtl number is greater than 1, δt < δ and when Prandtl number is less than 1, δt > δ. In real life scenario, the Prandtl number is equal to 0.71 thus the thermal boundary layer thickness is greater than the velocity boundary layer thickness.