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In which case are the velocity and thermal boundary layer equal?(a) Pr = 0(b) Pr = 1(c) Pr > 1(d) Pr < 1I 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

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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.



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