Question

The boundary layer thickness for an incompressible, laminar flow at a distance x with Reynolds number Rex is δ. Which is____

(a) Directly proportional to √x

(b) Inversely proportional to x

(c) Directly proportional to Rex

(d) Inversely proportional to Rex^2

The question was asked in an online quiz.

My enquiry is from Laminar Flow topic in section Incompressible Flow over Airfoils of Aerodynamics

Answer 1

Right option is (a) Directly proportional to √x

Explanation: The boundary layer thickness for an incompressible, laminar flow over a flat plate at zero angle of attack, at a distance x where Reynolds number is Rex is given by δ=\(\frac {5x}{\sqrt{Re_x}}\). Here Reynolds number is Rex=\(\frac {xVd}{u}\). Thus, the thickness is directly proportional to √x.