Question

Which of these is not a property of boundary layer?

(a) No – slip condition at the surface

(b) Temperature of fluid at the surface is equal to wall temperature

(c) Flow velocity increases along y – direction

(d) Thermal boundary layer is equal to velocity boundary layer

The question was asked in an interview.

This is a very interesting question from Boundary Layer Properties in portion Boundary Layers, Laminar & Turbulent Boundary Layers, Navier Stokes Solutions of Aerodynamics

Answer 1

Right option is (d) Thermal boundary layer is equal to velocity boundary layer

The explanation is: When there’s a viscous flow over a flat plate, there’s a boundary layer formation which has certain properties. On the surface, there’s a no slip condition. Apart from this the temperature of the fluid which is immediately at the surface has the same temperature as the surface which is known as the wall temperature. The velocity profile inside a boundary layer increases along the y – direction until it becomes equal to the freestream velocity. The only property that is incorrect is that the thermal boundary layer is equal to the velocity boundary layer.

The boundary of thermal layer is defined as the layer where the outer edge temperature becomes equal to the freestream temperature. Similarly, at the velocity boundary layer, the outer edge velocity is equal to the freestream velocity.