Question

1.Describe the differences, if any, between laminar and turbulent boundary layers.

2.Describe the different sub-layers required to model turbulent boundary layers.

3.Discuss the reason(s) for a similar pattern between wall shear stress and heat transfer coefficient profiles.

4.Describe the reason(s) why both turbulent momentum and thermal boundary layers approaching each other.

5.Describe the reason(s) why the unsteady governing equation terms must be kept when dealing with turbulence

Answer 1

Answer1)

A boundary layer may be laminar or turbulent. In a laminar boundary layer, any exchange of mass or momentum takes place only between adjacent layers on a microscopic scale which is not visible to the eye. A turbulent boundary layer, on the other hand, is marked by mixing across several layers of it. The mixing is now on a macroscopic scale. Packets of fluid may be seen moving across. Thus there is an exchange of mass, momentum and energy on a much bigger scale compared to a laminar boundary layer.

Answer 2)

(a)The viscous sublayer

(b)The inertial sublayer

(c)The turbulent boundary layer

(d)The ambient flow

Answer 3)

For fluids, the shear stress is directly proportional to the velocity gradient across the y-axis. Noting that the velocity profile remains unchanged in the hydrodynamically fully developed region, the wall shear stress also remains constant, for fluids of constant viscosity.

When fluid enters the tube with tube walls at a different temperature from the fluid temperature, the thermal boundary layer starts growing. After some distance downstream (entrance length), the temperature profile remains constant along the flow direction. Hence heat transfer coefficient remains constant for a thermally fully developed flow in a pipe, for constant heat flow (q) and surface temperature (Ts).

Answer 4)

The thermal boundary layer and velocity boundary layer are related by the Prandtl number, is called the momentum diffusivity and is called the thermal diffusivity; if it is less than unity, the momentum boundary layer (or velocity boundary layer) remains within the thermal boundary layer.

Answer 5)

This is because in turbulent flow, flow condition and everything rapidly changes with the time.