Heat Transfer problem:
 Explain why the skin friction becomes independent of Reynolds number for a rough wall at large Re

If the Reynolds number is large the viscosity effect is small. For practical values the inertia or density forces dominate, and the dragging force increases with the square of the velocity. However, although the viscosity is unimportant, it may still affect the very thin boundary layer, leading to the creation of turbulent flow.

Therefore, the skin friction becomes independent of Reynolds number for a rough wall at large Re.

Step-by-step explanation

solution:

If the Reynolds number is large the viscosity effect is small. For practical values the inertia or density forces dominate, and the dragging force increases with the square of the velocity. However, although the viscosity is unimportant, it may still affect the very thin boundary layer, leading to the creation of turbulent flow.

Therefore, the skin friction becomes independent of Reynolds number for a rough wall at large Re.

for example :

If the Reynolds Number is large, the viscosity effect is small. For the for us practical values the inertia or density forces dominate, and the parasite drag increases with the square of the velocity. However, although the viscosity is unimportant, it may still affect the very thin boundary layer, leading to the creation of turbulent flow. Thus the importance of the Reynolds Number is that it tells us the type of flow we can expect. It tells you whether you can hope for having laminar flow over the wing and other parts of your airplane. A low Reynolds Number gives laminar flow while a high Reynolds Number gives turbulent flow. For both a laminar and a turbulent boundary layer increasing Reynolds Number gives lower skin friction drag. However, because of the higher energy loss in the boundary layer, a turbulent layer always has higher skin friction drag.