Question

What fluid property is responsible for the development of the velocity boundary layer? For what kinds of fluids will there be no velocity boundary layer on a flat plate?

Short answer

Answer: Fluids with zero viscosity, such as ideal fluids and superfluids (e.g., helium-II at extremely low temperatures), will not have a velocity boundary layer on a flat plate. The fluid property responsible for the development of the velocity boundary layer is viscosity.

Step-by-step solution

Define the velocity boundary layer

The velocity boundary layer is the region of fluid flow adjacent to a solid surface where the fluid's velocity changes from zero at the surface (due to the no-slip condition) to the free-stream velocity away from the surface. In this region, viscous forces play a significant role in the flow behavior.

Identify the fluid property responsible for the development of the velocity boundary layer

The fluid property responsible for the development of the velocity boundary layer is viscosity. Viscosity is a measure of the fluid's resistance to flow and deformation, resulting from the interaction between fluid particles. It dictates how a fluid responds to shear stress, which determines the development of the velocity boundary layer.

Describe types of fluids with no velocity boundary layer on a flat plate

For a fluid to have no velocity boundary layer, it must have zero viscosity (an ideal fluid). In reality, there are no fluids with zero viscosity. However, some fluids, such as superfluids (e.g., helium-II at extremely low temperatures), exhibit behavior that comes close to having zero viscosity. These superfluids can flow without friction, and therefore, a velocity boundary layer is not observed on a flat plate.