The boundary layer is the layer of fluid in immediate vicinity of a surface that has significant viscosity effects. Which option best describes it?

• A. The layer near a surface with significant viscosity effects
• B. The region where fluid flows freely away from surfaces
• C. The core region where velocity is uniform
• D. The region where density changes rapidly

Answer: (A)

The boundary layer is the thin region adjacent to a solid surface where viscous effects are significant. Because of the no-slip condition, the fluid velocity goes to zero at the wall and rises to the free-stream value away from the surface, creating a steep velocity gradient within this layer. Outside of it, viscous effects are small and the flow can often be treated as inviscid. So the best description is the layer near a surface where viscosity matters. The other statements describe regions away from the wall, a uniform core, or rapid density changes, which are not defining features of the boundary layer.