Which boundary layer, when considering its velocity profile perpendicular to the flow, has the greatest change in velocity close to the surface?

• A. Laminar boundary layer
• B. Free stream
• C. Turbulent boundary layer
• D. Transitional boundary layer

Answer: (C)

Understanding how velocity changes with distance from a surface is key. The slope of the velocity profile right at the wall is the wall shear rate, related to the wall shear stress. In a fully developed turbulent boundary layer, vigorous eddy motion transports momentum toward the wall and away from it, increasing the wall shear and making the velocity rise from zero at the surface to the free-stream value over a very short distance. This creates a much steeper near-wall gradient than in a laminar boundary layer, where the rise is more gradual, and than in a transitional layer, which is intermediate. The free stream isn’t a boundary layer and doesn’t describe this near-wall change. So the greatest change in velocity close to the surface occurs in the turbulent boundary layer.