Which boundary layer condition is associated with the lowest skin friction drag?

• A. Laminar
• B. Turbulent
• C. Separated
• D. Transitional

Answer: (A)

The amount of skin friction drag comes from the shear stress in the boundary layer at the surface. A laminar boundary layer features smooth, orderly motion with layers sliding past one another, producing a relatively small velocity gradient right at the wall. Since wall shear stress is proportional to this gradient, laminar flow has the smallest surface shear and thus the lowest skin friction drag.

In a turbulent boundary layer, the near-wall region is highly mixed by eddies and bursts, which increases momentum transfer toward the wall and raises the effective viscosity felt there. This boosts the wall shear stress and markedly increases skin friction drag.

If the boundary layer separates due to an adverse pressure gradient, it can reduce friction locally on parts of the surface, but that typically comes with a large pressure drag from the resulting wake, so overall drag isn’t minimized. Transitional flow sits between laminar and turbulent, so its skin friction is intermediate.

Thus, laminar flow is associated with the lowest skin friction drag.