In the boundary layer, which type of airflow is desired to delay separation?

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

Answer: (B)

The boundary layer’s ability to stay attached to the surface under adverse pressure gradients is what determines separation. When the pressure climbs against the flow, the boundary layer slows and can detach if the gradient is strong enough. A turbulent boundary layer has more momentum in the near-wall region because of mixing from the outer flow into the inner layer. That extra momentum helps the fluid resist the adverse pressure pushing it away from the surface, so the flow remains attached longer and separation is delayed. A laminar boundary layer, being smoother and lacking that enhanced near-wall momentum, tends to separate sooner under the same conditions. Designers sometimes purposefully trip the boundary layer to turbulence in critical areas to prevent or postpone separation, even though that increases drag. Transitional flow sits between these two states, but the tendency is that turbulent flow better resists separation.