The boundary layer concept involves maintaining which type of flow to minimize drag?

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

Answer: (A)

In the boundary layer, drag mainly comes from viscous forces between the fluid and the surface. When the boundary layer is laminar, the flow moves in smooth, parallel layers with little mixing, so the shear stress at the wall is relatively small. That means the viscous or skin-friction drag is minimized. If the boundary layer becomes turbulent, the chaotic motions increase momentum transfer toward the wall, boosting wall shear and thus increasing skin-friction drag. Although a turbulent boundary layer can resist separation better in some adverse-pressure-gradient regions, the overall drag is typically higher. Therefore, the goal to minimize drag is to keep the boundary layer laminar as much as possible.