In the boundary layer, to delay flow separation you should aim for:

• A. Laminar airflow because it has low skin friction drag
• B. Turbulent airflow
• C. Separated flow
• D. Transitional flow

Answer: (A)

The key idea is how the state of the boundary layer affects its resistance to adverse pressure gradients that cause separation. A laminar boundary layer has less internal mixing and less near-wall momentum to work with when the pressure rises along the surface, so it tends to detach sooner under adverse gradients. A turbulent boundary layer, however, contains more mixing and higher momentum transfer toward the wall because of eddies, which helps the flow cling to the surface longer as the pressure increases. That stronger near-wall momentum makes turbulent boundary layers more capable of withstanding adverse pressure gradients and delaying separation, even though they come with higher friction in other contexts. So, to delay flow separation, promoting a turbulent boundary layer (rather than relying on laminar flow) is generally the better strategy, even though laminar flow has lower skin-friction drag.