What is the difference between laminar and turbulent boundary layers in terms of drag and energy dissipation?

What is the difference between laminar and turbulent boundary layers in terms of drag and energy dissipation?

• A. Laminar boundary layers have lower skin-friction drag but are more susceptible to separation; turbulent layers have higher skin-friction drag but better resistance to separation.
• B. Laminar boundary layers have higher skin-friction drag but are more resistant to separation.
• C. There is no difference in drag between laminar and turbulent boundary layers.
• D. Turbulent boundary layers have lower energy dissipation due to friction.

Answer: (A)

The main idea being tested is how the state of the boundary layer—laminar or turbulent—changes drag components and how energy is dissipated in the flow around a surface. A laminar boundary layer flows in an orderly, smooth layer where viscous shear near the wall is relatively small, so the skin-friction drag is low. However, if the flow encounters an adverse pressure gradient, a laminar layer has a harder time keeping attached to the surface and tends to separate earlier. When separation occurs, a large wake forms and pressure drag increases, so the overall drag can become substantial despite the low skin-friction portion. A turbulent boundary layer, in contrast, has many eddies and strong mixing, which increases momentum transfer toward the wall. This raises the skin-friction drag because the wall shear stress is higher. But that same turbulence helps the flow resist adverse pressure gradients and stay attached longer, reducing the likelihood of separation and the associated pressure drag. So, attached turbulent layers typically exhibit higher frictional drag yet better resistance to separation. Regarding energy dissipation, turbulence adds continuous chaotic motion that ultimately dissipates energy via viscosity at the smallest scales, so the boundary layer dissipates more energy overall than a laminar layer. Laminar flow dissipates energy mainly through smooth shear, which is lower. So, the described trade-off—laminar with lower skin-friction drag but greater separation risk, versus turbulent with higher skin-friction drag but better attachment—captures the essential difference, and explains why turbulent dissipation is higher.

The main idea being tested is how the state of the boundary layer—laminar or turbulent—changes drag components and how energy is dissipated in the flow around a surface.

A laminar boundary layer flows in an orderly, smooth layer where viscous shear near the wall is relatively small, so the skin-friction drag is low. However, if the flow encounters an adverse pressure gradient, a laminar layer has a harder time keeping attached to the surface and tends to separate earlier. When separation occurs, a large wake forms and pressure drag increases, so the overall drag can become substantial despite the low skin-friction portion.

A turbulent boundary layer, in contrast, has many eddies and strong mixing, which increases momentum transfer toward the wall. This raises the skin-friction drag because the wall shear stress is higher. But that same turbulence helps the flow resist adverse pressure gradients and stay attached longer, reducing the likelihood of separation and the associated pressure drag. So, attached turbulent layers typically exhibit higher frictional drag yet better resistance to separation.

Regarding energy dissipation, turbulence adds continuous chaotic motion that ultimately dissipates energy via viscosity at the smallest scales, so the boundary layer dissipates more energy overall than a laminar layer. Laminar flow dissipates energy mainly through smooth shear, which is lower.

So, the described trade-off—laminar with lower skin-friction drag but greater separation risk, versus turbulent with higher skin-friction drag but better attachment—captures the essential difference, and explains why turbulent dissipation is higher.