Which is more efficient: laminar or turbulent flow over an airfoil? Find out in this simple Computational Fluid Dynamics (CFD) study.

CFD Simulation of Laminar Flow Over An AirfoilVelocity Magnitude Contours

Background

For streamlined bodies, such as airplanes, maintaining laminar boundary layer airflow is a sure way to minimize drag and so improve efficiency and reduce fuel costs. However, given the speeds and conditions that airplanes encounter, trying to maintain laminar flow is difficult. In addition to a Reynolds number dependency, laminar boundary layers are very sensitive to bugs (the squidgy variety) and dirt on airfoil leading edges. These imperfections can cause a transition to turbulence and increased drag. Worse still, a specially designed laminar wing can exhibit abrupt changes in airplane handling when laminar flow breaks down. Most airplane designers instead settle for turbulent boundary layers, sacrificing efficiency for predictable performance characteristics.

CFD Simulation

Using the NACA 0012 airfoil as the basis for this study it is relatively easy in CFD to compare fully laminar and fully turbulent simulations and assess the performance in terms of lift and drag at various angles of attack below the stall condition.

Laminar Pressure Coefficient (Cp) Contours at 8 Degrees

Turbulent Pressure Coefficient (Cp) Contours at 8 Degrees

Laminar Velocity Magnitude Contours at 8 Degrees

Turbulent Velocity Magnitude Contours at 8 Degrees

Lift Comparison for Laminar and Turbulent Flow

There is very little difference between the lift for the laminar and turbulent simulations.

Drag Comparison for Laminar and Turbulent Flow

As expected, the turbulent simulation produces more drag than the laminar simulation for all angles of incidence.

Conclusion

Clearly, if the conditions for laminar flow could be consistently maintained, then a designer would favor the more efficient, low drag, laminar wing. Alas, the real world is messy (remember squidgy bugs and dirt) and so an optimized turbulent wing flow is usually sought.

Future

Boeing has proposed a leading-edge suction method to maintain laminar flow over the tail of the new 777X. It appears this method relies on tiny holes to suck air from the boundary layer and prevent transition to turbulence. A major issue Boeing designers will need to address is how to keep the holes free of bugs and dirt? Maybe they are looking at a NASA study on special coatings to reduce bug splats?

Notes

• NACA 0012 mesh courtesy of FreeCASE and also used in the Caedium tutorial "Transonic Flow Over the NACA 0012 Airfoil".
• Simulations assume incompressible flow at 10 m/s.
• Simulations set up and performed in Caedium Professional using the incompressible, steady-state RANS solver, without a turbulence model (laminar) and with the k-omega SST turbulence model (turbulent).