Airflow over airfoils generates lift and drag, and the amount of lift depends on the flow regime.
When airflow transitions to turbulent at high Reynolds numbers, the result is greater drag.
Typical laminar-to-turbulent flow transitions over airfoils occur at Reynolds numbers of approximately 500,000.
Air traffic wouldn’t be possible without laminar flow and airfoils. On an aircraft, an airfoil is the structure responsible for moderating lift and drag created by oncoming airflow. The flow regime is important in aircraft design and engineering, as it determines the amount of lift and drag acting on an airfoil. Together, these are the main determining factors used to design aircraft to reach engineering and performance goals once an aircraft is put into operation.
During flight, an aircraft would prefer to encounter laminar flow. The reasons for this are varied, but they relate to the main aerodynamic forces acting on an aircraft, and particularly on the airfoil itself. We’ll discuss these aspects of laminar flow over airfoils and examine the limits of laminar flow across airfoils in this article.
Why Laminar Flow Over Airfoils?
During flight, airflow across the wing of an aircraft creates drag and lift. Thrust exerted by the aircraft can create additional lift and drag due to skin friction along the surface of the craft, and particularly along the airfoil. At low Reynolds number flows, the airflow is laminar and sets up a boundary layer along the surface of the wing. What exactly constitutes a “low” Reynolds number depends on the shape and roughness of the wing, as is briefly discussed below. Regardless, we would prefer the flow rate across the aircraft to always be laminar.
Why is laminar flow preferred during flight? The answer relates to two of the four primary aerodynamic forces acting on an airfoil and the aircraft as a whole:
Drag in the boundary layer: Once the flow crosses into the turbulent regime, fluid flow in the boundary layer becomes turbulent and creates additional drag during flight. As a result, the aircraft may be limited in its top speed due to a drastic increase in drag during flight.
Higher speed: If the limit on laminar flow is higher, then the aircraft can generally sustain a higher top speed without turbulence than an aircraft with a lower laminar flow limit. This is quantified using the Reynolds number, as discussed below.
Avoiding the drag crisis: As the transition between laminar and turbulent flow occurs, a phenomenon known as the drag crisis causes a momentary drop in drag coefficient and drag force. The drag force eventually recovers and drag begins increasing as the craft’s speed continues increasing. Ensuring laminar flow avoids this problem and the increased turbulence acting on the aircraft.
This all means that, for two different wing designs, one wing may allow laminar flow while the other allows turbulent flow, even though the fluid flow parameters (flow rate, density, length scale) are the same for both systems. Depending on the required velocity, attack angle, and efficiency (drag vs. lift), a wing that generates excessive turbulent flow may need to be redesigned to allow faster cruising velocity without creating excessive drag due to onset of turbulence.
Limits of Laminar Flow on Airfoils
Just like in other systems involving fluid flow, the flow across an airfoil will eventually break and become turbulent. When this occurs, drag suddenly increases and greater thrust is required to maintain the speed of the aircraft. The exact limit of laminar vs. turbulent flow varies for different aircraft, but the typical limit should be approximately when the Reynolds number reaches 500,000. This just happens to be the critical Reynolds number for a flat plate, so we would expect a similar limit for a curved airfoil. The limit will also vary with attack angle; at high attack angle, the amount of drag acting on an aircraft could suddenly decrease as flow separation occurs and turbulent flow begins.
This situation involving laminar flow would often be examined using Bernoulli’s equation, but this equation cannot capture the limits of laminar flow and onset of turbulence at high Reynolds number. CFD simulations are needed to examine the limits of laminar flow and identify regions with high drag on the aircraft as part of the aircraft wing and body design.
As can be seen from the above results, the numerical scheme links the vorticity to the friction coefficient (see the proportional color scales above), so we can see where drag is highest around the body of the aircraft. To reduce drag, these areas of the craft could be targeted for redesign. In addition, the turbulent flow breaking across the airfoil surfaces can be analyzed as a function of various shape or form factors along the wing. Additional analysis steps can help engineers optimize the wing shape and orientation to ensure maximum efficiency during flight.
Aerodynamics engineers and systems designers who need to analyze flow behavior across their systems should use the complete set of CFD simulation applications from Cadence. The meshing features in Pointwise include everything needed to build highly accurate meshes for complex systems, and the Omnis simulation suite implements modern numerical approaches to solve the main fluid dynamics equations in 3D.