Separation bubbles and transition points affect the design of low Reynolds number airfoils.
Laminar separation bubbles are indicative of an adverse pressure gradient in the airfoil.
Minimizing drag from the laminar separation bubbles requires optimization of the point of transition.
Understanding variables like pressure distribution or velocity can be key to efficient low Reynolds number airfoil design with optimal lift and drag
The different airfoil geometries and their significant impact on the performance and efficiency of aerodynamic systems have been widely studied. A common aspect in these studies tends to be the impact of flow behavior on airfoils at high or low Reynolds numbers. These are important to analyze lift and drag in a system.
When approaching an airfoil design method, a low Reynolds number is associated with a laminar flow, which is the preferred flow regime. However, there are phenomena like the formation or bursting of separation bubbles or flow transition that affect design variables. In this article, we will take a look at the low Reynolds number as well as airfoil design variables and their effect on aerodynamic design and performance.
Implications of a Low Reynolds Number in Airfoil Design
Drag and lift are regular forces experienced during flight. The only concern is the extent to which they act along an airfoil. A low Reynolds number is generally desired, as it indicates a laminar flow regime, which is associated with a decrease in drag coefficient and an increase in lift. In the analysis of aerodynamic performance, a low Reynolds number is considered to be less than 100000. The formation of laminar separation bubbles is associated with this number, which affects laminar airfoil design.
Laminar Separation Bubbles
A low Reynolds number can be characterized based on the laminar separation bubbles. These bubbles are formed due to an adverse pressure gradient, which can be seen as a result of the deceleration of the fluid. The increase in surface pressure causes the separation of the laminar boundary layer from the surface of the airfoil. The free shear layer takes the form of eddies or vortices, inducing the transition of the flow to a turbulent one.
As this turbulence attaches to the airfoil surface again, it forms laminar separation bubbles. These separation bubbles can significantly increase drag and reduce lift in the aerodynamic system as it thickens the boundary layer. The drag created can have multiple times the effect than when created without a separation bubble, affecting the lift severely and, in worst cases, causing stalls. The formation of separation bubbles and their effects can vary for different Reynolds numbers.
- □ At the range of 50000< Re<10000, the reattachment point of the separated shear layer falls along the trailing edge of the airfoil. This means the formation of a bigger separation bubble.
- □ At 10000< Re<50000, the reattachment point falls further back off the trailing edge. The stability of the boundary layer increases the resistance to transition, further increasing the drag.
Given these adverse effects on the aerodynamic system, the designer must focus on locating the point of transition, which can be the key to controlling drag from the laminar separation bubbles on low Reynolds number airfoils.
Optimizing Low Reynolds Number Airfoils
Improving the performance of low Reynolds number airfoils is dependent on many factors including point of transition and pressure distribution.
Point of transition - The point where the flow transition begins determines the length of the laminar separation bubble, which, in turn, affects the drag coefficient. The airfoil profile must be optimized to control this transition point so as to avoid the formation of long separation bubbles. This can be done by adapting the use of:
- Transition ramps
- Boundary layer trips
Pressure distribution - The size of the separation bubbles can affect the pressure distribution along the upper surface of an airfoil. While a short bubble may not produce a drastic effect, long separation bubbles create an adverse pressure gradient extending over a large part of the airfoil, causing an increase in drag. The transition ramp enables a gradual transition of the flow without increasing the pressure or drag.
Angle of attack - By maintaining the critical angle of attack for ideal flow separation, a low Reynolds number airfoil can be optimized to have a high lift coefficient.
Using CFD Solvers for Low Reynolds Number Airfoil Optimization
Airfoil design using Pointwise
The ideal way to study and optimize low Reynolds number airfoil characteristics for a given flow condition is through the use of CFD tools. CFD simulation facilitates the analysis of flow behavior for low Reynolds number airfoils. Using the Reynolds-averaged Navier-Stokes (RANS) based model, the lift and drag coefficients, moment, and skin friction can be identified for a range of Reynolds numbers to help in the analysis of the pressure distribution and transition point in an airfoil. Based on this, airfoil geometry can be optimized to produce minimal drag, avoid separation bubbles, and make decisions on the addition of transition ramps or trips.
Aerodynamic engineers and designers can make different flow-related analyses for airfoil using the complete CFD simulation package from Cadence. Creating high-precision meshing with Fidelity Pointwise and high-fidelity simulation from Fidelity can help provide a numerical approach to visualizing and solving the dynamics behind low Reynolds number airfoil designs.