Airfoils at low Reynolds Numbers

The airfoil section is a key feature of various lifting surfaces including wings, blades, and hydrofoils. Given its fundamental technical importance, airfoil performance has been the focus of numerous investigations in fluid mechanics. Recent developments in wind turbines, small-scale gas turbines, and unmanned aerial vehicles brought about an increased interest to airfoil operation at low Reynolds numbers (below 500,000).

Airfoil performance at low Reynolds numbers differs significantly from that common to high Reynolds number flows. Airfoil operation at low Reynolds number generally suffers from low lift and high drag. This occurs because even at low angles of attack a laminar boundary layer on the upper surface of the airfoil often separates and an unstable shear layer forms that undergoes transition to turbulence.

Flow visualization images shown in Figure 1 illustrate two flow regimes common to airfoils operating low at Reynolds numbers: (i) boundary layer separation occurs without subsequent reattachment on the upper surface, so that a wide wake forms (Fig. 1a), and (ii) the separated shear layer reattaches to the airfoil surface, forming a separation bubble (Fig. 1b).

Our work combines experimental investigation and analytical modeling to gain insight into key flow phenomena, such as boundary layer separation, laminar-to-turbulent transition, and wake development. For example, using a high-speed flow visualization technique, the progression of this transition process is captured in the video below, which shows the shear layer periodically rolling up into vortices.

Understanding this process is critical as, given the correct flow conditions, mean flow reattachment can occur, thus forming a separation bubble and improving performance dramatically. The flow development is quantified using Particle Image Velocimetry (PIV), with a representative experimental arrangement seen in figure 2. This aspect of our work is aimed at aiding in the design of wind turbines and Unmanned Aerial Vehicles.

upstream flowa.

upstream flow with low Reynolds Numberb.

Figure 1. Airfoil operating at low Reynolds numbers. 

Flow visualization showing airfoil laminar boundary layer separation leading to shear layer formation and roll-up into vortices.

<--break-> 

PIV set-up for imaging the side-view of the laminar separation bubble

 Figure 2. PIV set-up for imaging the side-view of the laminar separation bubble.