Air jets for lift control in low Reynolds number flow

The main focus of current research concerns rapid aerodynamic flow control in low Reynolds number regimes. The particular industrial application is air flow over wind turbine blades. Due to structural reasons, and other practical reasons, the aerofoil sections chosen for inboard sections of the blade tend to be relatively thick. Since the wind speed, relative to the blade, decreases from tip to hub, the flow around these thick sections is especially prone to separation.

Control Methods

The proposed control method is to eject air from the suction surface of the aerofoil near the trailing edge. If the external flow can be deflected then the lift of the aerofoil can be controlled directly. This is similar to the idea of a 'jet-flap', though in this case the 'flap' is placed on the opposite surface of the aerofoil.

The advantages of using control jets, rather than a traditional flap, are: mechanical simplicity, low inertia, and the potential for boundary layer control. The disadvantage of using control jets is that they continually consume power when deployed and therefore may need to be relatively efficient to justify their use.

Data Acquisition

Data is collected using the Particle Image Velocimetry (PIV) method. A pulsed laser and digital camera are used to acquire images of the smoke-seeded flow in the wind tunnel. Two images of the flow (Figure 1) are captured in rapid succession. A cross-correlation between the images results in a vector-map as in Figure 2. The lift of the aerofoil, and other flow characteristics, can be calculated directly from this vector-map.

Air flow vector maps

Typically the performance of this type of control system is evaluated by comparing the lift, or lift increment, to the ratio of jet momentum over free-stream momentum. More efficient systems provide a larger lift increment per momentum ratio increment.