Vortex Generators

The first and most obvious aerodynamic device I thought I’d write about is the vortex generator.As its name suggests, vortex generators generate a vortex. I won’t go too much into what a vortex is (check out Wikipedia), however it’s basically the rotation of the air flow – the core of which acts almost like a solid rotating cylinder as it moves with the surrounding flow. As it’s moving with the surrounding flow, and rotating, there is more energy in the vortex than the surrounding flow – and can be thought of as ‘stiffer’ than the surrounding flow. This higher-energy ‘stiffer’ vortex core effectively delays the boundary layer detaching from the airfoil: thus, delaying stall. In practical terms, this means that the CL_max for a blade section is increased, however as more energy is put into the flow drag is also increased: reducing L/D. Some test data is shown here for the NACA 63-415 section:

So, in essence vortex generators are fit to a blade in order to delay the stall and get more lift out of the blade - however knowing that we are going to increase drag and reduce our L/D ratio (something we don't want to be doing). A perfectly designed blade shouldn't have vortex generators, however are typically fitted to optimise the aerodynamics after the prototype blade has been power-tested. They are always fitted on the suction side of the blade, and typically from about 1/3 to 2/3 of the blade's span at 10 to 30% chord, as stall begins inboard.

For pitch-controlled turbines, vortex generators are typically fitted when the power performance of the rotor isn't as good as expected. What happens here, is that the turbine will be running at higher pitch settings to try and get the right power. This can result in stall (starting inboard and moving out), to prevent this vortex generators are fitted to delay the stall and squeeze a little more power out. For stall-controlled turbines, vortex generators are again fitted if the power production is less than expected and the turbine isn't hitting rated power (as it's stalling before rated is achieved). Again, vortex generators in this application will delay stall of the rotor and allow more power to be squeezed out of the turbine - although by a different mechanism. From a power-curve perspective, this will increase the power around rated speed, however the increased drag will reduce the Cp at lower speeds due to the increased drag in the system.

Here is a picture of a stall-controlled blade I took, vortex generators clearly visible with vortex paths clearly visible as dust buildup on the surface.

Check more info out at a great Risoe report here: Risø-R-1193(EN) ‘Wind Tunnel Tests of the NACA 63-415 and a Modified NACA 63-415 Airfoil’. 2000.