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xlxc34 June 24, 2011 04:42

Modelling unsteady 2D aerofoil drag
Hi, I've posted about this before but only on specific modelling techniques...

I'm working on modelling fatigue testing of wind turbine blades. This video (of my finite element model so far) shows how it's done better than I can explain!

The blade is being excited at it's flapwise, or out of plane, natural frequency and it's edgewise (in plane) natural frequency at the same time. The blue and green arrows are the flapwise and edgewise excitation forces respectively, and the red arrows are the aerodynamic drag force (they are scaled up 10 times compared to the excitation forces so they are easier to see).

At the moment those forces are calculated by resolving the velocity of each node (the blade model uses beam elements, the ends of which are shown by the black lines on the blade in the video) into a flapwise and edgewise component, and then applying a force as below to the node.

F=-1/2*rho*Area around node*node velocity*abs(node velocity)*Cd.

We've done some 2D CFD with a moving mesh which shows that because of a leading edge vortex and the fact that the blade encounters it's own wake the value of Cd can be up to 5.

The way in which I'm modelling drag is not very satisfactory to me at all, so I've been looking into other ways of modelling it to get a proper fluid structure interaction. Those that I've looked at so far include:

-Smoothed particle hydrodynamics (tried to implement this in matlab and failed miserably, and I can't spend too long on the aerodynamics as I need to focus on the structural side of things... also didn't run anywhere near as quickly as it needs to).
-Vortex methods . This looked promising, but as I need to run long simulations I can't keep track of all the vortices.
- Morrisions equation - this helps slightly but doesn't do enough to get the 'drag coefficient' as high as it is in CFD.

So instead of asking for help on any of these sub categories, I just want to throw the problem out there and ask if anyone has any ideas for how I could model the drag better? They can be quick and dirty, so long as they are better than what I'm doing at the moment. Any ideas are welcome!

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