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xlxc34 January 14, 2011 05:56

Deformable mesh settings in Ansys CFX

I am working on a simulation tool for wind turbine blade fatigue testing.

The gif on this page is worth a thousand words...

I have written a beam element FEA code in MATLAB that allows a transient analysis to be performed. It works great, but in order to get the model to match up to physical test results I have to use a very high drag coefficient for the blade when simulating the damping that occurs due to air resistance. I have a colleague who is a CFD expert and he has agreed to do a bit of CFD analysis, however he is new to Ansys CFX. As he is working on this problem more or less in his spare time, I was wondering if anyone has any pointers for the analysis I've outlined below?

Aerofoil with 1m chord length moving up and down with simple harmonic motion. The frequency is 1Hz and the amplitude is 1m. The analysis is 2D.

I'm looking for help with time step size, and any settings that will get it working etc. Any help would be much appreciated!

ghorrocks January 15, 2011 07:44

Use adaptive timestepping going for 3-5 coeff loops per timestep. Then it will find its own timestep size.

Other than that you should read the literature on wind turbines to find out what the best mesh, turbulence model, setup etc to use for accurate models.

xlxc34 January 17, 2011 07:29

Thanks for the time step information, I've had the k-omega turbulence model reccomended by someone else... The analysis is 2D (it is too complicated otherwise because the aerodynamic damping affects the mechanical structures motion if you do a full 3D analysis) so I haven't managed to find any relevant literature unforunately, although the mesh used looks very similar to that used for steady state aerofoil calculations I've seen in some of my friends work. The guy who recommended the k-omega model also recommended that I use a boundary condition that allowed air to escape from the domain but then put it back in on the opposite wall. I can't remember the reason for this or even the name of the boundary condition in fluent (which was his area of expertise) but if anyone has any ideas on it I'll pass them on to the person who is doing the work for me. This youtube video effectively shows what I am trying to achieve, except the block would be an aerofoil that is moving up and down with simple harmonic motion for a few cycles.

edit: just realised the youtube vid has a link to a step by step guide on how to do moving mesh analysis in Ansys CFX!

ghorrocks January 17, 2011 17:39

You will probably find in CFX that the SST turbulence model does everything the k-omega model does and a whole heap more.

It sounds like you are talking about a periodic boundary condition. The use of these type of boundaries is covered in the CFX tutorials.

xlxc34 January 20, 2011 05:06

Thanks for your help, I think we might have got it working!

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