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 AUN July 11, 2011 02:33

6-dof rigid body solver

Dear All,

I am working on a HAWT Project. The geometry of the blade is available, however, the power curves have to be generated given different input wind speeds and directions.

For this purpose, I am applying the 6-DOF Rigid Body Solver in CFX.

At first I modelled the fluid domain as a cuboid with the blades subtracted from it. Not only did the 6-DOF take insanely long (Mesh 2.2 million) however, after a rotation of three degrees, the solver crashed due to volume negativity. I had kept mesh skewness low, and the mesh was fine in proximity of the blade. The raeson why this problem is occuring is unclear to me.

To counter this effect, I 'tried' doing the same as they have done in the Tutorial 'Decoupling Mesh Motion'. I created a subdomain in the fluid domain, a cylinder, ascribed to it the same motion as the Rigid Body and ignoring rotations at the outer domain inrerfaces. The mesh at the boundary of the subdomain was conformal. Mesh Size 1.9 million made in ICEM CFD. The same negative element volume problem persisted.

What can be done regarding this?

Regards,

Aun

 ghorrocks July 11, 2011 06:47

I do not think you are using the correct approach. Forget 6DOF and FSI.

The way to do this is to put the blades in a rotating frame of reference and assume a rotation speed. Do a simulation and get the net torque. If the net torque is positive (ie blades would accelerate) then do another simulation a bit faster. Keep iterating until you get the rotation speed which produces a torque which matches the load on the turbine.

Once you have a few points on your speed versus torque curve you can use non-linear curve fits to greatly accelerate finding the converged value.

This way you replace an extemely long and difficult to converge transient run with a series of simple steady state runs. Much simpler.

 AUN July 11, 2011 22:59

Dear Glenn,