Single moving reference frame vs Multiple reference frame for Wind Turbine CFD
 

Hi guys,

I am analysing torque output from a horizontal axis wind turbine using CFD. There are two approach in the literature, one is using a single rotating frame of reference/fluid domain which is rotating at a constant speed and the other one using multiple frames (MRF) which consists of two frames whereas the outer frame is stationary and the frame around the wind turbine blades is rotating.

I tried both approaches and they give similar results. SRF takes much more time to converge than MRF. What is the difference between these two approaches for wind turbine blade simulations in FLUENT and in general?

Thank you.

It's a symmetry consideration, mostly. For your wind turbine, consider the following 2 situations:

- the domain around the turbine is empty, and can be modeled as perfectly tangentially symmetric

- there is a building relatively close to the wind turbine on the east side, modifying the flow pattern significantly.


In the first situation, you can use single reference frame (assume the world rotates around your wind turbine) due to the symmetry. In the second case, however, this would imply the building is rotating around your wind turbine, which is clearly wrong! Hence, in the second case you need to use multiple reference frames to ensure that the position of the building compared to the turbine is fixed, while the air directly around it is implied to rotate.

For a horizontal turbine, MRF needs to be used also, because otherwise the ground would revolve around the turbine (and the shaft in case that is modeled).

Thank you for the response. I guess there will be no problem in using periodic boundary condition using MRF as well?.So, I will model one blade in a rotating domain and put it in a stationary domain (120 Degrees, circle). Define both as periodic (stationary and rotating). See attachment.

Thank you

Actually, the point is that any zone where a rotating reference frame is used, either SRF or MRF, will have an additional source term in the equations. Using a SRF will use that source term in all the cells of the domain. Using a MRF, in this case, will require a source term only in a relatively small amount of cells. As that source term depends on the local velocity, it will necessarily affect the convergence and the more cells have it the worst it will be.

Obviously, this doesn't mean that you have to always apply MRF. It also comes with its own problems. More often than not, if SRF can be applied, you should go with it. But you can find additional details in the manual.

If there is no interface in the domain, is it advisable to use mesh motion along with frame motion? Because if I am using only frame motion then there occurs no motion in animation. While using a frame with mesh motion ...or only mesh motion, there is the motion as I can see in animation. I find no relative literature which relates mesh or frame motion without interface.