I need Moving Wall Slip
I'm simulating vertical wind turbines using OF1.5-dev. The boundary condition for U I've been using for the turbine is MovingWallVelocity with a uniform value of (0 0 0). I think this is giving me unrealistic drag on my blades.
Is it possible to have a slip boundary with a MovingWallVelocity? |
Consider MRFSimpleFoam
I don't think that the movingWallVelocity is what you want. It assumes that the wall is moving relative to a stationary volume, such as the crown of a piston moving within the confines of a stationary cylinder. Thus the mesh is changing. I'm guessing that you want to keep the entire mesh stationary.
I think you need to consider a moving reference frame (MRF) around your (complete or just a blade?) VAWT, though I don't have any experience with it - have a look at MRFSimpleFoam. |
Quote:
Steve |
More Details
Ok, I see.
So you have a rotating (non-conformal) interface on the boundary between the rotating cylinder (containining your VAWT) and the stationary surrounding mesh, and you are running a time dependent simulation, right? Or are you performing some kind of pseudo MRF with the rotating cylinder as your reference frame and a time dependent, transformed onset velocity? I don't think I can help much either way, but maybe others can weigh in once they know more about your simulation details. Good luck. |
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My problem (and maybe it's not a problem) is that my only option for the boundary condition on the turbine itself is a no slip surface. Since I posted this, people have been telling me that is actually the correct boundary condition to use for modeling the real world and air does not actually slip on a surface no matter how smooth it is. Like I said, I'm not a CFD or fluid dynamics expert so I'm not sure. I just know I'm getting more drag on the blades than I would expect. |
Accurate Drag Prediction is Difficult
I think you are performing scenario one that I offered.
No slip is the correct boundary condition for a wall - moving or stationary. Accurate drag prediction is difficult:
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2) and 3) I don't know enough to do this, I used values I copied from someone else's turbine experiment, but I don't know that they are good values. However when I run with icoDyMFoam (no turbulance model) the results are fairly close to real life. I suspect I am using a wall function for a very rough wall. I don't even know which parameter that is. Where can I find documentation on turbulance models and TurbFoam (TurbDyMFoam)? 4) I ran a flat plate through air (flat side pushing into the air.) and got a Cd of about 1.8 - 1.9, which I thought was kind of high. http://www.youtube.com/watch?v=S-ucDIDEbv4 |
Turbulence Modeling
For general info on turbulence modeling try: http://www.cfd-online.com/Wiki/Turbulence_modeling
Specific to OpenFOAM there is the UserGuide.pdf that is part of the distribution. You might find the lid-driven cavity example useful with the calculation of k/epsilon based on turb. intensity and turb. length scale. The latest 1.6.x OpenFOAM release has reworked the wall functions (high-Re. Turb. Models) and that might be worth a look, if you haven't already. |
Cfx
I think you'd better use CFX for your problem. There is a special module for turbomachine in CFX.
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hi, I am facing the same problem, which is my moving solid body cannot stimulate any flow in the ambient stationery air domain. I set my BC of movingWall as movingWall velocity with $internal value (uniform 0 0 0). Is there any additional setting need to be done to generate some force or at least velocity and pressure contour surrounding it in the domain.
I would be grateful if can get some advice and suggestion here. Thanks =) |
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