# HAWT, should we use sliding meshes? or the UDF?

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 January 2, 2011, 23:30 HAWT, should we use sliding meshes? or the UDF? #1 Member   J.-H. Wang Join Date: Oct 2010 Posts: 72 Rep Power: 14 Hello, I'm wondering the actual physical mechanism of a wind turbine, HAWT literally. From a basic physics sense, we already knew that the fluid should do work on the turbine to generate the power. That is, despite the start condition of the turbine, if we consider the steady and stable output of the turbine, it should be fluids who accelerates the blades. The resulting torque is balanced with the friction and the resistance of the generator and therefore provides power. However, both the MRF or sliding mesh method are ACTIVE method from my point of view. In specific, they let the blades rotate actively and then the fluid will got disturbed, generating the colorful flow field you've seen on the screen. In other words, this is actually the blades doing work on the fluids, a compressor it is. Is it the right way to do it? I have seen a bunch of master thesis and even the official Fluent News (2001 or 2002 Spring) with the analysis on wind turbines utilize these methods. Is there any one who can explain why they did this? How can it be even possible for draining a sensible results? (it actually did...... But I am not comfortable with this weired mechanism..) Or should I did it another way, applying the UDF and write the governing equation for the rotating motion and let the pressure distribution drives the blade? It is obviously a much more complexmethod, but with a rather sensible physics. I think I got something wrong in there. Please help me with the confusion. Thank you. Last edited by f0208secretx; January 3, 2011 at 00:52.

 January 3, 2011, 01:07 #2 Senior Member   Travis Carrigan Join Date: Jul 2010 Location: Arlington, TX Posts: 161 Rep Power: 14 Hi, There is really no wrong way about doing this. Yes, you can go write a snazzy UDF and let the fluid pressure rotate the blades. In fact, this would be a great way to do it, you could see exactly how fast the blades would rotate for a given freestream velocity. But, this would only work if you wanted to analyze a single operating condition. However, it isn't wrong to let the blade work on the fluid and use a sliding grid (for these problems, stay away from SRF or MRF). For my masters thesis I used a sliding grid and it turned out just fine. Now, using this method doesn't allow you to determine the rotation speed for the wind turbine for a given freestream velocity, but what it does do is allow you to build up a performance curve (like efficiency vs. tip speed ratio). When you build up a performance curve for a wind turbine, you usually are searching for efficiency as a function of the tip speed ratio, building on the fact that most wind turbine designs are quantified using dimensionless values. The tip speed ratio can be changed by, you guessed it, either changing the freestream velocity or the rotation speed of the wind turbine! Therefore, by simply specifying the freestream velocity and allowing the fluid to work on the wind turbine, you can only analyze a single operating condition and cannot build up a performance curve. However, if you want to explore the performance of your wind turbine over several operating tip speed ratios, use a sliding mesh. Muratoglu and ptphongdk like this.

 January 3, 2011, 10:34 #4 Senior Member   Travis Carrigan Join Date: Jul 2010 Location: Arlington, TX Posts: 161 Rep Power: 14 Ok, seems you are digging pretty deep here. It isn't really nonsense to think of a turbine and a compressor as the same. Work is work, energy is energy. Whether the fluid works on the turbine or the turbine works on the fluid doesn't necessarily matter (Think of the energy equation, energy in=energy out). This is why we can allow the wind turbine to work on the fluid in a CFD simulation and still achieve accurate results. When I say single operating condition, I'm referring to a single tip speed ratio (if you are unfamiliar with the term tip speed ratio you can find it pretty much anywhere online). Once a HAWT achieves a quasi-steady state, it is operating at a single tip speed ratio. The rotor isn't being accelerated or decelerated anymore by the fluid, hence the term quasi-steady. This is not an over simplification, this is actually what happens for HAWTs. Now VAWTs are a little different, but I won't get into that. Now, if you were studying the startup behavior of a wind turbine, that would be much different. In that case the turbine would be accelerating through several tip speed ratios, however, it seems you are only interested in the steady state behavior. Just so you know, I don't believe the generator needs to be involved. Typically, the CFD process is purely aerodynamic. If you were using a UDF you would allow the fluid to act on the blades, accelerate the wind turbine to a steady-state, and determine the rotation speed. Then, post-CFD, you could determine if the generator could handle the rotation speed achieved by the blades and determine if you need any aerodynamic breaking or something similar. So like I mentioned before, if you were to allow the wind turbine to work on the fluid, you could examine various operating conditions and analyze the performance characteristics of the rotor. Ideally, this is what most people would like to accomplish.

 January 3, 2011, 10:45 #5 Senior Member   Travis Carrigan Join Date: Jul 2010 Location: Arlington, TX Posts: 161 Rep Power: 14 And one more thing. Typically when modeling a compressor you would use a SRF (single reference frame) or MRF, and this reference frame would a moving reference frame. Hence, the fluid looks as if it is rotating while the blades are stationary. Think of the alternate approach. Allow the blades to rotate in a sliding grid. Now the blades are working on the fluid. What's the difference? The only difference is the frame of reference. In the first SRF case, you can imagine that you are rotating with the blades, hence, the blades look as if they are stationary. In the second case where the blades are working on the fluid in a sliding mesh approach, you are now in the global reference frame watching the blades rotate. This is why both methods are pretty much equivalent. The only difference is the frame of reference. I think this should answer your question. I've attached a link to the FLUENT user guide for rotating reference frames, hope it helps. http://my.fit.edu/itresources/manual...ug/node412.htm

 January 4, 2011, 00:28 #6 Member   J.-H. Wang Join Date: Oct 2010 Posts: 72 Rep Power: 14 Thanks to the discussion. Yet you have made some clear mistakes in the difference between turbine and compressor. The two resulting flow field should be different. As you know the work equals to force times displacement, it implies that the tangential force component of the blade is in the direction of the blade motion (a pump) or opposite to it (a turbine). You are confusing with the concept of reference frame since the fluid will get accelerated by the compressor and decelerated by the turbine, no matter which reference frame are you using. Furthermore, the rotation direction of the wake flow for a turbine and a compressor are also opposite considering the same rotating direction of these two machines. (Is it even a wake flow for a compressor?) The former is caused by the reacting force exerted by the turbine blade, while the latter is the result of momentum transfer. There are totally different mechanism driving the flow pattern. Don't you care about that? Is it really doesn't matter when you are calculating the pressure distribution and the velocity field, and the resulting drag, lift and moment? I doubt that. There are definitely some tricks in there, and I would like to find it out. Or I couldn't apply the method comfortably.

 January 4, 2011, 04:12 #8 Member   J.-H. Wang Join Date: Oct 2010 Posts: 72 Rep Power: 14 Hi, I will simplify my question. An idealistic household fan and an idealistic windwill. You are saying that the flow field at the back of the windmill will be approxiamtely identical to the one blows from the fan (in front)? Is this what you are saying?

 January 4, 2011, 10:33 #10 Senior Member   Travis Carrigan Join Date: Jul 2010 Location: Arlington, TX Posts: 161 Rep Power: 14 And to answer your simplified question. Yes for an idealistic windmill (no losses) the velocity behind the rotor would be zero, as the windmill has extracted all the energy available in the wind (ideal case). And the velocity in front of a fan would be zero as well in the ideal case because the electricity driving the mechanical motion of the fan is being 100% converted into the kinetic energy of the air behind the fan. Edit ----- Also, forgot to mention, seems you are a little confused with the ideal case. For an ideal windmill, it extracts all the energy in the wind approaching the rotor and therefore the velocity behind the windmill is zero.

 February 19, 2012, 06:58 sliding mesh #12 Member   Join Date: Jan 2012 Posts: 58 Rep Power: 12 hello i am using sliding mesh for train moving through tunnel, but i am not able to make it work . can any body help me . my email is sheikhnasir39@gmail .com thanks