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 Jonathan February 11, 2009 06:31

wind turbine

what would you recommend me a MRF or a sliding mesh to do a research on a VAWT. Ive tried both but with dissapointing results. which are the boundary conditions i should give?

Thanks

 Bikini Girl with Machinegun February 11, 2009 08:35

Re: wind turbine

Do you really think that you will get better results than momentum-based or vortex methods (e.g. cloud-in-the-cell)?

What you have to do is: 1. Find experimental results for your VAWT. 2. Adjust the grids to get the best correlation with experiments. 3. If that doesn't work, then make the graphs in your report very small and just say that you have achieved excellent agreement in the caption. Nobody will notice.

That's the CFD way!

 Jonas Holdeman February 12, 2009 00:51

Re: wind turbine

The VAWT problem is one that I have thought a little about, but after several years I have never gotten around to working on it. My simple 2D approach would be to construct a circular grid centered on the axis. The air foils would be fixed in the grid. Then I would take a rotating coordinate system moving with the blades/foils. In this system, the blades are fixed and the direction of the wind would be rotating. I would then transform back to the stationary coordinate system for plotting/visualizing the flow. One may have to add a term for the coriollis force though.

 Jonathan February 12, 2009 03:53

Re: wind turbine

about the rotating system, does not fluent do it automatically? if not how should i do it?

 James February 12, 2009 09:06

Re: wind turbine

I am sure it is a difficult problem, as I assume you have a bi directional airfoil/flow with the flow separating twice a rev and the blade continually working at different angles of attack.

A place to start would be to ensure your grid/model choice could at least replicate the airfoil performance vs angle of attack in a steady solution.

Until you can do this you probably have no hope of solving the real problem.

 Jonas Holdeman February 12, 2009 10:36

Re: wind turbine

Sorry, but I cannot answer your question about Fluent as I have my own research code and method. I have experience with my method with regard to open boundary conditions at outflow, and enough confidence to invest the time if I had time. I believe I could impose uniform flow BC along the inflow part of the grid and no BC on outflow. I have calculated flow over a step with a very short mesh that cut through the first recirculation bubble. This can be seen in a paper at http://webpages.charter.net/jtholdeman/IJNMFart1.pdf in Section 5.2.3, Figure 21, on page 25. Admittedly, it would be a stretch, as this example is with a 1:3 domain aspect ratio while the VAWT mesh would have an effective aspect ratio of 1:1.

For those following this discussion who are not familiar with the vertical axis wind turbine, it is reported to be a strange device, indeed. It delivers torque from the drag of usually-symmetrical blades rather than from lift, and so only while it is turning. It is not self-starting. An advantage is that the generator can be at ground level, but then the blades are subject to fatigue failure because the loading on the blades alternates positive and negative at each rotation.

 Bikini Girl with Machinegun February 12, 2009 13:43

Re: wind turbine

Jonas wrote: It delivers torque from the drag of usually-symmetrical blades rather than from lift, and so only while it is turning. It is not self-starting. An advantage is that the generator can be at ground level, but then the blades are subject to fatigue failure because the loading on the blades alternates positive and negative at each rotation.

Savonius VAWT are primarily drag devices. They are self-starting, but limited in their range of tipspeed ratios (TSR).

Fixed pitch Darrieus VAWT are primarily lift devices. They are self-starting, however they are usually unable to accelerate beyond a TSR of 1.

Variable pitch Darrieus VAWT are self-starting, and the pitch regime can be "chosen" (using a suitable mechanical or electrical system) so that the vibrations of the blades are minimised.

VAWT can also be used in water, however cavitation and ventilation of the blades must be accounted for.

There are a collection of references at the end of the wiki page for VAWT that might be a useful starting place. See: http://en.wikipedia.org/wiki/Darrieus_wind_turbine

HTH.

 Jonathan February 13, 2009 02:36

Re: wind turbine

I'm going to explain you what we do so you can tell me what you think about it.

There are two areas.

In the middle a circle to which we substract the airfoils. We set it as a rotating moving mesh.

The other is a big square to which we substract a circle as big as the other one. We set it as stationary. One side is a velocity inlet and the other three are velocity outlets.

We set the circles as interfaces.

Do you think this is a good way??

Thank you very much

 Jonathan February 13, 2009 02:37

Re: wind turbine

sorry i wanted to tell that one side is a velocity inlet and the other 3 pressure outlets.

 Bikini Girl with Machinegun February 14, 2009 01:02

Re: wind turbine

Have you noticed that at very high tipspeed ratios the vortices shed from each wing are drawn back inside the "inner circle"? That is a very messy flow problem!

Also, do you have difficulties at TSR=1, when one of the blades is travelling directly into the ambient wind? In some models, this means that the Reynolds number is (effectively) equal to zero which can create problems.

But maybe CFD handles these cases quite naturally.

Personally, I suggest that you first try modelling a thin, finite aspect ratio wing and compare it to experiments. If you can't get good agreement, then there is no point in proceeding to more difficult cases.

If you are happy with that simple case, then try a finite aspect ratio wing undergoing a forced pitch variation. Again, if you cannot do that well, then don't bother trying an actual VAWT.

 star February 19, 2009 09:49

Re: wind turbine

jonathan, are you doing 2D analysis?

 Joan March 1, 2009 02:42

Re: wind turbine

http://www.partenovcfd.com

 dbale April 8, 2009 21:21

modelling and simulation procedure VAWT

hi i need to simulate VAWT using gambit and fluent but i am new to both software and will apprciate if some one can tell the procedures involved to set up the cfd simulation.

 bima_ap May 18, 2009 07:59

hi guys, I'm doing same research about VAWT and now I'm stuck in Fluent simulation. I use moving mesh and i always get the negative moment. It means my turbine doesn't produce power. I really confuse with this problem. Anybody have solution for me?

thanx

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