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Tremore March 3, 2013 12:47

VAWT - Strange Result
 
2 Attachment(s)
http://www.cfd-online.com/Forums/[IM...vector.png</a>http://www.cfd-online.com/Forums/mem...-pressure.htmlI performed I simulation of a VAWT, using ANSYS CFX with the following data:

BLADES
NACA0018
0,5m coord
Set angle 0

ROTOR blades

Diameter: 5m

SIMULATION TYPE
quasi 2D (5mm thin regions)
TSR: 3,8
INLET wind speed: 11 m/S
INTERFACE: rotating-static: STAGE.

MESH
300 000 elements: fitted near the blades(expecially in TE) and near the interface.

Everything look go in the right way , the solution goes at convergence in about 1500 interations but the results seems be completely wrongs.
if you Look at the velocity vectors:
Attachment 19501
https://docs.google.com/file/d/0B81t...it?usp=sharing :eek::eek:
http://www.cfd-online.com/Forums/[IM...ity-vector.png
https://docs.google.com/file/d/0B81t...it?usp=sharingand the pressure have a big step in the trasition beetween static and rotating domain:
Attachment 19500
https://docs.google.com/file/d/0B81t...it?usp=sharing

https://docs.google.com/file/d/0B81t...it?usp=sharinghttps://docs.google.com/file/d/0B81t...it?usp=sharinghttp://www.cfd-online.com/Forums/mem...-pressure.html
Someone have some idea why I have this kind of results?

Thank you in advance

ghorrocks March 3, 2013 18:05

Two initial comments:
* Please try to avoid jargon and acronyms. I have been on this forum a while so I know VAWT means vertical axis wind turbine, but many others may not.
* This is a general FAQ on accuracy: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

In this case I would be looking at mesh quality, and resolution.

Tremore March 4, 2013 07:10

5 Attachment(s)
Sorry for the acronimous.

I think is not a mesh problem.


Attachment 19519Attachment 19520Attachment 19521Attachment 19522Attachment 19523



I solve the problem using frozen rotor instead stage in the interface option
Now the pressure distribution look like reasonable and also the totale torque of the three blade of 500Nm/m (for blade meter) very near at the hand calculated value.
The streamline are always completely wrong but probably this one is the way of CFX to illustre the streamlines on an rotating domain.

https://docs.google.com/file/d/0B81t...it?usp=sharing

https://docs.google.com/file/d/0B81t...it?usp=sharing

After the first simulation I tried to carry our a 360 simulation using a transition simulation:
0,75s totale time
0,02083 step time
I change the interface option again from rotor frozen to stage and use for initial condition the result of the previus simulation, the solver start normally but after 10 interation I have a overflow error.

Tremore March 4, 2013 09:19

I think I solved the problem, now is working:
I reduce the time step do 0,0075s (every step is solved in one o two interations and then the total time is almost the same) and now I'm at step 60 without any problem
I have forgotten to reduce the recorded timestep and then the solution file dimesion will be very big. It is possible reduce it?

ghorrocks March 4, 2013 17:37

Use adaptive timestepping, homing in on 3-5 coeff loops per iteration, then you know the tiem step is set correctly. And set transient result files every so often (maybe 1s or whatever is appropriate) so you get a manageable result file size.


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