CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Strange Result

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   March 3, 2013, 12:47
Question VAWT - Strange Result
  #1
New Member
 
Daniel
Join Date: Mar 2013
Posts: 5
Rep Power: 4
Tremore is on a distinguished road
I 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:
Velocity vector.jpg
https://docs.google.com/file/d/0B81t...it?usp=sharing

and the pressure have a big step in the trasition beetween static and rotating domain:
pressure.jpg
https://docs.google.com/file/d/0B81t...it?usp=sharing


Someone have some idea why I have this kind of results?

Thank you in advance

Last edited by Tremore; March 3, 2013 at 13:35.
Tremore is offline   Reply With Quote

Old   March 3, 2013, 18:05
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,646
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
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.
ghorrocks is offline   Reply With Quote

Old   March 4, 2013, 07:10
Default
  #3
New Member
 
Daniel
Join Date: Mar 2013
Posts: 5
Rep Power: 4
Tremore is on a distinguished road
Sorry for the acronimous.

I think is not a mesh problem.


mesh1.jpgmesh2.jpgmesh3.jpgmesh4.pngmesh5.jpg



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.

Last edited by Tremore; March 4, 2013 at 08:33.
Tremore is offline   Reply With Quote

Old   March 4, 2013, 09:19
Default
  #4
New Member
 
Daniel
Join Date: Mar 2013
Posts: 5
Rep Power: 4
Tremore is on a distinguished road
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?
Tremore is offline   Reply With Quote

Old   March 4, 2013, 17:37
Default
  #5
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,646
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
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.
ghorrocks is offline   Reply With Quote

Reply

Tags
vawt wind turbine

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Strange Nut behaviour with K-OmegaSST nicolarre OpenFOAM Running, Solving & CFD 10 January 29, 2015 12:53
the result of the velocity and pressure drop is a little strange. yuhehuan FLUENT 0 December 18, 2012 20:48
How to display the whole result of a periodic model via one period result? dixylo FLUENT 2 February 23, 2012 05:11
Strange Result for Versteeg Testcase (2D, convection only, steady state) caramelo OpenFOAM Running, Solving & CFD 4 August 17, 2011 17:55
Strange Result of turboPassageRotating tutorial. Ohbuchi OpenFOAM Running, Solving & CFD 0 July 27, 2011 21:06


All times are GMT -4. The time now is 03:45.