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

Strange...can you tell me why???

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 30, 2011, 00:05
Question Strange...can you tell me why???
  #1
Senior Member
 
mohammad
Join Date: Dec 2010
Location: Seoul, South korea
Posts: 210
Rep Power: 7
mohammad is on a distinguished road
Hi guys,
I'm simulating the NREL wind turbine with CFX.
my model contains about 4 million nodes.
I decided to check the node sensitivity analysis to see the effects of node density near the blade....
As I increased the number of nodes inside the boundary layer( say roughly from 14 to 19 nodes) and also as an other time I increased the number of nodes in chordwise direction ( form 37 to 50), the results deviated from the first set of the results and they went far away ( say about 40%) from Experimental result....

*Experiment ( v=4.5): Thrust force=552.91
*model 1 ( 37 chordwise , 14 node inside boundary layer) :Thrust force=754
*model 2 ( 37 chordwise , 19 node inside boundary layer) :Thrust force=874
*model 3 ( 50 chordwise , 19 node inside boundary layer) :Thrust force=917

I tried to keep the quality of mesh constant during the changes and I'm wondering why the results are diverging like this???

can everybody help me please?
mohammad is offline   Reply With Quote

Old   April 30, 2011, 06:50
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,651
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Looks like you are miles away from an appropriate mesh density. I suspect you will have to be far finer than these models.
ghorrocks is offline   Reply With Quote

Old   May 1, 2011, 15:25
Default
  #3
Senior Member
 
Julio Mendez
Join Date: Apr 2009
Location: Greensboro, U.S.A
Posts: 104
Rep Power: 8
juliom is on a distinguished road
Send a message via Skype™ to juliom
I do agree with ghorrocks, the mesh quality is very poor, since you have different results qith each one!!!
juliom is offline   Reply With Quote

Old   May 1, 2011, 18:45
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,651
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Mesh quality does not necessarily mean different results. It can, but a good quality mesh with an inadequate resolution will also do this. It could be either (or both) quality or resolution.
ghorrocks is offline   Reply With Quote

Old   May 1, 2011, 22:49
Smile Thank you everybody.....
  #5
Senior Member
 
mohammad
Join Date: Dec 2010
Location: Seoul, South korea
Posts: 210
Rep Power: 7
mohammad is on a distinguished road
Hi
Now I have 2 other questions...
I'm trying to increase the no. of nodes near my blade...two times more, and it will make my model HUGE for my PC to run it...then I MUST reduce no. nodes somewhere else.
Question1: Do you think the mesh density away form the balde( say one span length away) is of the same importance as the near-blade node density....I don't think so...then to lower No. nodes, do you think I can reduce the mesh density one blade span length away from the blade. What can be the largest ,but safe, distance between two successive nodes in axial direction?

Question2: Which part is less influential for my case to change the mesh density within that par...[far away in upstream or Downstream]?

Regards
mohammad is offline   Reply With Quote

Old   May 1, 2011, 22:57
Default
  #6
Senior Member
 
mohammad
Join Date: Dec 2010
Location: Seoul, South korea
Posts: 210
Rep Power: 7
mohammad is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
Mesh quality does not necessarily mean different results. It can, but a good quality mesh with an inadequate resolution will also do this. It could be either (or both) quality or resolution.
Hi ghorrocks;
to give some more information, the quality of my model is 0.41 and the smallest angle is 24 deg.
I think the matter of my model should be resolution....I will try some more runs and i will write the results for more clarity ASAP.

Regards
mohammad is offline   Reply With Quote

Old   May 2, 2011, 01:20
Default
  #7
Senior Member
 
Raashid Baig
Join Date: Mar 2010
Location: Bangalore, India
Posts: 136
Rep Power: 7
cfd_newbie is on a distinguished road
Quote:
Originally Posted by mohammad View Post
Hi
Now I have 2 other questions...
I'm trying to increase the no. of nodes near my blade...two times more, and it will make my model HUGE for my PC to run it...then I MUST reduce no. nodes somewhere else.
Question1: Do you think the mesh density away form the balde( say one span length away) is of the same importance as the near-blade node density....I don't think so...then to lower No. nodes, do you think I can reduce the mesh density one blade span length away from the blade. What can be the largest ,but safe, distance between two successive nodes in axial direction?

Question2: Which part is less influential for my case to change the mesh density within that par...[far away in upstream or Downstream]?

Regards
ASAK Mohammad,
The cold heart truth about the NREL's Phase VI validation is that till now nobody has been able to solve this problem accurately and consistently using RANS. I have been advised by some very senior paper in this field (and also pointed out in many papers) is that LES or DES is the way forward. Other researchers have tried much better grids than what you are using and are still unable to to get good results. Regions both near and in the wake region are important.
My suggestion is that you should have a very good mesh and fine mesh (so that there is no doubt about the mesh quality) and see how good RANS results are and then you should move to DES or LES.
If the computational resources are a constraint than you should have better access to computational resources for this problem.
Raashid
cfd_newbie is offline   Reply With Quote

Old   May 2, 2011, 07:41
Default
  #8
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,651
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Nice answer Raashid. I am sorry Mohammad, but if your computer is unable to take larger meshes then it is quite likely you will never get a good answer to your simulation. That is why people make super computers to run CFD simulations - because you have to if you want accurate answers in many classes of simulation.
ghorrocks is offline   Reply With Quote

Old   May 2, 2011, 08:53
Smile
  #9
Senior Member
 
mohammad
Join Date: Dec 2010
Location: Seoul, South korea
Posts: 210
Rep Power: 7
mohammad is on a distinguished road
Thank you,both of you, for your good guidelines
mohammad is offline   Reply With Quote

Old   May 8, 2011, 01:15
Smile Yplus values
  #10
Senior Member
 
mohammad
Join Date: Dec 2010
Location: Seoul, South korea
Posts: 210
Rep Power: 7
mohammad is on a distinguished road
Hi Raashid, Glenn.

Just to add more information to this thread for more clarity, here I present the values of Yplus on my blades which I got( Up-blade and Down-blade in 4 pictures ).
As its obvious except at the "tip section" of both blades, the value of "Yplus " for nodes on the surface are good enough( PLEASE INFORM ME IF I'M EXAGGERATING). But still thsose changes of results happens when i change no of mesh in far-blade zone( see the opening of this thread).

The only matter remains unsolved is a proper choice turbulent model and MOST PROBABLY Raashid's suggestion about LES or DES is true...

I will run my model in transient mode with LES or DES and I will add more info to this thread.

Anybody, please correct me and give me more of your knowledge and experiences.

Regards
Attached Images
File Type: jpg untitled2.JPG (40.7 KB, 13 views)
File Type: jpg untitled.JPG (30.0 KB, 11 views)
File Type: jpg untitled3.JPG (25.8 KB, 14 views)
File Type: jpg untitled4.JPG (22.9 KB, 10 views)
mohammad is offline   Reply With Quote

Old   May 8, 2011, 08:32
Default
  #11
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,651
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
To establish if your mesh is fine enough you really should do a sensitivity analysis.
ghorrocks is offline   Reply With Quote

Reply

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
Strange behaviour when using LienCubicKE and NonlinearKEShih hani OpenFOAM Running, Solving & CFD 20 March 6, 2013 11:06
A Strange Problem in making Parallel (Ansys/CFX 12) a.sarami CFX 13 October 7, 2010 01:33
Strange Problem With updating the library farhagim OpenFOAM 0 August 10, 2010 12:34
Coordination Frame Problem - strange Luk_Fiz CFX 2 July 30, 2010 08:20


All times are GMT -4. The time now is 11:26.