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

Seperation and Y+ value

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

Reply
 
LinkBack Thread Tools Display Modes
Old   February 26, 2008, 14:46
Default Seperation and Y+ value
  #1
Steven
Guest
 
Posts: n/a
Hi

What is the importance of setting y+ of 1 with the sst model? The reason i ask is that when setting the inflation layer on my airfoil with a y+ of 1 ( 8 layers ) the airfoil will not stall, however if it is set by thickness of 10mm (5 layers) the stall shows and seperation occurs with recirculation. The tet mesh density conected to the y+ version is also higher. Why would the stall not show?

Hope that makes sense

Steve
  Reply With Quote

Old   February 26, 2008, 16:00
Default Re: Seperation and Y+ value
  #2
Steven
Guest
 
Posts: n/a
I should also add that in a previous 2D study with the same airfoil seperation did occur however in the 3d study it does not. Just to clear that up

Steve
  Reply With Quote

Old   February 26, 2008, 17:33
Default Re: Seperation and Y+ value
  #3
Glenn Horrocks
Guest
 
Posts: n/a
Hi,

Whether y+<=1 is required or not is problem dependant. Do a sensitivity study on your case to find out whether it is important or not.

Glenn Horrocks
  Reply With Quote

Old   February 26, 2008, 17:36
Default Re: Seperation and Y+ value
  #4
Steven
Guest
 
Posts: n/a
Thanks Glen

I think i should pose the question differently, I have carried out two simulations on an airfoil, the first was 2d with y+ of 1 and seperated as expected, however when a 3d simulation was carried out with y+ also one the flow does not seperate. Are there common reasons for this? Is sst suitable for 3d? Does it need to be transient? Or should is use a different model?

Thanks again

Steve
  Reply With Quote

Old   February 27, 2008, 04:49
Default Re: Seperation and Y+ value
  #5
Raj
Guest
 
Posts: n/a
did you used prism layers near the walls, Also Please check the mesh jump an refinement near to the area of interest,SST is sensitive in such cases
  Reply With Quote

Old   February 27, 2008, 05:23
Default Re: Seperation and Y+ value
  #6
Steven
Guest
 
Posts: n/a
Im using 8 layers of prism elements and the face spacing is set at min 0.9mm and max 5mm the area above the airfoil also has line spacings set at 5mm so there is not much expansion at all.

Steve
  Reply With Quote

Old   February 27, 2008, 17:48
Default Re: Seperation and Y+ value
  #7
Glenn Horrocks
Guest
 
Posts: n/a
Hi,

Are there common reasons? No, it is proplem dependant.

Is SST suitable for 3D? Yes.

Does it need to be transient? No. In fact there are limitations on the applicability of RANS turbulence models to transient flows. RANS models are based on steady state flows.

Should you use a different model? SST is probably about as good as you'll get without getting a lot more complex with things like LES or DES.

It looks like you have just shown in your case you need a mesh with y+ around 1 and that the normal logarithmic wall functions are not suitable in your case.

Glenn Horrocks
  Reply With Quote

Old   February 29, 2008, 06:30
Default Re: Seperation and Y+ value
  #8
Steven
Guest
 
Posts: n/a
Hi again!

This problem has me pulling my hair out! This is the current situation. I have gone back to an aifoil which is extruded by a small amount between two walls to simulate '2D' effects. It stalls and shows seperation, now i change the geometry so the encloseure is bigger in the z direction (hence making it 3D) with exactly the same parameters all over save chaging one wall to be a symetry plane. It no longer stalls!!! How can this be!? A finite wing (3D) should stall sooner than the 2D calcuation due to induced drag and downwash.

Any ideas as im at a dead end here

Steve
  Reply With Quote

Old   February 29, 2008, 08:02
Default Re: Seperation and Y+ value
  #9
Steven
Guest
 
Posts: n/a
Im an idiot. I dont know where i got the idea that finite wings stall before infinite wings. What a plonker!!!

Steve
  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
particle seperation devalpandya FLUENT 0 October 15, 2009 04:24
How to find the seperation region? Omer CFX 3 October 8, 2007 08:12
Seperation Line Gavin Brady Main CFD Forum 3 August 21, 2001 08:48
FLOWS WITH SEPERATION AND RECIRCULATION Anup Paul FLUENT 0 October 12, 2000 14:02
turbulance and seperation pravinpardeshi Main CFD Forum 2 December 26, 1998 10:54


All times are GMT -4. The time now is 16:32.