CFD Online Logo CFD Online URL
Home > Forums > FLUENT

skewness and convergence

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

Like Tree1Likes
  • 1 Post By zxaar

LinkBack Thread Tools Display Modes
Old   October 13, 2005, 02:44
Default skewness and convergence
Posts: n/a
Hi all,

I really want to know one thing. I have seen a reply by Xaar that it is becuase of the highly skewed elements, the continuity doesn't fall below the imposed limits. Can anybody tell quantitatively that what is the limiting % of high skewness beyond which the continuity or other parameters donot converge properly, preferably in internal flows? (sorry for asking this, as "quantitatively" is meaningless when you are dealing with Fluent and CFD!!) Sometimes we cannot avoid to have highly skewed elements.


  Reply With Quote

Old   October 13, 2005, 03:57
Default Re: skewness and convergence
Posts: n/a
let me explain a little about this issue when you apply continuity, what you need is values of variable (u,v,w) at face center. This value can be calculated from values of cells sharing this face. from these values you can get fluxes at face and sum of fluxes equal to zero is continuity. To couple pressure-velocity, you get one more parameter, that is p. so we have fluxes in terms of (u,v,w,p), but these fluxes do not sum to zero, so we calculate pressure correction. from this pressure correction we correct the velocity and thus fluxes.

now comes the cretical part, the way pressure correction equation is formed, it becomes poisson equation in terms of pressure correction variable. This poisson equation has, terms that require you to calculate gradients of (u,v,w,p) at the face center and normal to face , in a orthogonal cell system, these values can be calculated accurately, and thus satisfying the continuity is easy. But when the non-orthogonality increases, these terms introduce errors, and thus it becomes increasing difficult to get correct value as non-orthogonolity increases.

i hope you get the point,

now this also means one more thing, that if the cells are non orthogonal but the gradients are zero, or scalar value does not change, then, the errors will also be very very less, thus if you have cells in the portion where flow does not change much, they will not create problems. But if it is in sensetive portion where flow changes too much, it creates lot of problems in satisfying continuity.

i hope it clears.
sharonyue likes this.
  Reply With Quote

Old   October 13, 2005, 04:08
Default Re: skewness and convergence
Posts: n/a
Thanx Zxaar

Your reply gave me some light into how we should modify our geometry according to the nature of the flow...Thanx a lot

  Reply With Quote


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
Convergence Centurion2011 FLUENT 31 Yesterday 04:54
convergence problem when use pisoFoam, LES for wind tunnel case Forrest_Lei OpenFOAM 3 July 19, 2011 06:00
Removing cells with high skewness Parag Gadgil FLUENT 0 July 11, 2011 01:52
Removing cells with high skewness Parag Gadgil Main CFD Forum 0 July 9, 2011 03:46
Naca 0012 (compressible and inviscid) flow convergence problem bipulsaha FLUENT 1 July 6, 2011 07:51

All times are GMT -4. The time now is 16:18.