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

help on initial and boundary conditions

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

Reply
 
LinkBack Thread Tools Display Modes
Old   July 17, 2003, 21:41
Default help on initial and boundary conditions
  #1
tommewang
Guest
 
Posts: n/a
Can any body here experienced in solving lid-driven flow in a square cavity (just for viscous incompressible N-S equations) help me on setting the pressure boundary and initial conditions in detail, or details of the corresponding refferences? thank you very much.
  Reply With Quote

Old   July 18, 2003, 02:31
Default Re: help on initial and boundary conditions
  #2
JG
Guest
 
Posts: n/a
If you were a student, you should ask your professor for such a simple question.
  Reply With Quote

Old   July 18, 2003, 08:47
Default Re: help on initial and boundary conditions
  #3
Magesh
Guest
 
Posts: n/a
If you have a answer to his question you could respond..otherwise it is completely uncalled for you to suggest what he needs to do to get an answer to his question..and least of all who wants your opinion on whether the question is simple or hard!!!
  Reply With Quote

Old   July 18, 2003, 10:38
Default Re: help on initial and boundary conditions
  #4
id
Guest
 
Posts: n/a
thanks Magesh for your great reply.

well done
  Reply With Quote

Old   July 18, 2003, 12:59
Default Re: help on initial and boundary conditions
  #5
JG
Guest
 
Posts: n/a
Well, I certainly know the answer for this basic and simple question. Do you know that?
  Reply With Quote

Old   July 18, 2003, 18:17
Default Re: help on initial and boundary conditions
  #6
Tony
Guest
 
Posts: n/a
The original Navier-Stokes equations only need a reference pressure datum.
  Reply With Quote

Old   July 18, 2003, 21:34
Default Re: help on initial and boundary conditions
  #7
tommewang
Guest
 
Posts: n/a
thank every body. I know what non-slip condition means to pressure. and I suppose pressure at the left bottom node to be zero. and with pressure correction method, I let all the pressure to be zero at all nodes at the right begining. I am writing my own code. I began with this simple and basic example. I know the pressre correction process is the key one in such a method. My results of pressure distribution is not precision enough. if anybody have experience in making ones own code, please give me more detail suggestions and please do not like someone just laughing. thank again.
  Reply With Quote

Old   July 19, 2003, 12:04
Default Re: help on initial and boundary conditions
  #8
centaur_ks
Guest
 
Posts: n/a
which formulation u r using, r u using primitive variable formulation or r u using vorticity-stream function formulation?

for pressure boundary u dont need to do anything i guess, on the boundaries the gredient will be zero.I mean the pressure on the boundary node shd be equal to the first node near the boundary. Read Anderson's book or Versteeg / Malasekera for this.

J-
  Reply With Quote

Old   July 20, 2003, 05:44
Default Re: help on initial and boundary conditions
  #9
Jim Clancy
Guest
 
Posts: n/a
JG, you are an expert who should participate only in an advanced level forum. we don't need your answers and your harsh comments.
  Reply With Quote

Old   July 20, 2003, 06:03
Default Re: help on initial and boundary conditions
  #10
JK
Guest
 
Posts: n/a
Lets have a round of applause for JG.....The likes of you is not, in my opinion, welcome around here. This is a forum where simple ---> extremly hard questions can be posted. Respect all questions, you were at the beginning stage once.

J.
  Reply With Quote

Old   July 20, 2003, 14:34
Default Re: help on initial and boundary conditions
  #11
YOUSEF EL HASSADI
Guest
 
Posts: n/a
there is a very good reference for your problem the refrence is computantional methods for fluid dynamics by joe freizer good luck
  Reply With Quote

Old   July 20, 2003, 22:42
Default thank everybody for help
  #12
tommewang
Guest
 
Posts: n/a
Why I ask the question is that I used pressure correction method in my code making. And I let dp/dn=0 at all boundaries for the non-slip condition, and all p=0 at the right begining of my cal, and I fix p of the left bottom to be zero (or the mid point). For solving the problem efficiently, I began with an example of elleptic partial differential equations in a square domain with unit length. I compared my results of the problem with that of an existed difference code. although my results have the same trend with that of the difference method, there are errors of 15% at each point. and with the refinement of my element, the whole errors become smaller but the center point. I feel strange here. I think that is because it is a problem with one gaven stress point

(d/dx(du/dx)+d/dy(du/dy)=f(x,y); f(x,y)=0,or 2*JMAX**2(only at the mid point of the square domain,and JMAX means the total number divided in each direction), u(x,y)=0,at all bonudary).

Can any body help me with the problem or show me your answer of my example.
  Reply With Quote

Old   July 21, 2003, 21:18
Default Re: help on initial and boundary conditions
  #13
Jim Park
Guest
 
Posts: n/a
Maybe you can give us some more information?

Where are your variables located on your mesh? Velocities & pressure at the same points, velocities on cell edges, pressure in the center, or what?

What sort of algorithm are you using to solve the difference equations?

Patankar's famous techniques (and those from Imperial College - and earlier, from the Los Alamos Lab), use an iterative technique that solves the velocities, then determines the pressure correction from a Poisson equation derived from the continuity equation. Fixing the continuity throws the velocities out of balance, so the cycle is repeated until everything is balanced within an 'acceptable' tolerance. These are usually based on velocities located at cell edges, pressures in cell centers.

Good Luck!
  Reply With Quote

Old   July 22, 2003, 04:23
Default Re: help on initial and boundary conditions
  #14
tommewang
Guest
 
Posts: n/a
Dear Park: I used Cholesky defracterization to solve the pressure poisson equations formed in the process of standard Galerkin process (FEM). My poisson equations were deduced from continuity equation. I plan to set the three on the same point(nodes). for solving the pressure poisson equations efficiently, I began with the example I mentioned. As the pressure solving is the key part of the whole solving process, I think if it can provide precise correction to velocity at each time step, the answer will be right, for I plan to set all the u,v,p at the right begining to be zero. Am I right? thank again
  Reply With Quote

Old   July 22, 2003, 19:43
Default Re: thank everybody for help
  #15
Jim Park
Guest
 
Posts: n/a
A. "Am I right?"

Can't say for sure. I've not done any FEM. It is encouraging that your solutions show the correct trends. Perhaps there is still a small bug someplace in the code?

B. " ... dp/dn=0 at all boundaries for the non-slip condition ... "

Perhaps you mean to say "no-flow"? Continuity is about fluxes across boundaries. I think the no-slip condition is set by u = 0 on the top or bottom (u = Uo on the moving boundary) boundaries, v= 0 on the left and right in the appropriate momentum equations. But, regardless of the word (slip or flow), the condition dp/dn = 0 that you've applied for the pressure is the correct one.
  Reply With Quote

Old   July 22, 2003, 21:38
Default Re: thank everybody for help
  #16
tommewang
Guest
 
Posts: n/a
setting dp/dn=0 at the boundaries, I mean as the velocity are gaven, there is no need to modify u and v here and the pressure on the boundary equal its neighbour node at the normal direction. If you can, please give me more suggestions. Thank you from my heart.
  Reply With Quote

Old   July 23, 2003, 05:26
Default Re: help on initial and boundary conditions
  #17
tommewang
Guest
 
Posts: n/a
I have no ways to access the books mentioned here. and I am a poor one.If anyone can help, please give me suggestions in details.thank alot
  Reply With Quote

Old   July 25, 2003, 02:37
Default Re: thank everybody for help
  #18
rockinwind
Guest
 
Posts: n/a
Helle,tommewang,

What do you mean ' and I fix p of the left bottom to be zero (or the mid point) ' ? Do you mean keep p fix at these places all the time during the iteration?

dp/dn = 0 is enough to determine the pressure at the boundaries. The fixing of pressure at some points will lead to pressure drifting in the field.

  Reply With Quote

Old   September 1, 2003, 22:56
Default Re: help on initial and boundary conditions
  #19
kenn
Guest
 
Posts: n/a
pressure BC is still in confusion even in the most respected literature of J. of Computational Physics.

for example, the pressure BC on wall is not a vanishing Neumann at all, which is a good approximation in when Re is high but wrongk for low Re. I derived this case, and found P.M.Gresho's 1987's article supports the same view.

pressure BC on non-walls are not fully determined.

I even doubt all appraoches, that applying normal component of mometum equation on boundary is sort of incorrect, I mean, they are all just approximate BC.

  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
Wind turbine simulation Saturn CFX 34 October 16, 2014 05:27
transsonic nozzle with rhoSimpleFoam Unseen OpenFOAM Running, Solving & CFD 7 April 16, 2014 03:38
problem with Min/max rho tH3f0rC3 OpenFOAM 7 February 23, 2013 06:37
RPM in Wind Turbine Pankaj CFX 9 November 23, 2009 05:05
Convective Heat Transfer - Heat Exchanger Mark CFX 6 November 15, 2004 16:55


All times are GMT -4. The time now is 12:29.