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Old   June 26, 2013, 14:07
Default No Flow Problem
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Hi all,

I'm new to open foam and CFD in general and am trying to start very basic in terms of OpenFoam simulation. My first project is to start with an empty block of air (no geometry) with no flow.

My (probably misguided) way of doing this was to copy the initial cavity tutorial and modify it to have no flow by only using fixed walls (i.e. removing fixedWalls and frontAndBack in all boundaryField sections in the 0/ folder, as well as in the boundary section in blockMeshDict (I moved all the faces to fixedWalls). I made the velocity in transportProperties 0.0 for no flow. I tried to run with icoFoam, but just got this spit out over and over:

Code:
...

Time = 0.495

Courant Number mean: 0 max: 0
DILUPBiCG:  Solving for Ux, Initial residual = 0, Final residual = 0, No Iterations 0
DILUPBiCG:  Solving for Uy, Initial residual = 0, Final residual = 0, No Iterations 0
DILUPBiCG:  Solving for Uz, Initial residual = 0, Final residual = 0, No Iterations 0
DICPCG:  Solving for p, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 0, global = 0, cumulative = 0
DICPCG:  Solving for p, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 0, global = 0, cumulative = 0
ExecutionTime = 0.14 s  ClockTime = 0 s

Time = 0.5

Courant Number mean: 0 max: 0
DILUPBiCG:  Solving for Ux, Initial residual = 0, Final residual = 0, No Iterations 0
DILUPBiCG:  Solving for Uy, Initial residual = 0, Final residual = 0, No Iterations 0
DILUPBiCG:  Solving for Uz, Initial residual = 0, Final residual = 0, No Iterations 0
DICPCG:  Solving for p, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 0, global = 0, cumulative = 0
DICPCG:  Solving for p, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 0, global = 0, cumulative = 0
ExecutionTime = 0.15 s  ClockTime = 0 s

End
Is this my expected outcome? I would imagine that I would get some increase in No Iterations, but the rest stay 0.

Any help is much appreciated!
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Old   June 26, 2013, 14:26
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Dear swartzism
Before starting any numerical simulation you should understand the physics of your problem. the icoFoam solver calculates the velocity & pressure for a laminar flow.
so I have a question: If you do not have any flow then WHAT do you exactly want to solve??
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Old   June 27, 2013, 09:43
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Quote:
Originally Posted by cfdonline2mohsen View Post
Dear swartzism
Before starting any numerical simulation you should understand the physics of your problem. the icoFoam solver calculates the velocity & pressure for a laminar flow.
so I have a question: If you do not have any flow then WHAT do you exactly want to solve??
Thanks for the response!

The intention of this first overly simple simulation is to build my openFoam knowledge from the literal "ground" up. The intended progression is as follows:

  1. Start with an empty block of air (no geometry) with no flow
  2. Then move to uniform flow
  3. Then apply solid surface boundary conditions to one boundary for comparison against infinite flat plate solutions
  4. Then move to a NACA 0012 airfoil
I understand the physics of my problem, but I may not fully grasp the intended physics of icoFoam. So what I'm looking for in (1) is that there is no difference between the final solution and initial conditions. Since icoFoam handles laminar flow as you said, is there a different openFoam utility intended for gasses?



Thanks again
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Old   June 27, 2013, 14:36
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I believe what's happening is that since there is no flow, the tolerance parameter in fvSolution is automatically satisfied and it never iterates. Is it possible to neglect tolerance and relTol and iterate up to maxIter. This idea coming from 4.5.1.1 in http://www.openfoam.org/docs/user/fvSolution.php

Thanks
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Old   June 27, 2013, 16:41
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It looks fine, solving for zero flow to specified accuracy happens very fast, especially if you start from zero initial conditions.
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Old   June 27, 2013, 17:02
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Cool beans. I found a comprehensive list of which openFoam tools are used for specific problems. However, under compressible cases (i.e. what I was going for), I couldn't find a clear choice for air. Is there a standard?

Thanks for the replies
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Old   June 27, 2013, 20:15
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interesting job!
Do you want to understand how a solver works by this test only?i admire your creativity but there's better ways to do that.its not so hard that you may imagine.
I think you have exaggerated your problem.go and start with naca tutorial without any doubt.you don't need to invent the wheel from first.
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