# read the user guide again, some questions very simple.

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January 1, 2013, 23:36
read the user guide again, some questions very simple.
#1
Senior Member

Dongyue Li
Join Date: Jun 2012
Location: Torino, Italy
Posts: 742
Rep Power: 9
1.About p in icoFoam

1.1
Quote:
 In this case, as in most we encounter, the initial fields are set to be uniform. Here the pressure is kinematic, and as an incompressible case, its absolute value is not relevant, so is set to uniform 0 for convenience.
What is "pressure is kinematic"? Is it dynamic pressure?

1.2
Quote:
 In a closed incompressible system such as the cavity, pressure is relative: it is the pressure range that matters not the absolute values. In cases such as this, the solver sets a reference level by pRefValue in cell pRefCell. In this example both are set to 0. Changing either of these values will change the absolute pressure field, but not, of course, the relative pressures or velocity field.
Does this mean p in icoFoam is relative pressure? so imagine this: there is a long hole full of water, the more deeper , the bigger pressure. but if it is relative pressure, what will happen to this? Back to the icoFoam, at 0 dictionary,I set the p is uniform 0;if the geometry is a long hole with water in it. but the lid is not moving, after calculation, Does this p can show nonuniform?or just nothing happen with it.it is still uniform 0?

btw,is this p in icoFoam the same with the p in interFoam? it looks like p field in interFoam is normal.

2.

I figured it out~

3.

Quote:
 Time = 1.63 3 4 Courant Number mean: 0.108642 max: 0.818175 5 DILUPBiCG: Solving for Ux, Initial residual = 7.86044e-06, Final residual = 7.86044e-06, 6 No Iterations 0 7 DILUPBiCG: Solving for Uy, Initial residual = 9.4171e-06, Final residual = 9.4171e-06, 8 No Iterations 0 9 DICPCG: Solving for p, Initial residual = 3.54721e-06, Final residual = 7.13506e-07, 10 No Iterations 4 11 time step continuity errors : sum local = 6.46788e-09, global = -9.44516e-19, 12 cumulative = 1.04595e-17 13 DICPCG: Solving for p, Initial residual = 2.15824e-06, Final residual = 9.95068e-07, 14 No Iterations 3 15 time step continuity errors : sum local = 8.67501e-09, global = 7.54182e-19, 16 cumulative = 1.12136e-17 17 ExecutionTime = 1.02 s ClockTime = 1 s
Why there is two "Solving for p"? and what does this time step continuity errors mean?

4.

page 40
Quote:
 Let us assume the initial turbulence is isotropic, i.e. U′ 2x = U′ 2y =U′ 2z , and equal to 5% of the lid velocity and that l, is equal to 20% of the box width, 0.1
If my case is not this simple, how can I set k and epsilon correctly? for example: I have a stirred vessel, the Vtip is 4m/s, how can I know this two parameters in WALL,and IMPELLER and something elsewhere?

I have so many questions which look simple but are perplexed for me. if you can help me in patience, I would be very grateful,Thanks in advance.

Last edited by sharonyue; January 1, 2013 at 23:52.

 January 2, 2013, 03:53 #2 Senior Member   Nima Samkhaniani Join Date: Sep 2009 Location: Tehran, Iran Posts: 1,193 Blog Entries: 1 Rep Power: 16 1- p in icoFoam is actually (p/rho) 2-in incompressible. absolute pressure is not important, what you want is pressure gradient, so it is not important where you consider that reference point! 3- icoFoam or PisoFoam using Piso algorithm to solve for momentum Equation, read this thread, will help you why! there is two correction for pEquation http://openfoamwiki.net/index.php/Th...hm_in_OpenFOAM 4-its so tricky, you should look at papers how they evaluate these values __________________ Telegram channel (https://telegram.me/openfoam4Iranian) My Weblog (http://openfoam.blogfa.com/) Training Course on OpenFOAM at (http://www.isme.ir/)

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