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-   -   Pressure is fluctuating (https://www.cfd-online.com/Forums/openfoam/102581-pressure-fluctuating.html)

 Sargentini May 29, 2012 07:30

Pressure is fluctuating

Dear

I'm trying to simulate a laminar flow using Herschel Bulkley and "non-Newtonian solver."
I adopted nu0 = 1E6, tau0 = 1, k = 0.01 and n = 1 to verify the result gives the analytical value of a Bingham material.
The problem is that pressure is fluctuating too much, although the velocity gain a suitable profile.
Someone please have an idea what might be happening and how to fix this problem.

grateful
Sargentini

 Bernhard May 29, 2012 07:34

Maybe you can give some more relevant information, such as discretization and convergence criteria used? Also, which solver are you using?

 Sargentini May 29, 2012 08:08

Pressure is fluctuating

I'm using nonNewtonianIcoFoam.

fvSolution:
p
{
solver PCG;
preconditioner DIC;
tolerance 1e-06;
relTol 0;
}
U
{
solver PBiCG;
preconditioner DILU;
tolerance 1e-05;
relTol 0;
}
}
PISO
{
nCorrectors 4;
nNonOrthogonalCorrectors 2;
--------------------------------------------------------
fvSchemes
{
default Euler;
}
{
default Gauss linear;
}
divSchemes
{
default none;
div(phi,U) Gauss upwind;
}
laplacianSchemes
{
default none;
laplacian(nu,U) Gauss linear corrected;
laplacian((1|A(U)),p) Gauss linear corrected;
}
interpolationSchemes
{
default linear;
interpolate(U) linear;
}
{
default corrected;
}
fluxRequired
{
default no;
p ;
}
-------------------------------------------------------------
p
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
dimensions [0 2 -2 0 0 0 0];
internalField uniform 1;
boundaryField
{
inlet
{
}
outlet
{
type fixedValue;
value uniform 1;
}
fixedWalls
{
}
frontAndBack
{
type empty;
}

U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
dimensions [0 1 -1 0 0 0 0];
internalField uniform (0 0 0);
boundaryField
{
inlet
{
type fixedValue;
value uniform (0.1 0 0);
}
outlet
{
}
fixedWalls
{
type fixedValue;
value uniform (0 0 0);
}
frontAndBack
{
type empty;
}
---------------------------------------
Grateful for the support

 Sargentini May 29, 2012 11:52

Pressure is fluctuating ,Complementing

Complementing:

HerschelBulkley.C

);
return

(
min

(
nu0_,
(tau0_ + k_*rtone*pow(tone*sr(), n_))
/(max(sr(), dimensionedScalar ("VSMALL", dimless/dimTime,VSMALL
)))
)
);
}

 calim_cfd June 2, 2012 08:44

Hi!

maybe you could upload a residual plot so we can take a look at your "fluctuation"

your running a transient case (1st order euler scheme), plus you have 2 non ortoghonal correctors.. it means the pressure will iterate twice more than velocities..hence you'll c an oscillation which isnt an oscillation..but corrections from the solver. plus it's a transient case.. and you're suppose to have an oscillation like response... now how much it will oscillates it depends on the case..

might have said the obvious but setting things in OF sts makes you forget the basics

hope it helps
:)

 Sargentini June 2, 2012 15:45

Hi!

Thank you for your assistance. In fact, as I am a student in both subjects these issues are not so obvious to me.
In this case, I was wondering what the formula written in the file HerschelBulkley.H, which considers nu0 would be more appropriate to the situation with the initial flow tau0 Yield stress, but it is only indicated. How can I use it? The file can be changed? This file is only available for reading.

 calim_cfd June 2, 2012 20:25

Hi

im not on my desktop right now so i cant search through the files and be more specific.. but i guess you're using them nonNewtonianIcoFoam solver... try to examine the tutorial for this solver and change the viscosity model accordingly i might be of more assistnce once i get to my pc..

ps: c eh br?:rolleyes:

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