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Darcy Forchheimer: trying to understand the code and how to run with it? 

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February 12, 2015, 18:12 
Darcy Forchheimer: trying to understand the code and how to run with it?

#1 
Senior Member

Hi all,
I'm facing strange behavior using fvOptions and simpleFoam. I mean by setting extrapolated values from experimental data I don't have same pressure losses on my simulation, but lower. The case is incompressible with MRF source for momentum. here is th fvOption: Code:
MRF1 { type MRFSource; active true; selectionMode cellZone; cellZone MRF; MRFSourceCoeffs { nonRotatingPatches (wallStat boccaglio AMI_ROT1 AMI_ROT2 AMI_ROT3);/ origin (0 0 0); axis (0 0 1); omega 300; } } ….... porosity2 { type explicitPorositySource; active true; //yes; selectionMode cellZone; cellZone filtro; explicitPorositySourceCoeffs { type DarcyForchheimer; DarcyForchheimerCoeffs { d d [0 2 0 0 0 0 0] (1790 1790 1790); f f [0 1 0 0 0 0 0] (1640 1640 1640); coordinateSystem { type cartesian; origin (0.05 0.11 0.15); coordinateRotation { type axesRotation; e1 (1 0 0); e2 (0 1 0); //e3 (0 0 1); } } } } } In order to set the D (linear?)& F(square?) I made some manipulation on experimental data. Experimental data consists of pressure losses (static) at different mean velocities. For each static pressure, I divided the value by thickness of filter (10mm), then by plotting the pressure/meters losses vs. velocity I was able to extrapolate a(=F) & b(=D) values by tendency line representation on the graph, forcing the curve passing by point (0,0). 1st question: is it correct to divided pressure losses by thickness of filter? I mean I followed a tutorial (not openfoam) and I think this division take into account the number of cell in order to interpolate pressure and velocity values, is it correct? Further by digging into the code I see on DarcyForchheimer.H: Code:
public porosityModel { private: // Private data // Darcy coeffient XYZ components (usersupplied) [1/m2] dimensionedVector dXYZ_; // Forchheimer coeffient XYZ components (usersupplied) [1/m] dimensionedVector fXYZ_; // Darcy coefficient  converted from dXYZ [1/m2] List<tensorField> D_; // Forchheimer coefficient  converted from fXYZ [1/m] List<tensorField> F_; // Name of density field word rhoName_; ….. 2nd question: how openfoam calculates rho as I specify only nu 1.5e5 m^2/s in transport properties? Further into DarcyForchheimer.C: Code:
void Foam::porosityModels::DarcyForchheimer::calcTranformModelData() { if (coordSys_.R().uniform()) { forAll (cellZoneIDs_, zoneI) { D_[zoneI].setSize(1); F_[zoneI].setSize(1); D_[zoneI][0] = tensor::zero; D_[zoneI][0].xx() = dXYZ_.value().x(); D_[zoneI][0].yy() = dXYZ_.value().y(); D_[zoneI][0].zz() = dXYZ_.value().z(); D_[zoneI][0] = coordSys_.R().transformTensor(D_[zoneI][0]); // leading 0.5 is from 1/2*rho F_[zoneI][0] = tensor::zero; F_[zoneI][0].xx() = 0.5*fXYZ_.value().x(); F_[zoneI][0].yy() = 0.5*fXYZ_.value().y(); F_[zoneI][0].zz() = 0.5*fXYZ_.value().z(); F_[zoneI][0] = coordSys_.R().transformTensor(F_[zoneI][0]); } } else { forAll(cellZoneIDs_, zoneI) { const labelList& cells = mesh_.cellZones()[cellZoneIDs_[zoneI]]; D_[zoneI].setSize(cells.size()); F_[zoneI].setSize(cells.size()); forAll(cells, i) { D_[zoneI][i] = tensor::zero; D_[zoneI][i].xx() = dXYZ_.value().x(); D_[zoneI][i].yy() = dXYZ_.value().y(); D_[zoneI][i].zz() = dXYZ_.value().z(); // leading 0.5 is from 1/2*rho F_[zoneI][i] = tensor::zero; F_[zoneI][i].xx() = 0.5*fXYZ_.value().x(); F_[zoneI][i].yy() = 0.5*fXYZ_.value().y(); F_[zoneI][i].zz() = 0.5*fXYZ_.value().z(); } const coordinateRotation& R = coordSys_.R(mesh_, cells); D_[zoneI] = R.transformTensor(D_[zoneI], cells); F_[zoneI] = R.transformTensor(F_[zoneI], cells); } } 3rd question: can someone help me please? Thanks a lot! Bye 

February 12, 2015, 19:59 

#2 
Senior Member

Hi,
I work a little on angleExplicit case on porousSimpleFoam folder. If in 0 folder I keep only U & p file, computation starts anyway. I'm a little in confusion about this topic, as I thought rho was calculated by perfect Gas equation, but removing T file, computation start and terminate with no errors. In any case, I discovered a mismatch between my BCs and the tutorial case: in the tutorial case a BC for the porosity walls is used and set to slip, but if I change it into wall with fixed uniform value (0 0 0) it works anyway...so what's the difference with it? thank you for any help. Bye 

February 13, 2015, 10:50 

#3 
Senior Member

I test the DarcyForchHeimer on a test case based on measurments of pressure losses on a porous media using porousSimpleFoam and porosity dict:
https://www.dropbox.com/s/w27xlva4ftbw908/case.JPG?dl=0 it's 2D case of a channel of 21x2 meters with one porous zone of 1meter in the middle. As it's possibile to see, I calculated the D & F coefficient by plotting on a graph and drawing the tendecy line passing through (0,0). These values are used into porosity dict for the DarcyForchheimer model, but red lines show results of calculation, higly disagree with measruments, any suggestion? As it possible to see on the following picture, after the porous zone (fluid flows in x+ direction) velocity has a 1.06m/s value, while at inlet the BC is 0.88m/s: I was expecting a reduction on velocity after the porous media. https://www.dropbox.com/s/duxz3jeu4hjghjr/fdsf.png?dl=0 Can someone give me an explanation? Here is the case, with the mesh.unv https://www.dropbox.com/s/ixu1pdwevbpg88b/case.zip?dl=0 Thanks a lot 

February 13, 2015, 11:16 

#4 
Senior Member

Hi,
Why do you think that 5000 iterations is enough for the case to converge? 

February 13, 2015, 11:40 

#5 
Senior Member

Hi,
thanks for reply. Here's a plot of the residual; https://www.dropbox.com/s/sr7kftxrvk...nning.png?dl=0 I see residual floating over very low values; honestly I haven't checked a physical quantity, as well I haven't set up a turbulent case, but a laminar one. In any case, I can't see how velocity should raise up its value after the porous media. Should it be as for the conservativness of the FV method? I mean as for the BC applied, mass conservation has to be kept, so higher pressure before porous media gives a "bump" to the fluid, but as for conservativness the calculation leads to have same mass flow between inlet and outlet and, as for constant section area, velocity can't decrease. 

February 13, 2015, 11:56 

#6 
Senior Member

Hi,
here's a plot of the sum of inlet and outlet massflow. As you can see after few hundred iterations it comes to 0. https://www.dropbox.com/s/1r97qj99jj...sflow.png?dl=0 This should explain the local raise for velocity value. How can I the set up correctly the Darcy Forchheimer model? 

March 25, 2015, 10:22 

#7 
Senior Member

Hi all,
I don't know why, but the DarcyForchHeimer model didn't work for me. I found this useful link: http://www.geocities.co.jp/SiliconVa...fvOptions.html and by changing model to FixedCoeff, I was able to solve my problem. Anyway, these are the steps I performed to test the model on a simple case: a straight 2D channel of constant section with a predefined thickness for the porous zone. 1  I divided pressure losses (experimental data) by the thickness of the porous zone: call this A 2  Using excel, I plotted A vs. velocity and ask to have the alfa e beta coefficient for the tendency line. 3  in the fvOptionDict I set these values for alfa & beta for the FixedCoeff model. Code:
porosity1 { type explicitPorositySource; active true; //yes; selectionMode cellZone; cellZone porous; explicitPorositySourceCoeffs { type fixedCoeff; fixedCoeffCoeffs { alpha alpha [0 0 1 0 0 0 0] (25000 25000 25000); //linear term beta beta [0 1 0 0 0 0 0] (10000 10000 10000); //squared term coordinateSystem { type cartesian; origin (1.005 0.25 0); rho 1.205; coordinateRotation { type axesRotation; e1 (1 0 0); e2 (0 1 0); //e3 (0 0 1); } } } } } Postprocessing the results I had the matching between the experimental and numerical results (within a small tolerance). Hope this can help. Regards 

February 12, 2018, 01:54 

#8  
Member
Ramana
Join Date: Jul 2017
Location: India
Posts: 58
Rep Power: 8 
Quote:
I want to simulate flow and heat transfer from porous square cylinder to a flowing wind. I have defined the porous cylinder as porous blockage( similar to the one in PisoFaom tutorial) and now i want to modify the governing equation to make a userdefined solver. In predefined solver for porous media i.e PorousSimplefoam,There is no time derivative (as it is a steady state solver )and there is no entry to read porosity. I have few questions regarding momentum and energy equations 1). How and where to define porosity (1 for fluid region & (0<porosity<1) for porous zone) as i am using single domain approach(single set of governing equations are utilized to solve both fluid and porous zones). 2). what are the modifications required in momentum equation to include porosity effect in it. 2). Can i have access to your case files.the above link is not working Thanks in advance, Reagrds, S.V.Ramana 

February 12, 2018, 09:01 

#10  
Member
Ramana
Join Date: Jul 2017
Location: India
Posts: 58
Rep Power: 8 
Quote:
I think i am doing similar kind of simulation.In my simulation i have defined the porous cylinder as blockage and assigned porous resistance through Darcyforchheimer coefficients. In predefined solver for porous media i.e PorousSimpleFoam,There is no time derivative (as it is a steady state solver )and there is no entry to read porosity. I have a query regarding momentum and energy equations 1). How and where to define porosity (1 for fluid region & (0<porosity<1) for porous zone) as i am using single domain approach(single set of governing equations are utilized to solve both fluid and porous zones). I have defined porosity values in my porosityDict in "constant" folder , and porosity Index in "0" folder in my case but i don't know how to read these values in solverporosityDict.png i am new to OF ,i will be glad if you can help me to resolve this issue. Regards, S.V.Ramana 

February 12, 2018, 15:24 

#11 
Senior Member
Join Date: Aug 2015
Posts: 494
Rep Power: 14 
Check out this tutorial  https://github.com/OpenFOAM/OpenFOAM...htDuctImplicit. The solver, porousSimpleFoam, will read the properties from the porousProperties dictionary in the constant folder.


February 14, 2018, 04:40 

#12  
Member
Ramana
Join Date: Jul 2017
Location: India
Posts: 58
Rep Power: 8 
Quote:
Thanks for the quick reply.The above file reads DarcyForchheimer coefficients and not porosity values. Regards, S.V.Ramana 

February 14, 2018, 10:49 

#13 
Senior Member
Join Date: Aug 2015
Posts: 494
Rep Power: 14 
If you know the porosity, you should be able to estimate the Darcy coefficient. The Forchheimer coefficient can initially be set to zero.


February 21, 2018, 08:54 

#14 
Member
Anurag
Join Date: Aug 2014
Location: Germany
Posts: 57
Rep Power: 11 
You can use any solver that has support for fvOptions (not all solvers support it). In the fvOptions dictionary included below, for example, you can specify a source called as "porosity1" for the cellzone named "porous". This zone has to be defined in your mesh using the topoSet utility and an accompaying dictionary.
Code:
/** C++ **\  =========    \\ / F ield  OpenFOAM: The Open Source CFD Toolbox   \\ / O peration  Version: 4.1   \\ / A nd  Web: www.OpenFOAM.org   \\/ M anipulation   \**/ FoamFile { version 2.0; format ascii; class dictionary; location "constant"; object fvOptions; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // porosity1 { type explicitPorositySource; active yes; explicitPorositySourceCoeffs { selectionMode cellZone; cellZone porous; type DarcyForchheimer; DarcyForchheimerCoeffs { d d [0 2 0 0 0 0 0] (5.7e6 5.7e6 5.7e6); f f [0 1 0 0 0 0 0] (2.3 2.3 2.3); coordinateSystem { type cartesian; origin (0 0 0); coordinateRotation { type axesRotation; e1 (1 0 0); e2 (0 1 0); } } } } } //************************************************************************* // 

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