How to calculate CoG in interDyMFoam

anmartin · January 12, 2012, 16:40

Hello,
I would like to calculate the centre of gravity of liquid fraction in a sloshing simulation.
Are there any options to calculate it in OF or in Paraview?

Regards
Angel

sail · January 12, 2012, 18:58

Quote:

Originally Posted by anmartin

Hello,
I would like to calculate the centre of gravity of liquid fraction in a sloshing simulation.
Are there any options to calculate it in OF or in Paraview?

Regards
Angel

Hi angel.

i have never done it by myself, but i would use the threshold filter to selct just the cells with alpha_water > 0.5, and than use the calculator filter, in conjunction with each cell's coordinates and volumes, to obtain a new array with just one value.

good luck

gschaider · January 12, 2012, 20:16

Quote:

Originally Posted by anmartin

Hello,
I would like to calculate the centre of gravity of liquid fraction in a sloshing simulation.
Are there any options to calculate it in OF or in Paraview?

Regards
Angel

You mean by calculating the weighted average of position of all the cells? Like this:

sum(pos()*alpha1*vol())/(vol()*alpha1)

In swak4Foam there is a functionobject swakExpression where the line

expression "sum(pos()*alpha1*vol())/(vol()*alpha1)";

will print exactly that at every timestep (OK. you've got to add a bit more to the controlDict, but it is less than 20 lines)

anmartin · January 23, 2012, 09:53

Quote:

Originally Posted by gschaider

You mean by calculating the weighted average of position of all the cells? Like this:

sum(pos()*alpha1*vol())/(vol()*alpha1)

In swak4Foam there is a functionobject swakExpression where the line

expression "sum(pos()*alpha1*vol())/(vol()*alpha1)";

will print exactly that at every timestep (OK. you've got to add a bit more to the controlDict, but it is less than 20 lines)

Hello,
First at all, many thanks to Bernhard and Vieri for your quick response.
I’ve tried first, with swak4foam and after installed and read yours presentations, I finally discovered one easy way to obtain the cog that works for me, but not very smart, because in can’t remove the floating point exception.
This is my controlDict, with the different options that I have tried, and the results obtained, included the error:
libs (
"libsimpleSwakFunctionObjects.so"
"libswakFunctionObjects.so"
);
functions
{
liquid_COG //simpleFunctionObjects

{
type swakExpression;
valueType internalField;
setName liquidCog;
accumulations (sum);
expression "(pos()*alpha1*vol())/0.034167"; //works
/*# Time sum
0.01 (0.000309988 0.0903394 0.25)*/
verbose true;
outputControl outputTime;
}
liquid_COG1

{
type swakExpression;
valueType internalField;
setName liquidCog;
accumulations (sum);
expression "pos()"; //works
/*# Time sum
0.01 (-401.694 14571.8 22412.5)*/
verbose true;
outputControl outputTime;
}
liquid_COG2

{
type swakExpression;
valueType internalField;
setName liquidCog;
accumulations (sum);
expression "alpha1*vol()"; //works
/*# Time sum
0.01 0.034167*/
verbose true;
outputControl outputTime;
}
liquid_COG3

{
type swakExpression;
valueType internalField;
setName liquidCog;
accumulations (sum);
expression "pos()*alpha1*vol()"; //works
/*# Time sum
0.01 (1.05913e-05 0.00308663 0.00854175)*/
verbose true;
outputControl outputTime;
}
liquidCOG4

{
type swakExpression;
//functionObjectLibs ("libswakFunctionObjects.so"); Not needed if it is called in libs ("libsimpleSwakFunctionObjects.so" "libswakFunctionObjects.so");
valueType internalField;
setName liquid_Cog;
accumulations (sum);
//expression "(pos()*alpha1*vol())/(alpha1*vol())"; //Floating point exceptio
variables (
"posX=(pos().x>0.0001) ? pos().x : 0;"
"posT=vector(posX,pos().y,pos().z);"
);
expression "posT*alpha1*vol()/(alpha1*vol())"; //ERROR
/*Expression liquidCOG4 : #0 Foam::error:

rintStack(Foam::Ostream&) in "/home/anmartin/OpenFOAM/OpenFOAM-2.0.x/platforms/linux64Gcc45DPOpt/lib/libOpenFOAM.so"
#1 Foam::sigFpe::sigHandler(int) in "/home/anmartin/OpenFOAM/OpenFOAM-2.0.x/platforms/linux64Gcc45DPOpt/lib/libOpenFOAM.so"
#2 __restore_rt at sigaction.c:0
#3 Foam::tmp<Foam::GeometricField<Foam::Vector<double >, Foam::fvPatchField, Foam::volMesh> > Foam:

perator/<Foam::Vector<double>, Foam::fvPatchField, Foam::volMesh>(Foam::GeometricField<Foam::Vector<d ouble>, Foam::fvPatchField, Foam::volMesh> const&, Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh> const&) in "/home/anmartin/OpenFOAM/OpenFOAM-2.0.x/platforms/linux64Gcc45DPOpt/lib/libfiniteVolume.so"
#4 parserField::FieldValueExpressionParser:

arse() in "/home/anmartin3/OpenFOAM/anmartin3-2.0.x/platforms/linux64Gcc45DPOpt/lib/libswak4FoamParsers.so"
#5 Foam::FieldValueExpressionDriver:

arse(std::strin g const&) in "/home/anmartin3/OpenFOAM/anmartin3-2.0.x/platforms/linux64Gcc45DPOpt/lib/libswak4FoamParsers.so"
#6 Foam::swakExpressionFunctionObject::write() in "/home/anmartin3/OpenFOAM/anmartin3-2.0.x/platforms/linux64Gcc45DPOpt/lib/libsimpleSwakFunctionObjects.so"
#7 Foam::simpleFunctionObject::execute(bool) in "/home/anmartin3/OpenFOAM/anmartin3-2.0.x/platforms/linux64Gcc45DPOpt/lib/libsimpleFunctionObjects.so"
#8 Foam::functionObjectList::execute(bool) in "/home/anmartin/OpenFOAM/OpenFOAM-2.0.x/platforms/linux64Gcc45DPOpt/lib/libOpenFOAM.so"
#9 Foam::Time::run() const in "/home/anmartin/OpenFOAM/OpenFOAM-2.0.x/platforms/linux64Gcc45DPOpt/lib/libOpenFOAM.so"
#10 main in "/home/anmartin/OpenFOAM/OpenFOAM-2.0.x/platforms/linux64Gcc45DPOpt/bin/interDyMFoam"
#11 __libc_start_main in "/lib64/libc.so.6"
#12 Foam::UOPstream::write(char) in "/home/anmartin/OpenFOAM/OpenFOAM-2.0.x/platforms/linux64Gcc45DPOpt/bin/interDyMFoam"
Floating point exception*/
verbose true;
outputControl outputTime;
}
};

By the other hand, in paraview, it is possible to obtain de volume with meshQuality, and then pass celldata to point data and obtain the cellcenters. Then is possible using the calculator to multiply coord(x y z)*alpha*vol, but I don’t know how to make the summation of all cell values.

If anybody knows how it is possible to solve these questiongs, I will be very gratefully.
Best regards,

gschaider · January 23, 2012, 17:24