CFD Online Discussion Forums - Force and Force Coefficient

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Force and Force Coefficient

Hi Foamers
I have a problem with force and force coefficient plotting.
I add this to end of cavity tutorial controlDict file
forces
{
type forces;
functionObjectLibs ("libforces.so"); //Lib to load
patches (fixedWalls); // change to your patch name
rhoInf 1.225; //Reference density for fluid
CofR (0.25 0 0); //Origin for moment calculations
}
forceCoeffs
{
type forceCoeffs;
functionObjectLibs ("libforces.so");
patches (fixedWalls); //change to your patch name
rhoInf 1.225;
CofR (0 0 0);
liftDir (0 1 0);
dragDir (1 0 0);
pitchAxis (0.25 0 0);
magUInf 10.0;
lRef 1;
Aref 1;
}
and I solved it by icoFoam > log
and then when I type " gnuplot forceCoeffs -" I got this message
"forceCoeffs", line 10: warning: Skipping unreadable file "./forceCoeffs/0.0005/forceCoeffs.dat"
"forceCoeffs", line 10: warning: Skipping unreadable file "./forceCoeffs/0.0005/forceCoeffs.dat"
"forceCoeffs", line 10: No data in plot"
I think that I did not calculate force and force coefficient correctly.Can any one help me?
Thanks

Ata

Hello Ata,

the force.dat file is not in the right format for Gnuplot.

forces.dat has ( and ) which gnuplot can not deal with. you need to remove those.

Hi jakaranda
Thank you very much for your reply. Can you explain more for me.
I do not understand what I must to do?
Best regards

HEllo ata,

in order to hanve Gnuplot work you must remove all the parentheses.

Ru nthe following script

cat forces.dat | tr -d "("")" > forcesClean.dat

and that will create a forcesClean.dat file wihtout any parentheses which Gnuplot will read.

then wihtin gnuplot use plot "forcesClean.dat" using 1:2 to plot the numbers in the first 2 columns.

for Gnuplot info look at the following websites
http://www.duke.edu/~hpgavin/gnuplot.html
http://t16web.lanl.gov/Kawano/gnuplot/index-e.html

hope this helps

J

Dear all,

I am using this function that it is needed in controlDict in orer to get the lift, drag and moment coefficients for a computation of the flow around an airfoil. The results I get for the lift and drag coefficients are very good compared to the reference I am using for both lift and drag coefficients but they are very bad for the moment coefficients. I use CofR=0.25*chord.
Any idea why I am getting this bad results?

Thanks,

José

Hello,

i have two questions about the probes utility.

1. Is the return value of the probe just the cell midpoint value at the cell where the given probe point lies inside or is it interpolated?

2. What about movingMeshes? Are the probes fixed in space or are they moving with the grid? Do i have to make something on the probes relative/absolute like for velocities in the solver?

Thanks,
rupert

Quote:

Originally Posted by jms (Post 299702)

The moment coefficient in libforces.so, is possitive under the rule of the right hand, while in aeronautics, the pitching moment is possitive nose up. I think this may the cause of your differences.
Gonzalo

Hi jakaranda,
now I use the function of "force",This is my function as follows:
forces
{
type forceCoeffs;
functionObjectLibs ( "libforces.so" );
outputControl timeStep;
outputInterval 1;

patches
(
MAIN
//FLAP
//SLATUP
//SLATDOWN
//SLATWEI
);

pName p;
UName U;
rhoName rho;
log true;
rhoInf 1;
CofR ( 0 0 0 );
liftDir ( -0.104528463 0.994521895 0 );
dragDir ( 0.994521895 0.104528463 0 );
pitchAxis ( 0 0 1 );
magUInf 68;
lRef 0.457;
Aref 1.2933;
}
I have a question at "Aref"and "rhoRef".because now I compute an airfoil,so "lref" is the chord length,but I don't know how to compute the "ARef".And this a "compressible"case,so I also don't know how to define the "rhoInf".Thank you

Aref is the chord of the airfoil by the lenght in the "z" direction of your problem. If your mesh is a 2D one then Aref=chord*1. The density is computed using the ideal gas law, with the pressure and temperature at freestream conditions.

I wrote a code for plotting residuals, forces or both

You just gotta execute them at terminal:

Code:

./plotResiduals

./plotForces

./plotRF

plotResiduals:

Code:

#!/bin/bash



# Code for plotting residuals



foamLog log.simpleFoam >/dev/null



gnuplot -persist > /dev/null 2>&1 << EOF

set logscale y

set title "Residual vs. Iteration"

set xlabel "Iteration"

set ylabel "Residual"

plot "logs/Ux_0" with lines,\

"logs/Uy_0" with lines,\

"logs/Uz_0" with lines,\

"logs/p_0" with lines



EOF

plotForces:

Code:

#!/bin/bash



foamLog log.simpleFoam >/dev/null



gnuplot -persist > /dev/null 2>&1 << EOF

set logscale x

set title "Forces vs. Iteration"

set xlabel "Iteration"

set ylabel "Forces"

plot '<sed -e "s/[(,)]//g" ./postProcessing/forces/0/force_0.dat' using 1:2 with lines title 'Fx [N]',\

'<sed -e "s/[(,)]//g" ./postProcessing/forces/0/force_0.dat' using 1:3 with lines title 'Fy [N]',\

'<sed -e "s/[(,)]//g" ./postProcessing/forces/0/force_0.dat' using 1:4 with lines title 'Fz [N]'



EOF

plotRF:

Code:

#!/bin/bash



# Code for plotting residuals and forces



foamLog log.simpleFoam >/dev/null



cat ./postProcessing/forces/0/force_0.dat | tr -d "("")" > ./forceClean.dat



gnuplot -persist > /dev/null 2>&1 << EOF



set terminal wxt size 500,600



set multiplot

set size 1,0.5

set origin 0.0,0.5; 

set logscale y

set logscale x

set title "Residual vs. Iteration"

set xlabel "Iteration"

set ylabel "Residual";

plot "logs/Ux_0" with lines,\

"logs/Uy_0" with lines,\

"logs/Uz_0" with lines,\

"logs/p_0" with lines; 





set origin 0.0,0.0; 

unset logscale y

set title "Forces vs. Iteration"

set xlabel "Iteration"

set ylabel "Forces";

plot '<sed -e "s/[(,)]//g" ./postProcessing/forces/0/force_0.dat' using 1:2 with lines title 'Fx [N]',\

'<sed -e "s/[(,)]//g" ./postProcessing/forces/0/force_0.dat' using 1:3 with lines title 'Fy [N]',\

'<sed -e "s/[(,)]//g" ./postProcessing/forces/0/force_0.dat' using 1:4 with lines title 'Fz [N]'

unset multiplot



EOF