CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   OpenFOAM (http://www.cfd-online.com/Forums/openfoam/)
-   -   ForcesCoeffs (http://www.cfd-online.com/Forums/openfoam/68171-forcescoeffs.html)

ronaldo September 9, 2009 05:13

ForcesCoeffs
 
please anyone, can you help me to model the drag coeffient of a 2D cylinder using Openfoam! I use icoFoam with CrankNicholson o.5 time integration scheme and i became the result below:

Time = 2.4164
Courant Number mean: 0.000625962 max: 0.018118
DILUPBiCG: Solving for Ux, Initial residual = 8.58266e-05, Final residual = 5.68787e-12, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000135717, Final residual = 1.58934e-11, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.000318544, Final residual = 2.92359e-06, No Iterations 10
DICPCG: Solving for p, Initial residual = 6.1916e-06, Final residual = 9.84286e-07, No Iterations 47
DICPCG: Solving for p, Initial residual = 1.54247e-06, Final residual = 7.8343e-07, No Iterations 1
DICPCG: Solving for p, Initial residual = 7.89665e-07, Final residual = 7.89665e-07, No Iterations 0
time step continuity errors : sum local = 8.41203e-15, global = 1.5953e-15, cumulative = -1.07022e-10
DICPCG: Solving for p, Initial residual = 8.12457e-07, Final residual = 8.12457e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 8.12457e-07, Final residual = 8.12457e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 8.12457e-07, Final residual = 8.12457e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 8.12457e-07, Final residual = 8.12457e-07, No Iterations 0
time step continuity errors : sum local = 8.65482e-15, global = 1.59517e-15, cumulative = -1.0702e-10
ExecutionTime = 25803.5 s ClockTime = 77443 s
forces output:
forces(pressure, viscous)((3.36771 -0.00561273 9.48494e-20) (0.365782 -2.56185e-05 -1.76796e-22))
moment(pressure, viscous)((3.44194e-19 -8.53456e-18 -0.263345) (-8.875e-22 1.48343e-20 0.0157597))
forceCoeffs output:
Cd = 152.388
Cl = -0.230137
Cm = -1.91412e-17
Time = 2.4166
Courant Number mean: 0.000625962 max: 0.0181183
DILUPBiCG: Solving for Ux, Initial residual = 8.58272e-05, Final residual = 5.68766e-12, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000135717, Final residual = 1.5893e-11, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.000307771, Final residual = 3.04239e-06, No Iterations 14
DICPCG: Solving for p, Initial residual = 5.71835e-06, Final residual = 9.44935e-07, No Iterations 174
DICPCG: Solving for p, Initial residual = 1.83437e-06, Final residual = 8.73055e-07, No Iterations 1
DICPCG: Solving for p, Initial residual = 8.84112e-07, Final residual = 8.84112e-07, No Iterations 0
time step continuity errors : sum local = 9.41649e-15, global = -2.7658e-16, cumulative = -1.0702e-10
DICPCG: Solving for p, Initial residual = 8.94637e-07, Final residual = 8.94637e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 8.94637e-07, Final residual = 8.94637e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 8.94637e-07, Final residual = 8.94637e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 8.94637e-07, Final residual = 8.94637e-07, No Iterations 0
time step continuity errors : sum local = 9.5286e-15, global = -2.7731e-16, cumulative = -1.07021e-10
ExecutionTime = 25806.4 s ClockTime = 77452 s
forces output:
forces(pressure, viscous)((3.37787 -0.0101397 9.47906e-20) (0.365779 -2.56295e-05 -1.76913e-22))
moment(pressure, viscous)((3.4472e-19 -8.52938e-18 -0.624579) (-8.87876e-22 1.48437e-20 0.0157587))
forceCoeffs output:
Cd = 152.802
Cl = -0.41491
Cm = -1.91296e-17
Time = 2.4168
Courant Number mean: 0.000625962 max: 0.0181186
DILUPBiCG: Solving for Ux, Initial residual = 8.58271e-05, Final residual = 5.68736e-12, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000135717, Final residual = 1.58931e-11, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.000308555, Final residual = 2.98446e-06, No Iterations 6
DICPCG: Solving for p, Initial residual = 5.6599e-06, Final residual = 9.87842e-07, No Iterations 193
DICPCG: Solving for p, Initial residual = 1.97131e-06, Final residual = 8.93472e-07, No Iterations 1
DICPCG: Solving for p, Initial residual = 9.0792e-07, Final residual = 9.0792e-07, No Iterations 0
time step continuity errors : sum local = 9.67148e-15, global = -1.15687e-16, cumulative = -1.07021e-10
DICPCG: Solving for p, Initial residual = 9.19959e-07, Final residual = 9.19959e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 9.19959e-07, Final residual = 9.19959e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 9.19959e-07, Final residual = 9.19959e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 9.19959e-07, Final residual = 9.19959e-07, No Iterations 0
time step continuity errors : sum local = 9.79972e-15, global = -1.14472e-16, cumulative = -1.07021e-10
ExecutionTime = 25809.3 s ClockTime = 77462 s
forces output:
forces(pressure, viscous)((3.36459 -0.00234243 9.49148e-20) (0.365777 -2.56403e-05 -1.76832e-22))
moment(pressure, viscous)((3.41853e-19 -8.54044e-18 -0.00215479) (-8.87561e-22 1.48373e-20
0.0157578))
forceCoeffs output:
Cd = 152.26
Cl = -0.0966561
Cm = -1.91544e-17
Time = 2.417
Courant Number mean: 0.000625962 max: 0.018119
DILUPBiCG: Solving for Ux, Initial residual = 8.58282e-05, Final residual = 5.68722e-12, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000135717, Final residual = 1.58931e-11, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.000315884, Final residual = 2.95918e-06, No Iterations 8
DICPCG: Solving for p, Initial residual = 6.17492e-06, Final residual = 9.82022e-07, No Iterations 76
DICPCG: Solving for p, Initial residual = 1.57902e-06, Final residual = 7.6317e-07, No Iterations 1
DICPCG: Solving for p, Initial residual = 7.70554e-07, Final residual = 7.70554e-07, No Iterations 0
time step continuity errors : sum local = 8.20779e-15, global = 9.71976e-16, cumulative = -1.0702e-10
DICPCG: Solving for p, Initial residual = 7.8575e-07, Final residual = 7.8575e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 7.8575e-07, Final residual = 7.8575e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 7.8575e-07, Final residual = 7.8575e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 7.8575e-07, Final residual = 7.8575e-07, No Iterations 0
time step continuity errors : sum local = 8.36965e-15, global = 9.71832e-16, cumulative = -1.07019e-10
ExecutionTime = 25811 s ClockTime = 77468 s
forces output:
forces(pressure, viscous)((3.37494 -0.0119856 9.47646e-20) (0.365774 -2.56513e-05 -1.76846e-22))
moment(pressure, viscous)((3.43041e-19 -8.5271e-18 -0.772271) (-8.87594e-22 1.48385e-20 0.0157568))
forceCoeffs output:
Cd = 152.682
Cl = -0.490257
Cm = -1.91245e-17
Time = 2.4172
Courant Number mean: 0.000625962 max: 0.0181193
DILUPBiCG: Solving for Ux, Initial residual = 8.58281e-05, Final residual = 5.68692e-12, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000135718, Final residual = 1.58931e-11, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.000308469, Final residual = 2.89403e-06, No Iterations 6
DICPCG: Solving for p, Initial residual = 5.61796e-06, Final residual = 9.72727e-07, No Iterations 52
DICPCG: Solving for p, Initial residual = 1.42823e-06, Final residual = 6.66354e-07, No Iterations 1
DICPCG: Solving for p, Initial residual = 6.7216e-07, Final residual = 6.7216e-07, No Iterations 0
time step continuity errors : sum local = 7.1595e-15, global = 1.21748e-15, cumulative = -1.07018e-10
DICPCG: Solving for p, Initial residual = 6.85686e-07, Final residual = 6.85686e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 6.85686e-07, Final residual = 6.85686e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 6.85686e-07, Final residual = 6.85686e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 6.85686e-07, Final residual = 6.85686e-07, No Iterations 0
time step continuity errors : sum local = 7.30358e-15, global = 1.21697e-15, cumulative = -1.07017e-10
ExecutionTime = 25812.6 s ClockTime = 77472 s
forces output:
forces(pressure, viscous)((3.37343 -0.00605809 9.48554e-20) (0.365771 -2.56622e-05 -1.76854e-22))
moment(pressure, viscous)((3.41449e-19 -8.5356e-18 -0.298624) (-8.87587e-22 1.48391e-20 0.0157559))
forceCoeffs output:
Cd = 152.621
Cl = -0.248316
Cm = -1.91435e-17
Time = 2.4174
Courant Number mean: 0.000625962 max: 0.0181196
DILUPBiCG: Solving for Ux, Initial residual = 8.58283e-05, Final residual = 5.68666e-12, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000135718, Final residual = 1.58931e-11, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.000310674, Final residual = 2.96383e-06, No Iterations 6
DICPCG: Solving for p, Initial residual = 5.80902e-06, Final residual = 9.64652e-07, No Iterations 179
DICPCG: Solving for p, Initial residual = 1.91265e-06, Final residual = 8.68626e-07, No Iterations 1
DICPCG: Solving for p, Initial residual = 8.80374e-07, Final residual = 8.80374e-07, No Iterations 0
time step continuity errors : sum local = 9.37581e-15, global = 7.32662e-17, cumulative = -1.07017e-10
DICPCG: Solving for p, Initial residual = 8.93729e-07, Final residual = 8.93729e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 8.93729e-07, Final residual = 8.93729e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 8.93729e-07, Final residual = 8.93729e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 8.93729e-07, Final residual = 8.93729e-07, No Iterations 0
time step continuity errors : sum local = 9.51804e-15, global = 7.31233e-17, cumulative = -1.07016e-10
ExecutionTime = 25815.3 s ClockTime = 77481 s
forces output:
forces(pressure, viscous)((3.36891 -0.00403213 9.48363e-20) (0.365769 -2.56731e-05 -1.7698e-22))
moment(pressure, viscous)((3.40287e-19 -8.53412e-18 -0.136958) (-8.8798e-22 1.48492e-20 0.0157549))
forceCoeffs output:
Cd = 152.436
Cl = -0.165625

Why are the focesCoefficient still so high?
The function looks like:

type forceCoeffs;
functionObjectLibs ("libforces.so");
log yes;
outputControl timeStep;
outputInterval 1;
patches (patch7);
rhoInf 1.225;
CofR (0.08 0.055 0);// Cylinder-origin
liftDir (0 1 0);
dragDir (1 0 0);
pitchAxis (0 0.08 0);// Axis across the Cylinder
magUInf 2;// equal =Inlet_Velocity=2m/s
lRef 1;
writePrecision 3;

please could someone help me waht i did wrong? Itīs very important.

I also did the Simulation with simpleFoam but nothing change!
I would be grateful if anyone has a step by step guide on how i should model this problem, please note I'm not an expert on Openfoam, as I've only just started using it recently.

ronaldo September 10, 2009 09:35

Foreces Coefficient
 
Please could someone tell me what i can do to get a reasonable Value for Drag and Lift. (Look at the Thread from above).

Thanks

Don456 September 10, 2009 15:14

Hi,

you should add

lRef 118; // reference length
Aref 48; // reference area

These two values are needed to calculate both coefficients. You can also look the formula up to see that.

You simple have to adjust these two values for your case.
The length should be clear.
The reference area is mostly the projected area.

ronaldo September 11, 2009 03:54

118 and 48 ! how to find this valueplease?

Thanks

walex September 14, 2009 07:11

hi,

usually your reference length is the diameter of your cylinder. The reference area is the girthed area of the cylinder. In 2D you have to take the mesh extrusion in z direction as cylinder length I think. Have a look in the "points" file if you dont know the mesh extrusion. Or check the mesh via the command "checkMesh".

Did you also check if the forces are are calculated correctly? Because I am having a problem that my lift and drag values are wrong for a 2D airfoil simulation (absolute values, NO coefficients!). This of course leads to wrong force coefficient results.

Regards
Alex


All times are GMT -4. The time now is 15:30.