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September 9, 2009, 05:13 
ForcesCoeffs

#1 
Member
toto
Join Date: Jun 2009
Posts: 71
Rep Power: 8 
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 Cl = 0.165625Courant Number mean: 0.000625962 max: 0.018118 DILUPBiCG: Solving for Ux, Initial residual = 8.58266e05, Final residual = 5.68787e12, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 0.000135717, Final residual = 1.58934e11, No Iterations 1 DICPCG: Solving for p, Initial residual = 0.000318544, Final residual = 2.92359e06, No Iterations 10 DICPCG: Solving for p, Initial residual = 6.1916e06, Final residual = 9.84286e07, No Iterations 47 DICPCG: Solving for p, Initial residual = 1.54247e06, Final residual = 7.8343e07, No Iterations 1 DICPCG: Solving for p, Initial residual = 7.89665e07, Final residual = 7.89665e07, No Iterations 0 time step continuity errors : sum local = 8.41203e15, global = 1.5953e15, cumulative = 1.07022e10 DICPCG: Solving for p, Initial residual = 8.12457e07, Final residual = 8.12457e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 8.12457e07, Final residual = 8.12457e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 8.12457e07, Final residual = 8.12457e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 8.12457e07, Final residual = 8.12457e07, No Iterations 0 time step continuity errors : sum local = 8.65482e15, global = 1.59517e15, cumulative = 1.0702e10 ExecutionTime = 25803.5 s ClockTime = 77443 s forces output: forces(pressure, viscous)((3.36771 0.00561273 9.48494e20) (0.365782 2.56185e05 1.76796e22)) moment(pressure, viscous)((3.44194e19 8.53456e18 0.263345) (8.875e22 1.48343e20 0.0157597)) forceCoeffs output: Cd = 152.388 Cl = 0.230137 Cm = 1.91412e17 Time = 2.4166 Courant Number mean: 0.000625962 max: 0.0181183 DILUPBiCG: Solving for Ux, Initial residual = 8.58272e05, Final residual = 5.68766e12, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 0.000135717, Final residual = 1.5893e11, No Iterations 1 DICPCG: Solving for p, Initial residual = 0.000307771, Final residual = 3.04239e06, No Iterations 14 DICPCG: Solving for p, Initial residual = 5.71835e06, Final residual = 9.44935e07, No Iterations 174 DICPCG: Solving for p, Initial residual = 1.83437e06, Final residual = 8.73055e07, No Iterations 1 DICPCG: Solving for p, Initial residual = 8.84112e07, Final residual = 8.84112e07, No Iterations 0 time step continuity errors : sum local = 9.41649e15, global = 2.7658e16, cumulative = 1.0702e10 DICPCG: Solving for p, Initial residual = 8.94637e07, Final residual = 8.94637e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 8.94637e07, Final residual = 8.94637e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 8.94637e07, Final residual = 8.94637e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 8.94637e07, Final residual = 8.94637e07, No Iterations 0 time step continuity errors : sum local = 9.5286e15, global = 2.7731e16, cumulative = 1.07021e10 ExecutionTime = 25806.4 s ClockTime = 77452 s forces output: forces(pressure, viscous)((3.37787 0.0101397 9.47906e20) (0.365779 2.56295e05 1.76913e22)) moment(pressure, viscous)((3.4472e19 8.52938e18 0.624579) (8.87876e22 1.48437e20 0.0157587)) forceCoeffs output: Cd = 152.802 Cl = 0.41491 Cm = 1.91296e17 Time = 2.4168 Courant Number mean: 0.000625962 max: 0.0181186 DILUPBiCG: Solving for Ux, Initial residual = 8.58271e05, Final residual = 5.68736e12, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 0.000135717, Final residual = 1.58931e11, No Iterations 1 DICPCG: Solving for p, Initial residual = 0.000308555, Final residual = 2.98446e06, No Iterations 6 DICPCG: Solving for p, Initial residual = 5.6599e06, Final residual = 9.87842e07, No Iterations 193 DICPCG: Solving for p, Initial residual = 1.97131e06, Final residual = 8.93472e07, No Iterations 1 DICPCG: Solving for p, Initial residual = 9.0792e07, Final residual = 9.0792e07, No Iterations 0 time step continuity errors : sum local = 9.67148e15, global = 1.15687e16, cumulative = 1.07021e10 DICPCG: Solving for p, Initial residual = 9.19959e07, Final residual = 9.19959e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 9.19959e07, Final residual = 9.19959e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 9.19959e07, Final residual = 9.19959e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 9.19959e07, Final residual = 9.19959e07, No Iterations 0 time step continuity errors : sum local = 9.79972e15, global = 1.14472e16, cumulative = 1.07021e10 ExecutionTime = 25809.3 s ClockTime = 77462 s forces output: forces(pressure, viscous)((3.36459 0.00234243 9.49148e20) (0.365777 2.56403e05 1.76832e22)) moment(pressure, viscous)((3.41853e19 8.54044e18 0.00215479) (8.87561e22 1.48373e20 0.0157578)) forceCoeffs output: Cd = 152.26 Cl = 0.0966561 Cm = 1.91544e17 Time = 2.417 Courant Number mean: 0.000625962 max: 0.018119 DILUPBiCG: Solving for Ux, Initial residual = 8.58282e05, Final residual = 5.68722e12, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 0.000135717, Final residual = 1.58931e11, No Iterations 1 DICPCG: Solving for p, Initial residual = 0.000315884, Final residual = 2.95918e06, No Iterations 8 DICPCG: Solving for p, Initial residual = 6.17492e06, Final residual = 9.82022e07, No Iterations 76 DICPCG: Solving for p, Initial residual = 1.57902e06, Final residual = 7.6317e07, No Iterations 1 DICPCG: Solving for p, Initial residual = 7.70554e07, Final residual = 7.70554e07, No Iterations 0 time step continuity errors : sum local = 8.20779e15, global = 9.71976e16, cumulative = 1.0702e10 DICPCG: Solving for p, Initial residual = 7.8575e07, Final residual = 7.8575e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 7.8575e07, Final residual = 7.8575e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 7.8575e07, Final residual = 7.8575e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 7.8575e07, Final residual = 7.8575e07, No Iterations 0 time step continuity errors : sum local = 8.36965e15, global = 9.71832e16, cumulative = 1.07019e10 ExecutionTime = 25811 s ClockTime = 77468 s forces output: forces(pressure, viscous)((3.37494 0.0119856 9.47646e20) (0.365774 2.56513e05 1.76846e22)) moment(pressure, viscous)((3.43041e19 8.5271e18 0.772271) (8.87594e22 1.48385e20 0.0157568)) forceCoeffs output: Cd = 152.682 Cl = 0.490257 Cm = 1.91245e17 Time = 2.4172 Courant Number mean: 0.000625962 max: 0.0181193 DILUPBiCG: Solving for Ux, Initial residual = 8.58281e05, Final residual = 5.68692e12, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 0.000135718, Final residual = 1.58931e11, No Iterations 1 DICPCG: Solving for p, Initial residual = 0.000308469, Final residual = 2.89403e06, No Iterations 6 DICPCG: Solving for p, Initial residual = 5.61796e06, Final residual = 9.72727e07, No Iterations 52 DICPCG: Solving for p, Initial residual = 1.42823e06, Final residual = 6.66354e07, No Iterations 1 DICPCG: Solving for p, Initial residual = 6.7216e07, Final residual = 6.7216e07, No Iterations 0 time step continuity errors : sum local = 7.1595e15, global = 1.21748e15, cumulative = 1.07018e10 DICPCG: Solving for p, Initial residual = 6.85686e07, Final residual = 6.85686e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 6.85686e07, Final residual = 6.85686e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 6.85686e07, Final residual = 6.85686e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 6.85686e07, Final residual = 6.85686e07, No Iterations 0 time step continuity errors : sum local = 7.30358e15, global = 1.21697e15, cumulative = 1.07017e10 ExecutionTime = 25812.6 s ClockTime = 77472 s forces output: forces(pressure, viscous)((3.37343 0.00605809 9.48554e20) (0.365771 2.56622e05 1.76854e22)) moment(pressure, viscous)((3.41449e19 8.5356e18 0.298624) (8.87587e22 1.48391e20 0.0157559)) forceCoeffs output: Cd = 152.621 Cl = 0.248316 Cm = 1.91435e17 Time = 2.4174 Courant Number mean: 0.000625962 max: 0.0181196 DILUPBiCG: Solving for Ux, Initial residual = 8.58283e05, Final residual = 5.68666e12, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 0.000135718, Final residual = 1.58931e11, No Iterations 1 DICPCG: Solving for p, Initial residual = 0.000310674, Final residual = 2.96383e06, No Iterations 6 DICPCG: Solving for p, Initial residual = 5.80902e06, Final residual = 9.64652e07, No Iterations 179 DICPCG: Solving for p, Initial residual = 1.91265e06, Final residual = 8.68626e07, No Iterations 1 DICPCG: Solving for p, Initial residual = 8.80374e07, Final residual = 8.80374e07, No Iterations 0 time step continuity errors : sum local = 9.37581e15, global = 7.32662e17, cumulative = 1.07017e10 DICPCG: Solving for p, Initial residual = 8.93729e07, Final residual = 8.93729e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 8.93729e07, Final residual = 8.93729e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 8.93729e07, Final residual = 8.93729e07, No Iterations 0 DICPCG: Solving for p, Initial residual = 8.93729e07, Final residual = 8.93729e07, No Iterations 0 time step continuity errors : sum local = 9.51804e15, global = 7.31233e17, cumulative = 1.07016e10 ExecutionTime = 25815.3 s ClockTime = 77481 s forces output: forces(pressure, viscous)((3.36891 0.00403213 9.48363e20) (0.365769 2.56731e05 1.7698e22)) moment(pressure, viscous)((3.40287e19 8.53412e18 0.136958) (8.8798e22 1.48492e20 0.0157549)) forceCoeffs output: Cd = 152.436 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);// Cylinderorigin 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. 

September 10, 2009, 09:35 
Foreces Coefficient

#2 
Member
toto
Join Date: Jun 2009
Posts: 71
Rep Power: 8 
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 

September 10, 2009, 15:14 

#3 
New Member
Felix Apel
Join Date: Sep 2009
Posts: 15
Rep Power: 8 
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. 

September 11, 2009, 03:54 

#4 
Member
toto
Join Date: Jun 2009
Posts: 71
Rep Power: 8 
118 and 48 ! how to find this valueplease?
Thanks 

September 14, 2009, 07:11 

#5 
New Member
Join Date: Apr 2009
Posts: 17
Rep Power: 9 
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 

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