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Old   July 1, 2019, 15:55
Cool Forces & Moments
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Hi foamers,

I have a propeller rotating in water using the interFoam solver and I want to calculate the forces 💪🏻it creates for pushing the propeller “forward” and the moment it needs to rotate the propeller. I have added the respective code for “forces” in the controlDict.
Which density shall I use for rhoInf? 1000 for the density of water in kg/m3?
Is it correct to use rhoInf at all?

The numbers I get seem very small, in particular compared to the numbers one gets using paraView. See e.g. : https://m.youtube.com/watch?v=3fnnIhsoPkw

Any help? 😀
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Old   July 3, 2019, 09:26
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Hello,

I am using 'forces' to induce my tidal turbine rotation. I also use rhoinf (it depend of the openfoam version). For me, it is good to use it this way, but as long as I don't have any experimental data to validate my calculation, I can't assure you that it is good.

My values are relatively realistic anyway.
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Old   July 3, 2019, 16:44
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Hi are the lines of my controlDict:
functions
{
forces
{
type forces;
libs ("libforces.so");
patches ("propeller");
rhoInf 1000;//?Water?
rho rhoInf;
log on;
writeControl timeStep;
writeInterval 1;
dragDir (0 1 0); // The propeller is providing propulsion the y axis direction
CofR (0 0 0);
pitchAxis (0 1 0); //The propeller is turning around the y axis
}
}


I have now tested different values of rhoInf namely 1 and 1000:
Here are the results of rhoInf being 1:
forces: 0.01 (-1.234811e-01 2.370447e+01 3.877202e+00) (-1.234825e-01 2.370454e+01 3.877200e+00) (1.356078e-06 -6.690415e-05 2.919466e-06)
moments:0.01 (3.335276e-02 -8.774596e-01 4.002858e-02) (3.335276e-02 -8.774525e-01 4.002867e-02) (-2.840285e-09 -7.104506e-06 -9.874402e-08)

The bold values are the ones I am looking for.


With the paraview method and taking out the factor of 1000 mentioned therein I get:
paraView: y-Force: 23.1802; y-Torque: 0.866438


which is not equal but at least similar ;-o



With rhoInf set to 1000 I get:
forces:
0.01 (-1.221264e-01 2.363763e+01 3.880119e+00) (-1.234825e-01 2.370454e+01 3.877200e+00) (1.356078e-03 -6.690415e-02 2.919466e-03)

moments:
0.01 (3.334992e-02 -8.845570e-01 3.992993e-02) (3.335276e-02 -8.774525e-01 4.002867e-02) (-2.840285e-06 -7.104506e-03 -9.874402e-05)


The rhoInf only changes the exponent of the viscous part (see underlined). The rest is the same. This difference in the exponent of course changes the overall sum (bold).


I understand that the force is 23 Newton and the moment is 0.88 Nm (ignoring the sign). Is that correct?

Kondorfa
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Old   July 4, 2019, 03:27
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Hello, I still not an expert, but my first guest is that your simulation is not converged.

You have to remember that forces are calculated on surfaces. To get correct results, you have to properly resolve your boundary layer. Your y+ should be close to 1.

In my simulations, solver does only converge after 1 sec.
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Old   July 4, 2019, 17:05
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Thanks a lot for your patience and your answer.


For these tests I have choosen an easy case which converges fast. E.g. the propeller rotates at only 3 turns per second.



With the numbers as below to me this has converged numerically sufficient. At least I do not think that calculating much further the numbers will significantly change. Of course the fluid pattern has not fully established. But for checking the postprocessing this is not required. The open question is, whether it is correct to conclude that the Force is approx 23 N.
And what is the correct value for rhoInf? Which value do you use?


Kondorfa


# Force
# CofR : (0.000000e+00 0.000000e+00 0.000000e+00)
#
# Time (total_x total_y total_z)
6.060606e-06 (-4.077425e+02 7.524906e+04 5.285565e+03)
1.324355e-05 (1.190529e+01 4.061184e+02 4.195220e+01)
2.173248e-05 (5.945336e+00 2.526728e+02 4.926480e+00)
3.163623e-05 (2.419251e+00 1.719977e+02 2.408273e+00)
4.366221e-05 (8.754492e-01 1.185954e+02 2.558898e+00)
5.787474e-05 (2.486640e-01 8.731274e+01 2.818835e+00)
7.56404e-05 (-1.352938e-01 6.488167e+01 3.107598e+00)
9.6367e-05 (-3.374381e-01 5.264152e+01 3.290784e+00)
0.0001222752 (-4.737659e-01 4.317800e+01 3.450796e+00)
0.0001481835 (-5.337829e-01 3.993208e+01 3.511931e+00)
0.0001740917 (-5.857782e-01 3.748195e+01 3.579293e+00)
0.0002 (-6.231863e-01 3.560211e+01 3.645510e+00)
0.0002285714 (-6.490337e-01 3.336389e+01 3.693921e+00)
0.0002628571 (-6.446935e-01 3.118492e+01 3.775217e+00)
0.0002971429 (-6.442700e-01 3.045729e+01 3.817263e+00)
0.0003314286 (-6.421199e-01 2.989934e+01 3.866675e+00)
0.0003657143 (-6.188839e-01 2.948086e+01 3.893896e+00)
0.0004 (-5.949076e-01 2.913487e+01 3.921160e+00)
0.00044 (-5.361459e-01 2.833287e+01 3.943388e+00)
0.00048 (-5.052831e-01 2.809759e+01 3.970321e+00)
0.00052 (-4.747167e-01 2.798270e+01 3.985532e+00)
0.00056 (-4.368310e-01 2.780784e+01 4.009292e+00)
0.0006 (-3.982067e-01 2.765959e+01 3.998167e+00)
0.00064 (-3.825901e-01 2.751498e+01 4.032529e+00)
0.00068 (-3.630563e-01 2.742073e+01 4.041836e+00)
0.00072 (-3.198662e-01 2.723692e+01 4.051225e+00)
0.00076 (-2.816432e-01 2.715020e+01 4.122869e+00)
0.0008 (-2.772580e-01 2.703926e+01 4.028668e+00)
0.00084 (-2.652958e-01 2.690881e+01 4.039623e+00)
0.00088 (-2.302857e-01 2.679670e+01 4.068615e+00)
0.00092 (-2.241774e-01 2.667885e+01 4.016153e+00)
0.00096 (-2.241812e-01 2.660902e+01 4.002555e+00)
0.001 (-2.130352e-01 2.651556e+01 3.990783e+00)
0.00104 (-2.118037e-01 2.645224e+01 3.987552e+00)
0.00108 (-1.931005e-01 2.637711e+01 3.990010e+00)
0.00112 (-1.607454e-01 2.628973e+01 4.000401e+00)
0.00116 (-1.686689e-01 2.617561e+01 3.973989e+00)
0.0012 (-1.940931e-01 2.609576e+01 3.958251e+00)
0.00124 (-1.997818e-01 2.604007e+01 3.946675e+00)
0.00128 (-1.760962e-01 2.594077e+01 3.955421e+00)
0.00132 (-1.428295e-01 2.594016e+01 3.976938e+00)
0.00136 (-1.447842e-01 2.581453e+01 3.962098e+00)
0.0014 (-1.598393e-01 2.577422e+01 3.955240e+00)
0.00144 (-1.587930e-01 2.571177e+01 3.949451e+00)
0.00148 (-1.654464e-01 2.564786e+01 3.942436e+00)
0.00152 (-1.668325e-01 2.561404e+01 3.939127e+00)
0.00156 (-1.674913e-01 2.554379e+01 3.942435e+00)
0.0016 (-1.709034e-01 2.553740e+01 3.942493e+00)
0.00164 (-1.708857e-01 2.545446e+01 3.949876e+00)
0.00168 (-1.780076e-01 2.545701e+01 3.956893e+00)
0.00172 (-1.757443e-01 2.536630e+01 3.964563e+00)
0.00176 (-1.836860e-01 2.534140e+01 3.966867e+00)
0.0018 (-1.811174e-01 2.528315e+01 3.965668e+00)
0.00184 (-1.806363e-01 2.524554e+01 3.957817e+00)
0.00188 (-1.799537e-01 2.520789e+01 3.945742e+00)
0.00192 (-1.915971e-01 2.517754e+01 3.940630e+00)
0.00196 (-1.917564e-01 2.514050e+01 3.944114e+00)
0.002 (-1.938516e-01 2.510923e+01 3.954313e+00)
0.00204 (-1.869400e-01 2.505577e+01 3.967206e+00)
0.00208 (-1.852314e-01 2.505199e+01 3.975518e+00)
0.00212 (-1.911837e-01 2.504542e+01 3.999075e+00)
0.00216 (-1.833012e-01 2.500226e+01 3.977558e+00)
0.0022 (-1.651341e-01 2.495407e+01 3.963916e+00)
0.00224 (-1.774518e-01 2.498266e+01 3.990832e+00)
0.00228 (-1.623631e-01 2.494639e+01 4.007639e+00)
0.00232 (-1.654308e-01 2.490911e+01 3.993987e+00)
0.00236 (-1.639255e-01 2.489581e+01 3.964766e+00)
0.0024 (-1.543307e-01 2.486883e+01 3.971417e+00)
0.00244 (-1.521774e-01 2.481025e+01 3.978274e+00)
0.00248 (-1.422025e-01 2.487971e+01 4.003518e+00)
0.00252 (-1.330081e-01 2.480264e+01 3.986804e+00)
0.00256 (-1.317307e-01 2.482035e+01 3.953470e+00)
0.0026 (-1.292866e-01 2.477212e+01 3.950677e+00)
0.00264 (-1.221567e-01 2.477410e+01 3.954296e+00)
0.00268 (-1.190829e-01 2.475191e+01 3.958527e+00)
0.00272 (-1.155622e-01 2.476640e+01 3.960943e+00)
0.00276 (-1.101825e-01 2.473404e+01 3.962193e+00)
0.0028 (-7.511757e-02 2.473280e+01 3.943951e+00)
0.00284 (-1.051571e-01 2.472159e+01 3.945200e+00)
0.00288 (-9.458637e-02 2.466891e+01 3.940822e+00)
0.00292 (-9.468064e-02 2.467261e+01 3.936765e+00)
0.00296 (-1.080450e-01 2.470080e+01 3.930808e+00)
0.003 (-8.066478e-02 2.464647e+01 3.949196e+00)
0.00304 (-1.020765e-01 2.464177e+01 3.913811e+00)
0.00308 (-1.030523e-01 2.462232e+01 3.914364e+00)
0.00312 (-9.494711e-02 2.464476e+01 3.929620e+00)
0.00316 (-8.915453e-02 2.460373e+01 3.931839e+00)
0.0032 (-8.447564e-02 2.459823e+01 3.933259e+00)
0.00324 (-8.055588e-02 2.458816e+01 3.929759e+00)
0.00328 (-8.285644e-02 2.457368e+01 3.921854e+00)
0.00332 (-8.749936e-02 2.456023e+01 3.920900e+00)
0.00336 (-9.307550e-02 2.457179e+01 3.919106e+00)
0.0034 (-9.061347e-02 2.451458e+01 3.916361e+00)
0.00344 (-9.036654e-02 2.455208e+01 3.914261e+00)
0.00348 (-8.704535e-02 2.452846e+01 3.913956e+00)
0.00352 (-8.451089e-02 2.452898e+01 3.916603e+00)
0.00356 (-7.850596e-02 2.453115e+01 3.921673e+00)
0.0036 (-8.761673e-02 2.450545e+01 3.917618e+00)
0.00364 (-9.029589e-02 2.447802e+01 3.916744e+00)
0.00368 (-8.918187e-02 2.446552e+01 3.921540e+00)
0.00372 (-8.898744e-02 2.447200e+01 3.923910e+00)
0.00376 (-8.853120e-02 2.447066e+01 3.923882e+00)
0.0038 (-1.018015e-01 2.445901e+01 3.906192e+00)
0.00384 (-1.087505e-01 2.446004e+01 3.912024e+00)
0.00388 (-9.308557e-02 2.445500e+01 3.910021e+00)
0.00392 (-1.064410e-01 2.443863e+01 3.914424e+00)
0.00396 (-1.048726e-01 2.441762e+01 3.913825e+00)
0.004 (-1.133152e-01 2.441415e+01 3.917441e+00)
0.00404 (-1.099901e-01 2.440561e+01 3.923636e+00)
0.00408 (-1.121166e-01 2.438556e+01 3.923265e+00)
0.00412 (-1.193073e-01 2.439106e+01 3.911701e+00)
0.00416 (-1.258592e-01 2.439914e+01 3.921731e+00)
0.0042 (-1.227768e-01 2.436624e+01 3.919729e+00)
0.00424 (-1.333819e-01 2.437196e+01 3.918329e+00)
0.00428 (-1.362399e-01 2.434987e+01 3.912106e+00)
0.00432 (-1.507805e-01 2.434705e+01 3.892338e+00)
0.00436 (-1.486209e-01 2.433181e+01 3.896686e+00)
0.0044 (-1.416001e-01 2.434228e+01 3.918683e+00)
0.00444 (-1.480214e-01 2.432685e+01 3.921987e+00)
0.00448 (-1.470133e-01 2.430558e+01 3.897806e+00)
0.00452 (-1.430543e-01 2.431897e+01 3.912080e+00)
0.00456 (-1.414669e-01 2.429589e+01 3.912813e+00)
0.0046 (-1.267227e-01 2.430595e+01 3.928737e+00)
0.00464 (-1.280210e-01 2.427470e+01 3.922083e+00)
0.00468 (-1.306138e-01 2.426858e+01 3.914706e+00)
0.00472 (-1.338360e-01 2.426598e+01 3.904067e+00)
0.00476 (-1.356549e-01 2.426132e+01 3.891033e+00)
0.0048 (-1.341947e-01 2.422750e+01 3.895302e+00)
0.00484 (-1.286451e-01 2.425472e+01 3.907468e+00)
0.00488 (-1.320930e-01 2.424474e+01 3.910740e+00)
0.00492 (-1.313424e-01 2.424656e+01 3.905607e+00)
0.00496 (-1.131388e-01 2.423720e+01 3.884759e+00)
0.005 (-1.008101e-01 2.424099e+01 3.884015e+00)
0.00504 (-1.066232e-01 2.421565e+01 3.884388e+00)
0.00508 (-1.143227e-01 2.421255e+01 3.887188e+00)
0.00512 (-1.167957e-01 2.421758e+01 3.891449e+00)
0.00516 (-1.166141e-01 2.419600e+01 3.889291e+00)
0.0052 (-1.117581e-01 2.418685e+01 3.871507e+00)
0.00524 (-1.084209e-01 2.418795e+01 3.870558e+00)
0.00528 (-8.992227e-02 2.416866e+01 3.866820e+00)
0.00532 (-8.909618e-02 2.416892e+01 3.868328e+00)
0.00536 (-7.605800e-02 2.417159e+01 3.861574e+00)
0.0054 (-7.955895e-02 2.415755e+01 3.869127e+00)
0.00544 (-9.260971e-02 2.414967e+01 3.879278e+00)
0.00548 (-7.663181e-02 2.416058e+01 3.854357e+00)
0.00552 (-8.992216e-02 2.413754e+01 3.875726e+00)
0.00556 (-8.983359e-02 2.411711e+01 3.872654e+00)
0.0056 (-7.391904e-02 2.413737e+01 3.861973e+00)
0.00564 (-8.315521e-02 2.413709e+01 3.862536e+00)
0.00568 (-8.393618e-02 2.411833e+01 3.869148e+00)
0.00572 (-1.003163e-01 2.411603e+01 3.878165e+00)
0.00576 (-7.163382e-02 2.410972e+01 3.857549e+00)
0.0058 (-7.518136e-02 2.411408e+01 3.862147e+00)
0.00584 (-8.008970e-02 2.408991e+01 3.862685e+00)
0.00588 (-9.359382e-02 2.409207e+01 3.860491e+00)
0.00592 (-7.721352e-02 2.406422e+01 3.840504e+00)
0.00596 (-8.063702e-02 2.407430e+01 3.859906e+00)
0.006 (-7.813854e-02 2.407470e+01 3.845028e+00)
0.00604 (-7.771462e-02 2.406584e+01 3.841781e+00)
0.00608 (-7.778923e-02 2.406395e+01 3.839381e+00)
0.00612 (-8.322963e-02 2.407667e+01 3.826802e+00)
0.00616 (-8.407008e-02 2.404766e+01 3.827920e+00)
0.0062 (-8.698469e-02 2.404252e+01 3.830513e+00)
0.00624 (-9.364467e-02 2.405198e+01 3.850631e+00)
0.00628 (-8.507066e-02 2.403659e+01 3.838259e+00)
0.00632 (-8.488607e-02 2.402888e+01 3.842360e+00)
0.00636 (-8.397207e-02 2.402581e+01 3.857393e+00)
0.0064 (-9.053761e-02 2.402425e+01 3.837348e+00)
0.00644 (-1.156974e-01 2.402850e+01 3.827407e+00)
0.00648 (-1.101128e-01 2.402986e+01 3.844042e+00)
0.00652 (-1.129035e-01 2.400624e+01 3.843106e+00)
0.00656 (-1.169807e-01 2.399803e+01 3.843785e+00)
0.0066 (-1.442131e-01 2.399746e+01 3.860903e+00)
0.00664 (-1.263953e-01 2.400322e+01 3.857590e+00)
0.00668 (-1.294871e-01 2.399432e+01 3.854584e+00)
0.00672 (-1.282440e-01 2.397953e+01 3.853498e+00)
0.00676 (-1.470743e-01 2.399265e+01 3.857444e+00)
0.0068 (-1.477289e-01 2.398369e+01 3.852508e+00)
0.00684 (-1.233699e-01 2.398673e+01 3.851057e+00)
0.00688 (-1.273551e-01 2.397745e+01 3.852725e+00)
0.00692 (-1.430219e-01 2.396849e+01 3.878722e+00)
0.00696 (-1.324328e-01 2.398042e+01 3.862631e+00)
0.007 (-1.353191e-01 2.396032e+01 3.862677e+00)
0.00704 (-1.438789e-01 2.395181e+01 3.869170e+00)
0.00708 (-1.324386e-01 2.394131e+01 3.856840e+00)
0.00712 (-1.273769e-01 2.395864e+01 3.856976e+00)
0.00716 (-1.338045e-01 2.393075e+01 3.861292e+00)
0.0072 (-1.319358e-01 2.393638e+01 3.861332e+00)
0.00724 (-1.294395e-01 2.392787e+01 3.859419e+00)
0.00728 (-1.264272e-01 2.394455e+01 3.846619e+00)
0.00732 (-1.289554e-01 2.392585e+01 3.849966e+00)
0.00736 (-1.342763e-01 2.390573e+01 3.866554e+00)
0.0074 (-1.327289e-01 2.390547e+01 3.863578e+00)
0.00744 (-1.312816e-01 2.391442e+01 3.861178e+00)
0.00748 (-1.329248e-01 2.390608e+01 3.860791e+00)
0.00752 (-1.293662e-01 2.388736e+01 3.854104e+00)
0.00756 (-1.331283e-01 2.388268e+01 3.851313e+00)
0.0076 (-1.342113e-01 2.387933e+01 3.849304e+00)
0.00764 (-1.354190e-01 2.387557e+01 3.848005e+00)
0.00768 (-1.369932e-01 2.387165e+01 3.846318e+00)
0.00772 (-1.381389e-01 2.386352e+01 3.848392e+00)
0.00776 (-1.400545e-01 2.386256e+01 3.851066e+00)
0.0078 (-1.411138e-01 2.385553e+01 3.852302e+00)
0.00784 (-1.474722e-01 2.385618e+01 3.852019e+00)
0.00788 (-1.447774e-01 2.384228e+01 3.850815e+00)
0.00792 (-1.356541e-01 2.383065e+01 3.841898e+00)
0.00796 (-1.352089e-01 2.384194e+01 3.842502e+00)
0.008 (-1.388362e-01 2.383750e+01 3.845510e+00)
0.00804 (-1.427924e-01 2.381232e+01 3.844155e+00)
0.00808 (-1.378481e-01 2.382821e+01 3.843120e+00)
0.00812 (-1.378572e-01 2.381515e+01 3.841374e+00)
0.00816 (-1.322042e-01 2.380314e+01 3.833544e+00)
0.0082 (-1.328302e-01 2.381220e+01 3.833128e+00)
0.00824 (-1.297354e-01 2.380732e+01 3.857285e+00)
0.00828 (-1.254939e-01 2.379503e+01 3.838851e+00)
0.00832 (-1.269557e-01 2.378459e+01 3.838238e+00)
0.00836 (-1.293186e-01 2.379155e+01 3.846500e+00)
0.0084 (-1.297030e-01 2.379313e+01 3.848641e+00)
0.00844 (-1.313961e-01 2.378024e+01 3.852360e+00)
0.00848 (-1.308630e-01 2.377760e+01 3.843722e+00)
0.00852 (-1.256887e-01 2.377922e+01 3.870292e+00)
0.00856 (-1.263322e-01 2.377265e+01 3.867007e+00)
0.0086 (-1.275931e-01 2.376736e+01 3.853638e+00)
0.00864 (-1.205795e-01 2.377373e+01 3.870463e+00)
0.00868 (-1.182204e-01 2.376555e+01 3.872188e+00)
0.00872 (-1.145675e-01 2.375258e+01 3.872378e+00)
0.00876 (-1.182746e-01 2.375416e+01 3.872576e+00)
0.0088 (-1.358603e-01 2.374172e+01 3.884418e+00)
0.00884 (-1.182884e-01 2.374603e+01 3.865791e+00)
0.00888 (-1.187939e-01 2.373813e+01 3.868224e+00)
0.00892 (-1.255328e-01 2.374690e+01 3.872231e+00)
0.00896 (-1.118773e-01 2.372498e+01 3.863692e+00)
0.009 (-1.198626e-01 2.373759e+01 3.869109e+00)
0.00904 (-1.204947e-01 2.371791e+01 3.869637e+00)
0.00908 (-1.294135e-01 2.372220e+01 3.863224e+00)
0.00912 (-1.108551e-01 2.371619e+01 3.860775e+00)
0.00916 (-1.131630e-01 2.370988e+01 3.863286e+00)
0.0092 (-1.194906e-01 2.371328e+01 3.867689e+00)
0.00924 (-1.137930e-01 2.370827e+01 3.864445e+00)
0.00928 (-1.072844e-01 2.370776e+01 3.880201e+00)
0.00932 (-1.093590e-01 2.369348e+01 3.878338e+00)
0.00936 (-1.341570e-01 2.369239e+01 3.875176e+00)
0.0094 (-1.248711e-01 2.370411e+01 3.877488e+00)
0.00944 (-1.242245e-01 2.368577e+01 3.878969e+00)
0.00948 (-1.208227e-01 2.368357e+01 3.887700e+00)
0.00952 (-1.190785e-01 2.368104e+01 3.887420e+00)
0.00956 (-1.173159e-01 2.367626e+01 3.885619e+00)
0.0096 (-1.280203e-01 2.366827e+01 3.882641e+00)
0.00964 (-1.209613e-01 2.366540e+01 3.886750e+00)
0.00968 (-1.195285e-01 2.366517e+01 3.887946e+00)
0.00972 (-1.186802e-01 2.367897e+01 3.886969e+00)
0.00976 (-1.192829e-01 2.365918e+01 3.886349e+00)
0.0098 (-1.210357e-01 2.365141e+01 3.879632e+00)
0.00984 (-1.181038e-01 2.366471e+01 3.884000e+00)
0.00988 (-1.201178e-01 2.364193e+01 3.885397e+00)
0.00992 (-1.208503e-01 2.364411e+01 3.882169e+00)
0.00996 (-1.236625e-01 2.363550e+01 3.866223e+00)
0.01 (-1.221264e-01 2.363763e+01 3.880119e+00)
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Old   July 8, 2019, 03:15
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The solver you are using is a compressible solver (I believe) which means that the pressure term already includes density. The correct useage (according to https://www.openfoam.com/documentati...es-forces.html) therefore is to not use rhoInf. The link uses the line
Code:
rho    rho;
But I have no experience with compressible solvers, so cannot validate that for you.

As for your interpretation, yes. You have a ~23 N force in the y direction and a 0.88 Nm torque about the y axis
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Old   July 8, 2019, 16:41
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Hi yambanshee,


Thanks a lot for your confirmation about the values of the force and the moment.


Concerning the question of compressible and incompressible:
https://openfoamwiki.net/index.php/Main_OFSolvers
tells me:
interFoam Solver for 2 incompressible fluids, which tracks the interface and includes the option of mesh motion.



https://www.openfoam.com/documentati...interFoam.html
states:
interFoam v1906 (info seems uptodate!)
Solver for two incompressible, isothermal immiscible fluids using VOF phase-fraction based interface capturing.
K.
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Old   July 9, 2019, 03:56
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Quote:
Originally Posted by Kondorfa View Post
Hi yambanshee,


Thanks a lot for your confirmation about the values of the force and the moment.


Concerning the question of compressible and incompressible:
https://openfoamwiki.net/index.php/Main_OFSolvers
tells me:
interFoam Solver for 2 incompressible fluids, which tracks the interface and includes the option of mesh motion.



https://www.openfoam.com/documentati...interFoam.html
states:
interFoam v1906 (info seems uptodate!)
Solver for two incompressible, isothermal immiscible fluids using VOF phase-fraction based interface capturing.
K.
Ah. I'm not familiar with the solver. My mistake.
I made an assumption it was based on looking at the 0 folder of a tutorial example. It includes a file called p_rgh, which is different from the file p used in most incompressible solvers. Looking at the units inside of the file, it can be seen that p_rgh is indeed pressure as we know it, where as p in any other incompressible solver has units of p/rho.

This would mean that the interfoam solver already considered density in it's pressure calculation, and so the pressure component of the force does not need to be multiplied by density further, as would be the case in most incompressible solvers.

At least, this is my understanding. Like I said, I'm not familiar with interFoam at all.
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