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Offset on the force derived for each cylinder composing a float

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Old   October 14, 2020, 03:29
Default Offset on the force derived for each cylinder composing a float
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Constance Clément
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Hi everyone,

I work on the modelling of a fixed float in waves composed by several cylinders. The simulation is runned with waveFoam to propagate waves (based on interFoam to solve the interface). The mesh is obtained with snappyHexMesh tool. I thus get a patch for each cylinder called “cylinder_i” and a patch called “float” corresponding to the sum of every patch “cylinder_i”. To determine the force on a given patch I add to controlDict:

Code:
float
        {
             type       forces;
             libs       ("libforces.so");
             patches         ("float");
             rhoInf          1000;
             timeStart 0;
             log             on;
             writeControl    adjustableRunTime;
             writeInterval   0.01;
             CofR            (0 0   0);
        }
Cylinder_1
        {
             type       forces;
             libs       ("libforces.so");
             patches         ("Cylinder_1");
             rhoInf          1000;
             timeStart 0;
             log             on;
             writeControl    adjustableRunTime;
             writeInterval   0.01;
             CofR            (0 0   0);
        }
The problem is: the force on the cylinder has an offset. More precisely, the force derived for cylinder_1 oscillates around 75kN whereas the force for cylinder_2 oscillates around -64N. I do not understand this offset/shift. The force is derived integrating the pressure field around the cylinder and should oscillates around 0. (When I model only the cylinder_1 in waves, I get a force of the same amplitude but oscillating around 0).
What’s also surprising is that when I sum all the forces shifted derived for the cylinders, I get a total force oscillating around 0 and identical to the force derived with the patch “float”. Thus the offsets are compensated when they are summed. I do not understand this shift, it seems that it is due to the presence of the several cylinders connected. But with interFoam solver, no structural forces are modelled.

Have you encountered the same issue when post-treating the force on a specific part of a float ?

Thanks a lot,

Constance
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Tags
cylinder, force, interfoam, libforces.so, wavefoam

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