[geth03]

hi all,

i am currently learning how to derive the force on a rotating impeller.
for that purpose i wanted to check if i can reproduce the result which is calculated with the "libforces"-lib from OF Version 7 with paraview.
unfortunately i get different results, so maybe can help.

Geometry and Simulation-setup:
the case is pretty simple.
one blade impeller, the force is only calculated on one side of the impeller.
rotor and stator, with MRF in rotor-cellzone.
SimpleFoam is used here, and convergence is reached.

OF processing:
from one tutorial i copied the forces-file:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  7
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/

forces
{
    type          forces;

    libs          ("libforces.so");

    writeControl  timeStep;
    timeInterval  1;

    log           yes;

    patches       ("force.*");
    rho           rhoInf;     // Indicates incompressible
    log           true;
    rhoInf        1;          // Redundant for incompressible

    CofR          (0 0 0);    // Rotation around centre line of propeller
    pitchAxis     (0 1 0);
}


// ************************************************************************* //

Paraview:
1. foamToVTK
2. load only patch that i want to analyze
3. Force = pressure * surfaceArea(from filter)
4. Force_Vector = Force * surfaceNormal(from filter)
5. extract x-Component.
6. Integrate Variables and divide result by total Area -> total Force in x-direction

Results for pressure derived force, only x-component:
OF: -7.876775e-05
PV: 1.79e-06
signs do not match bc PV makes the surface normal the other way round, however the magnitude should match.

note: simpleFoam is an incompressible solver, so pressure is kinematic (divided by rho). i did not correct for rho in PV bc i think OF does not do it either. so forces calcuted are in reality Force/rho. however, both values should match in the end.

i hope anybody can help in this regard. thanks.


-> Fixed: https://www.youtube.com/watch?v=rzkd61rBavg&t=1s
nice tutorial