How to get the force on FSI body?

[Alex Lan]

Hi, I want to get the force that SPH particles applied to a FSI body or mesh. How can I get it?

Regards.

[Luning Bakke]

Hello Alex,

PrintFsiInfoToFile() can output the position, pose and forces and torques applied to the FSI body to a file. See demo_FSI_CylinderDrop.cpp as an example where the outputs are written to the DEMO_OUTPUT/FSI_Cylinder_Drop/fsi/FSI_body0.csv file.  

Though FSI does not have an API that returns the body force and torque directly to the user yet, you can implement your own in ChSystemFSI.cpp . You can take a look at the implementation in Add_Rigid_ForceTorques_To_ChSystem(), or Add_Flex_Forces_To_ChSystem(), to see how the force/torque are extracted. 

For example, if you want the force from SPH particles applied to the i-th rigid FSI body, your implementation can be, 

ChVector<float> ChSystemFsi::GetFsiBodyForce(int i) const {
    thrust::host_vector<Real3> forcesH = m_sysFSI->fsiGeneralData->rigid_FSI_ForcesD;
    return utils::ToChVector(forcesH[i]);
}

Note that the force is expressed in global coordinate. 

Thank you,

Luning

[Alex Lan]

Hi Luning,

I want to give a fixed torque between the wheel and axle in the single wheel test demo. The wheel should be driven and then balance with the force from SPH terrain. I used ChLinkMotorRotationTorque to implement it, but the torque seem not right. The wheel kept accelerate all the time. Is there any problem here?

Best regards,

Alex