Units in ComputeForces4
Hi,
We've been modifying DSPH4.0 so we can use the code for towing tank simulations. I've implemented a simple PID control system to adjust the forces and moments on any block in any or all DOFs. This code can print out the position/velocity/acceleration and the forces and moments for each block and DOF. To check that the dynamometer code works properly, we compare forces calculated by ComputeForces4 based on the particle ACE's with forces reported by the dynamometer. The match for Force.x between our dynamometer and ComputeForces4 is very good, almost dead on. But I can't account for the z forces reported by the ComputeForces4 utility.
To test this I ran the first few steps of CaseFloatingWavesVal, killed the process, and post-processed it with ComputeForces4. The results follow (sorry if the columns don't line up).
Part Time Np Forces.x Forces.y Forces.z Forces
0 0 1326 -2.53184E-09 0 -12990.31072 12990.31072
1 0.050017 1326 -0.001425202 0 -12974.88472 12974.88472
2 0.100022 1326 0.045110934 0 -12972.5027 12972.5027
Waves haven't yet hit the block, so we expect the x and y components to be 0, and expect the forces in the z direction should mass times gravity, so the force divided by the mass should be -9.81.
1. Are the force in this report given in newtons?
2. If so, then the force in the z direction should be:
Force.z = massbody * g = masspart*NP * g
where
NP=1326
massbody value="3.60672" units_comment="kg" />
masspart value="0.00272" units_comment="kg" />
or
g = Force.z / (masspart*NP)
Using point 2 for example, g = -12972.5/(0.00272*1326) = -3596.8 (incorrect)
If we let masspart = 1.0, then g = -12972.5/1326 = -9.78 (essentially correct)
From this I conclude that the ComputeForces4 utility calculates the total forces
on a block normalized by 1/masspart.
Is that correct? If so, is that true with the forces in the x and y direction as well?
Is it possible that the acceleration of gravity is not handled correctly in ComputeForces4
It is possible that our modifications to the code are causing the problem, but the fact that
the x forces match and the z forces do not lead us to suspect that there's a problem.
Thanks,
Richard Akers
We've been modifying DSPH4.0 so we can use the code for towing tank simulations. I've implemented a simple PID control system to adjust the forces and moments on any block in any or all DOFs. This code can print out the position/velocity/acceleration and the forces and moments for each block and DOF. To check that the dynamometer code works properly, we compare forces calculated by ComputeForces4 based on the particle ACE's with forces reported by the dynamometer. The match for Force.x between our dynamometer and ComputeForces4 is very good, almost dead on. But I can't account for the z forces reported by the ComputeForces4 utility.
To test this I ran the first few steps of CaseFloatingWavesVal, killed the process, and post-processed it with ComputeForces4. The results follow (sorry if the columns don't line up).
Part Time Np Forces.x Forces.y Forces.z Forces
0 0 1326 -2.53184E-09 0 -12990.31072 12990.31072
1 0.050017 1326 -0.001425202 0 -12974.88472 12974.88472
2 0.100022 1326 0.045110934 0 -12972.5027 12972.5027
Waves haven't yet hit the block, so we expect the x and y components to be 0, and expect the forces in the z direction should mass times gravity, so the force divided by the mass should be -9.81.
1. Are the force in this report given in newtons?
2. If so, then the force in the z direction should be:
Force.z = massbody * g = masspart*NP * g
where
NP=1326
massbody value="3.60672" units_comment="kg" />
masspart value="0.00272" units_comment="kg" />
or
g = Force.z / (masspart*NP)
Using point 2 for example, g = -12972.5/(0.00272*1326) = -3596.8 (incorrect)
If we let masspart = 1.0, then g = -12972.5/1326 = -9.78 (essentially correct)
From this I conclude that the ComputeForces4 utility calculates the total forces
on a block normalized by 1/masspart.
Is that correct? If so, is that true with the forces in the x and y direction as well?
Is it possible that the acceleration of gravity is not handled correctly in ComputeForces4
It is possible that our modifications to the code are causing the problem, but the fact that
the x forces match and the z forces do not lead us to suspect that there's a problem.
Thanks,
Richard Akers
Comments
For 3-D simualtions, units are N
Acceleration forces of the particles already includes gravity in Z-component!!!