Force on a submerged sphere


I am working on some test case with Damien. I am simulating the case of a submerged sphere (D=3.76cm, center 5cm below the free surface in a constant stream of 0.45m/s). I created the geometry using Salome.

  1. First problem : salome encounters issue when using dynamics (with 3 layers) to mesh inward. So I created 3 spheres by hand and meshed as if it was Lennard Jones BC. I still have some leaks with this technique. I am working on it. Particles enter the sphere probably because spacing is too large (so I reduced it).
    Another solution which appears to work ok was to create the smallest sphere (inner sphere of the 3 layers) and ask salome to mesh outward. On the inner sphere I reduced the dr such that the outward sphere has the correct dr (assuming particle are just moved following the local normals).

  2. Second problem : with a mesh with no leaks, I was able to simulate the case but obtained forces about 3 times larger than expected. The expected force should be from experiments and Code Saturne simulations around 0.048N. I measured 0.12N… Any idea why the force is so much larger? I have no turbulence in the model for now.
    How is the force evaluated in GPUSPH?

The goal afterward is to add a ‘paddle’ to this sphere and let it swim with a force model following a Hill’s like law.

Thanks in advance for help,


Hello @kermit,

concerning point 1., I’m afraid I’m not sure how SALOME handles the meshing of the spheres. This sounds like an issue in that software.

For 2., the forces are computed by adding up all the fluid/boundary interactions, using the same equations which are used for the fluid motion. Over/underestimation of the forces may be related to the particle mass. Does using a higher or lower resolution change the ratio between the computed and expected forces? Can you you try to set up the problem creating a problem file by hand and see if it makes any difference?