GenCase v 5.0.174 and calculation of moments of inertia
As we all know, moments of inertia are key design parameters for structures withstanding loads that cause rotations. In the context of naval and ocean engineering, think of ships, floating reservoirs, drilling platforms, whether subject to mutual contact or to wave slamming/sloshing.
Earlier last year, I possibly identified a limitation in the way GenCase version 4 computed the moments of inertia of floating objects. In particular, GenCase was not able to obtain the same diagonal, same-valued tensor of inertia for three identical cubes (picture at https://forums.dual.sphysics.org/uploads/305/D4L13U40FE4X.png). For the discovery process and outcomes of that investigation, please see the comment https://forums.dual.sphysics.org/discussion/comment/3215/#Comment_3215 and up there, with XML snippets and shell scripts attached.
In sum, the calculation of the moments of inertia was correct only if the ratio between the cube edge and the particle spacing is an integer number. Otherwise strange things occur – see in that post the spread of results for arbitrary particle spacing. Maintaining integer ratios between particle spacing and any dimension defining several floating objects is too restrictive for real-word applications. Therefore, if this is correct, GenCase 4 may have done well for academic test cases, but was hardly suited for real-world applications. As a consequence the same perplexity applies to DualSPHysics, since one does not really know what is going on.
I have repeated the same test with GenCase v 5.0.174 (16-04-2020) and I am afraid that the problem is still there. The cubes’ edges are 1 unit, and
- If the particle spacing is 0.125 units (1/8), the (principal) moments of inertia are the same within and across each cube (0.208333, 0.208333, 0.208333; pass).
- If the particles spacing is 0.120 units (1/8.33) two cubes have (principal) moments of inertia (0.192,0.192,0.192; pass) but the last one has (0.2148,0.192,0.2148; fail).
I understood that this issue had ended up in the to-do list. Not having heard of any progress in the meantime, it would be important to know whether you could replicate it in the first place. Would you please let me know?