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I implemented a rather simple SPH simulation using a cubic-spline-kernel and a simple non-iterative pressure solver as described in this PDF in equation 9. I followed algorithm 1 of that paper (including gravity).

The resulting particle behaviour is certainly fluid-like (with quite some compressibility as is expected from such a simple pressure solver). However as you can see in this screenshot......the particles are not evenly spread when in equilibrium, but instead arrange into small clusters of about 3 particle.

Is this normal behaviour ? It appears strange to me, so I wanted to make sure this is either correct or someone would have an idea what could be wrong here.

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  • $\begingroup$ It does not look correct, since the particles are quite clearly crystallising on the plane. $\endgroup$ – lr1985 Oct 31 '19 at 8:35
  • $\begingroup$ Welcome to scicomp! To me that looks like the particles accumulate in some local energy minima. What kind of interaction do they have with each other, what kind of forces are at play? As it is, we do not have enough information to help you. $\endgroup$ – MPIchael Oct 31 '19 at 10:27
  • $\begingroup$ the algorithm 1 in the mentioned paper has all the needed details. however i think i figured out what's the problem. see answer... $\endgroup$ – Lenny Nov 1 '19 at 10:17
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After some more research this effect seems to be known as "tensile instability". So in a sense this behaviour seems to be "correct" for a naive implementation of SPH. There are explicit measures against it like here for example.

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