<p>The screenshot shows the so-called <em>pairing instability</em>, which is one of the most frequent instability problems in SPH computations.</p>

<p>Pairing instability is the consequence of the application of bell-shaped kernel functions with too large smoothing radii. Since polynomial kernel functions of at least third order have an infection point, particles, which are getting too close to each other, experience lower and lower repulsive forces and gradually stick together. This can be overcome by choosing a suitable smoothing radius leading to a rather optimal number of neighbors, which depends on the applied kernel function but usually is around 25 in 2D.</p>

<p>You can read about the pairing instability and other issues of SPH simulations <a href="https://arxiv.org/abs/1111.1259" rel="nofollow noreferrer">here</a>. Pairing instability is briefly discussed on page 9.</p>
Smoothed Particle Hydrodynamics: Weird clustering of particles. Is that normal?
<p>I implemented a rather simple SPH simulation using a cubic-spline-kernel and a simple non-iterative pressure solver as described <a href="https://cg.informatik.uni-freiburg.de/publications/2014_EG_SPH_STAR.pdf" rel="nofollow noreferrer">in this PDF</a> in equation 9. I followed algorithm 1 of that paper (including gravity).</p>

<p>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 <a href="https://i.stack.imgur.com/K5WpD.png" rel="nofollow noreferrer">this screenshot</a> the particles are not evenly spread when in equilibrium, but instead arrange into small clusters of about 3 particle.</p>

<p>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.</p>