Triangular Constellations in Flows

etc15 Tracking Number 419

Particles advected on the surface of a fluid can exhibit fractal clustering. The local structure of a fractal set is described by its dimension $D$, which is the exponent of a power-law relating the mass ${\cal N}$ in a ball to its radius $\varepsilon$: ${\cal N}\sim \varepsilon^D$. It is desirable to characterise the {\em shapes} of constellations of points sampling a fractal measure, as well as their masses. The simplest example is the distribution of shapes of triangles formed by triplets of points, which we investigate for fractals generated by chaotic dynamical systems. The most significant parameter describing the triangle shape is the ratio $z$ of its area to the radius of gyration squared. We show that the probability density of $z$ has a phase transition: $P(z)$ is independent of $\varepsilon$ and approximately uniform below a critical flow compressibility $\beta_{\rm c}$, which we estimate. For $\beta>\beta_{\rm c}$ the distribution appears to be described by two power laws: $P(z)\sim z^{\alpha_1}$ when $1\gg z\gg z_{\rm c}(\varepsilon)$, and $P(z)\sim z^{\alpha_2}$ when $z\ll z_{\rm c}(\varepsilon)$.