Journal of Statistical Physics

, Volume 152, Issue 5, pp 934–953 | Cite as

A Model for the Shapes of Advected Triangles

  • Alain Pumir
  • Michael Wilkinson


Three particles floating on a fluid surface define a triangle. The aim of this paper is to characterise the shape of the triangle, defined by two of its angles, as the three vertices are subject to a complex or turbulent motion. We consider a simple class of models for this process, involving a combination of a random strain of the fluid and Brownian motion of the particles. Following D.G. Kendall, we map the space of triangles to a sphere, whose equator corresponds to degenerate triangles with colinear vertices, with equilaterals at the poles. We map our model to a diffusion process on the surface of the sphere and find an exact solution for the shape distribution. Whereas the action of the random strain tends to make the shape of the triangles infinitely elongated, in the presence of a Brownian diffusion of the vertices, the model has an equilibrium distribution of shapes. We determine here exactly this shape distribution in the simple case where the increments of the strain are diffusive.


Diffusion Advection 



We acknowledge R. Guichardaz for his comments on our manuscript. A.P. has been supported by the grant from A.N.R. “TEC 2”. We acknowledge the support of the European COST Action MP0806.


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Copyright information

© Springer Science+Business Media New York 2013

Authors and Affiliations

  1. 1.Laboratoire de PhysiqueEcole Normale Supérieure de LyonLyonFrance
  2. 2.Department of Mathematics and StatisticsThe Open UniversityMilton KeynesUK

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