Dispersion dans un écoulement de Stokes

We approach the geometric dispersion effects in a granular porous medium at large Peclet numbers by first considering the distribution of residence times around a single sphere in a uniform applied velocity field U. We get a logarithmic singularity of the dispersion in U Log U which is due to the slow flow near the stagnation points of the flow field. This feature is independent of the flow structure at large distances from the stagnation points. Thus the result can be applied to a random dilute array of spheres (fixed bed) On decrit des effets singuliers de la dispersion geometrique dans un milieu poreux granulaire aux grands nombres de Peclet