Transient and asymptotic Taylor--Aris dispersion of Brownian rods in arbitrary regular-polygonal ducts

Abstract

Taylor--Aris dispersion of Brownian rods in non-circular ducts is governed by a coupling absent from passive-scalar theory. Pressure-driven shear aligns the rods and makes translational diffusion tensorial, while duct geometry determines how this tensor is sampled across the cross-section. We formulate this problem for dilute rods in regular-polygonal ducts of arbitrary side number. At each cross-sectional point, a local shear-aligned Jeffery--Brownian closure gives four transport fields, namely two transverse diffusivities, a direct axial diffusivity and a signed shear--axial cross coefficient. Because the shear frame rotates through a polygon, these fields enter a conservative two-dimensional transverse operator rather than a radial scalar-diffusion problem. Its zero mode is a non-uniform invariant density, which replaces the area measure in the Taylor--Aris reduction and reduces, in the circular-pipe limit, to a weighting proportional to the inverse shear-direction diffusivity. The resulting cell problem separates the effects of rod alignment on streamline sampling and transverse relaxation. Alignment produces only a small, non-monotone shift in mean speed, but gives a larger enhancement of the Taylor coefficient by reducing transverse mixing. Normalization by the same-geometry spherical coefficient removes most passive shape dependence and exposes the approach to the fully aligned transverse-mixing limit. Finite regular polygons converge smoothly to the circular-pipe branch, whereas low-sided polygons retain distinct shear-sampling signatures. A biorthogonal spectral formulation resolves finite-time releases. Localized, multi-peaked and broad injections excite different non-zero transverse modes and exhibit different pre-asymptotic variance growth, but modal decay selects the common long-time Taylor--Aris coefficient given by the cell problem.