Longitudinal dispersion in a sandpack obtained numerically using dispersionEvaluationFoam. The asymptotic hydrodynamic dispersion varies with the Péclet number according to a power-law. The exponent is lower than Taylor–Aris dispersion but larger than a mere linear dispersion model. Below Pe ≈ 1, hydrodynamic dispersion is negligible and the value of the plateau corresponds to the sandpack tortuosity.