  Asymptotic multiple scale homogenisation allows to determine the effective
behaviour of a porous medium by starting from the pore-scale description, when
there is a large separation between the pore-scale and the macroscopic scale.
When the scale ratio is "small but not too small," the standard approach based
on first-order homogenisation may break down since additional terms need to be
taken into account in order to obtain an accurate picture of the overall
response of the medium. The effect of low scale separation can be obtained by
exploiting higher order equations in the asymptotic homogenisation procedure.
The aim of the present study is to investigate higher-order terms up to the
third order of the advective-diffusive model to describe advection-diffusion in
a macroscopically homogeneous porous medium at low scale separation. The main
result of the study is that the low separation of scales induces dispersion
effects. In particular, the second-order model is similar to the most currently
used phenomenological model of dispersion: it is characacterized by a
dispersion tensor which can be decomposed into a purely diffusive component and
a mechanical dispersion part, whilst this property is not verifed in the
homogenised dispersion model (obtained at higher P\'eclet number). The
third-order description contains second and third concentration gradient terms,
with a fourth order tensor of diffusion and with a third-order and an
additional second-order tensors of dispersion. The analysis of the macroscopic
fluxes shows that the second and the third order macroscopic fluxes are
distinct from the volume means of the corresponding local fluxes and allows to
determine expressions of the non-local effects.
