Bd. Wood et Ml. Kavvas, Ensemble-averaged equations for reactive transport in porous media under unsteady flow conditions, WATER RES R, 35(7), 1999, pp. 2053-2068
We present a method for deriving the ensemble-averaged reactive solute tran
sport equation for unsteady, non-divergence-free flow field conditions. Our
approach uses a cumulant expansion, Lie group theory, and time-ordered exp
onentials to develop the ensemble-averaged transport equation. The cumulant
expansion is in powers of a alpha tau(c), where alpha measures the magnitu
de of the perturbations of the transport and reaction operators and tau(c)
is the correlation time of these perturbations. Because the cumulant expans
ion avoids secular terms (terms in powers of time), the problem can be clos
ed by rationally truncating the expansion. The truncated terms can be shown
to be of lower order than those terms that are kept, provided that a parti
cular constraint (in terms of the Kubo number) is met. The use of Lie group
theory allows one to automatically combine the Eulerian and Lagrangian app
roaches. A particular time-ordered exponential that arises in the analysis
can be interpreted as a translation operator that possesses a well-defined
algebra. These translation operators appear in the second-order (covariance
) terms of the cumulant expansion, and their effect is to shift one of the
terms of the covariance functions relative to the other along the trajector
y formed by the ensemble-averaged velocity field. This approach does not re
quire neglecting the local dispersion tensor and has the advantage that no
integral transformations are conducted; therefore all results are expressed
in terms of real space variables.