_
6
i=1 fci
˜ri . To complete the discretization
of the convection problem, it is necessary to discretize the
time derivative in Eq. (33). To this end, we use a secondorder
accurate predictor/corrector Runge–Kutta technique to
advance the solution by one time increment _t:
 ̄W
j = Wnj
− _t
2_ξ
_
Fj+1/2
_
Wn
_
− Fj−1/2
_
Wn
__
,
Wn+1
j
= Wnj
− _t
_ξ
_
Fj+1/2
_
 ̄W
_
− Fj−1/2
_
 ̄W
__
.
For the source step, the convective terms in the compaction
model are set to zero [i.e., (∂/∂ξ )f(w) = 0 in Eq. (1)],
resulting in the following autonomous system of ODEs:
dw
dt
= g(w). (34)