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6. Dispersivity under Partially-Saturated Conditions
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r.
Pe
1
0.8
0.6
04 0.4
0.2
0
var L
var M
var H
0 0.2 0.4 Sw 0.6 0.8 1
Figure 6.8: The relation between fraction of percolating saturated pores and
saturation shown for three networks.
ate a “fast” flowing domain, since they are saturated and also connected to
inlet boundary which imposes boundary pressure on these series of pores. The
unsaturated pore system will create a “slow” flowing domain. The residence
time of solute will be much higher in the unsaturated pore system compared
to the saturated pore system, which could cause extra dispersion. It worth
mentioning that, for a given pore network, the value of coefficient of variation
for a fixed saturation is a constant irrespective of applied pressure gradients
(different pore-water velocities). This is because, although the higher pressure
gradient increases the velocities within pore throats and also increases the
variance of velocities within different pore throats, the increase in variance is
canceled out with the increase in the average velocity, and the coefficient of
variation remains constant. This is obviously not the case when the saturation
changes, since saturation changes will change the distribution of the phase
available to flow.
6.5.2 Mobile-Immobile Model (MIM)
Under unsaturated conditions, non-equilibrium effects may exist due to preferential
flow paths. There are experimental studies which can not be properly
described via the ADE model [Beven and Young, 1988]. Under such conditions,
modified forms of ADE should be used to take into account the effects of
preferential flow or bypassing in the pore system. Drainage of pores will cause
changes in velocity distributions and magnitudes of pore scale velocities. In
particular, zones of low velocity or practically stagnant zones may be created.
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