Fingering, Unstable and Intermittent Flow Regimes in Unsaturated Soils.
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Transcript of Fingering, Unstable and Intermittent Flow Regimes in Unsaturated Soils.
Fingering, Unstable and Intermittent Fingering, Unstable and Intermittent Flow Regimes in Unsaturated SoilsFlow Regimes in Unsaturated Soils
Unstable and intermittent flows in porous media
Large body of evidence for fingering and flow instability in soils and similar porous media.
Intermittency (irregular temporal flow pattern) associated with flow instability is less studied.
Unstable and intermittent flows are not amenable for continuum representation by Richards’ equation and alike.
Although causes of such irregular flow regimes are attributed to various factors such as: heterogeneity, water repellency, layering, pressure build-up, etc., it is possible to unify these factors by diagnostic dimensionless groups leading to simple criteria.
Flow instability and fingering in porous media
Flow fingering and instability in soils and similar media has been observed for layered media (fine over coarse texture), water repellent layers, DNAPLs displacement, air pressure build-up in confined domains, etc.
Experiments by Glass et al. [1989] show remarkable persistence of fingers across several infiltration events (as long as water content remains non-uniform).
• Raats [1973] criterion for flow instability:
Criteria for flow instability in porous media
which is reduced to (h0-hL)<0
(Ks/L2(s-i) is always >0)
0)hh()(L
K
dL
duL0
is2
s
i S00
z
L
0dL
du
• Flow instability sets in whenever the pressure head at the soil surface is smaller than at the wetting front.
• Examples:• Air compression ahead of the wetting front.• Rate of rainfall smaller than Ks.• Layered soil (fine over coarse).• Certain redistribution conditions.
• Based on linear hydrodynamic stability analysis, Philip [1975] generalized the criteria for flow instability :
• Flow instability sets in whenever the pressure gradient G at the wetting front opposes the flow.
Criteria for flow instability in porous media
i S00
z
L0
L
hhG L0
• An equivalent criterion was identified by Hillel and Baker [1988] for a layered soil:
• When the hydraulic conductivity of the underlying coarse layer is larger than the average flux in the fine layer above.• Nicholl et al. [1994] provided a similar criterion for fingering in
fractured rock (G was later expanded to included capillarity):
inins qcosggk
qGK
Flow instability and fingering in a dry fracture
• An initially stable flow becomes unstable as soon as infiltration ends and redistribution with a certain air-entry value begins (see “drainage” along upper boundary).
• Sequence leading to development of flow-capturing fingers and the appearance of isolated liquid bridges.
Nicholl et al., 1994, Water Resour. Res. 30:2533-2546
• Philip’s [1975] linear hydrodynamic stability analysis that predicts also the wavelength of most unstable disturbance was tested by White et al., [1976] in a Hele-Shaw cell with reasonable success.
Tests of criteria for flow instability
• Best results were obtained for immiscible fluids in a system with sharp boundaries (–type). Experiments in porous media resulted in only qualitative agreement.
Flow intermittency in porous media (sand)
• Prazak et al. studied intermittency (temporally irregular flow pattern) in sand columns subject to various initial conditions (wet & dry medium).
• The resulting intermittent flux was associated with relatively large influence of gravitational forces with Bo in the range of 0.03 to 0.01.
Prazak et al. 1992 (Water Resour. Res. 28:1849-1855)
Rivulet flow in structured porous media (aggregates)
• Dexter 1993 studied flow in a non-absorbing aggregate bed (made of stones of different sizes).
• Uniform flow at the surface using a spraying nozzle.
• Outflow was collected from 74 locations.
Dexter 1993 (Water Resour. Res. 29:1859-1862)
• The number of rivulets per cross-sectional area decreased with increasing depth suggesting “random walk” with rivulet “capturing”.
• Similar to droplet avalanche, the concentrated mass flow (main rivulet) leaves behind a “cleared” path.
• Flow stability in porous media is a result of interplay between gravitational, capillary, and viscous forces.
Flow instability in porous media
cos
singrBo
2
Capillary Viscous
Gra
vity
cos
vCa
Je=Bo/Ca
• Most flow instability phenomena in soils and similar porous media are induced by gravitational forces overcoming restraining capillary forces (no instability observed for upward or horizontal flow).
• Held and Illangasekare (1995) and later Berkowitz and Ewing (1998) proposed extension of Lenormand et al. (1988) stability diagram to include gravitational forces (Bo).
• The interplay between the various forces is illustrated in this empirical phase diagram.
• Higher Bond numbers act to stabilize flow regimes.
Unstable flow regimes in porous media
cos
singrBo
2
cos
vCa
M
Typical values of dimensionless groups for flow regimes in soils (Friedman, 2000)
• Prazak et al. [1992], Su et al. [1999] and other experimental evidence clearly show that instability in soils occurs around Bo>0.05 when gravity becomes 5% (or less!) of capillarity.
• This could provide a potentially useful criterion for limits of applicability of the Richards equation based on liquid and media properties (i.e.,for pores with radius larger than 0.6 mm).
cos
singbBo
2
cos
vCa
Bond Number - gravitational relative to capillary forces
Capillary Number – viscous relative to capillary forces
Bo~0.05 1000 larger than soils (Su et al., 1999)
Jeffery Number - gravitational relative to viscous forces
Je=Bo/CaJe~1000 with typical soils Je~1 (Su et al., 1999)
Forces in Unsaturated Fractures
Pore scale physical considerations
r2
r1• The conditions for liquid invasion from r1 to r2 with r2> r1 require the capillary head to be h1+h given by the difference between equilibrium capillary heads.
• An increase (less negative) in capillary head at the front!
21 r
1
r
1
g
2h
• At an interface between fine and coarse layers we can postulate the following local scenario for the onset of a perturbation in the wetting front:
2 hsmall
hsmall
hlarge
h2 h3h1
wetting front
3
h1>h2<h3
1
Force balance for a hemispherical perturbation
weight
h’
Capillaryforces
• Consider a local force balance for a suspended hemispherical perturbation in the wetting front (i.e., what keeps the liquid element from “falling” through the porous matrix?)
• The weight is Fg=g(2/3h3)
SummarySummarySummarySummary
Unstable flow regimes in soils are manifested in various forms from fingering, to rivulet and intermittent flow.
Onset of instability is often associated with reduction in restraining capillary forces relative to gravitational ones either due to changes in pore size, wettability, or diminishing gradients.
Numerous evidence shows that for Bo>0.05 flows become unstable, and consequently the Richards equation may no longer be applicable.