Biomedical Engineering Reference
In-Depth Information
(a)
(b)
(c)
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
0.4
0.5
0.6
0.7
0.8
0.9
1
f
FIGURE 10.6
Calculation of tortuosity in sediments based on path-lines. (a) Velocity mag-
nitudes squared (
u
2
=
u
x
+
v
y
). Light gray boxes show randomly placed fixed
solid matrices (porosity is 0.7). (b) Streamlines calculated from the velocity
field when the flow is induced by the action of gravity. (c) Comparison of
tortuosity values calculated from our model (cross symbols with error bars),
calculated after Weissberg relation (solid line) and after Koponen et al.
Coastal sediments are often sandy with uneven surfaces, above which
the flows is induced into oscillatory motion under surface gravity waves.
Gundersen and Jørgensen (1990) measured the vertical distribution of oxygen
at the sediment-water interface. They found that the time-averaged concen-
tration was indeed nearly constant except in a thin layer immediately above
the sediment surface, where the mean concentration decreased linearly with
decreasing elevation. Within this diffusive boundary layer (DBL), which was
about 0.6 mm thick, molecular diffusion model was used to estimate the ver-
tical flux of solutes. However, comparisons with actual flux measurements
suggested that such an “empirical diffusion coecient” would have to be a
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