Geoscience Reference
In-Depth Information
Fig. 2 Side view of the particle trajectories for channel without bifurcation: (a) as bed-load
transport and (b) as suspended transport
particles are spread over the lowest 0.10 m of the cross section at the inlet. Particles
fall onto the bottom, saltate, get into suspension again by the turbulence, and are
transported further. However, the plot shows that the particles all stay in the near-
bed region. The fall velocity is apparently too high for the particles to get into full
suspension. Fig.
2b
shows particle trajectories for suspended load (
D
1
¼
0.01 mm)
which are spread uniformly over the channel cross section at the inlet. The crossing
lines represent particle trackings at different
y
-coordinates. Some particles go up
and others go down. The driving force on that motion is turbulence.
For both particle diameters, the concentration profiles of sediment at the inlet
and the outlet are the same or nearly the same, which implies that those diameters
represent well bed-load and suspended transport.
3.1 Bifurcation 90
: Runs 2a/b (Q
1
/Q
2
¼
0.2)
and 2c/d (Q
1
/Q
2
¼
0.5)
The direction of the fluid motion in the bifurcation is shown by streaklines in Fig.
3
.
Streaklines visualize flow patterns by drawing continuous lines joining all fluid
particles which originate from the same point in the fluid. Figure
3a
shows streak-
lines that originate 0.01 m above the bottom at the inlet, and are released 1.93 m
above the bottom (i.e., 0.07 m under the surface) for discharge
Q
1
/
Q
2
¼
0.2.
The fluid motion into the branch is different for both layers. At the bottom, eight
streaklines are banded into the branch, whereas at the surface only four. This is
caused by the difference in flow velocity between the bed region and the surface
region. The driving force on the fluid motion into the branch, caused by the negative
pressure gradient, bends the fluid particles at the bottom more easily into the branch
because of their lower momentum (Fig.
4
).
A more detailed analysis of the shape of the streaklines in the branch shows that
the fluid moves up from the bottom in the left part of the channel, and down from
the surface in the right part of the channel, which indicates a spiral motion of the
fluid in the branch.
The behavior of the turbulence in the bifurcation region represented by kinetic
energy of turbulence differs in the lowest and the highest cell layer (on the bottom
