Geoscience Reference
In-Depth Information
a
0.8
0.8
Immobile
H
= 0
Entrainment
H
= 0
0.6
0.6
0.4
0.4
Q
1
Q
2
Q
3
Q
4
0.2
0.2
0
0
-0.5
0
0.5
1
1.5
-0.5
0
0.5
1
1.5
S
i
,0
S
i
,0
b
0.8
0.8
Immobile
H
= 2
Entrainment
H
= 2
0.6
0.6
0.4
0.4
0.2
0.2
0
0
-0.5
0
0.5
1
-0.5
0
0.5
1
S
i
,2
S
i
,2
Fig. 8 Distributions of (a)
S
i
,0
(
z
/
h
) and (b)
S
i
,2
(
z
/
h
)
It means the sediment motion is governed by the arrival of high-speed fluid streaks. But
the contributions from
Q
1and
Q
3 events are feeble (
S
1,0
0.2).
Examining the Reynolds shear stress contributions from more extreme events occur-
ring for hole-size
H
0.3 and
S
3,0
2giveninFig.
8b
, there remains a consensus that similar
characteristics of most energetic
Q
2and
Q
4 events prevail in the flow zone. Therefore,
the quadrant analysis reveals that in the near-bed flow, ejections and sweeps in
immobile beds rescind each other giving rise to the outward interactions, while sweeps
are the dominant mechanism towards the sediment entrainment.
ΒΌ
8 Closure
While plenty of researches have been carried out on the entrainment threshold of
different sizes of sediments and all the investigations bring us a step nearer to an
improved understanding of the sediment entrainment phenomenon in relation to the
turbulence characteristics, there remains an inadequate attention to many cases.
