Geoscience Reference
In-Depth Information
a
0.8
0.8
T
P
E
D
T
D
P
D
0.6
0.6
0.4
0.4
Immobile
Immobile
0.2
0.2
0
0
0
20
40
60
80
100
-60
-40
-20
0
20
40
b
0.8
0.8
0.6
0.6
0.4
0.4
Entrainment
Entrainment
0.2
0.2
0
0
0
20
40
60
80
100
-60
-40
-20
0
20
40
T
P
,E
D
T
D
,P
D
Fig. 7 Energy budget for (a) immobile and (b) entrainment threshold beds
z
/
h
becoming a constant. The most interesting feature lies on the near-bed distributions
of
P
D
in flows over entrainment threshold beds. It is apparent that the sediment
entrainment is associated with a drastic changeover of
P
D
to a large negative value
(
P
D
45). The negative value of
P
D
indicates a gain in turbulent production. It is
therefore comprehensible that in near-bed flow zone over entrainment threshold beds,
the turbulent dissipation exceeds the turbulent production and the pressure energy
diffusion becomes considerably negative indicating a sediment entrainment.
The fractional contributions
S
i
,
H
(
z
/
h
) towards the total Reynolds shear stress
production from different bursting events, for the hole-sizes
H
ΒΌ
0 and 2, are
shown in Fig.
8a, b
, respectively.
In Fig.
8a
, for immobile beds,
Q
2and
Q
4 events at the nearest point of the bed
contribute about 75% (
S
2,0
0.75) to the total Reynolds shear stress produc-
tion. On the other hand,
Q
1 events contribute moderately by 40% (
S
1,0
S
4,0
0.4), while
Q
3 events contribute minimal (
S
3,0
0.1). To be explicit, the arrival of low-speed
fluid streaks from the near-bed region is revoked by the arrival of high-speed fluid
streaks from the upper region. Thus, only a faster moving process is prevalent in the
formof outward interactions
Q
1. In contrast, for entrainment threshold beds,
Q
4 events
are the main mechanism to entrain sediments contributing about 90% (
S
4,0
0.9)
towards the Reynolds shear stress production, while
Q
2 events contribute relatively
less (
S
2,0
0.6). The tendency of
Q
4 events to dominate momentum transfer over a
sediment bed is therefore strongly dependent upon the motion of surface particles.
