Geoscience Reference
In-Depth Information
U (y) = flow velocity (m s 1 )
parallel to substrate
(a)
U (y)
Edge of boundary layer
U (y)
Laminar Transitional Turbulent
Fig. 1.6 (a) The nature of flow regimes
within a fluid medium. During laminar
flow, the flow streamlines run parallel to
the substrate, flow velocity is low and
viscosity high. Under turbulent flow, the
streamlines move in a series of random
eddies, flow velocity is high and viscosity
low. Intermediate flow is described as
being transitional. (Adapted from Allen
1985.) (b) The relationship between
flow regime and sediment bedform
development. As flow velocity increases
from lower to upper flow, the amount and
size of sediment that can be entrained
and transported increase, leading to a
change in sediment bedform structure.
(Adapted from Selley 1994.)
(b)
Antidunes
Plane bed
Froude number > 1
Froude number < 1
Dunes
Ripples
Increasing flow velocity
fluid viscosity, and is determined by the follow-
ing equation:
where U is the average current velocity, g is
acceleration due to gravity and D is the depth of
the channel. As flow velocity increases the Froude
number approaches 1, a value that separates
lower (
R
=
Udp /
μ
1) flow
regimes (Allen 1985). Flow velocities increase
from lower to upper flow and associated with
this is an increase in the amount and size of
sediment that can be entrained and transported.
This, in turn, will influence the structure of the
sedimentary bedforms that develop (Fig. 1.6b
and section 1.4.1).
A number of forces act upon a sediment lying
on a substrate surface (Fig. 1.7a), and these
influence the potential for entrainment. The key
force here is bed shear stress, which is related
to the velocity of flow. This represents the force
acting per unit area parallel to the bed and
exerts a fluid drag across the grain. If this drag
exceeds the frictional and gravitational forces
acting on the grain, then lift and entrainment
will occur. Sediment entrainment thresholds thus
<
1) flow regimes from higher (
>
where U is the particle velocity, d is the particle
diameter, p is the particle density and
is the fluid
viscosity. In the context of particles moving in a
fluid, a low Reynolds number (
μ
500) describes
fluid flow occurring in a laminar fashion, whereas
at high Reynolds numbers (
<
2000), fluid flow
is turbulent. Between these two values flow is
described as transitional (Allen 1985). Flow
turbulence increases both proportionally with
velocity and as bed surface roughness increases.
Another important coefficient in terms of fluid
dynamics (the Froude number) explains the ratio
between the force required to stop a moving
particle and the force of gravity. Within open
channels the Froude number ( F ) is determined as:
>
F
=
U /
gD
Search WWH ::




Custom Search