Transcript There are several equations useful in understanding hydraulics and

Ch-3. There are several equations useful
in understanding hydraulics and
sediments deposits.
Reynolds Number
 Froude Number


These numbers helps us to understand the
relationship between fluid flow, the type of
bedforms produced along the surface, and the
mechanism by which entrained particles move.
Reynolds Number
 addressed
the problem of how laminar flow
changes to turbulent flow.
 the transition from laminar to turbulent flow
occurs as velocity increases, viscosity
decrease, the roughness of the flow boundary
increases, and/or the flow becomes less
narrowly confined.
Froud Number
The Froud Number is the ratio
between fluid inertial forces and
fluid gravitational forces.
Froud Number
 It
compares the tendency of a moving fluid
(and a particle borne by that fluid) to
continue moving with the gravitational
forces that act to stop that motion.
 The force of inertia express the distance
traveled by a discrete portion of the fluid
before it comes to rest.
 Like reynolds Numbers, Froud numbers
are dimensionless.
The equation for the Froud
number is

Fr =


Fr=



Fr=
fluid inertial forces
.
gravitational forces in flow
flow velocity
.
(acceleration of gravity)x(force of inertia)
V ,
_/(gD)
The equation for the Froud
number is

Fr=
V ,

_/(gD)
V= velocity, D= depth, g= gravitational constant
The relationship among the bedforms or surface waves
(ripples and dunes) produced beneath moving currents
of wind or water, the flow streamlines within the current
itself, and surface waves developed on the upper
surface of the fluid changes with the Froud numbers; so
too does the type of the flow.
Froud number

When the Froud number is less than 1, the
velocity at which wave moves is greater than the
flow velocity, and waves can travel upstream (tranquil, streaming, or subcritical).
 When the Froud number exceeds 1, waves do
not flow upstream, (rapid, shooting, or
supercritical.
 So, Froud number of 1 represent the threshold
between tranquil and rapid flow.
 When
tranquil flow give way to rapid it
occurs in an smooth transition.
 When rapid flow suddenly decreases to a
tranquil flow, there is an abrupt change
known as Hydraulic jump (a sudden
increase in depth accompanied by much
turbulence, smoke).
Entrainment, transport, and
deposition of clasts.
Entrainment

Bernoulli’s principle- the
sum of velocity and
pressure on an object in a
flow must be constant if
the velocity increase then
the pressure must
decrease, and vice versa.
Entrainment

Two main forces are
involve in how
particles get picked
up:


Fluid drag force
Fluid lift force
How do sediments move,
once they have been lifte?

Traction- they are
rolled and dragged
along the base of a
moving fluid.


Saltation- they
abruptly leave the
bottom and are
temporarily
suspended.
Traction and
saltation load
together constitute
bedload.
How do sediments move,
once they have been lifte?


Supension
constitues a third
mode of
transportat.
It concist of those
grains that more or
less continually
float within the
moving fluid.


Clast size has an
important effect on
sediment
entrainment,
transport, and
settling velocity, the
factors that control
deposition.
This figure shows
the critical velocity
for movement of
quartz grains on a
plane bed.