Environmental Engineering Reference
In-Depth Information
As the turbulent plume travels further away from the source, the detailed source characteristics become
less important. The
far-field
(also called passive dispersal region) refers to the much larger region outside
the near field in which the diluted waste field is mainly advected by the ambient current and undergoes
passive turbulent diffusion by the ambient turbulence. The typical time and length scales in the far-field
are on the order of hours and kilometers (Choi and Lee, 2007; Jirka and Lee, 1994).
Near-field
—The near-field mixing characteristics of an effluent discharge can be discussed in relation
to a typical single submerged port outfall (Fig. 8.52). From an orifice of diameter
D
laid on the sea floor,
the effluent is discharged in the form of a buoyant jet with initial velocity
U
0
, relative density difference
ǻ
ȡ
0
/
ȡ
a
, and reduced gravitational acceleration
g'
= (ǻ
ȡ
0
/
ȡ
a
)
g
. Provided that the jet Reynolds number
U
0
D
/
Ȟ
, in which
Ȟ
is the kinematic viscosity, exceeds about 2000—a condition that is always met in
practice—the flow will be turbulent. The jet mixing is governed mainly by the kinematic momentum
flux,
M
=
U
0
2
ʌ
D
2
/4, and kinematic buoyancy flux,
B
=
U
0
g'
ʌ
D
2
/4 of the discharge. The volume flux
Q
=
U
0
ʌ
D
2
/4 plays a minor dynamic role and becomes important only for very shallow depths. Close to the
point of discharge, the high velocity shear induces turbulent entrainment, and the source fluid mixes with
the uncontaminated ambient water as it rises, gaining vertical momentum by buoyant acceleration. If
C
o
is the concentration of a conservative substance in the discharge, then the local dilution at a given point
can be defined in terms of its concentration
C
(
x
,
y
,
z
,
t
) as
S
=
C
0
/
C
. The centerline or minimum dilution
is defined as
S
c
=
C
o
/
C
c
, where
S
c
is the centerline concentration. The initial dilution—the minimum or
centerline dilution at the surface - is often used as an outfall design parameter. For a buoyant jet in a
stagnant fluid, the jet trajectory, the concentration, and velocity field are determined by two important
governing parameters: a jet densimetric Froude number,
F
o
=
U
0
/(
g'D
)
½
, the ratio of inertia to buoyancy
forces, and the relative depth
z
/
D
. For
F
o
~ 1, the discharge behaves like a pure source of buoyancy
(plume), and for
F
o
ĺ , the discharge resembles a pure momentum jet. For a buoyant jet, the flow is
jet-like near the discharge and plume-like at large
z
/
D
where the initial source kinetic energy has been
dissipated. Figure 8.54 shows examples of a jet and a plume in a laboratory experiment.
Fig. 8.54
Initial mixing of momentum jet and buoyant plume
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