D-R-A-F-T
This formulation is provided through the DD card, where the variables K1, K2 and K3
( 10 -5 above) are specified.
parallel and transverse to the direction of flow. Since the coefficients in the present
version of SED2D WES apply in the x - and y - directions, not necessarily in the
flow directions, these equations can be used only as a guide.
Fortunately, in most applications, effective diffusion is smaller than convection by
the calculated flow velocities, so a wrong choice does not affect the results very
much unless the chosen coefficient is far too large. The best approach then is to use
a moderately high value (say 50 m2/sec) during the first few runs, and then reduce
the coefficients until the run becomes numerically unstable. This will allow the user
to determine a range of values for which the model gives a converged solution. The
user can then perform sensitivity analyses to determine how the solution changes as
the effective diffusion is varied over this range. If the solution does not vary greatly
then the model is "insensitive" to this coefficient, and no further testing is needed. If
the solution varies widely as this coefficient is varied then the user must rely upon
validation of the model against field measurements in order to determine the
appropriate values. If no field data is available for comparison, the user should use
as small a value as possible, effectively de-emphasizing the importance of these
terms in the overall solution of the system of equations.
The PE card provides a method of specifying the effective diffusion in an automatic
fashion based on the Peclet number:
λu
Pe =
De
If the user specifies the Peclet number, then the effective diffusion is
λu
De =
Pe
which provides generally for the diffusion coefficient proportional to the current
velocity. However, as prescribed on the PE card there is a minimum value of De
based on a specified minimum velocity (VPEC).
The table below lists some previous applications and the effective diffusion
coefficients that were used.
Table Example Dispersion Coefficients
Current
Typical Element
Coefficient
m 2sec
Typical Location
Speed, mps
Size, km
Medium-size river
1 - 1.5
0.1 - 0.5
100
Open bay
0.5 - 1.0
0.75
100
Tidal river
0.2 - 1.0
0.1 - 0.3
5 - 10
Initial concentration: IC cards
The nodal concentrations at the first time-step are specified on the IC cards, or in a
file if a hot start is used.
60 Run Control
Users Guide To SED2D-WES
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