Hivel2D users manual
and
∆ l = the element length
∆ t = the time-step size
The time-step can then be gradually increased. If the steady-state solution is desired,
then a fairly large time-step can be used. If the interest is in unsteady results, then, for
accuracy, the CFL number should be held to a maximum of 2.
Appropriate boundary conditions must be supplied. For an inflow boundary condition
when the flow is supercritical, the user must specify both x- and y-components of flow
along with the depth. If the flow is subcritical here, then only the x- and y-components of
flow are read. If the flow is subcritical at the outflow boundary, then the tailwater
elevation must be specified. If the flow is supercritical at the outflow, then no boundary
conditions need to be given. This is done in the input file by specifying a tailwater
elevation that is lower than the bed elevation at the outflow boundary.
Starting the model is sometimes difficult when the flow is supposed to be supercritical
at the outflow boundary. The model attempts to converge to a solution, but it may not be
the desired solution. One way to avoid this is to start the model with a tailwater that is
slightly subcritical (if the starting conditions are not known). Then after the model settles
down, the boundary condition can be changed to supercritical. This takes a little
experience.
Users should note that the Manning's n applies to each element type as well as the
adjoining sidewalls. It is also used in conjunction with velocity, depth, and C to
determine a turbulent eddy viscosity estimate.
The input to the hydrodynamics file is made through the list in Figure A5 (Appendix
A). All variables are real except those beginning with I, J, K, L, M, or N, which are
integer variables. All input is free-field. Each variable is described in Table 2 in the order
in which it appears in the hydrodynamic input file. Units are designated by L and T, for
length and time, respectively. If no units are specified, the variable is dimensionless.
Hot Start File
This file is the last two time-steps of information from the previous model run. Since
the temporal derivative can be a second-order backward difference, two time-steps of old
information are needed. If this is the initial run, then this information will have to be
12
Chapter 3 Developing an Application
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