Virtual Links
A user can specify that a user defined conduit or pressure pipe has a section type of "Virtual" by setting the section type to "Virtual" in the property grid for conduits or the "Is virtual" property to "True" in the property grid for pressure pipes. The behavior of a virtual link depends on the active solver and whether the link is a conduit or pressure pipe. Gutters and channels cannot be virtual.
Virtual links pass the flow from the upstream node to the downstream nodes but do not always calculate hydraulic properties such as velocity and head loss. Virtual links usually have length but this is only to assist in plotting the link in a profile drawing. Depending on the solver, the rise of the virtual link may not be shown in the profile. In some solvers (e.g. GVF-convex), the "Is virtual = True" setting is ignored and hydraulic properties are calculated. The behavior of different virtual links is summarized in the table below.
Virtual Link Behavior
| Case | Implicit | Explicit | GVF-Convex | GVF-Rational |
|---|---|---|---|---|
| Pressure Pipe Not At Pump | No head loss or velocity calculation | Treated as not virtual | Treated as not virtual | Virtual not allowed |
| Pump Suction | No head loss or velocity calculation | No head loss or velocity calculation | Treated as not virtual | No head loss or velocity calculation |
| Pump Discharge | No head loss or velocity calculation | No head loss or velocity calculation | Treated as not virtual | No head loss or velocity calculation |
| Conduit Not Diversion | Must have control structure | Must have start control structure | Cannot use unless diversion | Cannot use unless diversion |
| Conduit As Diversion | Ignore as virtual | Must have start control structure | No head loss or velocity calculation | No head loss or velocity calculation |
Virtual links enable the same model file to be used with different solvers even though the solvers have very different ways of representing different physical facilities. The explicit solver internally represents pumps as links with essentially no length while the GVF solver represents pumps as points which must be connected to non-virtual pipes at each end. To make these two solvers compatible, in a model (e.g. SewerGEMS, SewerCAD, CivilStorm or StormCAD) which represents pumps as points, virtual links must be inserted on the suction and discharge side of pump nodes. These virtual pipes and the pump node are combined into a single effective link when the model is run in the explicit solver and the results are later applied to model elements.
Similarly, control structures (e.g. weirs, orifices) are represented as links in the explicit solvers but are properties of links in the implicit and GVF solvers. The control structures need to be associated with virtual links to work with the explicit solver.
For details on using virtual links as conduits or pressure pipes, see help topics Virtual Conduits’ and Virtual Pressure Pipes’