In such flows, the liquid and the gas are clearly separated, not interpenetrating (mixing), and the density ratio between them is quite large. Flow is generally governed by the force of gravity and inertia. Due to a low density and negligible viscosity, both the inertia and the viscous force of the gas are negligible, and the only influence of the gas is the pressure acting on the interface. Hence, the region of gas need not be analyzed, and the free surface is calculated as a boundary with constant pressure (for example, zero pressure by totally ignoring the air effect).

Open channel flow is computed by using the "volume of fluid" method. Surface tension and droplets or fluid separating into separate regions are not included in the calculations. Also, only a single fluid should be included in the model.

In addition to setting up all of the fluid flow conditions that are normally done, the following items must be considered:

• The mesh should include any volume that the fluid will occupy at any time. That is, mesh the fluid and the gas. The calculation will determine which portions are fluid and which are gas.
• The user must specify where the fluid (or gas) exists at the beginning of the analysis. In the results, the volume of fluid in each element is shown (in addition to the other results that fluid flow provides).

TYPICAL APPLICATIONS

• Canal operations
• Drainage systems
• Irrigation canal studies
• Liquid column gauges
• Low-flow channel reclamation projects
• Marine systems
• Marshland wildlife studies
• River discharge studies
• Shallow water studies
• Stream discharge monitoring
• Tow tank tests
• Water quality monitoring
• Water treatment facilities