Many different applications such as printers involve the periodic opening and closing of fluid-flow VALVES. This is generally a difficult problem to model as it implies a moving boundary condition for the part of the geometry that acts as an obstacle for the flow.

An alternative is to use a material property (viscosity), which is easy to vary over time. In this case, specify a very large viscosity (ideally it should be an infinite viscosity) that in effect stops the flow in regions where this large viscosity is present. To simulate the movement of this region of large viscosity, the model includes a logical expression in the domain settings.

The model describes a valve where it is possible to direct the flow into one of the two channels. Flow of varying degrees can also occur in both channels during the opening and closing stages.