Junction Transient Data

Many junction types have an area where active transient data can be specified. Active transient in this context means that the behavior of a component over time is being directly defined by the user. It is also possible for components to react passively during the simulation as well - for example, a relief valve opening under excess pressure.

Table 1: Junctions that support transient data

A table showing the possible transient types for the junctions in AFT Impulse.

The active transient data is entered on the junction's Transient tab.

The Transient tab inside the Valve Properties window.

Figure 1: Transient tab for a Valve

In most cases, transient data can be entered as absolute values or as a percentage of steady-state, as defined by the first options in Transient Data.

The first data point always needs to match the steady-state value which is usually entered on the Loss Model tab. If, for example in Figure 1, the steady-state data for Cv of 1000 is changed to 750, the transient data will also need to be changed.

Note: If a junction does not have any active transient behavior, there is no need to enter any data on the Transient Data tab.

Repeat Transient

If the transient data is periodic, you can enter the data for one cycle of the period and then tick the check box for Repeat Transient. This will cause the one cycle of transient data to be repeated once it has reached the end. The repetition will continue until the end of the simulation.

Transient Special Condition

There are instances where you will only wish to use the defined transient data in certain cases or scenarios. Instead of deleting the transient data, specify the Transient Special Condition as Ignore Transient Data. The data will be left in the window for future use, but will not activate during the run.

A third type of Transient Special Condition is available for Assigned Flow and Assigned Pressure junctions. This is the No Reflections (Infinite Pipe) option. In such a case, the flow or pressure is used only for steady-state calculations. During the transient the flow or pressure is not fixed, but varies in accordance with the concept that no transient reflections occur. All transient data at this junction will be ignored. For more information see Infinite Pipe.

Initiation of Transient

There are multiple ways that the data entered in Transient Data can be applied during the simulation. The default selection is Time for most junctions, meaning that the behavior vs simulation time is directly defined by the user.

It is also possible to initiate a transient based on a system event. This will execute the Transient Data when a user defined parameter reaches a certain setting. For example, a valve can be set to close only when a specific flow rate becomes too low.

See Initiating a Transient for detailed information.

Related Blogs

And, Not, Or - Starting Transient Events with Complex Logic