HydroGeoSphere/Timestep Control

Before we discuss the instructions which are available for controlling the behaviour of a transient solution, some background information is required. The preprocessor grok generates an array of target times, which are derived from the following sources:

• Times specified by the user to meet timestep constraints.
• Times specified by the user to meet output requirements.
• Times at which transient boundary condition values change.

This target time array is passed to HydroGeoSphere which uses it to produce timestep values.

As well, adaptive timestepping, as discussed in Section 5.5.2.1, can be used to adjust the timestep values based on changes in head, saturation and/or concentration as the solution progresses.

The instructions on this page can be used to modify the time-stepping behaviour of a transient solution.

Initial time

1. tinit Initial time.

Assigns a new value for the initial time, which defaults to zero. This is useful if you are restarting the simulation and want to index the times used to an earlier run.

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Initial timestep

1. val Initial timestep size.

Assigns a new value for the initial timestep, which defaults to 0.01 time units.

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Maximum timestep

1. val Maximum timestep size.

Assigns a new value for the maximum timestep size, which defaults to 10²⁵ time units.

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Minimum timestep

1. val Minimum timestep size.

Assigns a new value for the minimum timestep size, which defaults to 1 × 10⁻¹⁰ time units. If the timestep becomes smaller than this value as a result of the adaptive timestepping procedure, HydroGeoSphere will stop and issue a diagnostic message.

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Target times

1. target_time...end Target times.

Listed times are added to the current set of target times.

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Generate target times

1. tstart Start time [T].
2. delta Initial time step size [T].
3. tinc Time step multiplier.
4. dtmax Maximum time step size allowed [T].
5. tend End time [T].

New target times are generated from the start time tstart to end time tend by repeatedly adding the time step delta, which is increased each time by the multiplier tinc until it reaches a maximum size of dtmax.

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Output times

1. output_time...end Output time.

Listed times are added to the current set of output times (i.e. times for which you want detailed output). Note that these values will automatically become part of the target time array.

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