A Model Crashes

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This section summarises some common reasons why a model crashes.

Initial Condition / Boundary Condition Mismatch

It's a common situation. Modellers always try to avoid warming up a model and hope that putting a large flow inflow (eg. 10,000 m^3/s) into an otherwise still model will run smoothly. TUFLOW FV is a relatively resilient model, but it has its limits.

There are a number of ways to manage this type of instability:

  • Using a warmup boundary condition
  • Increasing the model stability limits

Model Warmup

Use a warmup of the boundary condition, transitioning from the initial state to the preferred boundary condition. If this extended simulation period produces excessive model run times, consider writing a restart file at the end of the warm up period. The restart file can then be used to set the intial conditions for any following simulations. The following command should be included in the warmup model control file (*.fvc):

Write restart dt == 

Depending on the version of TUFLOW FV, this will write a restart file (*.rst) to the same directory as the model control file (*.fvc), or to the log directory. Future simulations need to reference the restart file produced from the warmup model to initilise the initial conditions for the following simulations. The following command should be included in the following design simulation models:

Restart == <restart file name>

Model Stability Limits

Model stability limits can be increased by the following ways:

  • Decrease the minimum timestep limit
Timestep Limits == <min timestep (s), max timestep (s)>
  • Decrease the Courant-Friedrichs-Lewy condition value which is used to calculate the model timestep
CFL == <global maximum courant number>
  • Increasing the model water level and velocity stability limits
Stability Limits == <maximum WL, maximum velocity>