TUFLOW Message 3043

TUFLOW Message
ERROR 3043: Model unstable.

Alternate Message
ERROR 3043: Too many consecutive failed step attempts.

Message Type
ERROR

Description
When using adaptive timestepping, should a timestep need to be repeated more than 10 times consecutively, the solution stops.

Suggestions
If using the adaptive timestepping (the default), the HPC timestep is calculated using the hydraulic conditions from the end of the previous timestep. If the hydraulic conditions have changed significantly it is possible for one or more of the Nu, Nc, Nd control number criteria to be violated at one or more locations within the model. For example, a sudden change in rainfall from one timestep to the next (which occurs with stepped rainfall boundaries) would potentially cause a violation. The HPC solver, by default, treats a 20% exceedance of a control number as a violation and will implement a repeat timestep feature.

HPC uses a repeat timestep feature to maintain unconditional stability. The repeat timestep feature involves retaining the complete hydraulic solution from the previous (good) timestep. Should a control number anywhere within the model be exceeded by more than 20%, the solution reverts to the retained timestep, the timestep is reduced and then repeated. Should a timestep need to be repeated more than ten times consecutively, the solution stops. The simulation will also stop if the default minimum permissible timestep of 0.1 seconds has been reached. This value can be manually adjusted using 'Timestep Minimum' TCF command. These occurrences are rare and potentially an indication of poor model schematisation or poor data.

Each timestep is also tested for the occurrence of NaNs. A NaN is “Not a Number” and occurs due to undefined mathematical calculations such as a divide by zero or square root of a negative number. The occurrence of a NaN is also indicative of a sudden instability. Should a NaN occur, the repeat timestep feature is implemented.

To review and isolate the location of the minimum timestep, look at:

• Console window and .hpc.tlf file
• .hpc.dt.csv file (this file contains every timestep)
• _HPC.csv file output in the results folder (has the number of repeated timesteps for a time interval in the nRS_NaNs and nRS_HCNs column)
• _messages layer (location of repeated timesteps)
• “Minimum dt” map output (excellent for identifying the location of the minimum timestep adopted – add “dt” to “Map Output Data Types ==”)

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