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FESWMS dynamic state_unsteady criteria


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The numeric stability of both FESWMS and RMA2 are independent from courant criteria. The courant criteria generally imposes small timesteps on the numeric engine. FESWMS uses an implicit solver technique that does impose such limitations on the timestep size.

FESWMS uses a solver that works through multiple iterations adjusting the nodal values to be a solution for the boundary conditions as well as the shallow water equations. The solver always works from previously provided values and uses the finite element techniques to iteratively come to the correct solution. If you are not using a hot-start, the previous provided values for the first iteration is a flat water surface elevation (as entered in the model control tab) with zero velocity. If you provide a hot-start value, the values in the hot-start will be used as the initial values.

When solving an iteration, the hope is that the nodal values will converge to a solution. However, the values may also not find a solution and is said to "diverge." The way to minimize this is problem is to have the initially provided values close to the solution you are trying to solve for. There is a process called incremental loading where you provide boundary conditions close to the initial condition (flat water surface with zero velocity). You take the solution from this run and use it has a hotstart with a boundary condition somewhere between your first run and the boundary conditions you wish to solve. If the run converges, you can take another step towards your boundary conditions. If the run diverges, you have to take a smaller step from the last successful run.

The steering module will automate the process above when running a steady-state FESWMS model.

If you want to run a transient model or have wetting and drying, I would recommend using TUFLOW or SRH which are both more stable models.

I hope the above helps.


I would like to ask about numerical stability of unsteady modelling in FESWMS. I tried to find criteria in manual but without response. So which criteria is included in software? Courant criteria or something else?



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