HPC FAQ: Difference between revisions
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Not all HPC models will show an increase in run time when changing from the 2018 to the 2020 release - models that are controlled by the wave celerity or velocity control numbers and not the diffusion control number are likely to be similar in runtime. Some models will be even faster with the last 2020 release due to other improvements. However, especially where the cell size is smaller than the depth, the Wu approach is vastly superior to the Smagorinsky, and the more sophisticated Wu solution may start causing the diffusion control number (the third one displayed) to control the timestepping causing longer run times.
= Why is my HPC model getting unstable right at the beginning? =
If there are no wet cells at the beginning of the simulation, the adaptive timestep can get quite big. Once the flow increases rapidly and the solver could not reduce the big timestep to a sufficiently small timestep within the ten default trials. The below suggestions can be implemented to eliminate the instability:
* Specify initial water level for the whole model with "Set IWL" command or locally with "Read GIS IWL" command. The wet cells can limit the adaptive timestep through the [[HPC_Adaptive_Timestepping#HPC_2D_Timestep | Shallow Wave Celerity Number]], and prevent the HPC solver from using big timesteps.
* Use "Timestep Maximum" command to cap the maximum timestep to new get too high.
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