HPC Adaptive Timestepping: Difference between revisions
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In general terms:
* Courant number relates to velocity relative to the cell size. High velocities will trigger this as the control.
* Celerity Control number relates to water depth relative to cell size. Energy can pass through deeper water faster than shallow water, as such deep water will trigger this control.
* Diffusion control relates diffusion of momentum relating to the sub grid viscosity. Small cells in deep water will trigger this one.
The method TUFLOW HPC uses to calculate a timestep and achieve unconditional stability is as follows:
* The <font color="blue"><tt>Timestep </tt></font> command included in the TUFLOW Control File (TCF) is only used for the first calculation timestep. The specified value should be consistent with what would be appropriate for a TUFLOW Classic model (i.e. 1/2 to 1/5 the 2D cell size). Internally, TUFLOW HPC divides this value by 10 to apply a value that is suitable for an explicit solution scheme. All subsequent calculations are completed using the adaptive timestep approach outlined in the following
* 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.
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