TUFLOW 2D Cell Size Selection: Difference between revisions

 

 

==How much will my runtime change when changing cell size?==

Solution time for a model is dependent on a number of factors: the solution scheme used, the size and duration of the model, the nature of the hydraulics within the model, the model features selected, and the hardware that it is executed on. However, once a model is developed at a relatively coarse resolution, there are some guidelines that can assist with predicting what the solve time will be as the resolution is refined. These can be useful for estimating whether the refined model will run overnight or need a few days.<br>

 

However, there is one last and important caveat when refining cell size. The explicit algorithm used by TUFLOW HPC requires that the timestep is also subject to Peclet (diffusion) number control, not just Courant (velocity) number control. As Peclet number is inversely proportional to cell size squared, whereas Courant number is just inversely proportional to cell size, the model will transition from Courant number control to Peclet number control once the cell size becomes small enough. This means that the factor of 8 on time for a halving of cell size will eventually become a factor of 16. This was not a feature of TUFLOW Classic. In general, we do not recommend refining cell size well into the region where timestep control is completely dominated by the Peclet number (the Nd column in the HPC output). The flows in the primary channels are usually well converged by this point, and the 2D map outputs may not reveal a lot of additional detail in the velocity fields.<br>

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