HPC FAQ: Difference between revisions

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= I have been given a model developed in an older release and the results are different in a newer release. Why? =
If comparing a Classic model with HPC, check the first question [[HPC_FAQ#Will_TUFLOW_HPC_and_TUFLOW_Classic_results_match.3F | Will TUFLOW HPC and TUFLOW Classic results match?]] <br>
In addition to above, there are numerous reasons why model results would be different between different TUFLOW releases, whether it is the Classic or HPC regardlesssolver as follows:
* General improvements and fine-tuning of the solution scheme, especially for the more complex hydraulic physical terms and situations such as: sub-grid turbulence representation; treatment of shocks (e.g. hydraulic jumps); and transitioning between sub-critical and super-critical flow on steep slopes.
* Some new functionality can cause a significant change in results. For example, Sub-Grid Sampling (SGS) whereapplied ato an existing model usesthat used a too coarse cell resolution in high flow areas of highly variable topography (relative to the 2D cell size). SGS will greatly improve the model's ability to convey water accurately in thisthese situationsituations with vastly improved results convergence due to varying the cell size compared to not using SGS. Another example is the new default sub-grid turbulence scheme in the 2020 release of TUFLOW HPC that is cell size independent and allows modellers to use cell sizes much smaller than the depth across all scales from flume to large rivers.
* Changes to the default settings and values, e.g. different default eddy viscosity formulation and/or coefficients; improved data pre-processing approaches (e.g.such as sampling materials on cell mid-sides instead of cell centres). For backward compatibility the “Defaults ==” command is available to run old models on new releases to replicate past results (note, sometimes full backward compatibility can not be catered for, especially for several releases earlier).
* New features that use GIS attributes previously reserved (i.e. unused). If these attributes were not populated with the recommended “reserved” value (usually 0 or blank), then they can cause unpredictable results in later releases.
* Bug fixes noting that most bug fixes are input/output related and rarely affect the model's resultshydraulic calculations.
 
Generally, there should not be substantial differences as the fundamental equations being solved are unchanged and TUFLOW Classic and HPC solvers have always solved all the physical terms using a 2nd order spatial approach. The one exception is the sub-grid turbulence (eddy viscosity) representation, which is the most complex and challenging to solve of all the physical terms (many 2D schemes simply omit this term). If significant differences (>10% of depth change across the whole model) are observed then it’s most likely due to the first four dot points above. To identify the cause(s) for the change, the model can be run with the latest build and for past releases to identify which release(s) the significant changes occurred.
 
The changes for each release are documented in their release notes. Past releases and release notes are all available [https://www.tuflow.com/downloads/tuflow-classichpc-archive/. here].<br>