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=Frequently Asked Questions (FAQ)=
== Why are model results developed in an older release different to a newer release? ==
If comparing a Classic model with HPC, also check the <u>[[HPC_FAQ#Will_TUFLOW_HPC_and_TUFLOW_Classic_results_match.3F | Will TUFLOW HPC and TUFLOW Classic results match?]]</u> page in addition to this answer. <br>
In addition to the above, there are reasons why model results would be different between different TUFLOW releases, whether it is the Classic or HPC solver 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) applied to an existing model that 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 these situations with vastly improved results.
** 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 flow depth across all scales from flume to large rivers. For more information on differences between Smagorinsky scheme (HPC releases up to 2020) and the new Wu turbulence scheme (2020 onwards) see <u>[[HPC_FAQ#With_Wu_turbulence_scheme_being_the_new_default.2C_are_old_models_using_Smagorinsky_wrong.3F | here]]</u>.
* Changes to the default settings and values, e.g.:
**different default eddy viscosity formulation and/or coefficients,
**improved data pre-processing approaches such as sampling materials on cell mid-sides instead of cell centres,
** and many others.
** For backward compatibility the <font color="blue"><tt>Defaults</tt></font> <font color="red"><tt>== </tt></font> command is available to run old models on new releases to replicate past results (note, sometimes full backward compatibility cannot be catered for due to different code compiler and updates that can't be reverted, 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 hydraulic calculations.
* Change in timestepping can also produce a small change in results. HPC uses the Runge-Kutta 4th order integrator, which is usually fairly insensitive to time step provided the model is running stably. However when a region is filled by flow that only just overtops an embankment, a 10 mm difference in water levels upstream of the embankment can create a much larger difference in levels downstream. Hence small differences in time-stepping (along with many other aspects of model setup) can trigger local differences in model results.
* Model orientation (if changed) could also mean slight change in results. This is mostly given by interpolating values from different calculation points. Every cell has nine calculation points. Based on the model origin, all or most of the calculation points would have different topography elevation sampled, which translates to slightly different results.
* If using 1D channel, possibly different cells have been selected as HX boundary and might have different elevations. This can be reviewed in <u>[[Check_Files_1d_to_2d_bc | 1d_to_2d check file]]</u>.
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 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 in which release(s) the significant changes occurred, the model can be run with the latest build and for past releases. 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]. Once the exact release where the changes occurred is tracked down, individual features can be turned off to narrow down the cause.<br>
The recommendation is usually for new or reworked models to use the newest build to take advantage of the latest features and enhancements, some level of calibration might be required for reworked models. The new TUFLOW executable is not different from the previous ones in the meaning that any existing model should be re-calibrated if there are available calibration data. However, particularly if a model is already calibrated, using prior builds of TUFLOW or winding back default settings using <font color="blue"><tt>Defaults</tt></font> <font color="red"><tt>== </tt></font> command is considered reasonable for established models that are to be used for minor tasks and an update of the model would not be cost effective.<br>
== What is the difference between single and double precision and when should I use them? ==
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