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TUFLOW 2D Hydraulic Structures: Difference between revisions

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== Can I model bridge piers explicitly in 2D using very small cells? ==
It isn't recommended to explicitly model bridge piers by blocking out the pier faces in TUFLOW, or in any hydraulic modelling software based on solving Shallow Water Equations(SWE). ExplicitlyDue representingto smallthe 3-scaledimentiality of the flow and turbulence around a obstructionspier, suchcomputational asfluid bridgedynamics (CFD) approach is often required to simulate the flow around piers, inexplicitly. TUFLOWThe requirewake turbulence behind a simple-shape pier can be resolved to some extent using extremely fine mesh resolutionsin andTUFLOW (see calibration example to a flume experiment in the [https://www.tuflow.com/library/webinars/#structures webinar on Energy Losses at Structures]), however the predictions for head losses show notable sensitivities to the mesh size, the mesh design, and the choice of turbulence model. parameterThe (see)extremely fine mesh resolution also results in significantly higher computational costs.
 
due to the mesh resolution required and the 3-dimentiality of the flow and turbulence around the piers.
 
 
This results in significantly higher computational costs and often fails to accurately capture the complex flow phenomena. Additional head losses caused by drag characteristics are better represented through empirical coefficients rather than direct modelling.
 
For more information please see: <u>[[TUFLOW_2D_Hydraulic_Structures#Can_I_model_bridge_piers_explicitly_in_2D_using_very_small_cells.3F | Can I model bridge piers explicitly in 2D using very small cells?]]</u>
 
 
Small scale obstructions to the flow, such as trees, poles, piers, etc. cause additional head losses along a flow path due to their drag characteristics. Historically, form loss (or drag) coefficients for various profile shapes have been determined as a function of Reynold’s number through experimental testing. <br>▼
 
[[File:Flow round a cylinder.png]]
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''The point of flow separation around an object has a major bearing on the drag coefficient and is not reliably reproduced by 2D or 3D software.''
 
TheTherefore, the safest and strongly recommended approach with regard to establishing head losses and consequently flood levels, is to not resolve the obstructions in the mesh but instead model the effects of such obstructions with form loss (drag) coefficients (applied to selected mesh cells) that have been derived from physical testing. This approach has been shown to provide the most consistent results across various mesh resolutions. It also has the added benefit that, by avoiding small cells in the mesh, it will provide much more efficient run times for flow solvers.▼
Modelling 2D flow around profiles with the 2D or layered 3D form of the shallow water equations (SWE) as used by TUFLOW and other free-surface water flow solvers is no different in this regard. While mesh-resolved wakes behind the piers using a fine mesh can be seen in the results, the predictions for head losses show the same sensitivities (mesh size, mesh design, choice of turbulence model) as seen in 3D CFD.<br>
 
<!-- SG commented out, too much CFD info
▲The safest and strongly recommended approach with regard to establishing head losses and consequently flood levels, is to not resolve the obstructions in the mesh but instead model the effects of such obstructions with form loss (drag) coefficients (applied to selected mesh cells) that have been derived from physical testing. This approach has been shown to provide the most consistent results across various mesh resolutions. It also has the added benefit that, by avoiding small cells in the mesh, it will provide much more efficient run times for flow solvers.
▲Small scale obstructions to the flow, such as trees, poles, piers, etc. cause additional head losses along a flow path due to their drag characteristics. Historically, form loss (or drag) coefficients for various profile shapes have been determined as a function of Reynold’s number through experimental testing. <br>
 
(the possible exception being CFD software)
 
More recently, computational fluid dynamics (CFD) has been used to attempt to reproduce the velocity field in the wake of such objects. Although providing better results than 2D modelling, the results have not always agreed well with physical tests. In particular, the drag of a given profile depends on the exact location of flow separation points, which in turn depends on the ability of the CFD code to predict the laminar to turbulent transition in the boundary layer, which is many times smaller than the profile shape itself. In general, the form loss results from CFD models show significant sensitivity to mesh size, mesh design, and choice of turbulence model. Considerable caution needs to be exercised even for CFD modelling.
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== How to best convert flow constriction data (2d_fc or 2d_fcsh) into newer formats (2d_lfcsh or 2d_bg)? ==
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