TUFLOW 2D Hydraulic Structures: Difference between revisions
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What is the recommended method for representing a railway ballast area in TUFLOW? |
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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 in TUFLOW, or any other 2D or 3D software (the possible exception being CFD software), but the reasons why are a little complicated.
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. 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.<br>
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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>
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.
== What are the limitations of explicitly modelling bridge piers in TUFLOW? ==
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The form loss parameters can be transferred from the flow constriction (2d_fc or 2d_fcsh) to the first layer of the layered flow constriction (2d_lfcsh) or pier layer of the 2d_bg. Definition of the remaining form loss and blockage layer inputs should follow the guidance outlined in <u>[[TUFLOW_2D_Hydraulic_Structures#2D_Layered_Flow_Constriction_.282d_lfcsh.29 | 2D Layered Flow Constriction]]</u> and <u>[[TUFLOW_2D_Hydraulic_Structures#2D_BG_Shape_.282d_bg.29 | 2D BG Shape]]</u> paragraphs.<br>
When using floating pontoon (type FD in the 2d_fc or 2d_fcsh) different setup might need to be used for different events. For large events when floating pontoon becomes fixed at the top of the supporting piles, standard 2d_lfcsh setup can be used. Smaller events when the pontoon is floating at different heights might require more sensitivity testing of the structure parameters to find out a setup the matches the reality as close as possible.<br>
== Should I model bridges in 1D or 2D Domain? ==
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== What is the difference between downstream and upstream controlled flow? ==
Downstream control means a change in downstream water level will cause a change in upstream water level. Upstream control means the upstream water level is insensitive to the downstream water level and usually indicates the occurrence of supercritical flow.
== What FLC values should be used for 2d_bg bridge if hB/T is below 2 or above 6? ==
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* For hB/T ratios of greater than 6, the FLC is likely to continue to decrease, but probably not significantly. Clamping to the end value (FLC of 0.20) might be considered the more conservative approach (if the primary concern is flood levels upstream of the bridge).
== What is the recommended method for representing a railway ballast area in TUFLOW? ==
The recommended approach to representing a railway ballast area in TUFLOW is to model it as a soil layer with high infiltration and high hydraulic conductivity (both horizontal and vertical).
Define the soil layer thickness
* Set the bottom elevation of the soil layer approximately 1m below the surface to ensure it has a consistent thickness.
* Modify the soil elevation under the rail embankment
Adjust the soil layer to create a flat surface beneath the embankment.
* Use GIS layers to define the base elevation in this area.
* Ensure the elevation points align correctly with the embankment shape.
Specify the soil type
* Assign a distinct soil type to the embankment area using a polygon.
* Differentiate it from the surrounding soil types for better representation.<br>
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