TUFLOW 1D2D SX Advice: Difference between revisions
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= Introduction =
With the release of TUFLOW HPC (in 2017) and the continuous improvement of GPU hardware compute capability, there is an industry trend for models of higher resolution. As such, the 2D cell size in flood models is becoming smaller and smaller. Testing has shown in some instances reducing the 2D cell size may lead to model instability at 1D/2D SX locations. This
* 1D timestep selection <br>
* Use of a 1D Node to assign 1D/2D boundary condition cells <br>
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=Base Case=
A simple 1D/2D model has been constructed based on <u>[[
* Type = R (Rectangular)
* Number of barrels = 5
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* The 1D/2D link associated with the 1m 2D cell size is not stable.
* The flow rate results through the 1D culvert decrease with cell size. This is an issue. It indicates cell size convergence (i.e. consistent results independent of cell size) was not achieved. This is caused by the 2D storage volume associated with the 1D/2D connection being insufficient compared to the flow area associated with the 1D channel (12m width). As a result, the 2D boundary cell is the model feature limiting flow exchange between the 1D and 2D, rather than the dimension and hydraulics associated with the 1D structure.
''Please note that
=1D Timestep=
Selection of a 1D timestep that is too large can cause instability. The <u>[https://docs.tuflow.com/classic-hpc/manual/latest/ TUFLOW
During real world studies it is good practice to check the 1D timestep sensitivity
* Select a 1D timestep based on the smallest channel length and the expected flow velocity within your model. <br>
* Trial using a smaller 1D timestep to establish whether the problem is timestep related. <br>
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As of the 2017 release of TUFLOW, nodes within a 1d_nwk file can be used to specify the location of 1D/2D boundary cells and also automatically assign an estimate of the appropriate number of 2D cells for the connection.
For a culvert, this is done by snapping a 1d_nwk point feature to the end of the 1d_nwk line feature and setting the 1d_nwk point attributes
* Type = Node
* CONN_1D_2D = SX
2D cells will be automatically selected as a 1D/2D SX links. The number of cells that are associated with the 1D/2D SX link will be defined by the nodal storage of the 1D channel. The figure below shows the 1D/2D linking cells in the 5m, 2m and 1m cell size models using this method.<br>
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=SX Boundary Lines (2d_bc)=
2d_bc SX line features are often used to connect 1D structure to the 2D domain if the structure width is greater than one 2D cell wide (e.g. [[
The 1D/2D linking cells in the 5m, 2m and 1m cell size models are shown in the the figure below.<br>
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=Conclusion=
When the spatial resolution of a model is increased (i.e. cell size reduced) review of result sensitivity at 1D/2D SX link locations is recommended. This can be done quickly and easily by plotting 1D results and checking for unwanted oscillations. This page demonstrated some useful methods for stabilising 1D/2D boundary (SX) links, in particular where the 1D structure is large in comparison to the 2D cell size. Available options that were introduced included reviewing the 1D timestep, using 1D nodes to define the 1D/2D boundary link, SX boundary lines, SX storage factors and SX boundary polygons.
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