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TUFLOW 1D2D Boundary Configuration Guidance: Difference between revisions

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==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 manualManual]</u> includes some discussion on 1D timestep selection and courant number criterion. Conceptually a 1D timestep should be chosen to ensure a volume of water does not travel a distance longer than the shortest 1D channel within a model. For example, if the flow velocity and celerity is 5m/s and the 1D channel length is 10 metres, the 1D timestep should be less than 2 seconds. To provide some tolerance for faster flows associated with different flood events, a timestep of 1 second may be appropriate.
 
During real world studies it is good practice to check the 1D timestep sensitivity: <br>
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The modelled flow rates are shown in the figure below. AsThe canfigure be seendemonstrates, comparable peak flow rates are obtained regardless of the cell size (i.e. cell size convergence is demonstrated). The stability in the 1m cell size case has been improved markedly, though there are still some oscillations at low flows. <br>
 
[[File:1D2D Model Stability 005.png|500px]]<br>
 
==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. <u>[[Tute_M02_QGIS_1d2d_Link#2d_bc_Link_as_Line_ObjectTutorial_M02 |Tutorial Module02Module 02]]</u>). This configuration not only increases the number of 1D/2D linking cells, it also has the added benefit of defining the 1D/2D boundary cells at the approximate location of the inlet/outlet of the 1D culvert. An additional benefit of this approach is that the "width" of the linking cells remains similar in length and location irrespective of 2D cell size. <br>
 
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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TUFLOW HPC inflow boundaries may experience recirculation when a uniform elevation is defined along the length of the boundary. This problem may also occur along HX boundaries in 1D-2D linked models that outflow from the 1D to the 2D domain. This is not a common configuration, though may occur when using a 1D channel to convey the flow from a dam break or similar along a defined channel onto floodplains in the 2D domain.
 
Consider setting up the 1D-2D linking shown in Figurethe 1.5figure Thebelow; the HX boundary “HXb1” shown in red is connected at each end to the same 1D node, whereas the remaining HX boundaries shown in purple are connected at each end to different 1D nodes. In the case that the flow is leaving the 2D domain to enter the 1D channel, the boundary is usually stable without energy correction, however for the reverse case, under high flow velocity conditions, the boundary may display instability or recirculation. If this does occur the problem may be remedied via an energy correction, using the command
<font color="blue"><tt>HPC Boundary Approach </font> <font color="red">==</font> Method B</tt>. Note that this will likely cause water level lines to display some discontinuity between the 1D and 2D domain across the boundary.
 
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