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Guest Chris Nielsen

The advantages of a flexible mesh

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Guest Chris Nielsen

Why should I use a flexible mesh model? Why not fixed grid?

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Guest Chris Nielsen

TUFLOW FV is a flexible mesh model. Compared to other approaches (using fixed grids, etc) the design of the flexible mesh tends to have a greater influence on model performance. Thus, more time and effort should be spent preparing the model geometry. Over the life cycle of a modelling project, a well assembled mesh will save time (both the modellers and the computers).

 

The flexible mesh consists of a network of irregular triangular and quadrilateral elements. This has inherent advantages, including:

·          Mesh resolution can be adjusted according to the needs of the study (ie – fine resolution in the area of interest, coarser resolution in the regional extents). Thus, a range of spatial scales can be modelled without resorting to nesting.

·          Mesh alignment can fit bathymetric contours and boundary extents, optimising mesh resolution. This is particularly relevant in regions with complex bathymetric features.

·          Specific features, such as man-made developments, infrastructure etc can be included in the model mesh precisely.

 

To exploit these advantages, the mesh needs to be designed carefully and appropriately for the specific model application. There are a number of mesh generators available to construct a model mesh, however BMT uses the SMS package, provided by Aquaveo (see www.aquaveo.com/sms). We use SMS for the following reasons:

·          SMS (previously Fasttabs of Brigham Young University) has been a commercially available mesh generation package for decades – it has been extensively tested, improved and adjusted.

·          SMS strikes a good balance between manual and automatic mesh generation techniques; in our experience setting up a mesh still needs some manual inputs!

 

An illustrative comparison of fixed grid vs flexible mesh is shown in the following figure. 

 

post-5-0-91361000-1394170731_thumb.png

 

The flexible mesh shown has 438 elements with a typical cell size of 40 to 140 m. In the narrowest bend of the river the cells are smaller and elongated (ie longer in the direction of flow, shorter across the channel) – the cross-channel width in this location is the critical cell distance in this situation, because this resolution is necessary to accurately describe the bed forms and flow conditions. Thus, the required minimum cell width must be greater than 40m.
 
To achieve a similar degree of accuracy the corresponding fixed grid requires a cell size of 40 m. Within the computational domain (ie in the river channel) there are 1676 cells. This is around a four-fold increase in the number of active cells. 
 
Without parallelisation across multiple cores, the computational performance of a finite volume scheme is slower compared to a finite difference scheme; nevertheless, in situations such as that illustrated in the figure there are good reasons for opting for a flexible mesh approach. 

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