intro story D-Flow FM

 

D-Flow Flexible Mesh

D-Flow Flexible Mesh (D-Flow FM) is the new software engine for hydrodynamical simulations on unstructured grids in 1D-2D-3D. Together with the familiar curvilinear meshes from Delft3D 4, the unstructured grid can consist of triangles, pentagons (etc.) and 1D channel networks, all in one single mesh. It combines proven technology from the hydrodynamic engines of Delft3D 4 and SOBEK 2 and adds flexible administration, resulting in:

  • Easier 1D-2D-3D model coupling, intuitive setup of boundary conditions and meteorological forcings (amongst others).
  • More flexible 2D gridding in delta regions, river junctions, harbours, intertidal flats and more.
  • High performance by smart use of multicore architectures, and grid computing clusters.
An overview of the current developments can be found here.
 
The D-Flow FM - team would be delighted if you would participate in discussions on the generation of meshes, the specification of boundary conditions, the running of computations, and all kinds of other relevant topics. Feel free to share your smart questions and/or brilliant solutions! 

 

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We have launched a new website (still under construction so expect continuous improvements) and a new forum dedicated to Delft3D Flexible Mesh.

Please follow this link to the new forum: 
/web/delft3dfm/forum

Post your questions, issues, suggestions, difficulties related to our Delft3D Flexible Mesh Suite on the new forum.

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** PLEASE TAG YOUR POST! **

 

 

Sub groups
D-Flow Flexible Mesh
DELWAQ
Cohesive sediments & muddy systems

 


Message Boards

Derefine grid for improved Courant number

CL
Charlotte Lyddon, modified 4 Years ago.

Derefine grid for improved Courant number

Youngling Posts: 7 Join Date: 10/19/15 Recent Posts
Hello,

I am starting to run Delft3D-FLOW in the Severn Estuary, U.K., which is forced by hourly water level data at 1 open boundary. So far I have run the model over 5 days with a time step of 1, 0.5 and 0.1 min.

The diagnostic file returns the error message '‘WARNING No convergence in UZD at tstep … ‘ over 7000 times for a time step of 1 and 0.5 min. I think this is mainly due to the fact that the resolution of the grid in the upper estuary is very fine. When I ran the model at 0.1 minute it did not return any error messages and the Courant number for the grid was <5. However I feel this is a very small time step and greatly increases computational time.

I am looking for advice on whether it would be appropriate to derefine my grid in the upper estuary so I can run the model at a time step of 1 or above, and maintain a small Courant number? And if so, what would you recommend is the best way to go about doing this? Using the function 'Derefine grid locally' or 'Derefine grid' in RGFGRID changes the resolution of the whole grid. I am looking to edit only the upper estuary part of it (.grd and .dep files attached for interest).

Many thanks

Charlotte
Richard Measures, modified 4 Years ago.

RE: Derefine grid for improved Courant number

Jedi Knight Posts: 178 Join Date: 3/23/11 Recent Posts
Hi Charlotte,

When thinking about the grid resolution I usually think about the scale of features which need to be represented by the grid. In the upper part of your grid the bathymetry seems very simple so it is probably not necessary to have such a high resolution. There are a few different ways you could de-refine that area:
  • Domain decomposition - if you want to derefine part of a grid you will need domain decomposition as at some point you will have grids of different resolutions joining.
  • Generating a new grid using a different arrangement of splines - by changing the splines you could reduce the resolution in the upper estuary. This might mean that the grid is not quite so well boundary fitted (you will likely have to have some "staircase" edges in the reach where the number of cells across the channel reduces
  • Using a flexible mesh model
Of these three options my preference would be to try option no.2 as it is probably the simplest but each has advantages and disadvantages. See image below of an example where I have introduced staircasing along the edge of my model to control the resolution (river on left, estuary on right) if I hadn't done this the river would have had much higher resolution, limiting my timestep to be very short.

Hope that helps,

Richard
CL
Charlotte Lyddon, modified 4 Years ago.

RE: Derefine grid for improved Courant number

Youngling Posts: 7 Join Date: 10/19/15 Recent Posts
Hi Richard,

Thank you your very quick and comprehensive reply. Your advice is much appreciated! I will have a go at editing my splines.

Best wishes

Charlotte