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! 


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: 

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





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


Message Boards

Improving FLOW-Stability - Tidal circulation

Kyle Albert, modified 7 Years ago.

Improving FLOW-Stability - Tidal circulation

Youngling Posts: 2 Join Date: 2/26/13 Recent Posts
Hello, I have been working with a model in FLOW for several months. I have been able to run several versions of the model to completion, but I experience frequent crashes ("flow exited abnormally"). I would appreciate any suggestions regarding my input conditions, so that I can eliminate sources of instability. I have highlighted the input components that have been most unclear to me. Thanks!

Grid - 50 m res, 675 x 500, rectangular
Bathy - ~35000 depth and topo samples, depending on the .dep file I use, 2-4 smoothing operations with 0.5 smoothing factor
Time - Generally 1 month intervals, 2 min timestep
Boundaries - 2 water level open boundaries, astronomic forcing

Physical Parameters
- Roughness: White-Colbrook, 0.05; Free slip wall condition
- Horizontal Eddy Viscosity: 1.5 m2/s

Numerical Parameters
Depth specified at grid cell corners
Depth at grid cell center = MEAN
Depth at grid cell faces = MEAN
Threshold depth = 0.1 m
Marginal depth = -999 m
Smoothing time = 240 min
Advection Scheme = Cyclic

Any insight would be a big help.
Bert Jagers, modified 7 Years ago.

RE: Improving FLOW-Stability - Tidal circulation

Jedi Knight Posts: 201 Join Date: 12/22/10 Recent Posts
Hi Kyle,

What is the maximum water depth in the model domain?

You indicate that your grid resolution is 50 m and the time step is 2 minutes.
Even with a water depth of 10 m, and hence a free surface celerity of sqrt{gH} of about 10 m, the Courant number for free surface waves sqrt{gH}*dt/dx would get in the order of 20.
I don't know the characteristics of the model area, but if you are close to a shelf it may get much deeper quickly.
Even though Delft3D includes an implicit solver, there is a limit on the time step for accuracy and in the end for stability reasons. See section 10.4.5 and table 10.1 of the Delft3D-FLOW manual with an overview time step limitations.

Best regards,

Kyle Albert, modified 7 Years ago.

RE: Improving FLOW-Stability - Tidal circulation

Youngling Posts: 2 Join Date: 2/26/13 Recent Posts

Thanks for your help. The maximum sample depth is ~170 m. The average water depth is 13-14 m.

I will take a look in the manual, thanks for the reference!