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

Numerical instability caused by Q-H boundary?

RS
Rudy Schueder, modified 5 Years ago.

Numerical instability caused by Q-H boundary?

Padawan Posts: 52 Join Date: 10/8/13 Recent Posts
Hi all,

I have been getting some odd results for WL that manifest about a month into my simulation. My suspicion is that they are the result of some numerical instability because of the simplicity/steady state condition of the model. Please see the attached .pdf for a visualization of what I mean. The first two plots show discharge through (CS effluent) and WL at the effluent boundary (L2). Notice how they begin to go wonky at the beginning of May. The third plot is discharge through the influent boundary (CS influent). It remains constant as prescribed.

This is a description of the simplified model that gives me these odd WL readouts:
- 1 influent boundary (Pipe): constant total discharge prescribed across one grid cell
- 1 effluent boundary (Weir): Q-H relationship describing flow at WL between 0-1 m. WL should never go above 0.06 m. Prescribed at one grid cell
- both boundaries have a reflection parameter alpha = 86400 s^2
- modeling temperature and pollutants, no wind, waves, or sediment
- ts = 0.05 min
- I have attached a picture of the grid (cross sections removed for clarity) and the input file for further description

Does anyone have any experience with this kind of result? Does it in fact look like a numerical error? I would have expected WL to remain stable since all inputs of water are stable....

Any help is much appreciated and thank you in advance,

Rudy
RS
Rudy Schueder, modified 5 Years ago.

RE: Numerical instability caused by Q-H boundary?

Padawan Posts: 52 Join Date: 10/8/13 Recent Posts
Hi,

It appears that I am able to remove the "wiggle" in water level and discharge when I omit temperature as a modeled constituent. I am not sure why, but I expect it had something to do with the fact that I was specifying temperature in my effluent Q-H boundary condition. Although discharge was always out of the model domain (discharge was always negative), there were observed temperature influences at the effluent boundary in my model output. For instance, heat entered the model from the Q-H boundary at times when the prescribed boundary temperature was higher than the temperature of the adjacent cell within the domain.

Does anyone have any ideas as to why a temperature boundary condition would have affected temperature within the domain when water strictly exited through the aforementioned boundary? Perhaps diffusion processes from boundary into the domain?

Thank you,

Rudy