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

Very low values for depth averaged velocity

AD
A D, modified 2 Years ago.

Very low values for depth averaged velocity

Youngling Posts: 15 Join Date: 9/25/17 Recent Posts
I am running a flow model over a gulf. The gulf is surrounded entirely by land on three sides and there is only the sea boundary towards the south. I set this as my open boundary and input water level from astronomical tidal conditions. For the bathymetry I used the GEBCO dataset. The depth file varies from 2000m near the open boundary to about 10-30 meters inside a narrow channel between an island within the gulf and the main land.

My problem is that I have been getting very low values for the depth averaged velocities (in the order of < 0.1 m/s) when I open the com file in Quickplot. And I get these low velocities even within the channel. I know these values are wrong because the field measurements are significantly higher than this. The water level at the boundary conditions is about ~0.9 meters from the astronomical constituents, my grid sizes are about 4 km x 2km near the entrance of the gulf and come down to 250m x 250m within the channel. I set the roughness to a Manning coeffcient of 0.02 and used an eddy viscosity of 100 m^2/s. My timestep is 1 minute and I also set a smoothing time of 600 minutes.

Any ideas on what might be wrong with my model?
PS
Phil Shepperd, modified 2 Years ago.

RE: Very low values for depth averaged velocity

Padawan Posts: 33 Join Date: 5/31/11 Recent Posts
Hi. GEBCO is very coarse ..... 0.5 nautical miles I think. Is your channel being sufficiently resolved by this dataset?
AD
A D, modified 2 Years ago.

RE: Very low values for depth averaged velocity

Youngling Posts: 15 Join Date: 9/25/17 Recent Posts
Hey, thank you.

Yes I know GEBCO is pretty coarse. I used GEBCO for the entire gulf but within the channel I used some in situ data that I already have. Besides, I manually checked the depth after interpolation and it seemed okay to me.
AD
A D, modified 2 Years ago.

RE: Very low values for depth averaged velocity

Youngling Posts: 15 Join Date: 9/25/17 Recent Posts
My model works perfectly fine when I use some uniform depth like 20 meters or something. So it appears to be an issue with the bathymetry. But I don't see a way around this as it is wrong to set a uniform depth of 20 m for the entire gulf.
Qinghua Ye, modified 2 Years ago.

RE: Very low values for depth averaged velocity

Jedi Council Member Posts: 612 Join Date: 3/2/11 Recent Posts
Hi,

I would agree with you that it is due to the big bathymetry in the mouth. The velocity is proportional to the ratio of tidal prism volume/ the steady water volume. Thus with 20m uniform water depth it will give reasonable results.

Greetings,

Qinghua
AD
A D, modified 2 Years ago.

RE: Very low values for depth averaged velocity

Youngling Posts: 15 Join Date: 9/25/17 Recent Posts
Hi Qinghua,

Thank you. Yes that is the case. However, I am trying to tweak some parameters to get my values up by as much as possible.
Horizontal eddy viscosity seems to be having a decent effect so I am trying to create a space-varying file for that. The manual suggests creating a .edy file. But when I do that I get the following error.

forrtl: severe (66): output statement overflows record, unit -5, file Internal Formatted Write

The model works fine if I just set a uniform value for eddy viscosity. So, I am wondering if the format for my .edy file is wrong.
The way I created it is I just opened my bathymetry file (.dep file) and replaced the depth values with eddy viscosity values that I wanted and saved it with a .edy extension. Is the format wrong or am I missing something?

Thank you
AD
A D, modified 2 Years ago.

RE: Very low values for depth averaged velocity

Youngling Posts: 15 Join Date: 9/25/17 Recent Posts
Okay. Never mind this. It was because I had NaN values in the .edy file.

Cheers!