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

 


Message Boards

Some advice on using Delft3D to analyse a deep fjord

AE
Alistair Everett, modified 7 Years ago.

Some advice on using Delft3D to analyse a deep fjord

Youngling Posts: 2 Join Date: 2/22/13 Recent Posts
Hi all,

I intend to model a deep coastal fjord (up to ~1000m deep), the fjord is approximately 100km long and varies from 5-10km wide. The waters in the fjord are strongly stratified with warm salty water (~3-4C, 34psu) at depth and cool fresher water towards the surface (~0C, 10-20psu). Fresh water is released at depth in some areas of the fjord and therefore forms a vertically buoyant flow. I am particularly interested in modelling this flow with reasonable accuracy as it may play an important role in mixing and exchange within the fjord. It is also important that I can analyse flows into and out of the fjord at different depths and how they are affected by the volume of freshwater released, tides and other forcings.

I'm currently looking into a number of potential models which I could use for this and was just after some advice from others who have experience of using D3D on similar problems before I commit to one model. So, having looked through the D3D-Flow manual and doing a bit of experimentation, I have a number of questions which perhaps someone might be able to help me with:

Firstly, is D3D a suitable model for this kind of problem? Has anyone had experience of modelling a similar fjord like this with D3D (or another model)? Were your results reasonable and did you validate with any field data? Do you have any advice/warnings or problems I might encounter using D3D (outside the normal of course!)?

I understand that when using a sigma-coordinate system in D3D the assumption is made that the depth is much smaller than the horizontal length scale, I think I would be right in saying that this approximation is not valid for the dimensions given above, and therefore I would have to use the non-hydrostatic model in a Z-coordinate system? Does anyone have experience of using the non-hydrostatic pressure model with rapid convection, is it reasonably reliable? Are there any other areas of D3D-Flow which still rely on the shallow water approximation, even when using the Z-coordinate model, which might introduce error to this problem?

I have attempted (quickly) to introduce a horizontal fresh water input at depth within a fictitious domain filled with salty water, I was hoping this would have an initial horizontal velocity, but would then rise up to the surface some distance away, but this doesn't seem to behave as I would've expected, instead it generates surface waves directly over the point of input. I haven't been using D3D for long so I may simply have made a mistake, does anyone have experience of this? Is it possible to create horizontal inputs at depth along a boundary in D3D? For now I'd be happy to know it is possible, in which case I will spend more time investigating it!

Thanks in advance for any help anyone might be able to provide,

Alistair
Bert Jagers, modified 7 Years ago.

RE: Some advice on using Delft3D to analyse a deep fjord

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

Interesting case. I can't remember any other Delft3D fjord models, but there are some successful applications with narrow deep model areas.

Delft3D-FLOW is based on the shallow water equations in both sigma- and z-layer mode, so I wouldn't use that as main argument against the sigma-approach. The main restriction that the "shallow water assumptions" cause is that any waves that you model should be significantly greater than the water depth. I believe that this isn't a problem in your case which might possibly even be simulated using a model without a free surface (depending on the details of the research questions you want to solve).

The main argument should be that the sigma-layer approach isn't recommended in combination with steep topographic gradients as it can lead to spurious mixing of the water layers at the location of the gradients. The horizontal diffusion term is split into a diffusion along sigma-planes and an "anti-creep" correction for the deviation from the horizontal. That correction doesn't work well in case of steep topographic gradients. So, I agree with you that you should use the z-layer approach. Since you're interested in the development of a buoyant, you need indeed a non-hydrostatic model. It should be noted that Delft3D used in both hydrostatic and non-hydrostatic mode, an anisotropic turbulence model reflecting shallow water: the horizontal turbulence can be included using eddy viscosity or HLES, whereas the vertical turbulence can be included via k-eps equations. Extending k-eps to 3D might be considered for this kind of application.

I don't know about the details of your model set-up, but there have been successful simulations of buoyant plumes with the desired behavior that you describe. Why your model doesn't show this behavior I don't know. Maybe you can start with some simple square basins for some tests.
  • Did you model the fresh water inlet as a discharge with (horizontal) momentum? Delft3D switches to an upwind approach at locations with momentum discharges; under weakly forced conditions the effect of changing the numerical scheme can be significant. You can try to specify Upwsrc = #NO# in the mdf file to overrule this behavior.
  • Surface waves can also result if the time step is too big. Have you verified that the simulation mostly satisfies the time step criteria specified in Table 10.1 / Section 10.4.5 of the Delft3D-FLOW manual?
  • The default non-hydrostatic model performs a hydrostatic simulation step, followed by a non-hydrostatic correction step. This works well for weakly non-hydrostatic conditions, but might fail for a strongly buoyant yet. In that case, it might be better to use the full non-hydrostatic solver that is currently undocumented.


Hope this helps a bit,

Bert