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How does Delft3D-FM handle cells inside dry areas?

JS
Julius Schlumberger, modified 1 Month ago.

How does Delft3D-FM handle cells inside dry areas?

Youngling Posts: 12 Join Date: 12/20/20 Recent Posts
Hey all,
I have been reading the Flow-manual, but could not find an answer to below question.
So I was wondering if D-Flow will use the cells inside dry areas for the computations (require additional computational time) or whether it will skip them?

The background of the question: I have a urban model grid with many elements (around 3 million, of which 1 million lie inside of structures which i either want to delete or define as dry areas) but try to reduce the number of grid cells that are used for the computations. Basically I want to reduce the required time for a simulation.
Many elements fall inside of structures which should not bet considered. I only want to make sure that the momentum exchange between the wall and the flow is used. My current approach is to delete cells inside the structures while keeping at least one grid cell inside the structure polygons that will have the elevation of the building and make sure that there will be momentum exchange between the structures and the flow. When deleting cells inside the structures in the triangular mesh, this effects the orthogonality of the grid negatively, which cannot be resolved as I found here: https://oss.deltares.nl/web/delft3dfm/general1/-/message_boards/message/3753167
Thank you,
Julius
Sebastian Martijena, modified 1 Month ago.

RE: How does Delft3D-FM handle cells inside dry areas?

Youngling Posts: 11 Join Date: 11/18/15 Recent Posts
Dear Julius,
I did run a 2D model in the past with both scenarios, with and without dry areas. The model output (.map) with dry areas is smaller. Also, the grid topology looks different in that area (when you look at it with quickplot or QGIS).
Is not a direct answer, but hope it helps.
Have a nice day,
Sebastian.
MK
Michal Kleczek, modified 1 Month ago.

RE: How does Delft3D-FM handle cells inside dry areas?

Padawan Posts: 57 Join Date: 10/23/14 Recent Posts
Dear Julius,When you have a dry area that spans over few cells or dry points that are defined on some points, these cells are excluded from further computation. That means that are not taken into further numerical calculations and that should indeed decrease the "size" of your computation. 
Similar effect you can achieve by having an enclosure which defines "everything outside enclosure is a dry area".
I hope that answers your question. And indeed as Sebastian mentioned above the output should get smaller since you "disabled/removed" some cells from computation.

Cheers,
Michal
JS
Julius Schlumberger, modified 1 Month ago.

RE: How does Delft3D-FM handle cells inside dry areas?

Youngling Posts: 12 Join Date: 12/20/20 Recent Posts
Dear Michal, dear Sebastian,
thank you very much for your comments and explanations. That answers my question indeed.
Best, Julius