# 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!

### =======================================================

** PLEASE TAG YOUR POST! **

## Message Boards

### Reimann Boundary Conditios

LD

#### Reimann Boundary Conditios

Youngling Posts: 5 Join Date: 1/23/12
Hi,

i'm trying to implement the Reimann BC into a 3D z-coordinate model and i've read that the formulae to calculate it is Re = velocity +/- eta*sqrt(g/h), but taking a look into the matlab routine nesthd2.m of the open earth tools I found a acor factor (defined as -dps/depth) that multiplies Re. So I want to know what is this acor factor and if a need to include it when my BC is only Reimann ('cause in the Riemann + tangential BC I don't have to multiplicate Re by the acor factor).

Thanks,
Leonardo.

#### RE: Reimann Boundary Conditios

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

You are right, the acor is a calibration factor, which is not shown in the formula. You can choose to use it or not. But we use it to smooth out neighouring points.

Would you update us as well after you finished your Riemann + tangential BC case?

Thanks,

Qinghua
LD

#### RE: Reimann Boundary Conditios

Youngling Posts: 5 Join Date: 1/23/12
Hi Qinghua,

yes, I'm performing some simulations testing the Reimann and tangential boundary conditions. But I'm still in doubt of this smoothing (acor) and how use it. What's the criteria to smooth the Riemann BC and how to calculate it? And more, why I don't have to smooth the Riemann BC when I use the tangential BC?

By now, I implemented a 3D z-coordinate model based on Hycom-ncoda's solution for the boundary (for while, just for elevation and velocities) for a meso-scale grid (15°x20°). I'm using the same horizontal and vertical grid of the Hycom-ncoda model to avoid interpolation. The results looks fine for the velocities (for Ri and Ri+Tg BC) and the qualitative pattern for elevation looks good too, but the delft model overestimate the elevation and so comes another question that may be the cause of this error: how I define the signal into the calculation of the Riemann BC, I mean this depends on the signal of the normal velocity that cross the BC or it is defined as we find into the nesthd2.m routine (north and east BC the signal is + and it is + for the south and west BC).

I don't take a closer look into the results, comparing the Riemann BC and Ri+tg BC, but when I got it I'll reply here my findings.

Best regards and thanks for the answer,
Leonardo.
FM

#### RE: Reimann Boundary Conditios

Youngling Posts: 21 Join Date: 3/12/13
Hi Leonardo,

I also work on that but with ROMS output for generate boundary conditions.
I created my own matlab toolbox but I have the same question for the north east SOuth West signal).

I dont know if the positive magnitude was for inward current and negative for outward current or juste positve for eastward- northward and negative for westward and southward.
If someone has the answer...

Regards

Florian

#### RE: Reimann Boundary Conditios

Padawan Posts: 74 Join Date: 6/21/12
Florian or Leonardo, did you have any advance? I'm facing the same doubt.

Thanks.