# 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!

## Message Boards

### RE: simulation time step

PS

#### simulation time step

Youngling Posts: 4 Join Date: 3/11/13
Hallo all,

I have to do a long-term simulation with a sediment model and I use parallel computing. Right now I am varying the simulation time steps to get the simulation finished in a certain time.

Using the "Check Time Step" function within quickplot, I get a maximum allowed timestep for horizontal advection of 2.3 seconds. Then I used a time step of 1.2 seconds and got following lines in several of the tri-diag files:

*** MESSAGE Courant number for U-velocity in U-point equals 1.00 for (m,n,k) = (1,31,7), at nst = 249. Advised time step: 0.200E-01 minutes.
*** WARNING Negative concentrations for l, k, nst: 1 6 250
*** WARNING Negative concentrations for l, k, nst: 1 6 250

Can you explain what that means for my results and how these warnings can be avoided?

Thanks in advance and kind regards.
Patrick

#### RE: simulation time step (Answer)

Yoda Posts: 1224 Join Date: 1/3/11
Hi Patrick,

If the messages are local in time and space and are not in the region of main interest (time and space), you can ignore it. Otherwise you will have to adapt your model. Some tips:
• The time step predictor in Quickplot can not take into account all modelling aspects. May be there is one aspect that spoils it all, for example when the initial conditions do not meet the boundary conditions smooth enough. The reported warnings should help you to find the cause.
• May be the problem does not appear when not running in parallel. Then you can choose: parallel with a smaller time step or sequantial with a bigger timestep

Regards,