Boundary Numerical Instability  DFlow Flexible Mesh  Delft3D
intro story DFlow FM
DFlow Flexible MeshDFlow Flexible Mesh (DFlow FM) is the new software engine for hydrodynamical simulations on unstructured grids in 1D2D3D. 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:
An overview of the current developments can be found here. The DFlow 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!
=======================================================  Sub groups

Message Boards
Boundary Numerical Instability
Fernando Barreto, modified 7 Years ago.
Boundary Numerical Instability
Padawan Posts: 74 Join Date: 6/21/12 Recent Posts 00
I'm doing a simulation of a meso scale region to understand the baroclinic circulation of this place. My model contains 32 Zlayers and 3 bounds (north, east, south). To the boundary configuration I chose to impose current in the top and east boundary, and elevation in the south boundary. At this configuration I got bizarre instabilities in the boundary. It was generated a velocity jet in the interface between boundary types (elevation and current)
In another configuration I imposed current in north bound., and in the half of the east boundary; the remaning sections was elevation type (south and the rest of east). In this configuration was also generated a jet in the interface between bound types (fig). The jet can be seen in the east boundary.
Someone knows how to solve this. Can this be related with the zmodel? if not, any advice?
In another configuration I imposed current in north bound., and in the half of the east boundary; the remaning sections was elevation type (south and the rest of east). In this configuration was also generated a jet in the interface between bound types (fig). The jet can be seen in the east boundary.
Someone knows how to solve this. Can this be related with the zmodel? if not, any advice?
Attachments:
SA
Steven Ayres, modified 6 Years ago.
RE: Boundary Numerical Instability
Padawan Posts: 33 Join Date: 4/21/11 Recent Posts 00
Fernando,
Were you able to resolve the boundary problems in your model? I have something similar going on in my 18level Zmodel with salinity. The model is creating velocity jets in the middle of an open stage boundary. I assume this has something to do with the salinity constituent but I'm not sure how to go about fixing the problem.
Thanks,
Steve
Were you able to resolve the boundary problems in your model? I have something similar going on in my 18level Zmodel with salinity. The model is creating velocity jets in the middle of an open stage boundary. I assume this has something to do with the salinity constituent but I'm not sure how to go about fixing the problem.
Thanks,
Steve
Attachments:
Fernando Barreto, modified 6 Years ago.
RE: Boundary Numerical Instability (Answer)
Padawan Posts: 74 Join Date: 6/21/12 Recent Posts 00
Dear Ayres,
my problem was the input of different boundary conditions (velocity and wl) in adjacent boundaries. This was resolved when I used riemann boundary conditions. What's your model configuration? boundary type, dimension?
regards,
Fernando
my problem was the input of different boundary conditions (velocity and wl) in adjacent boundaries. This was resolved when I used riemann boundary conditions. What's your model configuration? boundary type, dimension?
regards,
Fernando
SA
Steven Ayres, modified 6 Years ago.
RE: Boundary Numerical Instability
Padawan Posts: 33 Join Date: 4/21/11 Recent Posts 00
I have water level boundaries for the tide boundaries and an inflowing river total discharge boundary. I think the problem is short wave reflections in the water level boundaries. I raised the alpha coefficient to 1000 and this helped extend the simulation time to a few days, but I still seem to get recirculation around the open boundary. I again raised the alpha coefficient to 10,000 and am testing now.
A formula is given in the manual to determine the alpha coefficient 
Equation 9.76, Water level boundary: alpha = Td * sqrt(H/g), where Td is the time it takes for a free surface wave to travel from the left boundary to the right boundary of the model area.
Since the model is around 30 miles (48 km) across the shortest open tide boundary, and assuming the tidal wave travels around 30 miles/hour or 13.4 m/s, Td would equal 3600 seconds. Using a typical depth of 200 meters at the open boundary, alpha would be on the order of 16,000. Does that sound right?
A formula is given in the manual to determine the alpha coefficient 
Equation 9.76, Water level boundary: alpha = Td * sqrt(H/g), where Td is the time it takes for a free surface wave to travel from the left boundary to the right boundary of the model area.
Since the model is around 30 miles (48 km) across the shortest open tide boundary, and assuming the tidal wave travels around 30 miles/hour or 13.4 m/s, Td would equal 3600 seconds. Using a typical depth of 200 meters at the open boundary, alpha would be on the order of 16,000. Does that sound right?
Fernando Barreto, modified 6 Years ago.
RE: Boundary Numerical Instability
Padawan Posts: 74 Join Date: 6/21/12 Recent Posts 00
Hi Steven,
When I use WL boundaries I usually put alpha as 1000. I didn't know how to calculate alpha, it's really to know. In Riemman we didn't use alpha, Riemann is a weaky reflective boundary condition, so I don't know a lot about alpha.
There are many possibilities to cause your error. Could you attach your model ?, I think it's easier to help.
Regards,
Fernando Barreto
When I use WL boundaries I usually put alpha as 1000. I didn't know how to calculate alpha, it's really to know. In Riemman we didn't use alpha, Riemann is a weaky reflective boundary condition, so I don't know a lot about alpha.
There are many possibilities to cause your error. Could you attach your model ?, I think it's easier to help.
Regards,
Fernando Barreto
SA
Steven Ayres, modified 6 Years ago.
RE: Boundary Numerical Instability
Padawan Posts: 33 Join Date: 4/21/11 Recent Posts 00
The problem was definitely related to boundary reflections. When I set alpha to 10000, the boundary velocity jets disappear.