intro story Coast / Estuary

Coast / Estuary

Coastal systems are among the most dynamic physical systems on earth and are subject to a large variety of forces. The morphodynamic changes occurring to coastlines worldwide are of great interest and importance. These changes occur as a result of the erosion of sediments, its subsequent transport as bed load or suspended load, and eventual deposition. 
 
Estuaries are partly enclosed water bodies that have an open connection to the coast. Estuaries generally have one or more branching channels, intertidal mudflats and/or salt marshes. Intertidal areas are of high ecological importance and trap sediments (sands, silts, clays and organic matter).
Within the Delft3D modelling package a large variation of coastal and estuarine physical and chemical processes can be simulated. These include waves, tidal propagation, wind- or wave-induced water level setup, flow induced by salinity or temperature gradients, sand and mud transport, water quality and changing bathymetry (morphology). Delft3D can also be used operationally e.g. storm, surge and algal bloom forecasting. 
 
On this discussion page you can post questions, research discussions or just share your experience about modelling coastal and/or estuarine systems with Delft3D FM. 
 

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

 

 

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Numerical instability near water level boundary caused by secondary flow?

SS
Steven Sandbach, modified 7 Years ago.

Numerical instability near water level boundary caused by secondary flow?

Youngling Posts: 7 Join Date: 3/28/11 Recent Posts
Dear Delft3D community

I am currently having stability issues with a water level boundary which is causing some of my simulations to crash. It appears to be related to the secondary flow field as this problem does not persists when I turn off secondary flow.

The setup is a model of the Columbia river estuary and extends from the river to beyond the estuary mouth. I have specified a combination of river discharge and astronomically forced water levels and Neumann boundaries (see image Mesh.jpg):
1. West boundary: astronomic water level boundary (black),
2. South and North boundaries: astronomic Neumann boundaries (red),
3. River: discharge boundary (red).

To reduce the computational load, I have derefined the mesh in the seaward part of the domain as in this application I am interested in the upper part of the Estuary. To illustrate the problem, I have included images of:

SecondaryFlow.jpg : The predicted water-level in the cells directly next to the water level boundary (black in mesh.jpg)
WaterLevel.jpg : The predicted secondary flow
Mesh.jpg: The morphologic grid

The simulation start time is 01-Jan-2011 00:00:00 so the presented results are for a simulation that has been running for 1 day from a 'cold' start. As stated, the water level instability only appears when secondary flow is turned on and it ultimately results in FLOW exiting due to large water level changes in the cells next to the boundary. This instability can be seen in 'WaterLevel.jpg' which is the predicted water level in the cells next to the outlet. The secondary flow 'instability' appears to be related to refining/derefining the mesh in the seaward part of the domain. I have tried refining the mesh in this area but this doesn't seem to make any difference.

Any advice is gladly accepted - thank you in advance.

Regards

Steve
Bert Jagers, modified 7 Years ago.

RE: Numerical instability near water level boundary caused by secondary flo (Answer)

Jedi Knight Posts: 201 Join Date: 12/22/10 Recent Posts
Dear Steven,

That secondary flow pattern doesn't look good; far too much spatial variability and curious stripes.
The only potential conflict that springs to mind is the combination of secondary flow and HLES.
Are you using HLES? If so,
  • try a constant (or smoothed spatially varying) viscosity (field) rather than HLES and see whether that also stops the problem.

If you're not using HLES then try the following:
  • set beta_c = 0 (switch off feedback of secondary flow onto main flow)
  • switch Equilibrium state on (this removes advection behaviour)

Hope one of these three options helps.

Success,

Bert
SS
Steven Sandbach, modified 7 Years ago.

RE: Numerical instability near water level boundary caused by secondary flo

Youngling Posts: 7 Join Date: 3/28/11 Recent Posts
Dear Bert,

Thanks for your reply. I have tried your suggestions and obtained the following results:

1. In the original simulations, I did not include HLES so that wasn't the cause of the problem.
2. I had already tried changing to the equilibrium secondary flow solution - that also did not solve the problem.
3. Setting beta_c = 0 does stop the oscillations. however, as I understand it, this is the effectively the same as turning it off (at least for a hydraulic simulation)?

Thanks

Steve