How to fix this error?

Hello Everybody,

I have created both curvilinear grid and bathymetry files in MIKE21C. I ran the model but i got an error. To fix the problem, when i change the initial condition, sometimes it says "high water level" or "low water level" and it does not get stable. The middle of the channel is the deepest point. When i check the output file, the water depths after and before the deepest point are logical but the model gets unstable at that point.

I also increased the number of elements and created a new grid and then a new bathymetry file, but i got the same error. I also changed other parameters such as, boundary conditions, Manning coefficient, but it did not work either.

Could you please guide me how to fix the problem?
Regard

Ali,

I am not familiar with non-uniform grids in MIKE, but I have experience with non-uniform 3-D meshes in other software.  What exactly are you attempting to model?  Can you include a screencap of the grid?  It is difficult, otherwise, to diagnose numerical instability.

My initial guess would be that your grid and/or time discretization are too large for the flow velocity you are attempting to model (see Courant Number, CFL criteria).  What values are you using?

Instability could also be caused by poor mesh/grid quality.  I'm not sure what freedoms MIKE gives you for non-uniform meshes, but an abrupt 'jump' in cell size can bring about numerical error, especially if a large gradient encounters such a cell (in this case, position or velocity, but the same is true with heat, pressure, shear stress, etc.)

Hope this helps.

Thanks Abowers for the answer,

Actually, i have created two different models with MIKE 21C for the Mississippi River. The first one is the Mississippi River with hundreds of KMs length , 700-1300 meters width and 30-60 meters depth along the channel. The second one is s small physical model in the lab. The length, width and depth of the real river are divided by 6000, 6000, and 400 respectively in order to create the physical model in the lab. I have created both of the models in MIKE21C.

In fact i want to see the effect of the scaling on the hydrodynamic of the channel. As i said, for the real river model i got three types of errors. I fixed them with decreasing the width and depth of the channel at the beginning of the channel.

I tried to do the same process for the smaller model ( 8-16 cm depth, 8-13 cm width, 30-40 meter length) but it does not work for that.

I have created rectangular meshes for both of them. Both orthogonality and aspect ratio conditions have been fulfilled and the meshes are correct.

For now, i do not know how to fix the error for the smaller model. I have changed all of the parameters and also i have increased mesh resolution but none of them does not work.

Again, thanks for your all consideration.

Regards