multi-block mesh problem
 

Hi all,

I am working on simulating a large pond with several inlets and outlets. Thus, I use multi-block mesh, but a velocity disturbance starts to grow on the inter-block boundaries and spread throughout the domain.

For finding the source of the problem, I decided to make the problem simple, so I simulated a 10m*10m*2m pool with 0.9 water elev. without any inlet or outlet. There is no wind, and no initial vel. It has 2 linked mesh blocks with sizes of 0.5m and 0.25m (Aspect ratio is 1 for both of them). All of the BCs were left as default (symmetry). We expect the velocity remains zero, but after 1000 sec the disturbance starts and after a few seconds it takes over the whole domain.

I already increased Inter-block boundary type coefficient (alphamb), but it didn’t help. Smaller mesh size and different turbulence model didn’t help either. Other configurations of mesh blocks (nested or overlapping) also cause the same problem

Does anyone have an idea why this simple simulation has this error?
Thank you!

ِDear user

That is due to interpolation error between blocks. By choosing 1:2 mesh size ratio, and increasing alphamb, and putting the interblock boundary somewhere with less p and v gradient you can only decrease it.
To me this is a big problem of Flow3D that needs to be resolved!!! F3D guys may have a better response for you.

Try alphamb = 1e-6 and alphamb = 1.0. See if the problem is resolved by setting one of these values.