Learning Forum

[narendrapatel111]

Hello, ##Fluent.

I am using pressure based solver, implicit scheme, VOF method (solving steady state open channel flow), k-e-RNG, and Coupled algo. Initially, I assigned Courant number = 10 and got flow field but residual for continuity is not dropping in the order of 10^-5. Also, I have mass imbalance problem (mass inflow is not matching with mass outflow). For coarser mesh, I didn't face mass imbalance problem but to improve results, I have refined mesh, and this unexpected problem showed up. I am using under relaxation factors 0.3. Is it due to inappropriate value of Courant number? Any suggestion to drop the residual of continuity and get mass balance?

[RK]

Hello, 

When you are coarse and fine mesh, can you give us the statistics so we can better assist you. Thanks. 

[narendrapatel111]

Hi,

It's a 3D problem, geometry is like cuboid. Assume width(x), streamwise (y), and height (z).  I am mainly interested for flow in y and z direction so I refine the mesh in y-z plane. I am simulating open channel flow using VOF.

For coarser mesh: element size in Y-Z plane was 3.6mm, Inflow and out flow matched, error was less than 1%.

For refined mesh: element size in Y-Z plane was 3.0mm, inflow -outflow mas imbalance is 10%. (outflow is less)

Residual for continuity is not dropping below 3.5x10^-4.

 

[RK]

Yes, probably courant number can be the factor here. 

[narendrapatel111]

I couldn't get desired mass balance. Error in mass balance decreased initially and then it started increasing. I reduced courant number to 2.

[DrAmine]

What about using Explicit Scheme and much more conservative settings: please start with default settings and use CFL number not larger than 2.