Mixed convection with large Buoyancy force - cannot converge
Hi
i am simulating the flow and heat transfer in a rectangular cavity, i want to get the temperature distribution in the steady state. It's size is 6m(length)*10m(width)*8m(height), the top surface (6m*10m) is considered as velocity inlet with constant and uniform velocity and temperature of V = 0.023m/s and T = 137 K, the four side walls are considered as constant and uniform temperature of Tw =177K. The outlet is located at the bottom of the cavity. Nitrogen is considered as the working fluid, and the boussinesq approximation for density is used. Due to symmetry, i modeled 1/4 of the cavity. The grid size is tested to get the Y+ between about 30 and 70. The Reynolds number and Grashof number for the above condition is calculated as about 10^4 and 10^14, respectively. RNG-ke model is used for turbulence. "Standard wall functions" is used for near wall treatment. Under the above boundary conditions, i can get the converged solution, as the residual for all variables decrease to less than 10^-4. Then i want to get the results for other inlet velocities, but i cannot get the correct results. 1. When i increase the inlet velocity to 2V or even higher, the residuals keep fluctuating at the level about 10^-1, and no converged solution can be get. I tried to increase the grid number, but it still does not work. 2. I decreased the inlet velocity to V/2, i can get the converged solution. But the results are obviously not correct, as the results show the average temperature in the cavity becomes a little smaller. This problem has confused me for a few weeks, as each calculation lasts for 2 days. i don't know if the models and procedure i used for such parameters are correct, or the problem itself is not stable? please give me some advise if you can help. Thanks a lot. |
I think the CFL number is too high when you increase your velocity to 2V. Try to change the time step and the mesh size to keep CFL number below 1.
|
Quote:
|
All times are GMT -4. The time now is 17:14. |