grid convergence study question
Dan Hofstetter Jan 11, 2015 9:23 PMHi all,
Does anyone have any tips or tricks for performing a grid convergence study in Flow Simulation? I need to try to determine the grid convergence index (GCI) for my simulation. It seems that to do it right, I have to disable all of the automatic settings. I have an airflow problem where I have set up the following parameters in the manual mesh settings:
Case  1  2  3  4  5  6 
Nx  70  140  168  202  242  280 
Ny  18  36  43  52  62  72 
Nz  22  44  53  63  76  88 
Total Cells  27,720  221,760  383,201  662,172  1,144,233  1,774,080 
However, the total cell count never works out to those numbers due to some of the mesh refinements that occur, such as finer mesh in corners. I solved steadystate simulations for each case, using global goals for average static pressure, turb dissipation, turb energy, Vx, Vy, Vz, and Mass of Fluid. I left all of the convergence criteria set to auto. For a grid study, would it be better for me to manually set the convergence criteria so each simulation solves to the same level of accuracy? Or are the automatic criteria good enough?
After all simulations finished, I compared Velocity at several points (using point parameters) and plotted the values from each case. What I am finding is that the values don't seem to be converging, and in many cases are oscillating as much as 20% from one case to the next even at the highest mesh levels used so far.
The room I am simulating is 36' long x 9 ft wide x 8 ft high, and the total flow rate is less than 2,000 cfm (see attached model). Should I really need more than 1/2million cells in a room of that size?
The next steps I need to accomplish are to perform a timestep sensitivity study, then run a transient simulation and try to achieve a timeaccurate gas decay solution that I can compare to measured data. Any help is appreciated!

Model.SLDPRT.zip 11.6 MB