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grid convergence study question

Question asked by Dan Hofstetter on Jan 11, 2015
Latest reply on Jan 15, 2015 by Jared Conway

Hi 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:

 

Case123456
Nx70140168202242280
Ny183643526272
Nz224453637688
Total Cells27,720221,760383,201662,1721,144,2331,774,080

 

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 steady-state 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. 

 

Graph.jpg

 

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 1/2-million cells in a room of that size?

 

The next steps I need to accomplish are to perform a time-step sensitivity study, then run a transient simulation and try to achieve a time-accurate gas decay solution that I can compare to measured data.  Any help is appreciated!

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