MULT mesh creation affecting output significantly?

[John S.]

I am trying to optimize computation time for a simple case with a OBST created by Blender FDS with voxels and a small vent burning to a prescribed mass flow rate.  I am calculating soot and CO yield as well.  I have changed radiative angles to 2 and I am using HT3D.

I have used MULT to create the mesh:

&MESH IJK=15,15,15, XB=-.075,0.0,-.075,0.0,0,.075, MULT_ID='Mesh' /

&MULT ID='Mesh', DX=0.075, DY=0.075, DZ=0.075, I_UPPER=1, J_UPPER=1, K_UPPER=1 /

I then ran two simulations, one with the above mesh and one with one big mesh but same mesh size and dimensions.  

The 8 mesh code sped up computation by a factor of two, but the results were significantly different.  All other things being identical, the thermocouple I had placed on the edge of the OBST recorded an average of 3.5C higher temperature for the 8 mesh case.  The plot of the Temp vs. Time was also much less turbulent for the 8 mesh case.  In addition, in the SMV simulation the flame in the 8 mesh case was broken in sections and very choppy with some particles appearing, completely different from the 1 mesh case.

My question is:  Can you cut a plume/flame mesh and expect accurate results?  Can we expect the 1 mesh case to be more accurate if the mesh is cut through a vital part of the simulation?  Or is 3.5C just an acceptable variation in temp?  Is dividing the mesh always going to introduce some error/variation?

I know people sometimes divide their mesh so the flame/plume is all in one mesh, that is what I am going to try next.

Also an aside:  I changed my OBST cylinder from a repeated object MULT generated cylinder (as in FDS guide) to one drawn with voxels in Blender FDS, and the temperature of the thermocouple on average changed by 14C.  Does anyone have an opinion as to which type of cylinder physics would more accurately model the flame?  Specifically with HT3D on all OBST?

Thanks for any help!

[fde]

I suggest to verify HRR if it is the same in both cases

Compare velocity and pressure errors. More mesh means more error is generated at mesh boundaries. If error is significantly different, you might try decreasing VELOCITY_TOLERANCE and other pressure related settings. 

[Kevin McGrattan]

The default pressure solver in FDS is not exact on multiple meshes. Therefore, there will always be differences between a single and multiple mesh case unless you use one of the alternative pressure solvers (GLMAT, UGLMAT). These solvers are more time consuming. I suggest you decrease the VELOCITY_TOLERANCE below its default value, and increase the MAX_PRESSURE_ITERATIONS to drive down the error in the default pressure solver.

[John S.]

Thanks for the quick response.  I will try those.

Any opinion on the difference between repeated objects cylinders with MULT (FDS Guide) vs. voxel based cylinders drawn in Blender FDS?  I saw a 14C average temperature difference when switching from the repeated objects to the Blender voxels.

[Leobardo Valera]

Hello everyone,

I have run similar experiments. Checking results with respect to numbers of meshes and I have realized that the HRRPUV changes depending on the number of meshes.

Best regards

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