# HVAC coarse mesh

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 November 12, 2009, 09:07 HVAC coarse mesh #1 New Member   fabio Join Date: Aug 2009 Posts: 17 Rep Power: 10 Hello all Im modeling an office environment with inlet air from the air conditioner at 17 and an return plenum duct. In this, people ate cilinders with the medium adult human area (1,8 m²), computers with heat flux and temperature walls taken from Energyplus simulations. But my simulation is diverging, i think because of my inlet-outlet boundary conditions and my timestep. I have three questions: 1) A coarse mesh can make my simulation diverge? 2) What should be the inlet-oulet conditions for the model? (I already tryed massflow-massflow, velocity-velocity, volocity-pressure) 3) What could be the bigger timestep that should not make the simulation diverge? (What always happen is the overflow error, due to mommentum and mass errors in the linear solver) Thanks 4 who´ll help!

December 3, 2009, 17:03
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Join Date: Jul 2009
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Quote:
 Originally Posted by fabioacfoz Hello all Im modeling an office environment with inlet air from the air conditioner at 17 and an return plenum duct. In this, people ate cilinders with the medium adult human area (1,8 m²), computers with heat flux and temperature walls taken from Energyplus simulations. But my simulation is diverging, i think because of my inlet-outlet boundary conditions and my timestep. I have three questions: 1) A coarse mesh can make my simulation diverge? 2) What should be the inlet-oulet conditions for the model? (I already tryed massflow-massflow, velocity-velocity, volocity-pressure) 3) What could be the bigger timestep that should not make the simulation diverge? (What always happen is the overflow error, due to mommentum and mass errors in the linear solver) Thanks 4 who´ll help!
Hi,

What solver are you using? Coarse meshes can give convergence errors, what's your discretisation scheme and what's your time step? These are very strongly related, ie use a small timestep, not longer than your air residence time.

As for boundary conditions, are they based on experimental or profile data? I noticed often a profile inlet boundary conditions and a pressure outlet works well

 December 3, 2009, 19:34 #3 New Member   fabio Join Date: Aug 2009 Posts: 17 Rep Power: 10 Yeah king, we discussed this on the other post I created. I found out that my mesh was too refined, what was forcing me to use very small timesteps, else the overflow error ocurrs. When I made a coarser mesh, i could use bigger timesteps and then run my simulation. But thanx, you really helped my end course work!

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