courant number for hvac simulation

Chilli83 · May 5, 2009, 04:50

Hi all

I have a little problem when setting up the Solver Control for a simulation with occupants in a large room with displacement ventilation. The simulation should be run as transient for around 1800 s with the use of a very rough mesh just to see what happen with the flow.

My question is then when I have to set up the timestep initialisation I choose "automatic" and then I have to define a lower and upper courant number? Should the default values of 5 and 10 respectively be used or is it not necesary to define them?

I get a max courant number of 999.999 for my simulation with a timestep of 1 s and a max inner loop of 10 when I don't "click"/choose some values for lower and upper courant number.

Kind regards
Michael

ghorrocks · May 5, 2009, 21:39

Hi,

Leave everything at the default values unless you have a good reason to change them - and know what it means!

When you are starting a new simulation and you have no idea what the fluid timescales in the flow are it is a good idea to use adaptive timestepping with a very wide maximum and minimum limits to allow the solver to find a good timestep size by itself. The best policy is generally to home in 3-5 iterations per time step.

I suspect you will need a timestep size much smaller than you are currently using, at least in the startup transient.

Glenn Horrocks

May 5, 2009, 04:50	courant number for hvac simulation	#1
Chilli83 New Member Michael Join Date: Apr 2009 Posts: 20 Rep Power: 18	Hi all I have a little problem when setting up the Solver Control for a simulation with occupants in a large room with displacement ventilation. The simulation should be run as transient for around 1800 s with the use of a very rough mesh just to see what happen with the flow. My question is then when I have to set up the timestep initialisation I choose "automatic" and then I have to define a lower and upper courant number? Should the default values of 5 and 10 respectively be used or is it not necesary to define them? I get a max courant number of 999.999 for my simulation with a timestep of 1 s and a max inner loop of 10 when I don't "click"/choose some values for lower and upper courant number. Kind regards Michael

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May 5, 2009, 21:39		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 18,018 Rep Power: 146	Hi, Leave everything at the default values unless you have a good reason to change them - and know what it means! When you are starting a new simulation and you have no idea what the fluid timescales in the flow are it is a good idea to use adaptive timestepping with a very wide maximum and minimum limits to allow the solver to find a good timestep size by itself. The best policy is generally to home in 3-5 iterations per time step. I suspect you will need a timestep size much smaller than you are currently using, at least in the startup transient. Glenn Horrocks