Buoyancy Reference Density [+Multicomponent Flow]

Felggv · May 18, 2012, 16:56

Hello everyone,

I'm concerned about BRD. As I'm modeling a air+water vapor flow, it only solves if the BRD is set to something between 0.5 and 1.23 (air ~ 25ºC and 1atm), otherwise the solver crashes.

I was wondering if someone could tell me why it happens... I'll study the influence of this variation (0.5~1.23) on the trajetory of the water vapor jet that comes from the inlet.

Should I always use the lighter fluid density? What it would change? If the program has the material properties on its library, why does he ask us to set the reference if we set the Initial Temperature and pressure on the domain editor?

Thanks in advance,

Felipe.

ghorrocks · May 19, 2012, 07:18

The BRD is the same in concept as all other reference parameters. The simulation models the deviation from the reference quantity so the modelled quantity is as small as possible to minimise round-off errors. So the BRD (and all other reference parameters) should be set to minimise the range the simulated parameters (density in this case) need to span in the simulation.

Therefore it is not a surprise that your simulation only converges for a small range of BRD around the actual density of your fluid. It is in this region that it has reduced the density round off error sufficiently that the numerics can converge.

Felggv · May 19, 2012, 14:29

Thank you once again Glenn! Now it makes sense to me.

I'll be back with more doubts later hahahaha.

Felipe.

May 18, 2012, 16:56	Buoyancy Reference Density [+Multicomponent Flow]	#1
Felggv Member Felipe Gobbi Join Date: Apr 2012 Location: Brazil Posts: 76 Rep Power: 14	Hello everyone, I'm concerned about BRD. As I'm modeling a air+water vapor flow, it only solves if the BRD is set to something between 0.5 and 1.23 (air ~ 25ºC and 1atm), otherwise the solver crashes. I was wondering if someone could tell me why it happens... I'll study the influence of this variation (0.5~1.23) on the trajetory of the water vapor jet that comes from the inlet. Should I always use the lighter fluid density? What it would change? If the program has the material properties on its library, why does he ask us to set the reference if we set the Initial Temperature and pressure on the domain editor? Thanks in advance, Felipe.

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May 19, 2012, 07:18		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	The BRD is the same in concept as all other reference parameters. The simulation models the deviation from the reference quantity so the modelled quantity is as small as possible to minimise round-off errors. So the BRD (and all other reference parameters) should be set to minimise the range the simulated parameters (density in this case) need to span in the simulation. Therefore it is not a surprise that your simulation only converges for a small range of BRD around the actual density of your fluid. It is in this region that it has reduced the density round off error sufficiently that the numerics can converge.

May 19, 2012, 14:29		#3
Felggv Member Felipe Gobbi Join Date: Apr 2012 Location: Brazil Posts: 76 Rep Power: 14	Thank you once again Glenn! Now it makes sense to me. I'll be back with more doubts later hahahaha. Felipe.