AIR or Water

HMR · March 31, 2011, 00:45

Hi,

I planning to do simulation on ventilation and cooling systems. For my numerical simulation if I choose air as a working fluid which fluid should I use during experimental validation time(air or water)?Project proposal advice me to use air as a working fluid during CFD simulation but water should be used during experimental validation time, in here I am bit confuse, I prepare geometry and meshing on CFX as per experimental setup specification but in CFX pre if I choose air as working medium then air inlet velocity to the domain in simulation will be not same as water velocity in the experimental testing tank.How can I compare different parameter based on air data with same as from water data.

Could anybody suggest me in this regards.

Thanks in advance.

HMR

ghorrocks · March 31, 2011, 06:34

I don't recall seeing water coming through a ventilation system recently.

Honestly, are you serious? If you are modelling the flow of air, then you use air as the working fluid. Surely that is obvious.

I can only imagine that your experimental results use a water analog model to scale down the geometry size but keep the Reynolds number the same.

HMR · April 4, 2011, 05:28

Hi Glen,

Well, I am new in this field, so I have so many questions, may be some question is not logical.

Anyway, thank you very much for your valuable information.

Could you please give me some little bit advice.

I briefly tell my plan, in my numerical simulation part, I will inject cold air at 25degree C (density A) into a room (size 1mx1mx1m) with hot air at 30 degree C(density B) through a round diffuser, here obviously A>B.

For validate above simulation I planned to use water at 25degree C (density X) which I will inject in to a fluid (density Y) in a testing tank(size 1mx1mx1m) through a nozzle which is similar in size and shape of above diffuser.

1. Could you please tell me whether density X will be greater than density Y or not. and
2. How can I made relation in between density A and density C, anything like dynamic similarity or any other way.

Sorry to bother you.

Regards

HMR

ghorrocks · April 4, 2011, 19:25

Just about all fluids decrease in density as temperature increases. There are not many exceptions.

You have not told us what fluid Y is. If it is a different fluid than fluid X then who knows what the density difference is.

Likewise, what is Fluid C?

This is all basic thermodynamics. If you cannot work this sort of stuff out you are not going to get far in CFD.

HMR · April 4, 2011, 22:06

Hi Glenn,

Thanks for your advice,

In my experimental part I will use saline water (water and salt mixer) as Fluid C and its density Y will be depends on Froude number and Reynolds number.

ghorrocks · April 5, 2011, 18:36

I think you will find the density of a water/salt mixture depends on the salt concentration mainly, and to a lesser extent on temperature. I cannot see where Reynolds number and Froude number could affect density.

HMR · April 7, 2011, 03:28

Hi Glenn,

Many many thanks for your advice

March 31, 2011, 00:45	AIR or Water	#1
HMR Member HMR Join Date: Jan 2011 Posts: 31 Rep Power: 15	Hi, I planning to do simulation on ventilation and cooling systems. For my numerical simulation if I choose air as a working fluid which fluid should I use during experimental validation time(air or water)?Project proposal advice me to use air as a working fluid during CFD simulation but water should be used during experimental validation time, in here I am bit confuse, I prepare geometry and meshing on CFX as per experimental setup specification but in CFX pre if I choose air as working medium then air inlet velocity to the domain in simulation will be not same as water velocity in the experimental testing tank.How can I compare different parameter based on air data with same as from water data. Could anybody suggest me in this regards. Thanks in advance. HMR

March 31, 2011, 06:34		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,705 Rep Power: 143	I don't recall seeing water coming through a ventilation system recently. Honestly, are you serious? If you are modelling the flow of air, then you use air as the working fluid. Surely that is obvious. I can only imagine that your experimental results use a water analog model to scale down the geometry size but keep the Reynolds number the same. zhaowei and cfd_newbie like this.

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April 4, 2011, 05:28		#3
HMR Member HMR Join Date: Jan 2011 Posts: 31 Rep Power: 15	Hi Glen, Well, I am new in this field, so I have so many questions, may be some question is not logical. Anyway, thank you very much for your valuable information. Could you please give me some little bit advice. I briefly tell my plan, in my numerical simulation part, I will inject cold air at 25degree C (density A) into a room (size 1mx1mx1m) with hot air at 30 degree C(density B) through a round diffuser, here obviously A>B. For validate above simulation I planned to use water at 25degree C (density X) which I will inject in to a fluid (density Y) in a testing tank(size 1mx1mx1m) through a nozzle which is similar in size and shape of above diffuser. 1. Could you please tell me whether density X will be greater than density Y or not. and 2. How can I made relation in between density A and density C, anything like dynamic similarity or any other way. Sorry to bother you. Regards HMR

April 4, 2011, 19:25		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,705 Rep Power: 143	Just about all fluids decrease in density as temperature increases. There are not many exceptions. You have not told us what fluid Y is. If it is a different fluid than fluid X then who knows what the density difference is. Likewise, what is Fluid C? This is all basic thermodynamics. If you cannot work this sort of stuff out you are not going to get far in CFD.

April 4, 2011, 22:06		#5
HMR Member HMR Join Date: Jan 2011 Posts: 31 Rep Power: 15	Hi Glenn, Thanks for your advice, In my experimental part I will use saline water (water and salt mixer) as Fluid C and its density Y will be depends on Froude number and Reynolds number.

April 5, 2011, 18:36		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,705 Rep Power: 143	I think you will find the density of a water/salt mixture depends on the salt concentration mainly, and to a lesser extent on temperature. I cannot see where Reynolds number and Froude number could affect density.

April 7, 2011, 03:28		#7
HMR Member HMR Join Date: Jan 2011 Posts: 31 Rep Power: 15	Hi Glenn, Many many thanks for your advice