Scale Down the Model

shahid nadeem · August 22, 2014, 02:53

Hello Dear Friends can anyone tell me how to specify the inlet and outlet boundary conditions when i have scalwd down the model to some smaller dimensions. Please guide me on this

flotus1 · August 22, 2014, 03:35

Why would you want to scale the model in a CFD analysis?
The same topic was discussed here:
http://www.cfd-online.com/Forums/mai...raft-wing.html

BTW: please dont cross-post the same question in different forums. This is usually considered as spam.

shahid nadeem · August 22, 2014, 05:32

Oh i was not aware of it thank you.
And thanks for the link.
And actually i have a pipe of diameter 2 Meter and Length 20 meter so i doubt it would get solved in my system so i was planning to scale my model to 1/4 and run simulation and from the scale down model find the pressure drop and multiply with if any factor and give the pressure drop value of the original model.
I can scale the geometry easily but my doubt is how should i change my boundary conditions like i have mass flow rate of 16 kg/s as inlet or 0.4 bar as pressure inlet and 0.01 milibar as pressure outlet which is the vaccum condition since the flow gets exits in the vaccum vessel. so i am confuse what value should i give as the pressure inlet for the scale down model so if you have any experience in this help me out.

flotus1 · August 22, 2014, 06:04

From your description of the model I dont see any chance to reduce the computational effort apart from making use of symmetries (if there are any).

"Scaling" the geometry, i.e. simulation only a fraction of the full 20 m length of the pipe would only make sense if there were no gradients in this direction.
But since you have a high pressure gradient between inlet and outlet, I dont think that you can make this assumption.

shahid nadeem · August 23, 2014, 00:38

Yes sir i have symmetry conditions i ll go with it. another small doubt the fluid gets exits in the vaccum vessel which is maintained at 0.01milibar so i should just define my outlet pressure as pressure outlet gauge pressure as 0.01milibar write or is there a way to defien the vaccum conditions for the outlet.

Thanks

flotus1 · August 23, 2014, 17:55

I dont know which software you are working with.
But the usual procedure for high pressure differences is setting the reference pressure to zero and working with absolute pressure values instead of gauge pressure.

August 22, 2014, 02:53	Scale Down the Model	#1
shahid nadeem Member Nazim Join Date: Oct 2013 Posts: 93 Rep Power: 12	Hello Dear Friends can anyone tell me how to specify the inlet and outlet boundary conditions when i have scalwd down the model to some smaller dimensions. Please guide me on this

August 22, 2014, 05:32	Pipe scale down	#3
shahid nadeem Member Nazim Join Date: Oct 2013 Posts: 93 Rep Power: 12	Oh i was not aware of it thank you. And thanks for the link. And actually i have a pipe of diameter 2 Meter and Length 20 meter so i doubt it would get solved in my system so i was planning to scale my model to 1/4 and run simulation and from the scale down model find the pressure drop and multiply with if any factor and give the pressure drop value of the original model. I can scale the geometry easily but my doubt is how should i change my boundary conditions like i have mass flow rate of 16 kg/s as inlet or 0.4 bar as pressure inlet and 0.01 milibar as pressure outlet which is the vaccum condition since the flow gets exits in the vaccum vessel. so i am confuse what value should i give as the pressure inlet for the scale down model so if you have any experience in this help me out.

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August 22, 2014, 03:35		#2
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,398 Rep Power: 46	Why would you want to scale the model in a CFD analysis? The same topic was discussed here: http://www.cfd-online.com/Forums/mai...raft-wing.html BTW: please dont cross-post the same question in different forums. This is usually considered as spam.

August 22, 2014, 06:04		#4
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,398 Rep Power: 46	From your description of the model I dont see any chance to reduce the computational effort apart from making use of symmetries (if there are any). "Scaling" the geometry, i.e. simulation only a fraction of the full 20 m length of the pipe would only make sense if there were no gradients in this direction. But since you have a high pressure gradient between inlet and outlet, I dont think that you can make this assumption.

August 23, 2014, 00:38		#5
shahid nadeem Member Nazim Join Date: Oct 2013 Posts: 93 Rep Power: 12	Yes sir i have symmetry conditions i ll go with it. another small doubt the fluid gets exits in the vaccum vessel which is maintained at 0.01milibar so i should just define my outlet pressure as pressure outlet gauge pressure as 0.01milibar write or is there a way to defien the vaccum conditions for the outlet. Thanks

August 23, 2014, 17:55		#6
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,398 Rep Power: 46	I dont know which software you are working with. But the usual procedure for high pressure differences is setting the reference pressure to zero and working with absolute pressure values instead of gauge pressure.