Single Flow Gravity effect

Jack_Mate · July 17, 2018, 07:49

Hello "CFDmates" how are you? So, here is the thing... I have a steady state, single phase, laminar, incompresible flow with a constant density and viscosity inside a microfluidics channel (my geometry has two parallel pipes, one at the upper side and one at the bottom side and they are connected to with a nozzle). I would like to consider the effect of the gravity in my model using the ANSYS CFX but without turn on the buoyancy option. How can I do it?

Thanks in advance!

evcelica · July 17, 2018, 14:14

It does not sound like gravity will have any effect at all, other than hydrostatic pressure on the walls, which you could just add in post processing.

Is the flow gravity driven or something?

You could set the the thermal expansion coefficient to zero (or a very small number 1e-30 if it rejects zero) then turn on buoyancy.
Sorry I don't fully understand what you are trying to model.

Jack_Mate · July 17, 2018, 15:08

Thank you for your reply! I do also believe that gravity plays a minor rolle in my case but I need to examine it. So, here are two images for better understanding

https://drive.google.com/file/d/12Su...ew?usp=sharing
https://drive.google.com/file/d/1NT0...ew?usp=sharing

I've been told that the fluid goes downwards because of the channel's geometry and not because of the gravity. Like in the outer space or something...

ghorrocks · July 17, 2018, 23:00

I am puzzled why you want to model buoyancy but do not want to turn the buoyancy model on. If you want to model buoyancy just turn the buoyancy model on.

If you don't like the built in buoyancy model you can always use a source term to do your own. The buoyancy term is just a simple source term.

Jack_Mate · July 18, 2018, 04:31

Hello! I just want to take into account the gravity without turning on the buoyancy since I do not have thermal phenomena and the fluid's density is constant.

ghorrocks · July 18, 2018, 07:15

If the fluid density is constant then gravity does nothing to the flow. All it does is add a hydrostatic head. If all you want is the pressure including the hydrostatic head then just turn buoyancy on and you will get it (in the absolute pressure variable). If gravity affects the flow somehow then you better explain how because nothing you have said so far would be affected by gravity.

Jack_Mate · July 18, 2018, 17:04

Thank you very much both of you! I did what evcelica said and my results were exactly the same! Now I have the proof I wanted!

July 17, 2018, 07:49	Single Flow Gravity effect	#1
Jack_Mate New Member Jack Join Date: Apr 2018 Posts: 5 Rep Power: 8	Hello "CFDmates" how are you? So, here is the thing... I have a steady state, single phase, laminar, incompresible flow with a constant density and viscosity inside a microfluidics channel (my geometry has two parallel pipes, one at the upper side and one at the bottom side and they are connected to with a nozzle). I would like to consider the effect of the gravity in my model using the ANSYS CFX but without turn on the buoyancy option. How can I do it? Thanks in advance!

July 17, 2018, 14:14		#2
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,188 Rep Power: 23	It does not sound like gravity will have any effect at all, other than hydrostatic pressure on the walls, which you could just add in post processing. Is the flow gravity driven or something? You could set the the thermal expansion coefficient to zero (or a very small number 1e-30 if it rejects zero) then turn on buoyancy. Sorry I don't fully understand what you are trying to model. Jack_Mate likes this.

July 17, 2018, 23:00		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	I am puzzled why you want to model buoyancy but do not want to turn the buoyancy model on. If you want to model buoyancy just turn the buoyancy model on. If you don't like the built in buoyancy model you can always use a source term to do your own. The buoyancy term is just a simple source term. Jack_Mate likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

July 18, 2018, 07:15		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	If the fluid density is constant then gravity does nothing to the flow. All it does is add a hydrostatic head. If all you want is the pressure including the hydrostatic head then just turn buoyancy on and you will get it (in the absolute pressure variable). If gravity affects the flow somehow then you better explain how because nothing you have said so far would be affected by gravity. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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July 17, 2018, 15:08		#3
Jack_Mate New Member Jack Join Date: Apr 2018 Posts: 5 Rep Power: 8	Thank you for your reply! I do also believe that gravity plays a minor rolle in my case but I need to examine it. So, here are two images for better understanding https://drive.google.com/file/d/12Su...ew?usp=sharing https://drive.google.com/file/d/1NT0...ew?usp=sharing I've been told that the fluid goes downwards because of the channel's geometry and not because of the gravity. Like in the outer space or something...

July 18, 2018, 04:31		#5
Jack_Mate New Member Jack Join Date: Apr 2018 Posts: 5 Rep Power: 8	Hello! I just want to take into account the gravity without turning on the buoyancy since I do not have thermal phenomena and the fluid's density is constant.

July 18, 2018, 17:04		#7
Jack_Mate New Member Jack Join Date: Apr 2018 Posts: 5 Rep Power: 8	Thank you very much both of you! I did what evcelica said and my results were exactly the same! Now I have the proof I wanted!