Single Flow Gravity effect

Jack_Mate · July 17, 2018, 06:49

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, 13:14

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, 14:08

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, 22:00

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, 03:31

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, 06:15

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, 16:04

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, 06:49	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, 13:14		#2
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,167 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, 22:00		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	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, 06:15		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	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.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Issues on the simulation of high-speed compressible flow within turbomachinery	dowlee	OpenFOAM Running, Solving & CFD	11	August 6, 2021 06:40
Review: Reversed flow	CRT	FLUENT	1	May 7, 2018 05:36
Eulerian multiphase flow- flow direction not comply with gravity	mejahan	CFX	11	April 17, 2017 04:19
Difference in flow distribution of single phase and multiphase analysis	shivasluzz	CFX	0	April 22, 2015 22:54
Free Surface Gravity Driven Flow	Rupp Carriveau	FLUENT	4	June 14, 2001 16:54

July 17, 2018, 14:08		#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, 03:31		#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, 16:04		#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!