Non Newtonian fluids: gravity or no gravity ?

MjG · April 1, 2018, 08:59

hello, I have a quick question for you. I have a square cross section (64μm^2) microfluidics channel. The fluid flow is parallel to the xz plane (with the yy' to be the vertical axis). My fluid is the blood and I used the Bird-Carreu formula for modeling it. Also my flow is pressure driven ( I have applied an inlet velocity and I set the outlet pressure to be 0 Pa). Do I have to take into account the gravity or not?

piu58 · April 1, 2018, 13:04

Of course you need to take into account the gravity.

BTW: A giraffe has an systolic pressure of around 400 to fight against the gravity. For not to get varices they has some kind of elastic stockings.

MjG · April 1, 2018, 13:16

Thank you for your answer! I was thought that the only case we can take gravity into account was when the fluid flows vertically in a pipe or when a pipe is tilted.

flotus1 · April 1, 2018, 13:17

I assume Uwe is just teasing you. A giraffe ist just about the opposite of what you are trying to simulate: macro-channel with gravity pointing in stream-wise direction vs. micro-channel with gravity pointing cross-stream.
The safest/easiest way to check (i.e. without making some estimates) is by just running both simulations and comparing the results.

MjG · April 1, 2018, 13:24

Yes I know that I have to do this eventualy ... but my laptop is not very good and it realy takes a lot to give me a result ... So I was hopping for someone to give me an answer. What parameter should I test? Is the pressure in the middle of the channel a good indicator? I am studying a FSI problem.

FMDenaro · April 1, 2018, 13:28

Gravity is important in a complete real circulation but if you are modelling such a small channel, the body force can be neglected compared to the pressure driven force.

April 1, 2018, 08:59	Non Newtonian fluids: gravity or no gravity ?	#1
MjG New Member macaffy Join Date: Jul 2017 Posts: 15 Rep Power: 8	hello, I have a quick question for you. I have a square cross section (64μm^2) microfluidics channel. The fluid flow is parallel to the xz plane (with the yy' to be the vertical axis). My fluid is the blood and I used the Bird-Carreu formula for modeling it. Also my flow is pressure driven ( I have applied an inlet velocity and I set the outlet pressure to be 0 Pa). Do I have to take into account the gravity or not?

April 1, 2018, 13:04		#2
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	Of course you need to take into account the gravity. BTW: A giraffe has an systolic pressure of around 400 to fight against the gravity. For not to get varices they has some kind of elastic stockings. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

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April 1, 2018, 13:16		#3
MjG New Member macaffy Join Date: Jul 2017 Posts: 15 Rep Power: 8	Thank you for your answer! I was thought that the only case we can take gravity into account was when the fluid flows vertically in a pipe or when a pipe is tilted.

April 1, 2018, 13:17		#4
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,400 Rep Power: 47	I assume Uwe is just teasing you. A giraffe ist just about the opposite of what you are trying to simulate: macro-channel with gravity pointing in stream-wise direction vs. micro-channel with gravity pointing cross-stream. The safest/easiest way to check (i.e. without making some estimates) is by just running both simulations and comparing the results.

April 1, 2018, 13:24		#5
MjG New Member macaffy Join Date: Jul 2017 Posts: 15 Rep Power: 8	Yes I know that I have to do this eventualy ... but my laptop is not very good and it realy takes a lot to give me a result ... So I was hopping for someone to give me an answer. What parameter should I test? Is the pressure in the middle of the channel a good indicator? I am studying a FSI problem.

April 1, 2018, 13:28		#6
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,782 Rep Power: 71	Gravity is important in a complete real circulation but if you are modelling such a small channel, the body force can be neglected compared to the pressure driven force.