Simulation of non-newtonian fluid giving bad results?

Kambrook14 · November 9, 2021, 20:29

Hi all,

I am attempting to simulate the flow of blood through an arterial system in the brain and then measure the wall shear at a point in a particular artery. My system contains 2 inlets with mass flow conditions and a large range of outlets with relative pressure conditions.

During simulation i get lots of messages saying that flow has reversed in the outlets and I am getting ridiculous results of a constant pressure through the entire system of 170MPa? Why would i be getting these ridiculous results with a very minuscule mass flow at the inlet?

Thanks in advance

LoGaL · November 10, 2021, 02:38

It maybe that the combination of pressure outlet and mass flow inlet is not feasible for the viscosity ( and pressure drop) associated. How are you selecting the non newtonian fluid properties? I bet my money on out of the chart viscosity values… I would expect blood to be ~4 times less viscous than water

Kambrook14 · November 10, 2021, 06:05

For the non-newtonian fluid (blood) i used a density from literature and used the carreau model for the viscosity, again with parameters from other literature

LoGaL · November 10, 2021, 06:25

Ok so we have density which is more or less the one of water and some model for viscosity right? Which values of viscosity you get? Could you plot a dynamic viscosity contour? It has to be basically less then water viscosity

Kambrook14 · November 11, 2021, 22:14

Shouldn't blood be more viscous than water rather than less? I re-ran the simulation this time without a non-newtonian model for viscosity, just a constant value and kept everything else the same. Regardless I am still running into the same issue of having a constant pressure at every point in the system as well as a wall shear stress of zero and reversed flows at the outlets

AlexanderZ · November 11, 2021, 22:58

which model are you using for density?

going back to your first question, you have abnormal pressure because of problem in case settings.
go step by step through each setting and check carefully.

try to find reference cases in google / youtube

LoGaL · November 12, 2021, 16:05

if you get the same issue using a normal viscosity model, alexander is right. It unfortunately has to be some stupid mistake around in the setup, we all make them.

You say mass flow inlet, pressure outlet. Constant density? (This one i am sure it is basically water). Is the operating pressure set to zero or to a number? Did you check mesh quality?

Concerning blood viscosity, shouldn't it be less, at least in tiny vessels?

Kambrook14 · November 14, 2021, 22:59

Thank you for all the help guys. I have gone back through my set up, and have changed the outlets to "outflow" rather than a pressure outlet and this has seemed to fix the problem. How does operating pressure affect my solution and what value should this be set too?

While the constant pressure has been fixed i am still getting reverse flow at the outlets and very unrealistic pressure values across the system (Between 1-2MPa). Could this be related to the operating temperature?

November 9, 2021, 20:29	Simulation of non-newtonian fluid giving bad results?	#1
Kambrook14 New Member New South Wales Join Date: Nov 2021 Posts: 4 Rep Power: 4	Hi all, I am attempting to simulate the flow of blood through an arterial system in the brain and then measure the wall shear at a point in a particular artery. My system contains 2 inlets with mass flow conditions and a large range of outlets with relative pressure conditions. During simulation i get lots of messages saying that flow has reversed in the outlets and I am getting ridiculous results of a constant pressure through the entire system of 170MPa? Why would i be getting these ridiculous results with a very minuscule mass flow at the inlet? Thanks in advance

November 11, 2021, 22:58		#6
AlexanderZ Senior Member Alexander Join Date: Apr 2013 Posts: 2,363 Rep Power: 34	which model are you using for density? going back to your first question, you have abnormal pressure because of problem in case settings. go step by step through each setting and check carefully. try to find reference cases in google / youtube __________________ best regards ****************************** press LIKE if this message was helpful

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November 10, 2021, 02:38		#2
LoGaL Senior Member Lorenzo Galieti Join Date: Mar 2018 Posts: 373 Rep Power: 12	It maybe that the combination of pressure outlet and mass flow inlet is not feasible for the viscosity ( and pressure drop) associated. How are you selecting the non newtonian fluid properties? I bet my money on out of the chart viscosity values… I would expect blood to be ~4 times less viscous than water

November 10, 2021, 06:05		#3
Kambrook14 New Member New South Wales Join Date: Nov 2021 Posts: 4 Rep Power: 4	For the non-newtonian fluid (blood) i used a density from literature and used the carreau model for the viscosity, again with parameters from other literature

November 10, 2021, 06:25		#4
LoGaL Senior Member Lorenzo Galieti Join Date: Mar 2018 Posts: 373 Rep Power: 12	Ok so we have density which is more or less the one of water and some model for viscosity right? Which values of viscosity you get? Could you plot a dynamic viscosity contour? It has to be basically less then water viscosity

November 11, 2021, 22:14		#5
Kambrook14 New Member New South Wales Join Date: Nov 2021 Posts: 4 Rep Power: 4	Shouldn't blood be more viscous than water rather than less? I re-ran the simulation this time without a non-newtonian model for viscosity, just a constant value and kept everything else the same. Regardless I am still running into the same issue of having a constant pressure at every point in the system as well as a wall shear stress of zero and reversed flows at the outlets

November 12, 2021, 16:05		#7
LoGaL Senior Member Lorenzo Galieti Join Date: Mar 2018 Posts: 373 Rep Power: 12	if you get the same issue using a normal viscosity model, alexander is right. It unfortunately has to be some stupid mistake around in the setup, we all make them. You say mass flow inlet, pressure outlet. Constant density? (This one i am sure it is basically water). Is the operating pressure set to zero or to a number? Did you check mesh quality? Concerning blood viscosity, shouldn't it be less, at least in tiny vessels?

November 14, 2021, 22:59		#8
Kambrook14 New Member New South Wales Join Date: Nov 2021 Posts: 4 Rep Power: 4	Thank you for all the help guys. I have gone back through my set up, and have changed the outlets to "outflow" rather than a pressure outlet and this has seemed to fix the problem. How does operating pressure affect my solution and what value should this be set too? While the constant pressure has been fixed i am still getting reverse flow at the outlets and very unrealistic pressure values across the system (Between 1-2MPa). Could this be related to the operating temperature?