interFoam: strange pressure with high fluid viscosity?

ckroener · April 1, 2010, 08:22

Hi,

I want to model a visco-plastic fluid (using OpenFOAM-1.6.x), therefore i need to handle high viscosities. When I take for example the damBreak tutorial and just increase the viscosity of the fluid to nu [ 0 2 -1 0 0 0 0 ] 1e3,
my pressure field gives very strange values (0.05s and 0.10s):

I have tried to use a finer grid, and to increade the number of nCorrectors, and to decrease the tolerance values for the pressure solver.

It seems to be a problem with the interface, if i change the viscosity of the second fluid to the same viscosity, my pressure field looks fine.

What should I change to get a stable pressure field?

Thanks,
ckroener

sega · April 1, 2010, 17:15

What does the velocity field look like in these cases?

ckroener · April 6, 2010, 04:54

This is how the velocity field looks like:

Time 0.05s, scale factor for vectors: 500

Time: 0.1s, scale factor 5:

I have uploaded the case here: http://geo.uni-bonn.de/members/kroen...roblem.tar.bz2

The problem seems to be in the air phase?

Tanks a lot!
ckroener

sega · April 7, 2010, 12:51

As far as I know the CSF model for calculating the forces due to surface tension may have some problems when dealing with high density ratios.

In addition with high viscosity ratio you may run in even more problems.

Keeping in mind that the Navier-Stokes-Equations are solved with weighted average values for density $\rho = \rho_1 \cdot \gamma + (1-\gamma)\rho_2$ and vicosity $\nu= \nu_1 \cdot \gamma + (1-\gamma)\nu_2$ I'm picturing the situation in my head like the following:

The water column ( $\gamma=1$ , the visco-plastic fluid in your case) starts to move and generates high values (due to the high density and viscosity) for most of the terms in the Navier-Stokes-Equations.
These "high" forces act on the air-phase ( $\gamma=0$ ) and since it incorporates very low (in comparison) values for density and viscosity has to maintain the force balance by creating high velocities.

Correct me if I'm wrong. I'm aware that this is not very usefull for you, but maybe it's good for some brainstorming ...

April 1, 2010, 08:22	interFoam: strange pressure with high fluid viscosity?	#1
ckroener New Member Christian Kröner Join Date: Nov 2009 Location: Bonn Posts: 7 Rep Power: 16	Hi, I want to model a visco-plastic fluid (using OpenFOAM-1.6.x), therefore i need to handle high viscosities. When I take for example the damBreak tutorial and just increase the viscosity of the fluid to nu [ 0 2 -1 0 0 0 0 ] 1e3, my pressure field gives very strange values (0.05s and 0.10s): I have tried to use a finer grid, and to increade the number of nCorrectors, and to decrease the tolerance values for the pressure solver. It seems to be a problem with the interface, if i change the viscosity of the second fluid to the same viscosity, my pressure field looks fine. What should I change to get a stable pressure field? Thanks, ckroener

April 1, 2010, 17:15		#2
sega Senior Member Sebastian Gatzka Join Date: Mar 2009 Location: Frankfurt, Germany Posts: 729 Rep Power: 20	What does the velocity field look like in these cases? __________________ Schrödingers wife: "What did you do to the cat? It's half dead!"

April 6, 2010, 04:54	the velocity field	#3
ckroener New Member Christian Kröner Join Date: Nov 2009 Location: Bonn Posts: 7 Rep Power: 16	This is how the velocity field looks like: Time 0.05s, scale factor for vectors: 500 Time: 0.1s, scale factor 5: I have uploaded the case here: http://geo.uni-bonn.de/members/kroen...roblem.tar.bz2 The problem seems to be in the air phase? Tanks a lot! ckroener

April 7, 2010, 12:51		#4
sega Senior Member Sebastian Gatzka Join Date: Mar 2009 Location: Frankfurt, Germany Posts: 729 Rep Power: 20	As far as I know the CSF model for calculating the forces due to surface tension may have some problems when dealing with high density ratios. In addition with high viscosity ratio you may run in even more problems. Keeping in mind that the Navier-Stokes-Equations are solved with weighted average values for density $\rho = \rho_1 \cdot \gamma + (1-\gamma)\rho_2$ and vicosity $\nu= \nu_1 \cdot \gamma + (1-\gamma)\nu_2$ I'm picturing the situation in my head like the following: The water column ( $\gamma=1$ , the visco-plastic fluid in your case) starts to move and generates high values (due to the high density and viscosity) for most of the terms in the Navier-Stokes-Equations. These "high" forces act on the air-phase ( $\gamma=0$ ) and since it incorporates very low (in comparison) values for density and viscosity has to maintain the force balance by creating high velocities. Correct me if I'm wrong. I'm aware that this is not very usefull for you, but maybe it's good for some brainstorming ... stoneforshizq and JuRe09 like this. __________________ Schrödingers wife: "What did you do to the cat? It's half dead!"

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