Learning Forum

[Tareq]

How to find the Stress tensor (xy component) in Ansys fluent?
After searching in Ansys fluent post-processing i could not find a way to find the Stress tensor (xy component), any suggestions please

[DrAmine]

Strain tensor components can be found under derivatives 

[Tareq]

I am looking for stress tensor not strain tensor, please do you know how to get the stress tensor and if you have example would be great

[DrAmine]

You just multiply by the viscosity and you get what you need. No I do not have any example.

[Tareq]

i am dealing with DEM model and the stress tensor value needed to find the viscosity, the viscosity is not known for the fluid, please do you have another method to find the stress tensor without using the viscosity equation

[DrAmine]

But the stress tensor or strain tensor is only available for the carrier phase and not for particulate phase. 

What do you want to do?

[Tareq]

I am studying the variation of viscosity with the increase of the particle percentage, but I can not measure the viscosity, each time I am measuring it I go the setting viscosity of the fluid-like it has no effect by the particles, and I read it can be measured by the stress tensor

 

please, do you have an idea of how to measure it?

[Rob]

The viscosity won't be affected by the particle volume fraction: it's still the fluid viscosity. If you're wanting to model a slurry using particles then you'll need to review what you're measuring: how is a slurry viscosity calculated/measured experimentally?

[Tareq]

Thank you for the answer

Actually the measurement principle is based on rheometer based on shear rate and shear stress, but in particles its not working because it will give the viscosity of the fluid without including the effect of the particles volume fraction

the paper below say exactly what I want to do :

Simulating the rheology of dense colloidal suspensions using dissipative particle dynamics
E. S. Boek* and P. V. Coveney

https://journals.aps.org/pre/abstract/10.1103/PhysRevE.55.3124

[DrAmine]

You can adjust the viscosity of the fluid to be non Newtonian shear thinning based on particle concentration in cell. Do not expect thst we will read the paper you are sharing

[Tareq]

it's not about the particle concentration only, it's about the momentum exchange between the fluid and particles, so this won't work. Still, the main question is how to get the stress tensor and by it, I can find the effect of particles on viscosity.

Please do you have an idea of how to get the stress tensor but without using the viscosity equation you mentioned before?

the paper abstract is for further information if you like to know more

[Rob]

In the experiment you (should) have correlations for shear v strain at different particle loading?  In CFD models for slurries we either model the fluid only as a nonNewtonian fluid or we model the particles in a (usually) constant viscosity fluid.  All the strain rate/stress values are for the fluid phase. 

[Tareq]

what we are modelling is same as "we model the particles in a (usually) constant viscosity fluid" and the correlation between the shear v strain is exist, but I have a viscosity from measurement at certain particles concentration and I made the simulation for the same situation based on rheometer setup but I do not know how to get the viscosity values that effected by the particles collision and momentum exchange, with any particles concentration I am getting only the constant viscosity fluid, but I need the effective viscosity that is different according to the concentration of the particle. the last paper said it's related to the stress tensor and tht why I am looking forward to get it

please how to measure it in ansys fluent?

[DrAmine]

There is no stress tensor written for particles. We only have strain components. You can still use a Non-Newtonian law for viscosity make it dependent on particle concentration and on the same time have the coupling between the particles and the continuous phase.

[Tareq]

The coupling is activated but as I mentioned "It's not about the particle concentration only, it's about the momentum exchange between the fluid and particles, so this won't work. Still, the main question is how to get the stress tensor and by it, I can find the effect of particles on viscosity", I found many papers about particle modelling but no one mentioned clearly how he found the effective viscosity values and my research depend on this now

[DrAmine]

What is now missing in all details provided by me and by rwoolhou? 

Are you referring to the so called apparent viscosity? You can calculate it by means of the viscosity of the carrier phase (your liquid) and particle volume fraction / concentration.

[Tareq]

till now I did not have a clear answer on how to find the effective viscosity, my question is how to find the effective viscosity in Ansys fluent in DEM

and this equation you mentioned  "the viscosity of the carrier phase (your liquid) and particle volume fraction / concentration." is different from the THE Einstein equation E = E0(1 + 2.5 V) to estimate the effective viscosity and again for the third time ""It's not about the particle concentration only, it's about the momentum exchange between the fluid and particles" 

Please let me know if you know the answer or if you know someone knows the answer

 

[DrAmine]

What is V in your formulation.

[Tareq]

V is the volume fraction of particles (solid), but still, the main challenge to finding the effective viscosity by particles momentum exchange...etc