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July 28, 2021, 04:26 
Herschel Bulkley Model in Openfoam

#1 
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mohammad
Join Date: Sep 2015
Posts: 274
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Hi everybody,
I am trying to simulate the particle settling in a NonNewtonian fluid, using HerschelBulkley model. As I know the HB model has been implemented in OF using the following equation for viscosity: Code:
return ( min ( nu0_, (tau0_ + k_*rtone*pow(tone*sr(), n_)) /(max(sr(), dimensionedScalar ("VSMALL", dimless/dimTime, VSMALL))) ) ); where is the final calculated viscosity, as strain rate, n power law index, k consistency index, and as yield stress in viscoplastic fluids. Here my question is regarding which can only be found in OF and describes the largest possible viscosity in the domain for plastic phase. Considering a settling particle, which starts to settle from zero velocity, I think it's an important factor, because my calculated viscosity based on the shear rate is always comparing with this parameter. So what you think about this parameter? Should it be kept as the highest possible value? If yes, for a settling particle, it fails to exceed the in initial iterations! So what is the highest value?!!! 

December 20, 2021, 17:01 

#2 
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MNM
Join Date: Aug 2017
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Thanks for asking this question Mohammad.
I also came across this same question and spent few hours looking for it . Hope the following info might help someone . The equations shown by you refer to the regularization approach (one of the ways to tackle the discontinuous behavior of non Newtonian fluids). The parameter refers to the Creeping viscosity or Yielding viscosity, and it represents the viscosity when the shear stress is smaller than yield stress. Hence, it should be as high as possible to accurately model the NonNewtonian behavior. Although, there is no standard rule to set this, I came across few papers that provided some rough approximations 1. Here ( https://www.researchgate.net/publica...zing_OpenfoamR) authors advised to take it somewhere from 10k 100k Pa.S (dont forget to convert it to kinematic viscosity) 2. In another paper (Eq 4 of Numerical study of the Bingham squeeze film problem https://www.sciencedirect.com/scienc...77025784800294), it is suggested to take its value 1000 times of dynamic viscosity. (again make sure to divide it by density b4 giving as input in FOAM) P.S.  Curious fellows can also refer to this awesome piece of work for a nice overview (Progress in numerical simulation of yield stress fluid flows https://link.springer.com/article/10...39701609859) 

December 20, 2021, 19:30 

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mohammad
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Quote:
Thanks SHUBHAM for your comprehensive explanation. BTW, I have seen in OF, whenever we consider a higher in the simulation, the time step should be lower, as the viscosity in the unyielded will be so high, and there will be a problem in terms of stability in this kind of simulation. So we need to have an optimum viscosity value for the simulation. The papers you mentioned are so valuable. However, I think will be a case sensitive parameter, that can't be set to a very high value, as we see in the literature. 

October 1, 2022, 11:28 

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