
[Sponsors] 
HerschelBulkley nonNewtonian viscosity model has term with sign error 

LinkBack  Thread Tools  Search this Thread  Display Modes 
March 1, 2011, 18:31 
HerschelBulkley nonNewtonian viscosity model has term with sign error

#1 
New Member
Paul Bryant
Join Date: Feb 2011
Posts: 13
Rep Power: 15 
The HerschelBulkley (HB) nonNewtonian viscosity model in OpenFOAM contains a term with a sign error. Outside of CFD, the HB model usually consists of a yield stress tao_y below which the shear rate sr is exactly zero, sr=0, and above which the stress tao is equal to tao_y plus a simple power law function of the shear rate, tao = tao_y + k*sr^n. The model in OpenFOAM allows for a transition to Newtonian behavior when the shear rate is very low, thereby eliminating the undesirable discontinuity in the function at zero stress. The Newtonian viscosity is nu0, and the transition occurs when the stress exceeds the threshold value given by the parameter tao0. Assuming that nu0 is not infinity, tao_y and tao0 are not the same but one can be calculated from the other by matching the Newtonian and nonNewtonian forms at the transition point resulting in: tao_y = tao0  k*(tao0/nu0)^n. This correction to tao0 can be found in the source code for the HS model but INCORRECTLY with a plus sign rather than the minus sign in front of the k. In the code from the file HerschelBulkley.C, the model gives the effective viscosity nu by dividing the above expression for tao by a factor of sr:
return (min(nu0_,(tau0_ + k_* rtone *( pow(tone * sr(), n_) + pow(tone*tau0_/nu0_,n_))) / (max(sr(), dimensionedScalar ("VSMALL", dimless/dimTime, VSMALL))))); Note that the stress and viscosity are both given in kinematic form (i.e. divided by the density). The factors tone and rtone are both equal to unity but have dimensions of time and inverse time and are included to get the units correct for the result. The CORRECTED VERSION with the sign change is: return (min(nu0_,(tau0_ + k_* rtone *( pow(tone * sr(), n_)  pow(tone*tau0_/nu0_,n_))) / (max(sr(), dimensionedScalar ("VSMALL", dimless/dimTime, VSMALL))))); Note that when sr=tao0/nu0, the two pow() functions will now cancel out and give the expected result nu=tao0/sr. Without the indicated sign correction, tao0 has no obvious physical interpretation, as it is neither the stress at the transition point between the two forms nor is it the stress that would occur if the power law form was used as the shear rate approaches zero. The correction will have little effect in cases when the value of nu0 is set to a very large value to approximate infinity. 

May 6, 2011, 06:46 

#2 
Senior Member
Robert Sawko
Join Date: Mar 2009
Posts: 117
Rep Power: 22 
This is just to say: you're absolutely right! Thanks for sharing.


May 7, 2011, 02:58 

#3 
Senior Member
Alberto Passalacqua
Join Date: Mar 2009
Location: Ames, Iowa, United States
Posts: 1,912
Rep Power: 36 
FYI, this was fixed some time ago: http://www.openfoam.com/mantisbt/view.php?id=153
__________________
Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats (current status: http://albertopassalacqua.com/?p=1541) OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

November 4, 2011, 04:01 
HerschelBulkley validation

#4 
Senior Member
Albrecht vBoetticher
Join Date: Aug 2010
Location: Zürich, Swizerland
Posts: 238
Rep Power: 16 
It seems like the HerschelBulkley viscosity model gets more and more applied in OpenFOAM. Did somebody already run a validation test case to compare the OpenFOAM implementation with the results of another CFD? I currently finish my PhD were I model shallow landslides and debris flows with the HerschelBulkley rheology law, all results fit the experiments but only if I choose the yield stress tao0 and consistency factor k exactly a hundred times smaller that recommended by the literature.


November 14, 2011, 08:47 

#5 
Senior Member
Albrecht vBoetticher
Join Date: Aug 2010
Location: Zürich, Swizerland
Posts: 238
Rep Power: 16 
...sorry that was a mistake in the preprocessor changing from [lb/ft^2] to [Pa]. Actually, the HerschelBulkley looks fine :)


June 18, 2013, 23:53 

#6  
Senior Member
Dongyue Li
Join Date: Jun 2012
Location: Beijing, China
Posts: 844
Rep Power: 18 
Quote:
This is where I cannot get it, just see the image: The red line is nu, black one is tao. Is this what you said? so is there anything wrong with my equation? but my equation is not implemented in openFOAM. THANKS! 

Tags 
herschelbulkley, nonnewtonian, viscosity models 
Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
CGNS Compiling  Diego  Main CFD Forum  17  December 21, 2014 01:40 
CGNS lib and Fortran compiler  manaliac  Main CFD Forum  2  November 29, 2010 06:25 
[swak4Foam] groovyBC: problems compiling: "flex: not found" and "undefined reference to ..."  sega  OpenFOAM Community Contributions  12  February 17, 2010 09:30 
compile errors of boundary condition "expDirectionMixed"  liying02ts  OpenFOAM Bugs  2  February 1, 2010 20:11 
Regarding Implementation of Hershel Bulkley Model in Viscosity correlation  Pathak  FLOW3D  0  August 24, 2009 06:34 