# coefficient of power-Law viscosity Model in OpenFOAM

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 February 23, 2014, 13:16 coefficient of power-Law viscosity Model in OpenFOAM #1 Senior Member   Daniel Join Date: Mar 2013 Posts: 213 Rep Power: 13 Hello foamers I have searched a lot but I was not able to find any proper explanation about coefficient of power-Law viscosity Model. I have used online curve fitting tools to determine constant coefficients in power-Law viscosity model based on my experimental data. These values are obtained for k and n: k = 0.0102 [Pa.s^(n)] n = 0.46 Now when it comes to OpenFOAM, it uses kinematic viscosity and after reading source code of power-Law viscosity model, I have found out that k has the same dimension as nu. I have divided my k value by rho and that gives me: k = 7.293e-6 However, when I use this value for k the solution diverges quickly. I have seen a few tutorials in which k has a very large magnitude like 2500. Am I missing something here? maybe I did not understand the model properly. best wishes regards

 March 23, 2014, 17:30 #2 Member   Nadish Saini Join Date: Feb 2014 Location: Raleigh, North Carolina Posts: 39 Rep Power: 4 Hello Daniel, Were you able to find a good explanantion for the power law model. I am also a little confused as to why k value is taken so large in some cases. If i take k value small, the solution is not converging. Please help!!

 July 7, 2014, 00:46 #3 New Member   Filippo Join Date: May 2014 Posts: 27 Rep Power: 4 Hi friends, Have you found a solution for this problem that you can share with me? I have the same problem too... Cheers, FS

 September 21, 2014, 03:06 #4 New Member   Alexa Join Date: Sep 2014 Posts: 5 Rep Power: 4 Dear friends, I have the same problem as you. Can someone post an explanation for this problem, plz? Thank you very much!!

 January 28, 2015, 09:05 #5 New Member   Jonas Join Date: Sep 2014 Posts: 6 Rep Power: 4 Same problem here. When I divide by density I get very high velocities...

 January 28, 2015, 15:29 #6 Member   Nadish Saini Join Date: Feb 2014 Location: Raleigh, North Carolina Posts: 39 Rep Power: 4 Hi, I had this problem some time back. Someone suggested to initiate the flow with constant viscosity. The k value is supposed to be large (if i remember correctly). So yeah, i ran a few time steps with constant viscosity and then switched to power law model and that allowed the simulation to run smoothly. Try that! wurst likes this.

 April 5, 2016, 04:23 #7 Senior Member   Ananthakrishnan.A.S Join Date: Feb 2012 Location: Mumbai (Bombay), India Posts: 157 Rep Power: 8 Hi was anyone able to find the explanation of the units for k in openfoam? From this thread Power-law viscosity model for blood i understand that openfoam multiples strain rate by one sec, so k does not have n in its power but how to eliminate sec from SI units. I get the values of k experimentally in pa.s^n. I dont know how to incorporate this into openfoam Thanks

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