UDF for describing non-newtonian behaviour of fiber suspension

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 June 11, 2010, 09:02 UDF for describing non-newtonian behaviour of fiber suspension #1 New Member   Join Date: Apr 2010 Posts: 10 Rep Power: 9 Sponsored Links Hi, I’m trying to model a fiber suspension in fluent by creating an UDF for the viscosity of the fluid. When the shear is low I want to use a Bingham model to describe the viscosity and when the flow becomes turbulent the viscosity should be equal to the turbulent viscosity. This is how the Bingham model I’m working with looks like: mu_a=mu+tau_0/gamma mu_a=apparent viscosity (=effective viscosity?) tau=stress tensor mu=shear viscosity gamma=second invariant of the rate of strain tensor (=shear rate?) tau_0=yield stress Since the function is not valid for gamma=0 I’m using an exponential regularization model: mu_a=mu+(1-exp(-m*gamma))*tau_0/gamma The code looks like this: #include "udf.h" DEFINE_PROPERTY(cell_viscosity,c,t) { real mu_a; real gamma=C_STRAIN_RATE_MAG(c,t); real tau_0=200; real mu_t=1; real m=200; real mu=0.001; real gamma_t; gamma_t=tau_0/(mu_t-mu); if (gamma

 June 14, 2010, 03:57 #2 Senior Member   Max Join Date: Mar 2009 Posts: 133 Rep Power: 10 Hello shib, 1) hooking your code in the material property panel you will define the molecular (=laminar) viscosity. That is, if turbulence is on Fluent will automatically compute a turbulent viscosity and sum up both to get the effective viscosity even in your laminar Bingham region. Looking at the turbulence-panel, there is an option to set the turbulent viscosity explicitly using a udf. I think that is what you need to suppress turbulent viscosity being added in your low-shear Bingham region. 2) yes, C_SRAIN_RATE_MAG gives the second invariant as needed cheers jotac likes this.

 June 15, 2010, 16:18 #3 New Member   Join Date: Apr 2010 Posts: 10 Rep Power: 9 Thanks Max! I managed to get some pretty good results when I set the turbulent viscosity to 0. Does anyone know how to plot the viscosity and the shear rate in CFD-post?

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