
[Sponsors] 
June 11, 2010, 09:02 
UDF for describing nonnewtonian behaviour of fiber suspension

#1 
New Member
Join Date: Apr 2010
Posts: 10
Rep Power: 9 
Sponsored Links
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+(1exp(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_tmu); if (gamma<gamma_t) mu_a=mu+(1exp(m*gamma))*tau_0/gamma; else mu_a=mu_t; return mu_a; } 1. One thing I’m wondering about is how it works when I define the viscosity. I understand that the effective viscosity is a sum of the laminar and turbulent viscosity. When I use the “DEFINE_PROPERTY(cell_viscosity,c,t)” function in my code does it define the total effective viscosity for the fluid or just the laminar or the turbulent viscosity? 2. Also I’m not sure if I’ve defined the shear rate (second invariant of the rateof strain tensor, gamma) correctly. Does anyone know if it’s correct? 3. Are there any other problems with the code or does it look ok (this is the first time I've written an UDF so I'm not sure I've used the right syntax and structure)? Thanks for the help! Last edited by shib; June 11, 2010 at 09:34. 

Sponsored Links 
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 turbulencepanel, 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 lowshear Bingham region. 2) yes, C_SRAIN_RATE_MAG gives the second invariant as needed cheers 

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 CFDpost? 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Dynamic Mesh UDF  Qureshi  FLUENT  7  March 23, 2017 08:37 
Fiber Suspension Solver laminar AdvaniFolgarTucker model  kerstin  OpenFOAM Running, Solving & CFD  5  November 21, 2014 23:49 
UDF parallel error: chipexec: function not found?????  shankara.2  Fluent UDF and Scheme Programming  1  January 16, 2012 23:14 
UDF...UDF...UDF...UDF  Luc SEMINEL  FLUENT  0  November 25, 2002 05:03 
UDF, UDF, UDF, UDF  Luc SEMINEL  Main CFD Forum  0  November 25, 2002 05:01 
Sponsored Links 