FLUENT - How to model viscosity (or other material property) as a function of the fluid residence time

It is sometimes desireable to have a material property such as viscosity which varies as a known function of residence time of the fluid in the system.

A user-defined scalar (UDS) is used to compute the fluid residence time. The UDS value is substituted for time in the material property equation. The steps are as follows:

1. Edit and compile the material property udf
2. Read case.... set # UDS > 0
3. At inlet, set UDS (value) = 0
4. Set diffusivity of UDS to a small value
5. Set source term of UDS to "rt_source" in the appropriate fluid zones
6. Hook the UDF for viscosity (or other property) in the material properties panel
7. Iterate.

-------------- UDFs follow --------------------------------------

#include "udf.h"

* UDF to compute fluid residence time

real source = C_R(c,t);
dS[eqn] = 0.0;
return source;

* UDF to calculate the material property value. The example shown here sets the
* material property value equal to the residence time, please edit as appropriate.


real res_time, mat_prop;
Thread *st = THREAD_SUPER_THREAD(t);

res_time = C_UDSI(c,st,0);
mat_prop = res_time;

return mat_prop;


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