[victorueda7]

Hi everybody, I'm trying to programme a viscosity model using Galindo y Rosales model in fluent 16.1, but when I run the simulation, it crashes and I get the next message:

"Divergence detected in AMG Solver: X momentum"

I write here my code:

#include "udf.h"
#include "mem.h"

DEFINE_PROPERTY(cell_viscosity,c,t)
{
real visco=1;
real mu_0=3.5191;
real mu_c=3.05642;
real gamma_c=7.94343;
real lambda_I=1.161901047;
real n_I=1.28837007;
real mu_max=60.6729;
real gamma_max=79.4275;
real lambda_II=0.0594804;
real n_II=1.83716288;
real lambda_III=0.00206575;
real n_III=0.62014834;
real gamma=C_STRAIN_RATE_MAG(c,t);
real den;
if (gamma==0)
visco=1;
if (gamma<=gamma_c){
den=lambda_I*((pow(gamma,2))/(gamma-gamma_c));
visco=mu_c+((mu_0-mu_c)/(1+pow(den,1.28837007)));
}
if ((gamma>gamma_c)&&(gamma<=gamma_max)){
den=lambda_II*((gamma-gamma_c)/(gamma-gamma_max))*gamma;
visco=mu_max+((mu_c-mu_max)/(1+pow(den,1.83716288)));
}
if (gamma>gamma_max){
den=lambda_III*(gamma-gamma_max);
visco=mu_max/(1+pow(den,0.62014834));
}
return visco;
}

I hope you could help me, thank u in advance.

[chienfm]

Hello Víctor Rueda García,

I don't think the error comes from your udf. First try your simulation with constant viscosity, if there is no error, then test your udf. Also, your If-condition in your UDF is not strict, you can simply test your code by print your value in C program.

[victorueda7]

Originally Posted by chienfm

Hello Víctor Rueda García,

I don't think the error comes from your udf. First try your simulation with constant viscosity, if there is no error, then test your udf. Also, your If-condition in your UDF is not strict, you can simply test your code by print your value in C program.

Yes, thank u very much. Indeed, yesterday after post It, I tried what you just have proposed and apparently, the problem is that the value I get in the first denominator is a complex number, but I don't know how to solve this, because I have checked the paper where the equations un my udf come from, and they are well implemented. Do u have any solution?

Thank u very much for your help 😊

[chienfm]

As I said, your if-condition is not strict, such as the case gamma == 0, it actually returns the value in second if-condition since gamma = 0 also satisfies gamma < gamma_c. Hence, you should use else-if in this case.

For example

#include "udf.h"
#include "mem.h"

DEFINE_PROPERTY(cell_viscosity,c,t)
{
	real visco=1;
	real mu_0=3.5191;
	real mu_c=3.05642;
	real gamma_c=7.94343;
	real lambda_I=1.161901047;
	real n_I=1.28837007;
	real mu_max=60.6729;
	real gamma_max=79.4275;
	real lambda_II=0.0594804;
	real n_II=1.83716288;
	real lambda_III=0.00206575;
	real n_III=0.62014834;
	real gamma=C_STRAIN_RATE_MAG(c,t);
	real den;
	if (gamma==0) 
	{
		visco=1;
	}
	else if (gamma<=gamma_c)
	{
		den=lambda_I*((pow(gamma,2))/(gamma-gamma_c));
		visco=mu_c+((mu_0-mu_c)/(1+pow(den,1.28837007)));
	}
	else if ((gamma>gamma_c)&&(gamma<=gamma_max))
	{
		den=lambda_II*((gamma-gamma_c)/(gamma-gamma_max))*gamma;
		visco=mu_max+((mu_c-mu_max)/(1+pow(den,1.83716288)));
	}
	else
	{
		den=lambda_III*(gamma-gamma_max);
		visco=mu_max/(1+pow(den,0.62014834));
	}
	return visco;
}

[pakk]

else if (gamma<=gamma_c)
    {
        den=lambda_I*((pow(gamma,2))/(gamma-gamma_c));
        visco=mu_c+((mu_0-mu_c)/(1+pow(den,1.28837007)));
    }

Double-check if you copied this correctly from the article.


If gamma<=gamma_c, then "den" becomes negative, so "visco" becomes imaginary. Maybe you missed a minus sign, or should have typed "gamma_c-gamma"?