Problem with using UDF to modify viscosity
 

Hi, I want to modify the viscosity in K-W SST model in Fluent 13.0. But I met a problem that my UDF cannot initiate and it shows that:
Error:
FLUENT received fatal signal (ACCESS_VIOLATION)
1. Note exact events leading to error.
2. Save case/data under new name.
3. Exit program and restart to continue.
4. Report error to your distributor.
Error Object: #f

I have tried initialise without using "compute from" ....but the error was still there, I pasted my UDF here, if anone can help me with that?

#include "udf.h"

DEFINE_TURBULENT_VISCOSITY(user_mu_t,c,t)
{
real mu_t;
real a;
real rho;
real k;
real d;

a=(C_R(c,t)-0.5542)/997.6458;
rho = 0.5542+a*a*997.6458;

k = C_K(c,t);
d = C_D(c,t);
mu_t = M_keCmu*rho*k*k/d;
return mu_t;
}

Specific dissipation rate C_O(c,t)
 

If you are working with k-w turbulence model, the dissipation rate C_D(c,t) will not be available in the UDFs as it is not calculated by the solver. Instead, specific dissipation rate is calculated C_O(c,t).

For the purpose of your udf calculation, you can covert \omega to \epsilon as under:
http://www.cfd-online.com/Wiki/Speci...ssipation_rate

Wow, thank you for your help, you really helped me a lot!
And I have one more question, when i use this UDF:

DEFINE_TURBULENT_VISCOSITY(user_mu_t,c,t)
{
real mu_t;
real a;
real rho;
real k;
real d;

a=(C_R(c,t)-0.5542)/997.6458;
rho = 0.5542+a*a*997.6458;

k = C_K(c,t);
d = C_D(c,t);
mu_t = M_keCmu*rho*k*k/d;
return mu_t;
}

with realizable k-epsilon model, the results diverged, but it worked well with the RNG-kepsilon model, are they actually different?

Hi teethfish,
Did you get your problem sorted?
I have some problems using the modified turbulent viscosity problem as well. I guess, although you hook a same udf to different turbulence models, there're something different more than the udf. The boundary condition, wall treatment, may be different. So a possible circumstance is the solution behavior is different between two distinct turbulence model with an identical udf.

I am still investigating this.

Sheng

Error occurs in UDF for DPM viscosity
 

Hello to you all,

I hope I post this in the right place, I did not found a more appropriate one...for an atomizer jet I have to write an UDF to express non- newtonian behaviour for the liquid phase. It is not possible to choose any non- newtonian models like cross or carreau or power- law for the DPM- material so I have to write this UDF. I want to realize the model of CROSS with this UDF.

I compiled this UDF and everything works fine but after exactly 9 iteration steps the same error as descriped above ocurrs:

Error:
Ansys received fatal signal (ACCESS VIOLATION)
1. Note exact events leading to error.
2. Save case/data under new name.
3. Exit program and restart to continue.
4. Report error to your contributor.
Error: Object f#

The code I use for realizing the UDF (and that causes this error) is:
------------------------------------------------------------------

#include "udf.h"
#include "dpm.h"


DEFINE_DPM_PROPERTY(cell_viscosity_CROSS_m2, c, t, p)
{
real mu_INF,mu_0,k,mu_CROSS,m;
double aux,SR;

SR=C_STRAIN_RATE_MAG(c,t);


k=0.01372;
m=0.7857;
mu_0=1.885;
mu_INF=0.05320;
mu_CROSS=DPM_MU(p);


aux=pow(SR,m);

mu_CROSS=mu_INF+(mu_0-mu_INF)/(1+k*aux);

return mu_CROSS;
}


I treid it out in steady, in transient, I changed the URF, I just interpreted it I compiled it, but nothing helps...I guess there is some error in the code that I do not see....actually my programming skills are quite bad and I do not have any experience at all with UDF.

I would really appreciate if someone could help me or has some advice for me. Greetings and thank you, have a nice weekend!

Check one of the pointers before actually access them. Try add the highlighted line to see whether it helps.

Hello Blackmask,

Thank you for your quick response and your advice. I tried it out but it did not work, the same error occurs at the same iteration step.

Maybe there is something wrong with the usage of the macro DPM_MU(p)? I tried to vary the posibilietes to include this (still I am not sure if I need it) but nothing changes...:-(

I did not see that your error occurs after several iterations. Did the calculation tends to diverge before the error occurred?

No, not at all...it neither brings any messages of divergences. The residuals are normal and tend to fall than to rise.

But as I found out, before this error occurs, there is brought a message "Advancing DPM injection..." and then the error occurs. So I guess it has directly to do with the particle model and with the code. When I set the viscosity to a constant value it works.

Has anyone any ideas?