Learning Forum

[James]

Hello!

I am trying to model fixed bed adsorption using Linear driving force model given by dq/dt = kf(q* - q) where kf is a constant q* = (KL*qm*C)/(1 + KL*C), where KL and qm are constants and C is the concentration. I defined C as a user defined scalar instead of using mass fraction for convenience with numbers (Does that make any difference?). The dq/dt equation is then applied in the source term as -(1-porosity)*rho*dq/dt. 

I put messages in my UDF to show me the values of the UDS and other variables calculated and they are absolutely ridiculous. An example is:

The value of q_i is now -38065484171715682240850117399674880.000000

q_i is the value of q at the previous time step.

Anyway, can someone look at my code and see where I am going wrong, please?

#include

#include

 

DEFINE_INIT(Initial,domain)

{

    Thread *t;

    cell_t c;

    thread_loop_c(t, domain)

    {

        begin_c_loop(c, t)

        {

            // Initially the contaminant concentration in solid phase is 0

            C_UDMI(c, t, 0) = 0.0; // q_i at the ith time step

            C_UDMI(c, t, 1) = 0.0; // q_i+1 at the i+1th time step

        }

        end_c_loop(c, t)

    }

}

DEFINE_ADJUST(define_q_ads, domain)  /*Adjust is the first step in the solution loop*/

{

    Thread *t;

    cell_t c;

    real dt;

    real rp = 1.1/1000;

    real De = 25;

    real qm = 154;

    real KL = 0.63;

    real kf = 15*De/(rp*rp);

    real qe;

   

    thread_loop_c(t, domain)

    {

        begin_c_loop(c, t)

        {

            dt = RP_Get_Real("physical-time-step");

            qe = (KL*qm*C_UDSI(c,t,0))/(1+KL*C_UDSI(c,t,0)); // C_UDSI contains the concentration in the liquid phase     

            C_UDMI(c, t, 1) = C_UDMI(c, t, 0) + kf*dt*((qe - C_UDMI(c, t, 0)));

            // q_i+1 = q_i + kf*dt*(qe - q_i)          

        }

        end_c_loop(c, t)

    }

}

DEFINE_SOURCE(m_src, c, t, dS, eqn) /*Source term calculation happens after the adjust function*/  

{

    real source;

    real eta = 0.77;

    real rho = 743;

    int ThreadID = THREAD_ID(t);

    source = -(1-eta)*rho*C_UDMI(c,t,1);

    Message("====================================================================\n");

    Message("The value of source in the cell thread %i is %f\n", ThreadID, source);

    Message("====================================================================\n");

    dS[eqn] = 0.0;

    return source;

}

DEFINE_EXECUTE_AT_END(execute_at_end)  /*Executes at the end of the iteration. Updates q_i for the next time step*/

{

    Domain* domain = Get_Domain(1);

    Thread* t;

    cell_t c;

    thread_loop_c(t, domain)

    {

        begin_c_loop(c, t)

        {

             C_UDMI(c, t, 0)=C_UDMI(c, t, 1);

             Message("The value of q_i is now %f\n",C_UDMI(c,t,0));

        }

        end_c_loop(c, t)

    }

}  

[Atharva Nagarkar]

Hello!

As part of the Ansys policy, employees cannot debug UDFs on the forum. However, I would suggest you to have a look at the following things.

For the macro C_UDSI, if you try to use the macro before you have specified the user-defined scalars in your Ansys Fluent model then an error will result.

3.2. Data Access Macros (ansys.com) -> Check section 3.2.11.3

Please also have a look at the definition of the macro C_UDMI and how the arguments are defined at the link below.

3.2. Data Access Macros (ansys.com) -> Check section 3.2.12.4

Additionally, this is the link from the Ansys manual for defining the source term. Make sure that you have correctly defined the dS[] array with respect to the dependent variable.

2.3. Model-Specific DEFINE Macros (ansys.com) -> Check section 2.3.43

Lastly, these are the time dependent macros which you can use to access variables for previous timesteps.

3.5. Time-Dependent Macros (ansys.com)

If you are not able to access the link, please refer to this forum discussion: Using Help with links (ansys.com)

Thanks!

[James]

I will take care next time, and thank you!