{"id":304712,"date":"2023-09-03T18:59:00","date_gmt":"2023-09-03T18:59:00","guid":{"rendered":"\/forum\/forums\/topic\/unnatural-values-calculated-by-udf\/"},"modified":"2023-09-03T18:59:00","modified_gmt":"2023-09-03T18:59:00","slug":"unnatural-values-calculated-by-udf","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/unnatural-values-calculated-by-udf\/","title":{"rendered":"Unnatural values calculated by UDF"},"content":{"rendered":"

Hello!<\/p>\n

I am trying to model fixed bed adsorption using Linear driving force model given by dq\/dt = kf(q* – <\/em>q) <\/em>where kf is a constant <\/em>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. <\/p>\n

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:<\/p>\n

The value of q_i is now -38065484171715682240850117399674880.000000<\/p>\n

q_i is the value of q at the previous time step.<\/p>\n

Anyway, can someone look at my code and see where I am going wrong, please?<\/p>\n

\n
#include <udf.h><\/div>\n
#include <math.h><\/div>\n
 <\/div>\n
DEFINE_INIT(Initial,domain)<\/div>\n
{<\/div>\n
    Thread *t;<\/div>\n
    cell_t c;<\/div>\n
    thread_loop_c(t, domain)<\/div>\n
    {<\/div>\n
        begin_c_loop(c, t)<\/div>\n
        {<\/div>\n
            \/\/ Initially the contaminant concentration in solid phase is 0<\/div>\n
            C_UDMI(c, t, 0) = 0.0; \/\/ q_i at the ith time step<\/div>\n
            C_UDMI(c, t, 1) = 0.0; \/\/ q_i+1 at the i+1th time step<\/div>\n
        }<\/div>\n
        end_c_loop(c, t)<\/div>\n
    }<\/div>\n
}<\/div>\n
DEFINE_ADJUST(define_q_ads, domain)  \/*Adjust is the first step in the solution loop*\/<\/div>\n
{<\/div>\n
    Thread *t;<\/div>\n
    cell_t c;<\/div>\n
    real dt;<\/div>\n
    real rp = 1.1\/1000;<\/div>\n
    real De = 25;<\/div>\n
    real qm = 154;<\/div>\n
    real KL = 0.63;<\/div>\n
    real kf = 15*De\/(rp*rp);<\/div>\n
    real qe;<\/div>\n
   <\/div>\n
    thread_loop_c(t, domain)<\/div>\n
    {<\/div>\n
        begin_c_loop(c, t)<\/div>\n
        {<\/div>\n
            dt = RP_Get_Real(“physical-time-step”);<\/div>\n
            qe = (KL*qm*C_UDSI(c,t,0))\/(1+KL*C_UDSI(c,t,0)); \/\/ C_UDSI contains the concentration in the liquid phase     <\/div>\n
            C_UDMI(c, t, 1) = C_UDMI(c, t, 0) + kf*dt*((qe – C_UDMI(c, t, 0)));<\/div>\n
            \/\/ q_i+1 = q_i + kf*dt*(qe – q_i)          <\/div>\n
        }<\/div>\n
        end_c_loop(c, t)<\/div>\n
    }<\/div>\n
}<\/div>\n

<\/p>\n

DEFINE_SOURCE(m_src, c, t, dS, eqn) \/*Source term calculation happens after the adjust function*\/  <\/div>\n
{<\/div>\n
    real source;<\/div>\n
    real eta = 0.77;<\/div>\n
    real rho = 743;<\/div>\n

<\/p>\n

    int ThreadID = THREAD_ID(t);<\/div>\n
    source = -(1-eta)*rho*C_UDMI(c,t,1);<\/div>\n

<\/p>\n

    Message(“====================================================================\\n”);<\/div>\n
    Message(“The value of source in the cell thread %i is %f\\n”, ThreadID, source);<\/div>\n
    Message(“====================================================================\\n”);<\/div>\n
    dS[eqn] = 0.0;<\/div>\n
    return source;<\/div>\n
}<\/div>\n

<\/p>\n

DEFINE_EXECUTE_AT_END(execute_at_end)  \/*Executes at the end of the iteration. Updates q_i for the next time step*\/<\/div>\n
{<\/div>\n
    Domain* domain = Get_Domain(1);<\/div>\n
    Thread* t;<\/div>\n
    cell_t c;<\/div>\n
    thread_loop_c(t, domain)<\/div>\n
    {<\/div>\n
        begin_c_loop(c, t)<\/div>\n
        {<\/div>\n
             C_UDMI(c, t, 0)=C_UDMI(c, t, 1);<\/div>\n
             Message(“The value of q_i is now %f\\n”,C_UDMI(c,t,0));<\/div>\n
        }<\/div>\n
        end_c_loop(c, t)<\/div>\n
    }<\/div>\n
}  <\/div>\n<\/div>\n","protected":false},"template":"","class_list":["post-304712","topic","type-topic","status-closed","hentry","topic-tag-adsorption","topic-tag-udf"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t