[DigiHctrGT]

Hi all,

I'm trying to set a UDF to set the bubble diameter for an Eulerian simulation while simulating the turbulence under the Dispersed model (as the density ratio is way beyond 1).

While a similar UDF works perfectly for the Mixture model (for turbulence), it does not work with the modifications I've deemed necessary (i.e., pointing to the liquid-primary phase thread using the super_thread macro, as suggested in another thread from this Forum).

The previous formulation resulted in SIGSEV error.

I managed to solve the issue:
1. Mixture and liquid threads need to be defined,
2. Mixture is pointed by using the THREAD_SUPER_THREAD(t) macro
3. Liquid phase is pointed by using the conventional THREAD_SUB_THREAD(mix, 0) - 0 because that's the ID of the continuous phase.

Thus, the UDF looks like:


#include "udf.h"
#include "math.h"

Thread *mix; /* Define the mixture thread */
Thread *liq; /* Define the liquid thread */

DEFINE_PROPERTY(calderbank_db, c, t)
{
/* Accessing the liquid domain */
mix = THREAD_SUPER_THREAD(t); /* Pointing to mixture thread */
liq = THREAD_SUB_THREAD(mix, 0); /* Pointing to liquid phase */

/* Define the variables */
real db; /* Bubble diamter placeholder */

/* Get local conditions from the dispersed (gas) phase */
real eps = C_D(c, liq); /* Obtain turbulence dissipation from liquid phase */
real eG = C_VOF(c, t); /* Volume fraction of the gas phase */

/* Liquid properties */
double density_liq = 998.2; /* Density of liquid phase (kg/m³) */
/* Ensure epsilon is positive to avoid division errors */
if (eps <= 0.0)
{
eps = 0.0000000001; /* Small value to prevent singularity */
}
if (eG > 0.3)
{
db = 0.0000001; /* almost zero db */
}

/* Calderbank's model (1958) for pure liquid */
/* db = C1 + C2 * ((surf_tension / rhoL)^0.6 / eps^0.4) * eG^0.5 */
db = 0.0009 + 4.15 * pow(0.072 / density_liq, 0.6) * 1 / (pow(eps, 0.4)) * pow(eG, 0.5);

return db;
}