ANSYS FLUENT 12.0 UDF Manual - 2.4.2 DEFINE_EXCHANGE

You can use DEFINE_EXCHANGE_PROPERTY to specify UDFs for some phase interaction variables in multiphase models. These include net heat transfer rates between phases, and drag and lift coefficient functions. Below is a list of user-defined functions that can be specified using DEFINE_EXCHANGE_PROPERTY for the multiphase models in ANSYS FLUENT. Note that there are some phase interaction variables such as vaporization pressure and surface tension coefficient (cavitation parameters) that are defined using DEFINE_PROPERTY. See Section 2.3.16 for details.

**Table 2.4.2:** `DEFINE_EXCHANGE_PROPERTY` Variables
Mixture Model	Eulerian Model
drag exchange coefficient	net heat transfer rate drag coefficient lift coefficient

Note that all of the arguments to a DEFINE macro must be placed on the same line in your source code. Splitting the DEFINE statement onto several lines will result in a compilation error.

Argument Type	Description
`symbol name`	UDF name.
`cell_t c`	Cell index.
`Thread *mixture_thread`	Pointer to the mixture-level thread.
`int second_column_phase_index`	Identifier that corresponds to the pair of
	phases in your multiphase flow that you are
	specifying a slip velocity for. The identifiers
	correspond to the phases you select in the
	Phase Interaction dialog box in the graphical user
	interface. An index of `0` corresponds to the
	primary phase, and is incremented by one for
	each secondary phase.
`int first_column_phase_index`	See `int second_column_phase_index`.

Function returns
`real`

There are five arguments to DEFINE_EXCHANGE_PROPERTY: name, c, mixture_thread, second_column_phase_index, and first_column_phase_index. You supply name, the name of the UDF. c, mixture_thread, second_column_phase_index, and first_column_phase_index are variables that are passed by the ANSYS FLUENT solver to your UDF. Your UDF will need to return the real value of the lift coefficient, drag exchange coefficient, heat or mass transfer to the solver.

The following UDF, named custom_drag, can be used to customize the default Syamlal drag law in ANSYS FLUENT. The default drag law uses 0.8 (for void

0.85) and 2.65 (void

0.85) for bfac. This results in a minimum fluid velocity of 25 cm/s. The UDF modifies the drag law to result in a minimum fluid velocity of 8 cm/s, using 0.28 and 9.07 for the bfac parameters.

/******************************************************************
   UDF for customizing the default Syamlal drag law in ANSYS FLUENT  
*******************************************************************/
   
#include "udf.h"

#define pi 4.*atan(1.)
#define diam2 3.e-4

DEFINE_EXCHANGE_PROPERTY(custom_drag,cell,mix_thread,s_col,f_col)
{
 Thread *thread_g, *thread_s;
 real x_vel_g, x_vel_s, y_vel_g, y_vel_s, abs_v, slip_x, slip_y,
      rho_g, rho_s, mu_g, reyp, afac,
      bfac, void_g, vfac, fdrgs, taup, k_g_s;

 /* find the threads for the gas (primary) */
 /* and solids (secondary phases)          */

 thread_g = THREAD_SUB_THREAD(mix_thread, s_col);/* gas phase  */
 thread_s = THREAD_SUB_THREAD(mix_thread, f_col);/* solid phase*/

 /* find phase velocities and properties*/

 x_vel_g = C_U(cell, thread_g);
 y_vel_g = C_V(cell, thread_g);

 x_vel_s = C_U(cell, thread_s);
 y_vel_s = C_V(cell, thread_s);

 slip_x = x_vel_g - x_vel_s;
 slip_y = y_vel_g - y_vel_s;

 rho_g = C_R(cell, thread_g);
 rho_s = C_R(cell, thread_s);

 mu_g = C_MU_L(cell, thread_g);

 /*compute slip*/
 abs_v = sqrt(slip_x*slip_x + slip_y*slip_y);

 /*compute Reynold's number*/

 reyp = rho_g*abs_v*diam2/mu_g;

 /* compute particle relaxation time */

 taup = rho_s*diam2*diam2/18./mu_g;

 void_g = C_VOF(cell, thread_g);/* gas vol frac*/

 /*compute drag and return drag coeff, k_g_s*/

 afac = pow(void_g,4.14);

 if(void_g<=0.85)
  bfac = 0.281632*pow(void_g, 1.28);
 else
  bfac = pow(void_g, 9.076960);

 vfac = 0.5*(afac-0.06*reyp+sqrt(0.0036*reyp*reyp+0.12*reyp*(2.*bfac-
              afac)+afac*afac));
 fdrgs = void_g*(pow((0.63*sqrt(reyp)/
                            vfac+4.8*sqrt(vfac)/vfac),2))/24.0;

 k_g_s = (1.-void_g)*rho_s*fdrgs/taup;

 return k_g_s;
}

The following UDF, named heat_udf, specifies a coefficient that when multiplied by the temperature difference between the dispersed and continuous phases, is equal to the net rate of heat transfer per unit volume.

#include "udf.h"

#define PR_NUMBER(cp,mu,k) ((cp)*(mu)/(k))
#define IP_HEAT_COEFF(vof,k,nu,d) ((vof)*6.*(k)*(Nu)/(d)/(d))

static real
heat_ranz_marshall(cell_t c, Thread *ti, Thread *tj)
{
 real h;
 real d = C_PHASE_DIAMETER(c,tj);
 real k = C_K_L(c,ti);
 real NV_VEC(v), vel, Re, Pr, Nu;

 NV_DD(v,=,C_U(c,tj),C_V(c,tj),C_W(c,tj),-,C_U(c,ti),C_V(c,ti),C_W(c,ti));
 vel = NV_MAG(v);

 Re = RE_NUMBER(C_R(c,ti),vel,d,C_MU_L(c,ti));
 Pr = PR_NUMBER (C_CP(c,ti),C_MU_L(c,ti),k);
 Nu = 2. + 0.6*sqrt(Re)*pow(Pr,1./3.);

 h = IP_HEAT_COEFF(C_VOF(c,tj),k,Nu,d);
return h;
}

DEFINE_EXCHANGE_PROPERTY(heat_udf, c, t, i, j)
{
  Thread *ti = THREAD_SUB_THREAD(t,i);
  Thread *tj = THREAD_SUB_THREAD(t,j);
  real val;

  val = heat_ranz_marshall(c,ti, tj);
  return val;
}

After the UDF that you have defined using DEFINE_EXCHANGE_PROPERTY is interpreted (Chapter 4) or compiled (Chapter 5), the name of the argument that you supplied as the first DEFINE macro argument (e.g., heat_udf) will become visible and selectable in the Phase Interaction dialog box in ANSYS FLUENT. See Section 6.3.2 for details.

2.4.2 DEFINE_EXCHANGE_PROPERTY

2.4.2 `DEFINE_EXCHANGE_PROPERTY`