书上看到的一个流化床模拟，代码我能看懂，但是为什么要用两个气固交换系数，书上是气相选择的custom_drag_syam，固相选择custom_drag_ihme。还有一个问题，第一个用的是syamlal-o’brien模型，但是第二个我查了下，不是常用的几个模型啊，没搞懂，书上也没说。大神帮忙看下，非常感谢。。。。
其中UDF代码如下：

/* This is a routine for customizing default Syamlal drag law in Fluent 6.
   The default drag law uses 0.8 (for void<=0.85) and 2.65 (void>0.85) for
   bfac. This is for a min fluid vel of 25 cm/s. The current drag law has been
   tuned for a min fluid vel of 8 cm/s and uses 0.28 and 9.07 for these
   parameters. */

#include "udf.h"
#include "sg_mphase.h"

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

DEFINE_EXCHANGE_PROPERTY(custom_drag_syam, 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).
These phases appear in columns 2 and 1 in the Interphase panel respectively*/

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 reynolds 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;

}

DEFINE_EXCHANGE_PROPERTY(custom_drag_ihme, 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, cd, eg,
      void_g, k_g_s;

/* find the threads for the gas (primary) and solids (secondary phases).
These phases appear in columns 2 and 1 in the Interphase panel respectively*/

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 reynolds number*/

reyp = rho_g*abs_v*diam2/mu_g;

cd = (24./(reyp+SMALL)) + 5.48*pow((reyp+SMALL),-0.573) + 0.36;

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

eg = pow(void_g,-2.65);

k_g_s = (3./4.)*(cd*(1.-void_g)*abs_v*rho_g*eg)/diam2;

return k_g_s;

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