DEFINE_DPM_BC(bc_reflect,p,t,f,f_normal,dim)
{
  real alpha;  /* angle of particle path with face normal */
  real vn=0.;
  real nor_coeff = 0.;
  real tan_coeff = 0;
  real normal[3];
  int i, idim = dim;
  real NV_VEC(x);


  for (i=0; i<idim; i++)
      normal[i] = f_normal[i];

  if(p->type==DPM_TYPE_INERT)
    {
      alpha = M_PI/2. - acos(MAX(-1.,MIN(1.,NV_DOT(normal,P_VEL(p))/
                                  MAX(NV_MAG(P_VEL(p)),DPM_SMALL))));
      if ((NNULLP(t)) && (THREAD_TYPE(t) == THREAD_F_WALL))
        F_CENTROID(x,f,t);
      
       /* 定义壁面反射系数 */
      nor_coeff=0.993-0.0307*alpha+4.75e-4*alpha*alpha-2.61e-6*alpha*alpha*alpha
      tan_coeff=0.998-0.029*alpha+6.43e-4*alpha*alpha-3.56e-6*alpha*alpha*alpha

      /* calculate the normal component, rescale its magnitude by 
         the coefficient of restitution and subtract the change */

      /* Compute normal velocity. */
      for(i=0; i<idim; i++)
        vn += P_VEL(p)[i]*normal[i];

      /* Subtract off normal velocity. */
      for(i=0; i<idim; i++)
        P_VEL(p)[i] -= vn*normal[i];

      /* Apply tangential coefficient of restitution. */
      for(i=0; i<idim; i++)
        P_VEL(p)[i] *= tan_coeff;

      /* Add reflected normal velocity. */
      for(i=0; i<idim; i++)
        P_VEL(p)[i] -= nor_coeff*vn*normal[i];  

      /* Store new velocity in P_VEL0 of particle */
      for(i=0; i<idim; i++)
        P_VEL0(p)[i] = P_VEL(p)[i];
      
      return PATH_ACTIVE;
    }

  return PATH_ABORT;
}