UDF中如何定义一个Y方向的单位向量呢？

求是创新

RT，如何定义一个全局变量的单位向量呢，要放在下面程序的那个位置呢？
#include "udf.h"
static real v_prev = 0.0;

DEFINE_CG_MOTION(piston,dt,vel,omega,time,dtime)
{
    Thread *t;
    face_t f;
    real NV_VEC(A);
    real force, dv;
    /* reset velocities */
    NV_S(vel, =, 0.0);
    NV_S(omega, =, 0.0);
    if (!Data_Valid_P())
      return;
    /* get the thread pointer for which this motion is defined */
    t = DT_THREAD(dt);
    /* compute pressure force on body by looping through all faces */
    force = 0.0;
    begin_f_loop(f,t)
      {
         F_AREA(A,f,t);
         force += F_P(f,t) * NV_MAG(A);
      }
    end_f_loop(f,t)
    /* compute change in velocity, that is, dv = F * dt / mass  velocity update using explicit Euler formula */


    dv = dtime * force / 50.0;
    v_prev += dv;
    Message ("time = %f, x_vel = %f, force = %f\n", time, v_prev,
      force);
    /* set x-component of velocity */
    vel[0] = v_prev;
}

gearboy

放在static real v_prev = 0.0;之后，DEFINE_CG_MOTION之前。