Help! How to get the gradient of void fraction
 

Hello, I use Eulerian multiphase model in FLUENT (version 6. 3.33)and tried to get the gradient of the void fraction. I found someone had the same problem before and tried his method:
if (NULL != THREAD_STORAGE(thread,SV_VOF_G))
{v_s = C_VOF_G(cell, thread)[0];}
But it did not work!!!!!!!!!!! I checked the THREAD_STORAGE, it is always == NULL.
Does anyone have some idea how to get it? Any suggestion is appreciated.