gradient of swirl velocity in 2D model
 

I have a 2D swirl axis symmetric model and I need components of swirl velocity gradient.
I have writen C_W_G(c,t)[0] but it doesn't work and an error massage appears.
nevertheless C_W(c,t) and also C_V_G(c,t)[0] works correctly.
WHAT should i do now?

THANK U