Gradient question
 

(1). How does FLUENT calculate the gradient of a variable at cell and face? Especially, at boundary face. (2). I am trying to calculate the second gradient of species mass fraction. Can i apply UDS to do that as following: I define two UDS-0,1 which are not solved by FLUENT, and then C_UDSI(c,t,0)=C_YI(c,t,i) C_UDSI(c,t,1)=C_UDSI_G(c,t,0) The second gradient of species mass fraction shoule be C_UDSI_G(c,t,1) Is it right way to calculate second gradient? If it is at boundary face, is this way still right?