The rate of change of a quantity f with respect to time at a fixed point of a fluid, ∂f /∂t ; it is related to the individual derivative df /dt through the expression ∂f /∂t = df /dt - V ⋅ ∇f, where f is a thermodynamic property f (x,y,z,t ) of the fluid, V the vector velocity of the fluid, and ∇ the del operator.