In steady flow the velocity field is constant with respect to time, v = v (x) = v (x (t)), so v itself is not directly a function of time t. It is only when the parcel moves through x that the cross sectional area changes: v depends on t only through the cross-sectional position x (t). d v d t = d v d x d x d t = d v d x v = d d x ( v 2 2 ) .