So, if this is place then what will happen; that means, since we can find a phi were q is equal to grad phi, and it satisfy these equation then we can say that this phi is call the velocity potential. So, what we have observe phi is the phi is the velocity potential, now what we have observed when the flow irrotational; that means, when the flow is irrotational we can always find we can find a phi such that q is a grad phi.