Let { e s , e n , e b } be a Frenet–Serret orthonormal basis which consists of a tangential unit vector, a normal unit vector, and a binormal unit vector to the streamline, respectively. Since a streamline is a curve that is tangent to the velocity vector of the flow, the left-hand side of the above equation, the convective derivative of velocity, can be described as follows: u ⋅ ⋅ ∇ ∇ u = u ∂ ∂ ∂ ∂ s ( u e s ) ( u = u e s , ∂ ∂ / ∂ ∂ s ≡ ≡ e s ⋅ ⋅ ∇ ∇ ) = u ∂ ∂ u ∂ ∂ s e s + u 2 R e n ( ∵ ∵ ∂ ∂ e s ∂ ∂ s = 1 R e n ) ,