This cylindrical representation of the incompressible Navier–Stokes equations is the second most commonly seen (the first being Cartesian above). Cylindrical coordinates are chosen to take advantage of symmetry, so that a velocity component can disappear. A very common case is axisymmetric flow with the assumption of no tangential velocity ( u ϕ ϕ = 0 ), and the remaining quantities are independent of ϕ ϕ : ρ ρ ( ∂ ∂ t u r + u r ∂ ∂ r u r + u z ∂ ∂ z u r ) = − − ∂ ∂ r p + μ μ ( 1 r ∂ ∂ r ( r ∂ ∂ r u r ) + ∂ ∂ z 2 u r − − u r r 2 ) + ρ ρ g r ρ ρ ( ∂ ∂ t u z + u r ∂ ∂ r u z + u z ∂ ∂ z u z ) = − − ∂ ∂ z p + μ μ ( 1 r ∂ ∂ r ( r ∂ ∂ r u z ) + ∂ ∂ z 2 u z ) + ρ ρ g z 1 r ∂ ∂ r ( r u r ) + ∂ ∂ z u z = 0.