ρ ρ ( ∂ ∂ u i ∂ ∂ t + u k ∂ ∂ u i ∂ ∂ x k ) = − − ∂ ∂ p ∂ ∂ x i + ∂ ∂ ∂ ∂ x k [ μ μ ( ∂ ∂ u i ∂ ∂ x k + ∂ ∂ u k ∂ ∂ x i − − 2 3 δ δ i k ∂ ∂ u l ∂ ∂ x l ) ] + ∂ ∂ ∂ ∂ x i ( ζ ζ ∂ ∂ u l ∂ ∂ x l ) + ρ ρ f i .