The equation for the pressure field p is an example of a nonlinear Poisson equation: Δ Δ p = − − ρ ρ ∇ ∇ ⋅ ⋅ ( v ⋅ ⋅ ∇ ∇ v ) = − − ρ ρ Tr ⁡ ⁡ ( ( ∇ ∇ v ) ( ∇ ∇ v ) ) . Notice that the above trace is not sign-definite.