With the irrotational assumption, namely, the flow velocity can be described as the gradient ∇ φ of a velocity potential φ. The unsteady momentum conservation equation becomes ∂ ∂ ∇ ∇ ϕ ϕ ∂ ∂ t + ∇ ∇ ( ∇ ∇ ϕ ϕ ⋅ ⋅ ∇ ∇ ϕ ϕ 2 ) = − − ∇ ∇ Ψ Ψ − − ∇ ∇ ∫ ∫ p 1 p d p ~ ~ ρ ρ ( p ~ ~ ) which leads to ∂ ∂ ϕ ϕ ∂ ∂ t + ∇ ∇ ϕ ϕ ⋅ ⋅ ∇ ∇ ϕ ϕ 2 + Ψ Ψ + ∫ ∫ p 1 p d p ~ ~ ρ ρ ( p ~ ~ ) = constant