The Bernoulli equation for unsteady potential flow is used in the theory of ocean surface waves and acoustics. For an irrotational flow, the flow velocity can be described as the gradient ∇ φ of a velocity potential φ. In that case, and for a constant density ρ, the momentum equations of the Euler equations can be integrated to: ∂ ∂ φ φ ∂ ∂ t + 1 2 v 2 + p ρ ρ + g z = f ( t ) ,