i'm a little bit confused. if the velocity potential Φ = Φ(x,y,t). and it says Φ satifies laplace's equation. does that mean ∂²Φ/∂x² + ∂²Φ/∂y² = 0 or ∂²Φ/∂x² + ∂²Φ/∂y² + ∂²Φ/∂t² = 0. does the time variable get included?? i'm thinking it isn't but i'm not exactly sure.