“…ρ u(x, y, t), v(x, y, t) u(x, y, t) v(x, y, t) dx dy (1) where ρ is the mass per unit area, and V (e) is the area of an element. u(x, y, t) and v(x, y, t) are the spatialand time-varying components of the displacement vector field in the x and y direction respectively.…”