Kelvin has established the remarkable theorem that Laplacian flow has less kinetic energy than any other motion consistent with the same normal motion of the boundaries of the fluid. The proof is given by Lamb (Art. 45, p. 47).
A practical criterion can be immediately inferred from this
theorem in connection with the foregoing example (Art. 1.13):
the effective added mass, 
, is a minimum in the
case of Laplacian flow around a cylinder. In the more
usual case where turbulent flow exists in the neighborhood
of the cylinder, we should expected to find that
for a circular cylinder.