  This paper presents some novel contributions to the theory of inviscid flow
regarding the forces exerted on a body moving through such a fluid in two
dimensions. It is argued that acceleration of the body corresponds to vorticity
generation that is independent of the instantaneous velocity of the body and
thus the boundary condition on the normal velocity. The strength of the vortex
sheet representing the body retains a degree of freedom that represents the net
effect of the tangential boundary condition associated with the viscous flow
governed by the higher-order Navier-Stokes equations. This degree of freedom is
the circulation of the vorticity generated by the acceleration of the body.
Equivalently, it is the net circulation around a contour enclosing the body and
any shed vorticity. In accordance with Kelvin's circulation theorem, a non-zero
value of the circulation around this system is necessarily communicated to
infinity. This contrasts with the usual acceptance of the theorem as requiring
this circulation to be zero at all times; a condition that is incapable of
capturing the effect of newly generated vorticity on the body surface when it
accelerates. Additionally, the usual boundary condition of continuity of normal
velocity is relaxed to allow for fluid entrainment into surfaces of
discontinuity that represent the mass contained within the viscous layers of
the physical problem. The generalized force calculation is presented in detail.
The importance of the vorticity generation due to body acceleration is
demonstrated on some modeled problems relevant to biological propulsion.
