  Inviscid computations are presented of a self-propelled virtual body
connected to a combined heaving and pitching foil that uses continuous and
intermittent motions. It is determined that intermittent swimming can improve
efficiency when the dimensionless heave ratio is $h^* < 0.7$ while it degrades
efficiency for $h^* \geq 0.7$. This is a consequence of the physical origins of
the force production for pitch-dominated ($h^* < 0.5$) and heave-dominated
($h^* > 0.5$) motions. Based on insight derived from classic unsteady thin
airfoil theory, it is discovered that pitch-dominated motions are driven by
added-mass-based thrust production where self-propelled efficiency is maximized
for high reduced frequencies, while heave-dominated motions are driven by
circulatory-based thrust production where self-propelled efficiency is
maximized by low reduced frequencies. Regardless of the dimensionless heave
ratio the reduced frequency is high for small amplitude motions, high Lighthill
numbers, and low duty cycles and vice versa. Moreover, during intermittent
swimming, the stopping vortex that is shed at the junction of the bursting and
coasting phases becomes negligibly weak for $h^* < 0.5$ and small amplitude
motions of $A^* = 0.4$. This study provides insight into the mechanistic
trade-offs that occur when biological or bio-inspired swimmers continuously or
intermittently use combined heaving and pitching hydrofoils.
