Aquatic organisms capable of undergoing extensive volume variation of their body during locomotion can benefit from increased thrust production. This is enabled by making use of not only the expulsion of mass from their body, as documented extensively in the study of pulsed-jet propulsion, but also from the recovery of kinetic energy via the variation of added mass. We use a simplified mechanical system, i.e. a shape-changing linear oscillator, to investigate the phenomenon of added-mass recovery. Our study proves that a deformable oscillator can be set in sustained resonance by exploiting the contribution from shape variation alone which, if appropriately modulated, can annihilate viscous drag. By confirming that a body immersed in a dense fluid which undergoes an abrupt change of its shape experiences a positive feedback on thrust, we prove that soft-bodied vehicles can be designed and actuated in such a way as to exploit their own body deformation to benefit of augmented propulsive forces.
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