Ultra-fast escape maneuver of an octopus-inspired robot

Abstract

Abstract. We design and test an octopus-inspired flexible hull robot that demonstrates outstanding fast-starting performance. The robot is hyper-inflated with water, and then rapidly deflates to expel the fluid so as to power the escape maneuver. Using this robot we verify for the first time in laboratory testing that rapid size-change can substantially reduce separation in bluff bodies traveling several body lengths, and recover fluid energy which can be employed to improve the propulsive performance. The robot is found to experience speeds over ten body lengths per second, exceeding that of a similarly propelled optimally streamlined rigid rocket. The peak net thrust force on the robot is more than 2.6 times that on an optimal rigid body performing the same maneuver, experimentally demonstrating large energy recovery and enabling acceleration greater than 14 body lengths per second squared. Finally, over 53% of the available energy is converted into payload kinetic energy, a performance that exceeds the estimated energy conversion efficiency of fast-starting fish. The Reynolds number based on final speed and robot length is Re ≈ 700, 000. We use the experimental data to establish a fundamental deflation scaling parameter σ * which characterizes the mechanisms of flow control via shape change. Based on this scaling parameter, we find that the fast-starting performance improves with increasing size.

Biomimetic Survival Hydrodynamics and Flow Sensing

“…The normally streamlined mantle becomes quite bluff; flow around a similarly shaped rigid body geometry would incur large energy penalties in the form of flow separation drag and increased added mass force. However, the flexible, rapidly deflating mantle completely alters the dynamics of the flow, inducing mechanisms of separation elimination and added mass energy recovery, as investigated in , and demonstrated experimentally in Weymouth, Subramaniam & Triantafyllou (2015). The core issue in determining the propulsive performance of the shrinking body is the evolution of the boundary layer vorticity at the external surface of the body as it undergoes large deformations.…”

“…where C(t) is a constant over the surface and does not contribute to the force or generation of vorticity (Weymouth, Subramaniam & Triantafyllou 2015). For a rapidly shrinking body the first term implies very low pressure at the front of the sphere (θ = 0) and very high pressure at the back (θ = π).…”

“…As shown in Weymouth, Subramaniam & Triantafyllou (2015), it is insightful to compare a shrinking deformable body to the application of suction on a rigid body boundary layer, as both processes induce a normal velocity to the body surface, although shrinking induces it without…”

Added mass energy recovery of octopus-inspired shape change

“…When plotted against the non-dimensional shrinking parameter σ * , we see that all recovery ratios closely follow the same curve, confirming that σ * is indeed a characteristic parameter of the energy recovery shrinking problem. We also note that our definition here of the non-dimensional shrinking parameter as σ * =ȧ max U √ Re 0 , whereȧ max is the maximum shrinking speed of the body, U is the characteristic forward velocity of the motion, and Re 0 is the Reynolds number based on the initial body size, is a more appropriate definition than that previously introduced in Weymouth et al (2015), where the deflation scaling parameter is defined as σ * =V AU √ Re, whereV is the body volume rate of change, A is the frontal area, U is the forward velocity, and Re is the Reynolds number. For example, in the two-dimensional simulations presented in this work of a circle shrinking to an elongating ellipse, with the major axis elongating as the inverse of the shrinking minor axis of the ellipse, the area is always constant so that the parameter as defined in Weymouth et al (2015) is always zero.…”

“…We note that energy recovery is significant, provided that flow separation is prevented; these results are entirely consistent with the results of Weymouth & Triantafyllou (2013) that include acceleration and jet effects. Weymouth et al (2015) provide an experimental validation that shape change can result in additional thrust due to jet momentum flux, while they introduce a deflation scaling parameter, σ * , based on an analogy with separation prevention through porous suction flow.…”