Jet propulsion of a squid-inspired swimmer in the presence of background flow

Qiang Zhu,Yang Luo,Guang Pan,Qing Xiao

doi:10.1063/5.0042575

Abstract

Inspired by recent studies of a squid-like swimmer, we propose a three-dimensional jet propulsion system composed of an empty chamber enclosed within a deformable body with an opening. By prescribing the body deformation and jet velocity profile, we numerically investigate the jet flow field and propulsion performance under the influence of background flow during a single deflation procedure. Three jet velocity profiles, i.e., constant, cosine and half cosine, are considered. We find that the maximum circulation of the vortex ring is reduced at a higher background flow velocity. This is because stronger interaction between the jet flow and background flow makes it harder to feed the leading vortex ring. Regarding thrust production, our analysis based on conservation of momentum indicates that with the constant profile the peak thrust is dominated by the time derivative of the fluid momentum inside the body, while momentum flux related thrust accounts for the quasi-steady thrust. For the cosine profile, its peak is mainly sourced from momentum flux associated with the unsteady vortex ring formation. No prominent thrust peak exists with the half cosine profile whose thrust continuously increases during the jetting. For all the three jet velocity profiles, added-mass related thrust attributed to body deformation enhances the overall thrust generation non-negligibly. Under the present tethered mode, the background flow has negligible influence on the thrust attributed to momentum flux and momentum change of the fluid inside the body. However, it indeed affects the over pressure-related thrust but its effect is relatively small. The overall thrust declines due to the significantly increased drag force at large incoming flow speed despite the rise of added-mass related thrust. Unsteady thrust involving vortex ring formation becomes more important in the overall thrust generation with an increased background flow velocity, reflected by larger ratios of the unsteady impulse to jet thrust impulse.

Highlights

Jet propulsion utilized by squids and other cephalopods is a unique aquatic locomotion method different from the commonly used fin oscillation and body undulation modes
We start with a constant jet velocity profile for different maximum equivalent stroke ratios m at the same incoming velocity U0 of the background flow, i.e., the time history of the spatially averaged jet speed is Vj (t ) = Vjm, t
Using a three-dimensional, unsteady, viscous and compressible Navier-Stokes fluid solver based on cell-centered finite volume method, we numerically study squid-like jet propulsion through body deformation

Summary

Introduction

Jet propulsion utilized by squids and other cephalopods is a unique aquatic locomotion method different from the commonly used fin oscillation and body undulation modes. A novel cephalopod-inspired jet propulsion system was proposed by Bi and Zhu (2018) They focused on a single bursting cycle and found the optimal speed coincides with the critical stroke ratio. Their numerical model was based on the potential-flow theory, where the viscosity effect was not included so that the accuracy of the results was compromised. The background flow would affect the vortex ring formation and the jet propulsion performance (Anderson and Grosenbaugh, 2005) This effect was considered in a numerical study by Jiang and Grosenbaugh (2006).

Problem formulation

Numerical model and approach

Results

Conclusions