MHz-Order Surface Acoustic Wave Thruster for Underwater Silent Propulsion.

Naiqing Zhang,James Friend,Yue Wen

doi:10.3390/mi11040419

Abstract

High frequency (MHz-order) surface acoustic waves (SAW) are able to generate intense fluid flow from the attenuation of acoustic radiation in viscous fluids as acoustic streaming. Though such flows are known to produce a force upon the fluid and an equivalent and opposing force upon the object producing the acoustic radiation, there is no convenient method for measuring this force. We describe a new method to accomplish this aim, noting the potential of these devices in providing essentially silent underwater propulsion by virtue of their use of the sound itself to generate fluid momentum flux. Our example employs a 40 MHz SAW device as a pendulum bob while immersed in a fluid, measuring a 1.5 mN propulsion force from an input power of 5 W power to the SAW device. Supporting details regarding the acoustic streaming profile via particle image velocimetry and an associated theoretical model are provided to aid in the determination of the propulsion force knowing the applied power and fluid characteristics. Finally, a simple model is provided to aid the selection of the acoustic device size to maximize the propulsion force per unit device area, a key figure of merit in underwater propulsion devices. Using this model, a maximum force of approximately 10 mN/cm was obtained from 1 W input power using 40 MHz SAW in water and producing a power efficiency of approximately 50%. Given the advantages of this technology in silent propulsion with such large efficiency and propulsion force per unit volume, it seems likely this method will be beneficial in propelling small autonomous submersibles.

Highlights

Efficient underwater propulsion has long been essential to the operation of autonomous underwater vehicles (AUV) [1,2]
In spite of ample research on surface acoustic waves (SAW)-induced acoustic streaming reported in the literature over the years, the ability of acoustic streaming to produce a reaction force upon the device that is the source of the acoustic streaming has not been investigated in detail
The purpose of this work is to both provide a simple method for measuring the force generated by acoustic streaming and to present a possible means for silent underwater propulsion for small to microscale submersible craft

Summary

Introduction

Efficient underwater propulsion has long been essential to the operation of autonomous underwater vehicles (AUV) [1,2]. As the size of submersibles decreases to the millimeter and smaller scales, electrical motors are all that remain to drive the propulsion mechanisms in submersibles, even though propellers are well known to suffer from poor efficiency at such small scales and produce torque steer that is difficult for a small craft to overcome [5]. Notwithstanding this issue, Li et al [6] recently demonstrated acoustic-induced propulsion by a lead zirconate titanate (PZT) piezoelectric element used to drive a propeller as an underwater piezoelectric thruster. Waterjet propulsion was briefly introduced in [3], regardless of scale, most waterjet devices still employ propellers, enclosing them within a tube instead of leaving them in the open flow with modest improvement in propulsive efficiency at small scales

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