Self-propulsion of flapping bodies in viscous fluids: Recent advances and perspectives

Flapping-powered propulsion is used by many animals to locomote through air or water. Here we review recent experimental and numerical studies on self-propelled mechanical systems powered by a flapping motion. These studies improve our understanding of the mutual interaction between actively flapping bodies and surrounding fluids. The results obtained in these works provide not only new insights into biolocomotion but also useful information for the biomimetic design of artificial flyers and swimmers.

The effects of caudal fin deformation on the hydrodynamics of thunniform swimming under self-propulsion

To investigate the effects of the caudal fin deformation on the hydrodynamic performance of the self-propelled thunniform swimming,we perform fluid-body interaction simulations for a tuna-like swimmer with thunniform kinematics.The 3-D vortices are visualized to reveal the role of the leading-edge vortex(LEV)in the thrust generation.By comparing the swimming velocity of the swimmer with different caudal fin flexure amplitudes fa,it is shown that the acceleration in the starting stage of the swimmer increases with the increase of fa,but its cruising velocity decreases.The results indicate that the caudal fin deformation is beneficial to the fast start but not to the fast cruising of the swimmer.During the entire swimming process,the undulation amplitudes of the lateral velocity and the yawing angular velocity decrease as fa increases.It is found that the formation of an attached LEV on the caudal fin is responsible for generating the low-pressure region on the surface of the caudal fin,which contributes to the thrust.Furthermore,the caudal fin deformation can delay the LEV shedding from the caudal fin,extending the duration of the low pressure on the caudal fin,which will cause the caudal fin to generate a drag-type force over a time period in one swimming cycle and reduce the cruising speed of the swimmer.

Self-propulsion of a three-dimensional flapping flexible plate

The self-propulsion of a 3-D flapping flexible plate in a stationary fluid is numerically studied by an immersed boundarylattice Boltzmann method for the fluid flow and a finite element method for the plate motion. When the leading-edge of the plate is forced to heave sinusoidally, the entire plate starts to move freely as a result of the fluid-structure interaction. Based on our simulation and analysis on the dynamical behaviors of the flapping flexible plate, we have found that the effect of plate aspect ratio on its propulsive properties can be divided into three typical regimes which are related to the plate flexibility, i.e. stiff, medium flexible, and more flexible regime. It is also identified that a suitable structure flexibility, corresponding to the medium flexible regime, can improve the propulsive speed and efficiency. The wake behind the flapping plate is investigated for several aspect ratios to demonstrate some typical vortical structures. The results obtained in this study can provide some physical insights into the understanding of the propulsive mechanisms in the flapping-based locomotion.

Coupling of RANS and BEM Solvers for Simulation of Propeller Behind the Hull in Full Appended Model

To design a propeller for ship power plant,the interaction between ship hull and propeller must be taken into account.The main concern is to apply the wake effect of ship stern on the propeller performance.In this paper,a coupled BEM(Boundary Element Method)/RANS(Renolds-Averaged Navier−Stokes)solver is used to simulate propeller behind the hull in the self-propulsion test.The motivation of this work is to develop a practical tool to design marine propulsion system without suffering long computational time.An unsteady boundary element method which is also known as panel method is chosen to estimate the propeller forces.Propeller wakes are treated using a time marching wake alignment method.Also,a RANS code coupled with VoF equation is developed to consider the ship motions and wake field effects in the problem.A coupling algorithm is developed to interchange ship wake field to the potential flow solver and propeller thrust to the RANS code.Based on the difference between hull resistance and the propeller thrust,a PI controller is developed to compute the propeller RPM in every time step.Verification of the solver is carried out using the towing tank test report of a 50 m oceanography research vessel.Wake factor and trust deduction coefficient are estimated numerically.Also,the wake rollup pattern of the propeller in open water is compared with the propeller in real wake field.

Untethered Micro/Nanorobots for Remote Sensing:Toward Intelligent Platform

Untethered micro/nanorobots that can wirelessly control their motion and deformation state have gained enormous interest in remote sensing applications due to their unique motion characteristics in various media and diverse functionalities.Researchers are developing micro/nanorobots as innovative tools to improve sensing performance and miniaturize sensing systems,enabling in situ detection of substances that traditional sensing methods struggle to achieve.Over the past decade of development,significant research progress has been made in designing sensing strategies based on micro/nanorobots,employing various coordinated control and sensing approaches.This review summarizes the latest developments on micro/nanorobots for remote sensing applications by utilizing the self-generated signals of the robots,robot behavior,microrobotic manipulation,and robot-environment interactions.Providing recent studies and relevant applications in remote sensing,we also discuss the challenges and future perspectives facing micro/nanorobots-based intelligent sensing platforms to achieve sensing in complex environments,translating lab research achievements into widespread real applications.

Structural and Operational Optimization of A Flapping Fin Used as A Self-Propulsor for AUV Propulsion

Marine mammals could directly harvest energy from waves and obtain propulsive force through oscillating flapping fins or horizontal tail flukes,which in many cases have been observed and proved to be substantial.The propulsion generated by the flapping fin has been analyzed by many researchers from both the theoretical and experimental prospects;however,the structural and operational optimization of a flapping fin for the optimal propulsion performance has been less studied,such as the investigation of the effects of the phase difference between heave and pitch motion,maximum oscillation angle,fin shape,oscillation centre of the fin and the operating sea state on the generated propulsion.In this paper,the flapping fin is used as a self-propulsor to propel an autonomous underwater vehicle(AUV)for propulsion assistance.For the optimization design of the flapping fin,its propulsion effect is numerically investigated with different structural parameters and under various operation conditions using computational fluid dynamics(CFD)approaches.Verification and validation study have been implemented to quantify the numerical uncertainties and evaluate the accuracy of the proposed CFD method.Then,a series of case studies are thoroughly conducted to investigate the effects of different structural parameters and operational conditions on the generated propulsion of a flapping fin by CFD simulations.The simulation results demonstrate that different structural parameters and operation conditions would significantly impact the magnitude and distribution state of the fluid pressure around the flapping fin surface,thus,affect the propulsion performance of the fin.The findings in this study will provide guidelines for the structural and operational optimization design of a flapping fin for self-propulsion of mobile platforms.

The Calculations of Propeller Induced Velocity by RANS and Momentum Theory

In order to provide instructions for the calculation of the propeller induced velocity in the study of the hull-propeller interaction using the body force approach,three methods were used to calculate the propeller induced velocity：1） Reynolds-Averaged Navier-Stokes（RANS） simulation of the self-propulsion test,2） RANS simulation of the propeller open water test,and 3） momentum theory of the propeller.The results from the first two methods were validated against experimental data to assess the accuracy of the computed flow field.The thrust identity method was adopted to obtain the advance velocity,which was then used to derive the propeller induced velocity from the total velocity field.The results computed by the first two approaches were close,while those from the momentum theory were significantly overestimated.The presented results could prove to be useful for further calculations of self-propulsion using the body force approach.

Numerical Study on Flow Around Modern Ship Hulls with Rudder-Propeller Interactions

Reducing the fuel consumption of ships presents both economic and environmental gains. Although in the past decades,extensive studies were carried out on the flow around ship hull, it is still difficult to calculate the flow around the hull while considering propeller interaction. In this paper, the viscous flow around modern ship hulls is computed considering propeller action. In this analysis, the numerical investigation of flow around the ship is combined with propeller theory to simulate the hull-propeller interaction. Various longitudinal positions of the rudder are also analyzed to determine the effect of rudder position on propeller efficiency. First, a numerical study was performed around a bare hull using Shipflow computational fluid dynamics(CFD) code to determine free-surface wave elevation and resistance components.A zonal approach was applied to successively incorporate Bpotential flow solver^ in the region outside the boundary layer and wake, Bboundary layer solver^ in the thin boundary layer region near the ship hull, and BNavier-Stokes solver^in the wake region. Propeller open water characteristics were determined using an open-source MATLAB code Open Prop, which is based on the lifting line theory, for the moderately loaded propeller. The obtained open water test results were specified in the flow module of Shipflow for self-propulsion tests. The velocity field behind the ship was recalculated into an effective wake and given to the propeller code that calculates the propeller load. Once the load was known, it was transferred to the Reynolds-averaged Navier-Stokes(RANS) solver to simulate the propeller action. The interaction between the hull and propeller with different rudder positions was then predicted to improve the propulsive efficiency.

Proliferation-mediated asymmetric nanoencapsulation of singlecell and motility differentiation

The biointerface engineering of living cells by creating an abiotic shell has important implications for endowing cells with exogenous properties with improved cellular behavior,which then boosts the development of the emerging field of living cell hybrid materials.Herein,we develop a way to perform active nanoencapsulation of single cell,which then endows the encapsulated cells with motion ability that they do not inherently possess.The emerging motion characteristics of the encapsulated cells could be self-regulated in terms of both the motion velocity and orbits by different proliferation modes.Accordingly,by taking advantage of the emergence of differentiated moving abilities,we achieve the self-sorting between mother cells and daughter cells in a proliferated Saccharomyces cerevisiae cell community.Therefore,it is anticipated that our highlighted study could not only serve as a new technique in the field of single-cell biology analysis and sorting such as in studying the aging process in Saccharomyces cerevisiae,but also open up opportunities to manipulate cell functionality by creating biohybrid materials to fill the gap between biological systems and engineering abiotic materials.

Effect of flexibility on unsteady aerodynamics forces of a purely plunging airfoil

Introducing flexibility into the design of a vertically flapping wing is an effective way to enhance its aerodynamic performance.As less previous studies on the aerodynamics of vertically flapping flexible wings focused on the lift generated in a wide range of angle of attack·a 2D numerical simulation of a purely plunging flexible airfoil is employed using a loose fluid–structure interaction method.The aerodynamics of a fully flexible airfoil are firstly studied with the flexibility and angle of attack.To verify whether an airfoil could get aerodynamic benefit from the change in structure,partially flexible airfoil with rigid leading edge and flexible trailing edge were further considered.Results show that flexibility could always reduce airfoil drag while lift and lift efficiency both peak at moderate flexibility.When freestream velocity is constant,lift is maximized at a high angle of attack about 40°while this optimal angle of attack reduces to 15°in drag-balanced status.The airfoil drag reduction,lift augmentation as well as efficiency enhancement mainly attribute to the passive pitching other than the camber deformation.Partially deformed airfoil with the longest length of moderate flexible trailing edge can achieve the highest lift.This study may provide some guidance in the wing design of Micro Air Vehicle(MAV).

Mesoporous silica as micro/nano-carrier： From passive to active cargo delivery, a mini review

Mesoporous silica has been widely explored for biomedical applications due to its unique structure and good biocompatibility. In particular it exhibits superior properties as micro/nano-carriers in the biomedical field. We explore their potentials in controlled drug/gene co-delivery and photodynamic therapy for cancer treatment both in vitro and in vivo. By incorporating mesoporous silica nanoparticles（MSNP） with two-dimensional nanomaterial, graphene oxide nano-sheet, we utilize MSNP in cellular bio-imaging with squaraine dye. Meanwhile, through delicate combination between mesoporous silica micro/nano carriers with catalytic/bio-catalytic reactions, we manage to achieve self-propelled micro/nano-motors based on mesoporous silica that are capable of transporting cargos in an active manner. Especially, enzyme powered mesoporous silica motors can be powered by physiologically available fuels such as glucose and urea,which are advantageous for future biomedical use. Motion control on both velocity and movement direction provides a powerful tool for targeted drug delivery. Therefore, such mesoporous silica based active carriers pave way to the solution of targeted drug delivery for cancer treatment in future nano-medicine field.

Fundamentals and applications of enzyme powered micro/nano-motors

Micro/nanomotors(MNMs)are miniaturized machines that can convert many kinds of energy into mechanical motion.Over the past decades,a variety of driving mechanisms have been developed,which have greatly extended the application scenarios of MNMs.Enzymes exist in natural organisms which can convert chemical energy into mechanical force.It is an innovative attempt to utilize enzymes as biocatalyst providing driving force for MNMs.The fuels for enzymatic reactions are biofriendly as compared to traditional counterparts,which makes enzyme-powered micro/nanomotors(EMNMs)of great value in biomedical field for their nature of biocompatibility.Until now,EMNMs with various shapes can be propelled by catalase,urease and many others.Also,they can be endowed with multiple functionalities to accomplish on-demand tasks.Herein,combined with the development process of EMNMs,we are committed to present a comprehensive understanding of EMNMs,including their types,propelling principles,and potential applications.In this review,we will introduce single enzyme that can be used as motor,enzyme powered molecule motors and other micro/nano-architectures.The fundamental mechanism of energy conversion process of EMNMs and crucial factors that affect their movement behavior will be discussed.The current progress of proof-of-concept applications of EMNMs will also be elaborated in detail.At last,we will summarize and prospect the opportunities and challenges that EMNMs will face in their future development.

Hybrid undulatory kinematics of a robotic Mackerel(Scomber scombrus):Theoretical modeling and experimental investigation

Fishes that use undulatory locomotion occasionally change their inherent kinematics in terms of some natural behavior.This special locomotion pattern was vividly dubbed "hybrid kinematics" by biologists recently.In this paper,we employed a physical model with body shape of a Mackerel(Scomber scombrus),to use the three most typical undulatory kinematics:anguillform,carangiform and thunniform,to investigate the hydrodynamic performance of the so-called "hybrid kinematics" biological issue.Theoretical models of both kinematics and hydrodynamics of the physical model swimming were developed.Base on this model,the instantaneous force produced by fish undulatory body and flapping tail were calculated separately.We also quantitatively measured the hydrodynamic variables of the robotic model swimming with the three undulatory kinematics on an experimental apparatus.The results of both theoretical model and experiment showed that the robot with thunniform kinematics not only reaches a higher speed but also is more efficient during steady swimming mode.However,anguilliform kinematics won the speed race during the initial acceleration.Additionally,the digital particle image velocimetry(DPIV) results showed some difference of the wake flow generated by the robotic swimmer among the three undulatory kinematics.Our findings may possibly shed light on the motion control of a biomimetic robotic fish and provide certain evidence of why the "hybrid kinematics" exists within the typical undulatory locomotion patterns.