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.

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.

基于实际预压载作业的自升式平台环境载荷图谱绘制新思路

The environmental load chart is an important technical support required for the jack-up drilling platform to facilitate its adaptation to different operating waters and ensure the safety of operation.This chart is a crucial part of the platform operation manual.The chart data are closely related to external factors such as water depth,wind,wave,and current conditions of the working water,as well as to the structural characteristics of the platform itself and the number of variable loads.This study examines the platform state under extreme wind,wave,and current conditions during preloading.In addition,this study focuses on the difference between the ultimate reaction force of the pile leg during preloading and the reaction force of the pile leg without considering any environmental load before preloading.Furthermore,the relationship between the difference and the new reaction force of the pile leg caused by the combination of different environmental conditions is established to facilitate the construction of a new form of environmental load chart.The newly formed chart is flexible and simple;thus,it can be used to evaluate the environmental adaptability of the platform in the target well location and provides the preloading target demand or variable load limit according to the given environmental constraints.Moreover,the platform can perform personalized preloading operations,thereby improving its capability to cope with complex geological conditions,such as reducing punch-through risks.This condition reduces the load on jacking system devices and increases its service life.

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.

Study on General Performance of Jack-up Under Elevated Condition

Method of checking for jack-up elevated performance including leg structure strength, fixation system or jacking system beating capacity, pre-load requirements, spud can beating capacity and overturning stability is suggested in this paper. As an example, a jack-up with truss legs is analyzed by finite element analysis method. This paper may be helpful to the rig owners, operators and designers.

Design of A Bionic Spudcan and Analysis of Penetration and Extraction Performances for Jack-up Platform

The mechanisms of soil structure interaction have drawn much attention in the past years in the installation and operation of jack-up platform. A bionic spudcan produced by biomimetic of egg and snail shell is proposed, and the performance of the penetration and extraction are analyzed by numerical method. The geometric contour of egg and snail shell is measured, and its mathematical model is established respectively. According to the structure of existing spudcan of jack-up platform, three kinds of typical biomimetic spudcan are designed. Furthermore, numerical analysis models of biomimetic spudcan are established respectively to study the soil structure interaction mechanism in the process of penetration and extraction, and contrastive analysis of resistance characteristics are carried out. To conclude, the results show that the biomimetic spudcan facilitates the platform installation, and it is also beneficial to the improvement of the bearing capacity of spudcan.

The Calculating Model of Structural Analysis for Jack-Up Rig Legs

The selection or establishment of structural calculating model is a matter of primary importance in structural analysis. By analysing the irrationality of the calculating models used by some research organizations at home and abroad in structural analyses for truss jack-up rig legs, this paper introduces a new calculation model which is more reasonable. This model can simulate better the load situation of a rig reflecting more precisely the effects of the leg loads caused by the spring support of gearboxes of the jacking system and the secondary bending moment raised by hull displacement under extreme conditions. The rig conversion was completed in 1989. In July 1991 the rig safely stood the frontal hitting of Typhoon with the wind speed of 52 m / s, which shows that the checking on the structure is correct.

Prediction Methods of Spudcan Penetration for Jack-up Units

Jack-up units are extensively playing a successful role in drilling engineering around the world, and their safety and efficiency take more and more attraction in both research and engineering practice. An accurate prediction of the spudcan penetration depth is quite instrumental in deciding on whether a jack-up unit is feasible to operate at the site. The prediction of a too large penetration depth may lead to the hesitation or even rejection of a site due to potential difficulties in the subsequent extraction process; the same is true of a too small depth prediction due to the problem of possible instability during operation. However, a deviation between predictive results and final field data usually exists, especially when a strong-over-soft soil is included in the strata. The ultimate decision sometimes to a great extent depends on the practical experience, not the predictive results given by the guideline. It is somewhat risky, but no choice. Therefore, a feasible predictive method for the spudcan penetration depth, especially in strata with strong-over-soft soil profile, is urgently needed by the jack-up industry. In view of this, a comprehensive investigation on methods of predicting spudcan penetration is executed. For types of different soil profiles, predictive methods for spudcan penetration depth are proposed, and the corresponding experiment is also conducted to validate these methods. In addition, to further verify the feasibility of the proposed methods, a practical engineering case encountered in the South China Sea is also presented, and the corresponding numerical and experimental results are also presented and discussed.

Soil-Structure Interaction Analysis of Jack-up Platforms Subjected to Monochrome and Irregular Waves

As jack-up platforms have recently been used in deeper and harsher waters, there has been an increasing demand to understand their behaviour more accurately to develop more sophisticated analysis techniques. One of the areas of significant development has been the modelling of spudean performance, where the load-displacement behaviour of the foundation is required to be included in any numerical model of the structure. In this study, beam on nonlinear winkler foundation （BNWF） modeling--which is based on using nonlinear springs and dampers instead of a continuum soil media--is employed for this purpose. A regular monochrome design wave and an irregular wave representing a design sea state are applied to the platform as lateral loading. By using the BNWF model and assuming a granular soil under spudcans, properties such as soil nonlinear behaviour near the structure, contact phenomena at the interface of soil and spudcan （such as uplifting and rocking）, and geometrical nonlinear behaviour of the structure are studied. Results of this study show that inelastic behaviour of the soil causes an increase in the lateral displacement at the hull elevation and permanent unequal settlement in soil below the spudcans, which are increased by decreasing the friction angle of the sandy soil. In fact, spudeans and the underlying soil cause a relative fixity at the platform support, which changes the dynamic response of the structure compared with the case where the structure is assumed to have a fixed support or pinned support. For simulating this behaviour without explicit modelling of soil-structure interaction （SSI）, moment- rotation curves at the end of platform legs, which are dependent on foundation dimensions and soil characteristics, are obtained. These curves can be used in a simplified model of the platform for considering the relative fixity at the soil- foundation interface.

Collision Analysis Between Spudcan and Seabed During the Process of Jack-up Platform Lowering Jack-up Legs

In the leg-lowering process,the offshore jack-up platform is in a floating state,and the spudcan may collide with the seabed due to the platform motion in waves,thereby threatening the safety and stability of the platform.This paper first analyzed the hydrodynamic response of a jack-up platform under different sea states.Then a finite-element model was established which considered the material and geometrical nonlinearity in the structure and the nonlinear interaction between soil and structure to investigate the collision response between the spudcan and seabed during lowering jack-up legs.The results show that the dynamic response of the platform decreases and gradually tends to be stable as the wave period increases.The motion response of the platform reach the largest value when the wave period is 8 s and the wave direction is 0°.The collision angle between the spudcan and seabed has a large influence on the collision response,while the velocity of leg-lowering has little influence on the collision response.The collision response is also significantly affected by soil conditions.For clay,the increase in undrained shear strength and Young’s modulus will increase the impact force.

Discussion on Key Points of Wind Load of Jack-up Unit

The anti-overturning ability and structure safety of jack-up unit in in-place condition are often affected by environment loads, especially wind load. According to the MODU rule, the projected area method is used for calculating the wind load. However, the calculated results are conservative and not good for structure optimization design. In this paper, a 400 ft jack-up is studied as an example. Based on the wind tunnel test and numerical simulation method, some key points of wind load calculation, such as shielding effect, lift effect and shape coefficient of component, are discussed. The study shows that the points mentioned above, which are ignored in the MODU rule calculation, result in the conservative result.

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.

Uplift Resistance Calculation and Sensitive Analysis of Jack-up Wind Installation Vessels

The uplift resistance calculation is an important basis for the construction decisions of the jack-up wind installation vessel and the design of the jacking system,and determines the operation risk and reliability in the installation process of the wind turbine. The influence factors of the pile shoe's penetration depth and uplift resistance are analyzed,and the calculation model and flow of the uplift resistance are given. Based on a construction example,the influence rules are analyzed for the change of the pile shoe's structural parameters on the penetration depth and uplift resistance.The analysis results show that the penetration depth is more sensitive to the width of the pile shoe,and the height has greater influence on the uplift resistance than the length and width of the spud. With the increase of the height,the uplift resistance may increase rapidly.Although the decreases of the length,width and height of the pile shoe may reduce the uplift resistance,the penetration depth may increase in the meantime. This will increase the pulling pile time and reduce the construction efficiency. So the parameters of the pile shoe should be optimized according to the adaptable geology condition so as to obtain the optimal uplift resistance and working efficiency.

Analysis of spudcan-footprint interaction in a single soil with nonlinear FEM

The footprints that remain on the seabed after offshore jack-up platforms completed operations and moved out provide a significant risk for any futurej ack-up installation at that site. Detrimental horizontal and/or rotational loads will be induced on the base cone of the jack-up platform leg （spudcan） in the preloading process where only vertical loads are normally expected. However, there are no specific guidelines on design of spudcan re-installation very close to or partially overlapping existing footprints. This paper presents a rational design approach for assessing spudcan-footprint interaction and the failure process of foundation in a single layer based on nonlinear finite element method. The rela- tionship between the distance between the spudcan and the footprint and the horizontal sliding force has been obtained. Comparisons of simulation and experimental results show that the model in this paper can deal well with the combined problems of sliding friction contact, fluid-solid coupling, and convergence difficulty. The analytical results may be useful to jack-up installation workovers close to existing footprints.

Bearing Capacity and Critical Punch-Through Depth of Spudcan on Sand Overlying Clay

Spudcan may experience punch-through failure on strong over weak layered soils, such as sand overlying clay. A large deformation finite element method （LDFE） is used to simulate the penetration process of spudcan into sand overlying clay. The sand is simulated by smoothed hyperbolic Mohr-Coulomb model, and the clay is simulated by a simple elasto-plastic model which obeys Tresca yield criterion. According to the LDFE results of a large amount of cases, the effects of the strength, unit weight and thickness of the top sand layer, as well as the effect of the strength of the underlying clay on the spudcan punch-through behavior, are investigated. The critical depth occurring punch-through and the critical bearing capacity are presented in charts. Fitting equations to calculate the critical punch-through depth and the critical bearing capacity are proposed for the convenience of engineering practice.

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.

Failure mechanisms of a spudcan penetrating next to an existing footprint

Reinstallation of mobile jack-up rigs next to existing footprints is a problematic operation because the spudcan located near the footprints is subjected to eccentric and/or inclined loading conditions. Geotech- nical centrifuge studies have measured these loads for combinations of changing footprint geometry, footprint soil properties and the offset of the reinstallation from the footprint centre. These tests have been of full model spudcans in order to accurately measure the combined loads developed. They have not provided information on the mechanisms of failure occurring during this complex installation. Ob- servations from a visualisation test, where a half spudcan is penetrated against a transparent window in a geotechnical centrifuge, are reported in this paper. The mechanisms of failure at different stages during the nenetrntirm are nr^nted

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.

舵桨相互作用下现代船舶绕流数值研究（英文）

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“potential flow solver”in the region outside the boundary layer and wake,“boundary layer solver”in the thin boundary layer region near the ship hull,and“Navier-Stokes solver”in the wake region.Propeller open water characteristics were determined using an open-source MATLAB code OpenProp,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.

Dynamic evaluation of jack-up platform structure under wave, wind, earthquake and tsunami loads

Nowadays,the demand for using jack-up platforms to carry out a large percentage of deep-water oil and gas exploration is steadily increasing.The response of jack-up platforms to the severe dynamic loads that may be encountered during the structure life is not examined enough.Therefore,this study attempts to investigate the response of jack-up platforms performance under the effect of dynamic loads due to wave,wind,earthquake and tsunami forces using the finite element method for two models with the lowest and highest hull elevations.The jack-up platform is located in the Gulf of Mexico.Earthquake ac-celerations are applied to the model in high and moderate seismic levels.In addition,tsunami waves are applied to the platform in three different directions at 0°,45°and 90°.This study utilised Airy’s linear wave approach to assess the surface elevations and wave kinematics.The reference wind velocity is 10 knots at 10 m over the mean water level.Results indicate that the dynamic response of the structure is affected by the height of the platform and by the increase of the platform hull elevation.The combination of the El-Centro earthquake,dead and live loads provides the major impact on the platform at the lowest(70 m)and highest(85 m)hull elevations.The comparison of all result proves that the jack-up platform hull under high earthquake intensity and tsunami waves with 45°has experienced maximum deforma-tion.Moreover,raising the deck will increase the response of the dynamic load and displacements but will negatively affect the platform.