Generalizing J_2 flow theory: Fundamental issues in strain gradient plasticity

It has not been a simple matter to obtain a sound extension of the classical J2 flow theory of plasticity that incorporates a dependence on plastic strain gradients and that is capable of capturing size-dependent behaviour of metals at the micron scale. Two classes of basic extensions of classical J2 theory have been proposed： one with increments in higher order stresses related to increments of strain gradients and the other characterized by the higher order stresses themselves expressed in terms of increments of strain gradients. The theories proposed by Muhlhans and Aifantis in 1991 and Fleck and Hutchinson in 2001 are in the first class, and, as formulated, these do not always satisfy thermodynamic requirements on plastic dissipation. On the other hand, theories of the second class proposed by Gudmundson in 2004 and Gurtin and Anand in 2009 have the physical deficiency that the higher order stress quantities can change discontinuously for bodies subject to arbitrarily small load changes. The present paper lays out this background to the quest for a sound phenomenological extension of the rateindependent J2 flow theory of plasticity to include a de- pendence on gradients of plastic strain. A modification of the Fleck-Hutchinson formulation that ensures its thermo- dynamic integrity is presented and contrasted with a comparable formulation of the second class where in the higher or- der stresses are expressed in terms of the plastic strain rate. Both versions are constructed to reduce to the classical J2 flow theory of plasticity when the gradients can be neglected and to coincide with the simpler and more readily formulated J2 deformation theory of gradient plasticity for deformation histories characterized by proportional straining.

Study on Mooring Design and Calculation Method of Ocean Farm Based on Time-Domain Potential Flow Theory

In order to calculate the mooring force of a new semi-submerged Ocean Farm quickly and accurately,based on the unsteady time-domain potential flow theory and combined the catenary model,the control equation of mooring cable is established,and the mooring force of the platform under the wave spectrum is calculated.First of all,based on the actual situation of the ocean environment and platform,the mooring design of the platform is carried out,and the failure analysis and sensitivity analysis of the single anchor chain by the time domain coupling method are adopted:including different water depth,cycle,pretension size,anchor chain layout direction and wind speed,etc.The analysis results confirm the reliability of anchoring method.Based on this,the mooring point location of the platform is determined,the force of each anchor chain in the anchoring process is calculated,and the mooring force and the number of mooring cables are obtained for each cable that satisfies the specification,the results of this paper can provide theoretical calculation methods for mooring setting and mooring force calculation of similar offshore platforms.

Some developments of hot accretion flow theory in the past ten years

The hot accretion flow model was re-discovered in 1994 by Narayan and collaborators.Intensive theoretical works have been conducted and significant progresses have been achieved.In this paper,we review several developments in the past ten years.This mainly includes the finding of outflow and convection and its dynamical effect on inflow;the direct electron heating by viscous dissipation;the effect of large scale toroidal magnetic fields in the inner region of the accretion flow;and the effect of global Compton scattering.Their observational applications are also introduced very briefly.

Gao's interacting shear flows( ISF) theory and its inferences and their applications

Gao＇s viscous/in-viscid interacting shear flows （ISF） theory, proposed by professor Gao Zhi in Institute of Mechanics, China Academy of Science, and its inferences and their applications in computational fluid dynamics （CFD） are reviewed and some subjects worthy to be studied are pro- posed in this paper. The flow-field and motion law of ISF, mathematics definition of strong viscous shear layer flow in ISF, ISF equations, wall-surface compatibility criteria （Gao＇s criteria ）, space scale variety law of strong viscous shear layer reveals flow mechanism and local space small scale triggered by strong interaction that cause some abnormal severe local pneumatic heating phenomenon in hypersonic flow. Gao＇s ISF theory was used in near wall flow, free ISF flow simulation and design of computing grids, Gao＇s wall-surface criteria were used to verify calculation reliability and accuracy of near wall flows, ISF theory approximate analytical result of shock waves-boundary layer interac- tion and ISF equations were used to obtain the numerical exact solution of local area flow （ such as stationary point flow）. Some new subjects, such as, improving near-wall turbulent models according to the turbulent flow simulation satisfying the wall-criteria and illustrating relation between grid-con- vergence based on the wall criteria and other convergence tactics, are suggested. The necessity of applying Gao＇s ISF theory and wall criteria is revealed. Difficulties and importance of hypersonic vis- cous/in-viscid interaction phenomenon were also emphasized.

THE THEORY OF CURRENT TRANSIENTS AT TUBULAR ELECTRODES IN A FLOWING FLUID

A theoretical equation is developed which describes the response of the current transients to a constant potential at tubular electrodes for a reversible electrode reaction in the flowing fluid.

Comparisons of Wave Force Model Effects on the Structural Responses and Fatigue Loads of a Semi-Submersible Floating Wind Turbine

The selection of wave force models will significantly impact the structural responses of floating wind turbines.In this study,comparisons of wave force model effects on the structural responses and fatigue loads of a semi-submersible floating wind turbine(SFWT)were conducted.Simulations were performed by employing the Morison equation(ME)with linear or second-order wave kinematics and potential flow theory(PFT)with first-or second-order wave forces.A comparison of regular waves,irregular waves,and coupled wind/waves analyses with the experimental data showed that many of the simulation results and experimental data are relatively consistent.However,notable discrepancies are found in the response amplitude operators for platform heave,tower base bending moment,and tension in mooring lines.PFT models give more satisfactory results of heave but more significant discrepan-cies in tower base bending moment than the ME models.In irregular wave analyses,low-frequency resonances were captured by PFT models with second-order difference-frequency terms,and high-frequency resonances were captured by the ME models or PFT models with second-order sum-frequency terms.These force models capture the response frequencies but do not reasonably predict the response amplitudes.The coupled wind/waves analyses showed more satisfactory results than the wave-only analyses.However,an important detail to note is that this satisfactory result is based on the overprediction of wind-induced responses.

Traffic flow velocity disturbance characteristics and control strategy at the bottleneck of expressway

In the three-phase traffic flow studies, the traffic flow characteristic at the bottleneck section is a hot spot in the academic field. The controversy about the characteristics of the synchronized flow at bottleneck is also the main contradiction between the three-phase traffic flow theory and the traditional traffic flow theory. Under the framework of three-phase traffic flow theory, this paper takes the on-ramp as an example to discuss the traffic flow characteristics at the bottleneck section.In particular, this paper mainly conducts the micro-analysis to the effect of lane change under the two lane conditions, as well as the effect of the on-ramp on the main line traffic flow. It is found that when the main road flow is low, the greater the on-ramp inflow rate, the higher the average speed of the whole road section. As the probability of vehicles entering from the on-ramp increases, the flow and the average speed of the main road are gradually stabilized, and then the on-ramp inflow vehicles no longer have a significant impact on the traffic flow. In addition, this paper focuses on the velocity disturbance generated at the on-ramp, and proposes the corresponding on-ramp control strategy based on it, and the simulation verified that the control strategy can reasonably control the traffic flow by the on-ramp, which can meet the control strategy requirements to some extent.

Shale oil development techniques and application based on ternary-element storage and flow concept in Jiyang Depression,Bohai Bay Basin,East China

The ternary-element storage and flow concept for shale oil reservoirs in Jiyang Depression of Bohai Bay Basin,East China,was proposed based on the data of more than 10000 m cores and the production of more than 60 horizontal wells.The synergy of three elements(storage,fracture and pressure)contributes to the enrichment and high production of shale oil in Jiyang Depression.The storage element controls the enrichment of shale oil;specifically,the presence of inorganic pores and fractures,as well as laminae of lime-mud rocks,in the saline lake basin,is conducive to the storage of shale oil,and the high hydrocarbon generating capacity and free hydrocarbon content are the material basis for high production.The fracture element controls the shale oil flow;specifically,natural fractures act as flow channels for shale oil to migrate and accumulate,and induced fractures communicate natural fractures to form complex fracture network,which is fundamental to high production.The pressure element controls the high and stable production of shale oil;specifically,the high formation pressure provides the drive force for the migration and accumulation of hydrocarbons,and fracturing stimulation significantly increases the elastic energy of rock and fluid,improves the imbibition replacement of oil in the pores/fractures,and reduces the stress sensitivity,guaranteeing the stable production of shale oil for a long time.Based on the ternary-element storage and flow concept,a 3D development technology was formed,with the core techniques of 3D well pattern optimization,3D balanced fracturing,and full-cycle optimization of adjustment and control.This technology effectively guides the production and provides a support to the large-scale beneficial development of shale oil in Jiyang Depression.

Generalised energy conservation law for chaotic phenomena

Chaotic phenomena are increasingly being observed in all fields of nature,where investigations reveal that a natural phe nomenon exhibits nonlinearities and attempts to reveal their deep underlying mechanisms.Chaos is normally understood as“a state of disorder”,for which there is as yet no universally accepted mathematical definition.A commonly used concept states that,for a dynamical system to be classified as chaotic,it must have the following properties:be sensitive to initial conditions,show topological transitivity,have densely periodical orbits etc.Revealing the rules that govern chaotic motion is thus an important unsolved task for exploring nature.W e present herein a generalised energy conservation law governing chaotic phenomena.Based on two scalar variables,viz.generalised potential and kinetic energies defined in the phase space describing nonlinear dynamical systems,we find that chaotic motion is periodic motion with infinite time period whose time-averaged generalised potential and kinetic energies are conserved over its time period.This implies that,as the averaging time is increased,the time-averaged generalised potential and kinetic energies tend to constants while the time-averaged energy flows,i.e.,their rates of change with time,tend to zero.Numerical simulations on reported chaotic motions,such as the forced van der Pol system,forced Duffing system,forced smooth and discontinuous oscillator,Lorenz’s system,and Rossler's system,show the above conclusions to be correct according to the results presented herein.This discovery may indicate that chaotic phenomena in nature could be controlled because,even though their instantaneous states are disordered,their long-time averages can be predicted.

The interaction between multiple bubbles and the free surface

The flow is assumed to be potential, and a boundary integral method is used to solve the Laplace equation for the velocity potential to investigate the shape and the position of the bubble. A 3D code to study the bubble dynamics is developed, and the calculation results agree well with the experimental data. Numerical analyses are carried out for the interaction between multiple bubbles near the free surface including in-phase and out-of-phase bubbles. The calculation result shows that the bubble period increases with the decrease of the distance between bubble centres because of the depression effect between multiple bubbles. The depression has no relationship with the free surface and it is more apparent for out-of-phase bubbles. There are great differences in dynamic behaviour between the in-phase bubbles and the out-of-phase bubbles due to the depression effect. Furthermore, the interaction among eight bubbles is simulated with a three-dlmensional model, and the evolving process and the relevant physical phenomena are presented. These phenomena can give a reference to the future work on the power of bubbles induced by multiple charges exploding simultaneously or continuously.

Wave Slamming on An OWSC Wave Energy Converter in Coupled Wave-Current Conditions with Variable-Depth Seabed

Coastal wave energy resources have enormous exploitation potential due to shorter weather window,closer installation distance and lower maintenance cost.However,impact loads generated by depth variation from offshore to nearshore and wave-current interaction,may lead to a catastrophic damage or complete destruction to wave energy converters(WECs).This objective of this paper is to investigate slamming response of a coastal oscillating wave surge converter(OWSC)entering or leaving water freely.Based on fully nonlinear potential flow theory,a time-domain wave-current-structure interaction model combined with higher-order boundary element method(HOBEM),is developed to analyze the coupled hydrodynamic problem.The variable-depth seabed is considered in the model to illustrate the shallow water effect on impact loads and free surface profiles in coastal zone.A domain decomposition approach is utilized to simulate the overlapping phenomenon generated by a jet falling into water under gravity effect.Through a series of Lagrangian interpolation methods,the meshes on boundaries are rearranged to avoid the mismatch between element size on free surface and body surface.The present model is validated against the existing experimental and numerical results.Simulations are also provided for the effects of wave-current interaction and uneven local seabed on the slamming responses.It is found that the length of the splash jet increases for a following current and decreases for an opposing current,and that the slamming response of the OWSC device is sensitive to the geometric features of the uneven seabed.

Width effects on hydrodynamics of pendulum wave energy converter

Based on two- and three-dimensional potential flow theories, the width effects on the hydrodynamics of a bottom-hinged trapezoidal pendulum wave energy converter are discussed. The two-dimensional eigenfunction expansion method is used to obtain the diffraction and radiation solutions when the converter width tends to be infinity. The trapezoidal section of the converter is approximated by a rectangular section for simplification. The nonlinear viscous damping effects are accounted for by including a drag term in the two- and three-dimensional methods. It is found that the three- dimensional results are in good agreement with the two-dimensional results when the converter width becomes larger, especially when the converter width is infinity, which shows that both of the methods are reasonable. Meantime, it is also found that the peak value of the conversion efficiency decreases as the converter width increases in short wave periods while increases when the converter width increases in long wave periods.

STUDY ON VARIATION OF SETTING AND STOPPING PRESSURES OF SAFETY VALVE WITH STRUCTURAL MODIFICATION

The possibility of pressure control with the structural change of a safety valve is investigated Safety valve is commonly used as safety devices for numerous applications which include boilers,ships,industrial plants,and piping Setting and stopping pressures of a safety valve, p set and p sto ,are traditionally adjusted with a fine tuning of seat ring and valve ring heights, h sr and h vr However, it is not easy to achieve the proper setting and stopping pressures of a safety valve in practice The depth of inside and outside grooves in a valve, d i and d o are modified and their effects on setting and stopping pressures of a safety vlave are tested The most appropriate values appear 1 0 mm in d i and 0 5～1 0 mm in d o,respectively The valve ring height, h vr ,shows that the best results can be achieved at 2 3 mm for setting pressures of 0 1～0 4 MPa and 1 0 mm for setting pressures of 0 5～1 0 MPa The stopping pressures increases with the increase of seat ring height, h sr , upto certain h sr value and then becomes independent to the seat ring height This implies that there exists the optimum h sr ,which provides the largest flow rate and the proper stopping pressure Stopping pressures of a safety valve are adjusted with the seat ring and valve ring heights This study,however,demonstrated that the modification of value grooves also changes setting and stopping pressures of a safety valve Therefore,the proper selection in dimensions of the inside and outside grooves should be considered for the safety valve design

Study on Wave Added Resistance of Ships in Oblique Waves Based on Panel Method

When the ship is sailing at sea,wave added resistance has great influence on the rapidity and economy of the ship.With the increasing pressure of energy and environmental protection,IMO has proposed the EEDI formula of the newly built ships,which restricts the energy consumption standard of civil ships more strictly.Therefore,a panel method based on three dimensional potential flow theory is proposed to study the problem of wave added resistance in this paper.Firstly,the method solves the motion responses of the ship in the time domain,and then calculates the wave added resistance of the ship by near-field pressure integration method.The wave added resistance of S175 container ship in head and oblique waves are calculated and compared with the experimental data,and the accuracy of the proposed method are verified.At last,the influence of Froude number and wave direction angle on wave added resistance is studied.The proposed method provides an approach of satisfactory accuracy and efficiency for the development of high-performance new ship forms,optimization of ship hull lines,comprehensive performance evaluation of ships and practical navigation guidance.

Calculating buoy response for a wave energy converter-A comparison of two computational methods and experimental results

When designing a wave power plant, reliable and fast simulation tools are required. Computational fluid dynamics （CFD） software provides high accuracy but with a very high computational cost, and in operational, moderate sea states, linear potential flow theories may be sufficient to model the hydrodynamics. In this paper, a model is built in COMSOL Multiphysics to solve for the hydrodynamic parameters of a point-absorbing wave energy device. The results are compared with a linear model where the hydrodynamical parameters are computed using WAMIT, and to experimental results from the Lysekil research site. The agreement with experimental data is good for both numerical models.

Hydrodynamic Response of A Fully Coupled TLP Hull-TTR System with Detailed Modeling of A Hydraulic Pneumatic Tensioner and Riser Joints

Tension Leg Platform(TLP)in deepwater oil and gas field development usually consists of a hull,tendons,and top tension risers(TTRs).To maintain its top tension,each TTR is connected with a tensioner system to the hull.Owing to the complicated configuration of the tensioners,the hull and TTRs form a strong coupled system.Traditionally,some simplified tensioner models are applied to analyze the TLP structures.There is a large discrepancy between their analysis results and the actual mechanism behaviors of a tensioner.It is very necessary to develop a more detailed tensioner model to consider the coupling effects between TLP and TTRs.In the present study,a fully coupled TLP hull-TTR system for hydrodynamic numerical simulation is established.A specific hydraulic pneumatic tensioner is modeled by considering 4 cylinders.The production TTR model is stacked up by specific riser joints.The simulation is also extended to analyze an array of TTRs.Different regular and irregular waves are considered.The behaviors of different cylinders are presented.The results show that it is important to consider the specific configurations of the tensioner and TTRs,which may lead to obviously different response behaviors,compared with those from a simplified model.

Study on the solid-liquid suspension behavior in a tank stirred by the long-short blades impeller

We investigated the solid–liquid suspension characteristics in the tank with a liquid height/tank diameter ratio of 1.5 stirred by a novel long-short blades(LSB) impeller by the Euler granular flow model coupled with the standard k–ε turbulence model. After validation of the local solid holdup by experiments,numerical predictions have been successfully used to explain the influences of impeller rotating speed,particle density, particle size, liquid viscosity and initial solid loading on the solid suspension behavior,i.e. smaller particles with lower density are more likely to be suspended evenly in the liquid with higher liquid viscosity. At a low impeller rotating speed(N), increase in N leads to an obvious improvement in the solid distribution homogeneity. Moreover, the proposed LSB impeller has obvious advantages in the uniform distribution of the solid particles compared with single Rushton turbine(RT), dual RT impellers or CBY hydrofoil impeller under the same power consumption.

An evolving trajectory-from PD,logistics,SCM to the theory of material flow

The growing interest in the material flow(MF)theory has invoked much interesting research in recent years.Although the MF theory is relatively new,a review of the related literature from a historical perspective shows that MF theory represents a new stage of the evolutionary development of interrelated subjects such as Physical Distribution(PD),Logistics,and Supply Chain Management(SCM).The purpose of this paper is to provide a summative review of the evolution of the subjects of PD,Logistics,and SCM,and their new development,MF theory.The paper aims at tracing how concepts and findings in PD,Logistics,SCM,and MF have been developed and have evolved.The study shows that PD evolved to Logistics in middle of the 1980s;starting from the late 1990s,Logistics has evolved to SCM;and today PD,Logistics,and SCM can be considered to be under the umbrella provided by a new theory called MF theory.This paper points out that MF theory is a necessity to deal with the overwhelming complexity of material flow systems in the global economy of the twenty-first century.

BASIC THEORY AND MATHEMATICAL MODELING OF URBAN RAINSTORM WATER LOGGING

In this paper, a mathematical model for the urban rainstorm water logging wasestablished on the basis of one- and two-dimensional unsteady flow theory and the technique ofnon-structural irregular grid division. The continuity equation was discretized with the finitevolume method. And the momentum equations were differently simplified and discretized for differentcases. A method of ''special passage'' was proposed to deal with small-scale rivers and open channels.The urban drainage system was simplified and simulated in the model. The method of ''open slot'' wasapplied to coordinate the alternate calculation of open channel flow and pressure flow in drainagepipes. The model has been applied in Tianjin City and the verification is quite satisfactory.

Lattice Boltzmann Modeling of Viscous Elementary Flows

A lattice Boltzmann method is developed for modeling viscous elementary flows.An adjustable source term is added to the lattice Boltzmann equation,which can be tuned to model different elementary flow features like a doublet or a point source of any strength,including a negative source(sink).The added source term is dimensionally consistent with the lattice Boltzmann equation.The proposed model has many practical applications,as it can be used in the framework of the potential flow theory of viscous and viscoelastic fluids.The model can be easily extended to the three dimensional case.The model is verified by comparing its results with the analytical solution for some benchmark problems.The results are in good agreement with the analytical solution of the potential flow theory.