Dynamic Response of 6MW Spar Type Floating Offshore Wind Turbine by Experiment and Numerical Analyses

The floating offshore wind turbine(FOWT) is widely used for harvesting marine wind energy. Its dynamic responses under offshore wind and wave environment provide essential reference for the design and installation. In this study,the dynamic responses of a 6 MW Spar type FOWT designed for the water depth of 100 m are investigated by means of the wave tank experiment and numerical analysis. A scaled model is manufactured for the experiment at a ratio of65.3, while the numerical model is constructed on the open-source platform FAST(Fatigue, Aerodynamics,Structures, and Turbulence). Still water tests, wind-induced only tests, wave-induced only tests and combined windwave-current tests are all conducted experimentally and numerically. The accuracy of the experimental set-up as well as the loading generation has been verified. Surge, pitch and heave motions are selected to analyze and the numerical results agree well with the experimental values. Even though results obtained by using the FOWT calculation model established in FAST software show some deviations from the test results, the trends are always consistent. Both experimental and numerical studies demonstrate that they are reliable for the designed 6 MW Spar type FOWT.

Modelling of temperature and strain rate dependent behaviour of pearlitic steel in block braked railway wheels

Block braked railway wheels are subjected to thermal and rolling contact loading.The thermal loading results in high temperatures and thermal stresses which cause slow time dependent processes such as creep,relaxation and static recovery of the wheel material.At the same time,the rolling contact loading implies a very fast mechanical load application.This paper is focused on material modeling of pearlitic steel for a wide range of loading rates at elevated temperatures.The starting point is a viscoplasticity model including nonlinear isotropic and kinematic hardening.The Delobelle overstress function is employed to capture strain rate dependent response of the material.The model also includes static recovery of the hardening to capture slower viscous(diffusion dominated)behaviour of the material.Experiments for the pearlitic wheel steel ER7 in terms of cyclic strain-controlled uniaxial tests with hold-time,uniaxial ratchetting tests including rapid cycles and biaxial cyclic tests with tension/compression and torsion are used to calibrate the material model.These experiments were performed under isothermal conditions at different temperatures.In the ratchetting tests,higher loading rates are obtained and these have been used to calibrate the high strain rate response of the viscoplasticity model.The paper is concluded with a numerical example of a block braked wheel where the importance of accounting for the viscoplasticity in modelling is highlighted.

Propagation of Acoustic Waves Caused by the Accelerations of Vibrating Hand-Held Tools in Viscoelastic Soft Tissues of Human Hands and a Mechanobiological Picture for the Related Injuries

As is well known, hand-arm vibration syndrome (HAVS), or vibration-induced white finger (VWF), which is a secondary form of Raynaud’s syndrome, is an industrial injury triggered by regular use of vibrating hand-held tools. According to the related biopsy tests, the main vibration-caused lesion is an increase in the thickness of the artery walls of the small arteries and arterioles resulted from enlarged vascular smooth muscle cells (VSMCs) in the wall layer known as tunica media. The present work develops a mechanobiological picture for the cell enlargement. The work deals with acoustic variables in solid materials, i.e., the non-equilibrium components of mechanical variables in the materials in the case where these components are weakly non-equilibrium. The work derives an explicit expression for the infinite-time cell-volume relative enlargement. This enlargement is directly affected by the acoustic pressure in the soft living tissue (SLT). In order to reduce the enlargement, one can reduce either the ratio of the acoustic pressure in the SLT to the cell bulk modulus or the relaxation time induced by the cell osmosis, or both the characteristics. Also, a mechanoprotective role of the above relaxation time in the cell-volume maintenance is noted. The above mechanobiological picture focuses attention on the pressure in an SLT and, thus, modeling of propagation of acoustic waves caused by the acceleration of a vibrating hand-held tool. The present work analyzes the propagation along the thickness of an infinite planar layer of an SLT. The work considers acoustic modeling. As a general viscoelastic acoustic model, the work suggests linear non-stationary partial integro-differential equation (PIDE) for the weakly non-equilibrium component of the average normal stress (ANS) or, briefly, the acoustic ANS. The PIDE is, in the exponential approximation for the normalized stress-relaxation function (NSRF) reduced to the third-order linear non-stationary partial differential equation (PDE), which is of the Zener type. The unique advantage of the PIDE is that it presents a compact model for the acoustic ANS in an SLT, which explicitly includes the NSRF, thereby enabling a consistent description of the lossy-propagation effects inherent in SLTs. The one-spatial-coordinate version of this PDE in the planar SLT layer with the corresponding boundary conditions is considered. The relevance of these settings is motivated by a conclusion of other authors, which is based on the results of the frequency-domain simulation in three spatial coordinates. The boundary-value problem at arbitrary value of the stress-relaxation time (SRT) and arbitrary but sufficiently regular shape of the external acceleration is analytically solved by means of the Fourier method. The obtained solution is the steady-state acoustic ANS and allows calculation of the corresponding steady-state acoustic pressure as well. The derived analytical representations are computationally implemented. Propagation of the pressure waves in the SLT layer at zero and different nonzero values of the SRT, and the single-pulse external acceleration is presented. They complement the zero-SRT and zero-SRT-asymptote results with the results for various values of the SRT. The obtained pressure values are, at all of the space-time points under consideration, meeting the condition for the adequateness of the linear model. In the case where the SRT is zero, the results well agree with the ones obtained by using the simulation software package LS-DYNA. The dependence of the damping of acoustic variables in an SLT on the SRT in the present third-order case significantly generalizes the one in the second-order linear systems. The related resonance effect in the waves of the acoustic pressure propagating in an SLT is also discussed. The effects of the NSRF-originated memory function provided by the present third-order PDE model are necessary for proper simulation of the pressure, which is of special importance in the aforementioned mechanoboiological picture. The results obtained in the work present a viscoelastic acoustic framework for SLTs. These results open a way to quantitatively specific evaluation of technological strategies for reduction of the vibration-caused injuries or, loosely speaking, achieving “zero’’ injury.

基于细观损伤模型的NVA船用钢临界孔洞体积分数（英文）

NVA mild steel is a commonly used material in the shipbuilding industry.An accurate model for description of this material’s ductile fracture behaviour in numerical simulation is still a challenging task.In this paper,a new method for predicting the critical void volume fraction fc in the Guson-Tvergaard-Needleman(GTN)model is introduced to describe the ductile fracture behaviour of NVA shipbuilding mild steel during ship collision and grounding scenarios.Most of the previous methods for determination of the parameter fc use a converse method,which determines the values of the parameters through comparisons between experi-mental results and numerical simulation results but with high uncertainty.A new method is proposed based on the Hill,Bressan,and Williams hypothesis,which reduces the uncertainty to a satisfying extent.To accurately describe the stress-strain relationship of materials before and after necking,a combination of the Voce and Swift models is used to describe the material properties of NVA mild steel.A user-defined material subroutine has been developed to enable the application of the new parameter deter-mination method and its implementation in the finite element software LS-DYNA.It is observed that the model can accurately describe structural damage by comparing the numerical simulation results with those of experiments;thus,the results demon-strate the model’s capacity for structural response prediction in ship collision and grounding scenario simulations。

减少海洋航运环境排放的新方法

Maritime shipping is a strategic sector with a strong international vocation and management.The need to define regulations valid for many different countries without generating disparities of treatment slowed down the formulation of environmental regulations,especially for atmospheric emissions.In particular,regulations pertaining to the reduction of sulphur compounds allowed two distinct approaches:the use of low-sulphur fuels or exhaust gas cleaning systems,the so-called Scrubbers.The actual implementation of these solutions presents specific concerns either related to the toxicity of atmospheric by-products and to the fuel cost or to the generation of polluting washwaters that may need treatment before discharge.In this paper we analyzed the potential environmental benefit deriving from the use of a distillate fuel,not compliant with current IMO Sulphur Regulations,together with a Scrubber.The pilot-scale experimental results indicated that a limited amount of water and/or scrubber volume is needed to reduce sulphur emissions below regulations on maritime shipping,especially with the addition of NaOH reaching a water-saving between 25%-33%compared to the use of pure seawater.Experiments indicated that scrubber washwater PAHs emissions are within the available water quality standards indicated by EU and USA guidelines.A bottom-up analysis on heavy metals concentration shed light on the prominent role of metal-parts corrosion on the washwater emissions.Taking into account for corrosion phenomena,the actual heavy metals concentration in the washwater deriving from scrubbing was normally below the water quality standards.

船舶航线优化中任意浪向的航速损失预测模型

This paper proposes a semi-empirical model to predict a ship’s speed loss at arbitrary wave heading.In the model,the formulas that estimate a ship’s added resistance due to waves attacking from different heading angles have been further developed.A correction factor is proposed to consider the nonlinear effect due to large waves in power estimation.The formulas are developed and verified by model tests of 5 ships in regular waves with various heading angles.The full-scale measurements from three different types of ships,i.e.,a PCTC,a container ship,and a chemical tanker,are used to validate the proposed model for speed loss prediction in irregular waves.The effect of the improved model for speed loss prediction on a ship’s voyage optimization is also investigated.The results indicate that a ship’s voyage optimization solutions can be significantly affected by the prediction accuracy of speed loss caused by waves.

Cavitation research with computational fluid dynamics:From Euler-Euler to Euler-Lagrange approach

Unsteady cavitating flow often contains vapor structures with a wide range of different length scales,from micro-bubbles to large cavities,which issues a big challenge to precisely investigate its evolution mechanism by computational fluid dynamics(CFD)method.The present work reviews the development of simulation methods for cavitation,especially the emerging Euler-Lagrange approach.Additionally,the progress of the numerical investigation of hot and vital issues is discussed,including cavitation inception,cloud cavitation inner structure and its formation mechanism,cavitation erosion,and cavitation noise.It is indicated that the Euler-Lagrange method can determine cavitation inception point better.For cloud cavitation,the Euler-Lagrange method can reveal the source of microbubbles and their distribution law inside the shedding cloud.This method also has advantages and great potential in assessing cloud cavitation-induced erosion and noise.With the ever-growing demands of cavitation simulation accuracy in basic research and engineering applications,how to improve the Euler-Lagrange method’s stability and applicability is still an open problem.To further promote the application of this advanced CFD simulation technology in cavitation research,some key issues are to be solved and feasible suggestions are put forward for further work.

A review of aerial manipulation of small-scale rotorcraft unmanned robotic systems

Small-scale rotorcraft unmanned robotic systems(SRURSs) are a kind of unmanned rotorcraft with manipulating devices. This review aims to provide an overview on aerial manipulation of SRURSs nowadays and promote relative research in the future. In the past decade, aerial manipulation of SRURSs has attracted the interest of researchers globally. This paper provides a literature review of the last 10 years(2008–2017) on SRURSs, and details achievements and challenges. Firstly, the definition, current state, development, classification, and challenges of SRURSs are introduced. Then, related papers are organized into two topical categories: mechanical structure design, and modeling and control. Following this, research groups involved in SRURS research and their major achievements are summarized and classified in the form of tables. The research groups are introduced in detail from seven parts. Finally, trends and challenges are compiled and presented to serve as a resource for researchers interested in aerial manipulation of SRURSs. The problem,trends, and challenges are described from three aspects. Conclusions of the paper are presented,and the future of SRURSs is discussed to enable further research interests.

Experimental analysis of tip vortex cavitation mitigation by controlled surface roughness

This study presents results of experiments where roughness applications are evaluated in delaying the tip vortex cavitation inception of an elliptical foil.High-speed video recordings and laser doppler velocimetry(LDV)measurements are employed to provide further details on the cavitation behavior and tip vortex flow properties in different roughness pattern configurations.The angular momentum measurements of the vortex core region at one chord length downstream of the tip indicate that roughness leads to a lower angular momentum compared with the smooth foil condition while the vortex core radius remains similar in the smooth and roughened conditions.The observations show that the cavitation number for tip vortex cavitation inception is reduced by 33%in the optimized roughness pattern compared with the smooth foil condition where the drag force increase is observed to be around 2%.During the tests,no obvious differences in the cavitation inception properties of uniform and non-uniform roughness distributions are observed.However,the drag force is found to be higher with a non-uniform roughness distribution.

Hydrodynamic mechanisms of aggressive collapse events in leading edge cavitation

Transient cavities generated from unsteady leading-edge cavitation may undergo aggressive collapses which are responsible for cavitation erosion.In this paper,we studied the hydrodynamic mechanisms of these events in the leading edge cavitation fonned over a modified NACA0009 hydrofoil using experimental and numerical methods.In the experimental investigation,high-speed visualization(HSV)and paint test are employed to study the behavior of the cavitating flow at σ=1.25,α=5°,U∞=20 m/s.In the numerical part,the same cavitating flow is simulated using an inviscid density-based compressible solver with a barotropic cavitation model.The numerical results are first compared with the experimental HSV to show that the simulation is able to reproduce the main features of the cavitating flow.Then,as the compressible solver is capable of capturing the shock wave upon the collapse of cavities,the location of collapse events with high erosion potential are determined.The location of these collapse events are compared with the paint test results with a qualitatively good agreement.It is clearly observed,in both the experiments and the numerical simulation,that there exists four distinct regions along the hydrofoil with higher risks of erosion:(1)A very narrow strip at the leading edge,(2)an area of accumulated collapses at around 60 percent of the sheet cavity maximum length,(3)an area around the closure line of the sheet cavity with the highest erosion damage,and(4)a wide area close to the trailing edge with dispersed collapse events.A combined analysis of the experimental and numerical results reveals that the small-scale structures generated by secondary shedding are more aggressive than the large-scale cloud cavities(primary shedding).It is also observed that the high risk of cavitation erosion in regions 2 and 3 is mainly due to the collapses of the small cavity structures that are formed around the sheet cavity closure line or the rolling cloud cavity.

An IDDES study of the near-wake flow topology of a simplified heavy vehicle

The complex wake flow of a GTS(ground transportation system)model contributes to large percentage of the aerodynamic drag force.Therefore,predicting accurate wake flow will help carry out the drag reduction strategies.In this paper,the near-wake flow topology of the GTS was studied at Re=2.7×104 to assess the capability of a hybrid RANS/LES(Reynolds-averaged Navier–Stokes/large eddy simulation)approach,known as IDDES(improved delayed detached eddy simulation).The current study also aims to understand the effects of different computational parameters,e.g.the spatial resolution,time step,residual level,discretization scheme and turbulence model,on this asymmetrical wake flow configuration.A comparison of IDDES with previous water channel tests,wellresolved LES,partially averaged Navier–Stokes and URANS(unsteady RANS)was included to better understand the benefits of this hybrid RANS/LES approach.The results show that on the medium and fine grids,the IDDES produces an asymmetrical flow topology(known as flow state I)in the near-wake of the vertical midplane,as reported in previous studies.The recommended parameters for the time step(1×10^(–4 )s)and residual level(1×10^(–4))provide sufficient accuracy of wake predictions to show good agreement with experiments.For the convective term of the momentum equation in IDDES,the bounded central difference discretization scheme is proposed to be adopted for discretization.Additionally,URANS cannot accurately capture this asymmetrical flow field.IDDES proves to be capable of predicting thewake flowfield of this simplified heavy vehicle with high accuracy.All obtained conclusions can provide references for the aerodynamic drag reduction of the GTS.

A comparison of two wave energy converters’power performance and mooring fatigue characteristics-One WEC vs many WECs in a wave park with interaction effects

The production of renewable energy is key to satisfying the increasing demand for energy without further increasing pollution.Harnessing ocean energy from waves has attracted attention due to its high energy density.This study compares two generations of floating heaving point absorber WEC,WaveEL 3.0 and WaveEL 4.0,regarding their power performance and mooring line fatigue characteristics,which are essen-tial in,e.g.,LCoE calculations.The main differences between the two WECs are the principal dimensions and minor differences in their geometries.The DNV software SESAM was used for simulations and anal-yses of these WECs in terms of buoy heave motion resonances for maximising energy harvesting,motion characteristics,mooring line forces,fatigue of mooring lines,and hydrodynamic power production.The first part of the study presents results from simulations of unit WEC in the frequency domain and in the time domain for regular wave and irregular sea state conditions.A verification of the two WECs’motion responses and axial mooring line forces is made against measurement data from a full-scale installation.In the second part of the study,the influence of interaction effects is investigated when the WECs are installed in wave parks.The wave park simulations used a fully-coupled non-linear method in SESAM that calculates the motions of the WECs and the mooring line forces simultaneously in the time domain.The amount of fatigue damage accumulated in the mooring lines was calculated using a relative tension-based fatigue analysis method and the rainflow counting method.Several factors that influence the power performance of the wave park and the accumulated fatigue damage of the mooring lines,for example,the WEC distance of the wave park,the sea state conditions,and the direction of incoming waves,are simulated and discussed.The study’s main conclusion is that WaveEL 4.0,which has a longer tube than WaveEL 3.0,absorbs more hydrodynamic energy due to larger heave motions and more efficient power production.At the same time,the accumulated fatigue damage in the moorings is lower compared to WaveEL 3.0 if the distance between the WECs in the wave park is not too short.Its motions in the hor-izontal plane are larger,which may require a larger distance between the WEC units in a wave park to avoid losing efficiency due to hydrodynamic interaction effects.

Safety benefit of cooperative control for heterogeneous traffic on-ramp merging

The safety of heterogeneous traffic is a vital topic in the oncoming era of autonomous vehicles(AVs).The cooperative vehicle infrastructure system(CVIS)is considered to improve heterogeneous traffic safety by connecting and controlling AVs cooperatively,and the connected AVs are so-called connected and automated vehicles(CAVs).However,the safety impact of cooperative control strategy on the heterogeneous traffic with CAVs and human-driving vehicles(HVs)has not been well investigated.In this paper,based on the traffic simulator SUMO,we designed a typical highway scenario of on-ramp merging and adopted a cooperative control method for CAVs.We then compared the safety performance for two different heterogeneous traffic systems,i.e.AV and HV,CAV and HV,respectively,to illustrate the safety benefits of the cooperative control strategy.We found that the safety performance of the CAV and HV traffic system does not always outperform that of AV and HV.With random departSpeed and higher arrival rate,the proposed cooperative control method would decrease the conflicts significantly whereas the penetration rate is over 80%.We further investigated the conflicts in terms of the leading and following vehicle types,and found that the risk of a AV/CAV followed by a HV is twice that of a HV followed by another HV.We also considered the safety effect of communication failure,and found that there is no significant impact until the packet loss probability is greater than 30%,while communication delay’s impact on safety can be ignored according to our experiments.

Behavioral adaptation of drivers when driving among automated vehicles

Purpose–This paper aims to explore whether drivers would adapt their behavior when they drive among automated vehicles(AVs)compared to driving among manually driven vehicles(MVs).Understanding behavioral adaptation of drivers when they encounter AVs is crucial for assessing impacts of AVs in mixed-traffic situations.Here,mixed-traffic situations refer to situations where AVs share the roads with existing nonautomated vehicles such as conventional MVs.Design/methodology/approach–A driving simulator study is designed to explore whether such behavioral adaptations exist.Two different driving scenarios were explored on a three-lane highway:driving on the main highway and merging from an on-ramp.For this study,18 research participants were recruited.Findings–Behavioral adaptation can be observed in terms of car-following speed,car-following time gap,number of lane change and overall driving speed.The adaptations are dependent on the driving scenario and whether the surrounding traffic was AVs or MVs.Although significant differences in behavior were found in more than 90%of the research participants,they adapted their behavior differently,and thus,magnitude of the behavioral adaptation remains unclear.Originality/value–The observed behavioral adaptations in this paper were dependent on the driving scenario rather than the time gap between surrounding vehicles.This finding differs from previous studies,which have shown that drivers tend to adapt their behaviors with respect to the surrounding vehicles.Furthermore,the surrounding vehicles in this study are more“free flow’”compared to previous studies with a fixed formation such as platoons.Nevertheless,long-term observations are required to further support this claim.