Effect of low-speed waterjet pressure on the rock-breaking performance of unsubmerged cavitating abrasive waterjet

Unsubmerged cavitating abrasive waterjet(UCAWJ)has been shown to artificially create a submerged environment that produces shear cavitation,which effectively enhances rock-breaking performance.The shear cavitation generation and collapse intensity depend on the pressure difference between the intermediate high-speed abrasive waterjet and the coaxial low-speed waterjet.However,the effect of the pressure of the coaxial low-speed waterjet is pending.For this purpose,the effect of low-speed waterjet pressure on rock-breaking performance at different standoff distances was experimentally investigated,and the effects of erosion time and ruby nozzle diameter on erosion performance were discussed.Finally,the micromorphology of the sandstone was observed at different locations.The results show that increased erosion time and ruby nozzle diameter can significantly improve the rock-breaking performance.At different standoff distances,the mass loss increases first and then decreases with the increase of low-speed waterjet pressure,the maximum mass loss is 10.4 g at a low-speed waterjet pressure of0.09 MPa.The surface morphology of cavitation erosion was measured using a 3D profiler,the increase in both erosion depth and surface roughness indicated a significant increase in the intensity of the shear cavitation collapse.At a low-speed waterjet pressure of 0.18 MPa,the cavitation erosion surface depth can reach 600μm with a roughness of 127μm.

SOME CHARACTERISTICS OF LOW-SPEED STREAKS UNDER SHEARED AIR-WATER INTERFACES

The characteristics of low-speed fluid streaks occurring under sheared air-water interfaces were examined by means of hydrogen bubble visualization technique. A critical shear condition under which the streaky structure first appears was determined to be u(tau) approximate to 0.19 cm/s. The mean spanwise streak spacing increases with distance from the water surface owing to merging and bursting processes, and a linear relationship describing variation of non-dimensional spacing <(<lambda>+)over bar> versus y(+) was found essentially independent of shear stress on the interface. Values of <(<lambda>+)over bar>, however, are remarkably smaller than their counterparts in the near-wall region of turbulent boundary layers. Though low-speed streaks occur randomly in time and space, the streak spacing exhibits a lognormal probability distribution behavior. A tentative explanation concerning the formation of streaky structure is suggested, and the fact that <(<lambda>+)over bar> takes rather smaller values than that in wall turbulence is briefly discussed.

POD analysis of the instability mode of a low-speed streak in a laminar boundary layer

The instability of one single low-speed streak in a zero-pressure-gradient laminar boundary layer is investigated experimentally via both hydrogen bubble visualization and planar particle image velocimetry(PIV) measurement. A single low-speed streak is generated and destabilized by the wake of an interference wire positioned normal to the wall and in the upstream. The downstream development of the streak includes secondary instability and self-reproduction process, which leads to the generation of two additional streaks appearing on either side of the primary one. A proper orthogonal decomposition(POD) analysis of PIV measured velocity field is used to identify the components of the streak instability in the POD mode space: for a sinuous/varicose type of POD mode, its basis functions present anti-symmetric/symmetric distributions about the streak centerline in the streamwise component, and the symmetry condition reverses in the spanwise component. It is further shown that sinuous mode dominates the turbulent kinematic energy(TKE) through the whole streak evolution process, the TKE content first increases along the streamwise direction to a saturation value and then decays slowly. In contrast, varicose mode exhibits a sustained growth of the TKE content,suggesting an increasing competition of varicose instability against sinuous instability.

High-frequency characterization of high-speed modulators and photodetectors in a link with low-speed photonic sampling

We propose a low-speed photonic sampling for independent high-frequency characterization of a Mach–Zehnder modulator(MZM)and a photodetector(PD)in an optical link.A low-speed mode-locked laser diode(MLLD)provides an ultrawideband optical stimulus with scalable frequency range,working as the photonic sampling source of the link.The uneven spectrum lines of the MLLD are firstly characterized with symmetric modulation within the interesting frequency range.Then,the electro-optic modulated signals are down-converted to the first Nyquist frequency range,yielding the self-referenced extraction of modulation depth and half-wave voltage of the MZM without correcting the responsivity fluctuation of the PD in the link.Finally,the frequency responsivity of the PD is self-referenced measured under null modulation of the MZM.As frequency responses of the MZM and the PD can be independently obtained,our method allows self-referenced high-frequency measurement for a high-speed optical link.In the proof-of-concept experiment,a 96.9 MS/s MLLD is used for measuring a MZM and a PD within the frequency range up to 50 GHz.The consistency between our method and the conventional method verifies that the ultra-wideband and self-referenced high-frequency characterization of high-speed MZMs and PDs.

Experimental investigation on vibration characteristics of the medium-low-speed maglev vehicle-turnout coupled system

The steel turnout is one of the key components in the medium–low-speed maglev line system.However,the vehicle under active control is prone to vehicle–turnout coupled vibration,and thus,it is necessary to identify the vibration characteristics of this coupled system through field tests.To this end,dynamic performance tests were conducted on a vehicle–turnout coupled system in a medium–low-speed maglev test line.Firstly,the dynamic response data of the coupled system under various operating conditions were obtained.Then,the natural vibration characteristics of the turnout were analysed using the free attenuation method and the finite element method,indicating a good agreement between the simulation results and the measured results;the acceleration response characteristics of the coupled system were analysed in detail,and the ride quality of the vehicle was assessed by Sperling index.Finally,the frequency distribution characteristics of the coupled system were discussed.All these test results could provide references for model validation and optimized design of medium–low-speed maglev transport systems.

Numerical study on evolution of subharmonic varicose low-speed streaks in turbulent channel flow

The evolution of two spanwise-aligned low-speed streaks in a wall turbulent flow, triggered by the instability of the subharmonic varicose （SV） mode, is studied by a direct numerical simulation （DNS） method in a small spatial-periodic channel. The results show that the SV low-speed streaks are self-sustained at the early stage, and then transform into subharmonic sinuous （SS） low-speed streaks. Initially, the streamwise vortex sheets are formed by shearing, and then evolve into zigzag vortex sheets due to the mutual induction. As the intensification of the SV low-speed streaks becomes prominent, the tilted streamwise vortex tubes and the V-like streamwise vortex tubes can be formed simultaneously by increasing ＋~. When the SV low-speed streaks break down, new zigzag streamwise vortices will be generated, thus giving birth to the next sustaining cycle of the SV low-speed streaks. When the second breakdown happens, new secondary V-like streamwise vortices instead of zigzag streamwise vortices will be generated. Because of the sweep motion of the fluid induced by the secondary V-like streamwise vortices, each decayed low-speed streak can be divided into two parts, and each part combines with the part of another streak, finally leading to the formation of SS low-speed streaks.

Optimizing the Exhaust System of Marine Diesel Engines to Improve Low-speed Performances and Cylinder Working Conditions

Proper design of exhaust systems in marine high-power turbocharged diesel engines can contribute to improve the low-speed performance of these engines and make the working conditions of the cylinders more uniform.Here a high-power marine 16-cylinder V-type turbocharged diesel engine is simulated using the GT-Power software.The results reveal the differences induced by different exhaust system structures,such as an 8-cylinder-inpipe exhaust system with single/double superchargers and a 4-cylinder-in-pipe exhaust system with a single supercharger.After a comparative analysis,the 8-cylinder type with double superchargers is determined to be the optimal solution,and the structure of the exhaust system is further optimized.The simulations show that the optimized maximum exhaust temperature difference among cylinders is reduced by 66%.Finally,the simulation results and the optimized performance of the designed exhaust system are verified through experiments.

Numerical simulation on evolution of subharmonic low-speed streaks in minimal channel turbulent flow

The evolution of low-speed streaks in the turbulent boundary layer of the minimum channel flow unit at a low Reynolds number is simulated by the direct numer- ical simulation （DNS） based on the standard Fourier-Chebyshev spectral method. The subharmonic sinuous （SS） mode for two spanwise-aligned low-speed streaks is excited by imposing the initial perturbations. The possibilities and the physical realities of the turbulent sustaining in the minimal channel unit are examined. Based on such a flow field environment, the evolution of the low-speed streaks during a cycle of turbulent sus- taining, including lift-up, oscillation, and breakdown, is investigated. The development of streamwise vortices and the dynamics of vortex structures are examined. The results show that the vortices generated from the same streak are staggered along the streamwise direction, while the vortices induced by different streaks tilt toward the normal direction due to the mutual induction effect. It is the spatial variations of the streamwise vortices that cause the lift-up of the streaks. By resolving the transport dynamics of enstrophy, the strength of the vortices is found to continuously grow in the logarithmic layer through the vortex stretching mechanism during the evolution of streaks. The enhancement of the vortices contributes to the spanwise oscillation and the following breakdown of the low-speed streaks.

Dynamics of low-speed streak evolution and interaction in laminar boundary layer

The present paper presents an experimental effort on the regeneration process of two low-speed laminar streaks in a zero-pressure-gradient laminar boundary layer. Two vertical thin wires separated by a spanwise distance of 30 mm are used to introduce disturbances of two rolls of transitional Karmain vortex street to the downstream boundary layer. Both hydrogen bubble visualization and particle image velocimetry （PIV） measurement show that two lowspeed streaks are induced through leading-edge receptivity process. As these streaks develop in the downstream, two additional low-speed streaks begin to appear outboard of the flank of the original two, together with complex dynamics of streak splitting and merging. A flow pattern of four streaks aligned along the spanwise direction occurs finally in the far downstream. It is found that besides the mechanisms of streak breakdown, the streak interaction is also an important factor characterizing the instability of low speed streaks and their regeneration process.

Research on Feature Extraction Method for Low-Speed Reciprocating Bearings Based on Segmented Short Signal Modulation Signal Bispectrum Slicing

Bearing condition monitoring and fault diagnosis (CMFD) can investigate bearing faults in the early stages, preventing the subsequent impacts of machine bearing failures effectively. CMFD for low-speed, non-continuous operation bearings, such as yaw bearings and pitch bearings in wind turbines, and rotating support bearings in space launch towers, presents more challenges compared to continuous rolling bearings. Firstly, these bearings have very slow speeds, resulting in weak collected fault signals that are heavily masked by severe noise interference. Secondly, their limited rotational angles during operation lead to a restricted number of fault signals. Lastly, the interference from deceleration and direction-changing impact signals significantly affects fault impact signals. To address these challenges, this paper proposes a method for extracting fault features in low-speed reciprocating bearings based on short signal segmentation and modulation signal bispectrum (MSB) slicing. This method initially separates short signals corresponding to individual cycles from the vibration signals based on encoder signals. Subsequently, MSB analysis is performed on each short signal to generate MSB carrier-slice spectra. The optimal carrier frequency and its corresponding modulation signal slice spectrum are determined based on the carrier-slice spectra. Finally, the MSB modulation signal slice spectra of the short signal set are averaged to obtain the overall average feature of the sliced spectra.

中国人工智能领先之源:基于ROAR创新政策框架

人工智能具备通用技术的诸多特征,使其很有可能成为新一轮科技革命和产业变革的助推器,从而开启新一轮经济长波。当前正处于第六次技术革命浪潮的导入期,人工智能的技术轨道还未成型,这是中国引领该技术发展的绝佳机会。中国的人工智能应用场景和数据资源丰富,具备发展人工智能技术的优势,但也面临诸多挑战。笔者基于ROAR创新政策框架的视角提出,中国在发展人工智能技术过程中,要充分发挥国家作为创新发动机的作用,以发展人类增强型人工智能为使命导向,建立和完善组织和制度支撑体系,引导人工智能技术进入良性发展轨道。

从ROAR和OpenDOAR调查中国的机构典藏发展现状

逐一访问了中国在ROAR上注册的开放存取典藏库及OpenDOAR收录的中国机构典藏库,从典藏库类型、建置地区、建置机构、典藏库URL、提供的记录数及全文篇数等方面进行了统计分析;在此基础上,对大陆和台湾地区的机构典藏发展特点进行了对比研究,以期引起内陆高校图书馆及科研院所等单位对机构典藏发展的重视。

Simulation Study of Disastrous Waves Along Indonesian Offshore and Coast Under Roaring Forties and Tropical Cyclones

Indonesian offshore and coastal areas are vulnerable to swells from Roaring Forties and cyclone disasters.However,the understanding of the characteristics and propagation mechanisms of local disastrous waves is insufficient,posing a threat to the construction,maintenance,and protection of coastal structures.This study establishes a multiple nested wave model based on the third-generation wave model WAVEWATCHⅢ.This model includes sole forcing of Roaring Forties and combined forcing of Roaring Forties and cyclone Ernie to simulate the influence of disastrous waves under the Roaring Forties and tropical cyclones in the Indonesian offshore zone and coasts.The following results are obtained.The Indonesian offshore is prevailed by relatively stable southern to southwestern dominant swells and small wind waves under the impacts of the Roaring Forties without cyclone winds.Long propagating swells originated from the Roaring Forties dominate in nearshore coastal waters with deformed directions and strength because of the shoaling effect.

Adaptive Dead Zone Compensation Control Method for Electro-Hydrostatic Actuators Under Low-Speed Conditions

This study proposes an accurate dead zone compensation control method for electro-hydrostatic actuators(EHAs)under low-speed conditions.Specifically,the nonlinear dead zone characteristics under low-speed conditions are summarized based on numerous EHA experiments.An adaptive compensation function(ACF)is then constructed for the dead zone.Next,this study proposes an adaptive dead zone compensation control method for EHAs by integrating the ACF with a virtual decomposition controller(VDC)based on the established EHA model.The stability of the proposed control method is also proven.Finally,the proposed control method is verified using an EHA platform.The test results show that the dead zone trajectory tracking errors of EHAs are significantly reduced when combined with the ACF.Furthermore,since most EHAs are controlled by adjusting the motor speed,the method presented in this study is simpler and easier to use than methods that employ flow compensation.

The Lions Roar Again

THE Cameroonian players went wild with joywhen the referee blew the final whistle, ending the match against their Egyptian opponents with an unexpected 2-1 win. Thus ended the 31st edition of the African Cup of Nations （ACN）, which took place in Gabon from January 14 to February 5, 2017.

New Park a Roaring Success for Tigers

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基于GT-Power的排气轰鸣噪声控制与优化研究

针对某1.5T三缸发动机车型排气尾管噪声在1 280 r/min、1 550 r/min转速下出现轰鸣声的问题,对其排气系统消声器结构进行优化设计,从而达到消除该轰鸣声的目标。分析了排气尾管轰鸣声的产生原因,并解释了轰鸣产生的机理,得出该轰鸣声是消声器在96~116 Hz下消声量不足的结论。通过GT-Power软件基于传递损失仿真结果优化了排气系统消声器结构,并建立发动机模型,与优化后的排气系统模型进行噪声仿真,仿真结果满足优化要求。在整车半消声室中进行了排气尾管噪声测试试验,测试结果验证了分析结果的准确性,且优化方案排气尾管轰鸣声消失,中高转速下噪声降低2 dB(A)

Interfacial friction induced capillary flow within nanofiber-supported ionic liquid droplets

As global economic growth increases,the demand for energy sources boosts.While fossil fuels have traditionally satisfied this demand,their environmental influence and limited reserves require alternatives.Fossil fuel combustion contributes substantially to greenhouse gas emissions,with a pressing need to halve these emissions by 2030 and target net-zero by 2050.Renewable energy sources,contributing currently to 29%of global electricity,are viewed as promising substitutes.With wind energy's potential,Zheng's team developed a novel method to harness even low wind speeds using well-aligned nanofibers and an innovative“drop wind generator”.This system,combining moisture-saturated ionic liquid 3-Methyl-1-octylimidazolium chloride with specific nanofiber arrays,exploits wind-inducedflows for energy conversion.This study highlights the vast untapped potential of low-speed wind as a sustainable energy source potentially for electronics.

Capillary Property of Entangled Porous Metallic Wire materials and Its Application in Fluid Buffers:Theoretical Analysis and Experimental Study

Strong impact does serious harm to the military industries so it is necessary to choose reasonable cushioning material and design effective buffers to prevent the impact of equipment.Based on the capillary property entangled porous metallic wire materials(EPMWM),this paper designed a composite buffer which uses EPMWM and viscous fluid as cushioning materials under the low-speed impact of the recoil force device of weapon equipment(such as artillery,mortar,etc.).Combined with the capillary model,porosity,hydraulic diameter,maximum pore diameter and pore distribution were used to characterize the pore structure characteristics of EPMWM.The calculation model of the damping force of the composite buffer was established.The low-speed impact test of the composite buffer was conducted.The parameters of the buffer under low-speed impact were identified according to the model,and the nonlinear model of damping force was obtained.The test results show that the composite buffer with EPMWM and viscous fluid can absorb the impact energy from the recoil movement effectively,and provide a new method for the buffer design of weapon equipment(such as artillery,mortar,etc.).

Characteristics of Particulate Matter Emissions for Low-Sulfur Heavy Oil Used in Low-Speed Two-Stroke Diesel Engines of Ocean-Going Ships

In this work,particulate matter(PM) emissions from a large two-stroke,low-speed marine diesel engine were investigated when the engine was operated with low-sulfur heavy fuel oil(HFO) at various loads.Particle samples were collected in situ from the engine exhaust to determine the detailed physical and chemical properties.The nanostructure and morphology of the nanoparticles were analyzed using transmission electron microscopy images(TEM).The results show that volatile organic carbon(OC) accounts for more than 80% in the HFO particles and leads to an increase in particle size.The thermodynamic conditions of a low-speed engine favor the behavior of capturing the soluble organic components.A large number of spherical char HFO particles with aerodynamic diameters of 0.2 μm-0.5 μm and a suspected inner metal core were detected.The two peak aerodynamic diameters of the HFO nanoparticles are 15 nm and 86 nm.The morphological differences among the HFO nanoparticles in varied engine conditions represent the formation process from primary nascent particles to mature graphitized particles caused by thermodynamics.The above study will be valuable for understanding the characteristics of PM emissions from low-sulfur HFO to achieve the ship PM emissions reduction target.