Numerical investigation of cavitating flow behind the cone of a poppet valve in water hydraulic system

Computational Fluid Dynamics (CFD) simulations of cavitating flow through water hydraulic poppet valves were performed using advanced RNGk-epsilon turbulence model. The flow was turbulent, incompressible and unsteady, for Reynolds numbers greater than 43000. The working fluid was water, and the structure of the valve was simplified as a two dimensional axisymmetric geometrical model. Flow field visualization was numerically achieved. The effects of inlet velocity, outlet pressure, opening size as well as poppet angle on cavitation intensity in the poppet valve were numerically investigated. Experimental flow visualization was conducted to capture cavitation images near the orifice in the poppet valve with 30° poppet angle using high speed video camera. The binary cavitating flow field distribution obtained from digital processing of the original cavitation image showed a good agreement with the numerical result. Project supported by the National Natural Science Foundation of China (No. 59835160) and Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry of China (No. 50175097)     

过冷流动沸腾热传递的分形模型

提出过冷流动沸腾热传递的分形模型,根据加热表面活化点的分形分布得到了过冷流动沸腾热流密度的表达式。从该模型中发现过冷流动沸腾热流密度是壁面过热度、流体的过冷度、流体的主流速度与流体的接触角、流体物理特性的函数关系,并且没有增加新的经验常数。模型预测的结果与实验数据进行了比较,两者是极好的吻合。     

Dynamic balancing of dual-rotor system with very little rotating speed difference

Unbalanced vibration in dual-rotor rotating machinery was studied with numerical simulations and experiments. A new method is proposed to separate vibration signals of inner and outer rotors for a system with very little difference in rotating speeds. Magnitudes and phase values of unbalance defects can be obtained directly by sampling the vibration signal synchronized with reference signal. The balancing process is completed by the reciprocity influence coefficients of inner and outer rotors method. Results showed the advantage of such method for a dual-rotor system as compared with conventional balancing. Project supported by the National Natural Science Foundation of China (No. 59875078), and Zhejiang Provincial Natural Science Foundation of China (No. 502092)     

Exact solutions for steady flows of second-grade fluids

This paper aims to investigate exact solutions for a second-grade fluid flow with the inverse method. By assuming the relation between the vorticity field and the streamfunction, the exact solutions of the motion of plane second-grade fluids are investigated and obtained. The solutions obtained include simple Couette flows, slit jet flows and uniform flows over a series of distributed obstacles. Project supported by the National Natural Science Foundation of China (Grant No.10472063)     

Reaction thrust of water jet for conical nozzles

Clear knowledge on the reaction thrust of water jet is valuable for better design of water jet propulsion system. In
this paper, theoretical, numerical and experimental studies were carried out to investigate the effects of the nozzle geometry
as well as the inlet conditions on the reaction thrust of water jet. Comparison analyses reveal that the reaction thrust has a
direct proportional relationship with the product of the inlet pressure, the square of flow rate and two-thirds power exponent
of the input power. The results also indicate that the diameter of the cylinder column for the conical nozzle has great influence
on the reaction thrust characteristics. In addition, the best values of the half cone angle and the cylinder column length exist
to make the reaction thrust reach its maximum under the same inlet conditions．     

外电场作用下柔性模型水的分子动力学模拟

1 Introduction Water is the most common liquid in nature and it isalso i mportant in numerous chemical and biologicalprogresses . With the availability of computers , MonteCarlo (MC) and molecular dynamics (MD) si mulationshave beenintroduced and efforts have been devoted toexamining the microscopic structure and dynamicproperties of liquid water utilizing computer si mula-tions[1-8].It is known that liquid water is totally con-nected by random tetrahedral hydrogen-bonds net- work[1 ,2]. Ho…     

Numerical study of heat transfer characteristics of downward supercritical kerosene flow inside circular tubes

The heat transfer characteristics of China RP-3 aviation kerosene flowing in a vertical downward tube with an inner diameter of 4 mm under supercritical pressures are numerically studied. A ten-species surrogate model is used to calculate the thermophysical properties of kerosene and the re-normalization group (RNG) k-ε turbulent model with the enhanced wall treatment is adopted to consider the turbulent effect. The effects of mass flow rate, wall heat flux, inlet temperature, and pressure on heat transfer are investigated. The numerical results show that three types of heat transfer deterioration exist for the aviation kerosene flow. The first type of deterioration occurred at the tube inlet region and is caused by the development of the thermal boundary layer, while the other two types are observed when the inner wall temperature or the bulk fuel temperature approaches the pseudo-critical temperature. The heat transfer coefficient increases with the increasing mass flow rate and the decreasing wall heat flux, while the inlet bulk fluid temperature only influences the starting point of the heat transfer coefficient curve plotted against the bulk fluid temperature. The increase of inlet pressure can effectively eliminate the deterioration due to the small variations of properties near the pseudo-critical point at relatively high pressure. The numerical heat transfer coefficients fit well with the empirical correlations, especially at higher pressures (about 5 MPa).     

Strengthening an in-service reinforcement concrete bridge with prestressed CFRP bar

Carbon fiber reinforced polymer （CFRP） bars were prestressed for the structural strengthening of 8 T-shaped reinforced concrete （RC） beams of a 21-year-old bridge in China. The ultimate bearing capacity of the existing bridge after retrofit was discussed on the basis of concrete structures theory. The flexural strengths of RC beams strengthened with CFRP bars were controlled by the failure of concrete in compression and a prestressing method was applied in the retrofit. The field construction processes of strengthening with CFRP bars including grouting cracks, cutting groove, grouting epoxy and embedding CFRP bars, surface treating, banding with the U-type CFRP sheets, releasing external prestressed steel tendons-were introduced in detail. In order to evaluate the effectiveness of this strengthening method, field tests using vehicles as live load were applied before and after the retrofit. The test results of deflection and concrete strain of the T-shaped beams with and without strengthening show that the capacity of the repaired bridge, including the bending strength and stiffness, is enhanced. The measurements of crack width also indicate that this strengthening method can enhance the durability of bridges. Therefore, the proposed strengthening technology is feasible and effective.     

Hydrodynamics of Continuous Spiral Dryer with Rotatory Conical Sleeves: Experiments versus CFD Simulations

In this study, a continuous and airtight twinspiral dryer was developed in accordance with the characteristics and challenges in the process of disposing polysilicon slurry. Computational fluid dynamics(CFD)simulations were used to investigate the flow field in the rotating twin-spiral continuous dryer and an original discrete phase model was also elaborated to compare with the cold-modeling experimental results. The corresponding flow field was obtained using the available inlet velocity of 0.05-0.3 m/s and the rotational speed of the inner cone of 12-44 r/min, the residence time distribution, and tracked particles trajectory. Results showed that the residence time of the tracer particles in the cone cylinder was about 15.8-25.4% of the time spent out of it, and the particle's residence time was much shorter in contrast to the rotational speed and inlet velocity. The external ribbon had a larger influence on the fluid, thereby leading to a larger velocity in the region outside the cone compared to that in the region inside the cone. In addition, the appearance of the vortex and boundary layer separation at the back of the ribbon and the spoke bar had secondary diversion effects on the fluid. Furthermore, the inlet velocity had little influence on the flow field while the rotational speed of the cone greatly affected the flow field. Hence, the CFD simulations showed good agreement with the experimental results.     

靠背式测速探头感受特性的数值模拟研究

本文针对在流场中速度、方向的测量问题,提出一种新型的靠背式测速探头的设计方案,并通过数值的手段模拟了靠背式测速探头的不同切向角对气流流向的感受特性。研究发现,当气流方向与靠背式测速探头的迎流面之夹角为15°时,靠背式测速探头比例系数值会出现极值点。这一现象的发现为在流动测量中对方向进行识别提供了依据。     

Numerical simulation of interaction between laminar flow and elastic sheet

A numerical simulation of the interaction between laminar flow with low Reynolds number and a highly flexible elastic sheet is presented. The mathematical model for the simulation includes a three-dimensional finitevolume based fluid solver for incompressible viscous flow and a combined finite-discrete element method for the three-dimensional deformation of solid. An immersed boundary method is used to couple the simulation of fluid and solid. It is implemented through a set of immersed boundary points scattered on the solid surface. These points provide a deformable solid wall boundary for the fluid by adding body force to Navier-Stokes equations. The force from the fluid is also obtained for each point and then applied on the boundary nodes of the solid. The vortex-induced vibration of the highly flexible elastic sheet is simulated with the established mathematical model. The simulated results for both swing pattern and oscillation frequency of the elastic sheet in low Reynolds number flow agree well with experimental data.     

亚超声遥控电路的制作与故障分析

亚超声遥控电路作为电子测量、电子制作课程的实训项目,其电路不仅实用性强,而且综合了电容降压、整流滤波、选频放大、双稳态翻转等.亚超声控制电路适用于实训教学,但是亚超声电路如果制作或使用不当,会产生一些故障,如何分析判断解决这些电路故障,保证电路的安全持续使用,是相关人员所必须解决的问题.     

Experimental Study of Sediment Incipience Under Complex Flows

Sediment incipience under flows passing a backward-facing step was studied. A series of experiments were conducted to measure scouring depth, probability of sediment incipience, and instantaneous flow velocity field downstream of a backward-facing step. Instantaneous flow velocity fields were measured by using Particle Image Velocimetry (PIV), and an image processing method for determining probability of sediment incipience was employed to analyze the experimental data. The experimental results showed that the probability of sediment incipience was the highest near the reattachment point, even though the near-wall instantaneous flow velocity and the Reynolds stress were both much higher further downstream of the backward-facing step. The possible mechanisms are discussed for the sediment incipience near the reattachment point. Partially supported by National Natural Science Foundation of China (No.10602017), Maritime Research Center and DHI-NTU Center of Nanyang Technological University, Singapore.     

Wind-induced stochastic response of controlled tall buildings by complex-mode analysis

This paper deals with the along-wind dynamic response of tall buildings passively controlled by Cross-Tensioned Spring Damper (CTSD) mechanisms. In order to estimate the dynamic response more accurately and efficiently, the building and its CTSD controllers were considered as a coupled non-classical damping system and a new stochastic response analysis method, complex-modal state-space method, was proposed based on the combination of complex modes of the structural system rather than on the real-modal decomposition as usual. As numerical examples, the displacement response of a 2-degree-of-freedom system to white-noise input is first considered and the validity and accuracy of the proposed method are demonstrated through comparison with the exact results. The wind-induced dynamic responses of a tall building controlled by two CTSD mechanisms are then analyzed. The calculation results showed that the proposed method is much more appropriate and accurate than the conventional real-modal decomposition method in the analysis of dynamic responses of passively controlled tall buildings. Project (59478034) supported by National Natural Science Foundation of China (NSFC).     

Studies on vibration characteristics of a pear using finite element method

The variation of the vibration characteristics of a Huanghua pear was investigated using finite element simulations. A new image processing technique was used to obtain the unsymmetrical and un-spherical geometrical model of a pear. The vibration characteristics of this type of pear with the correlation of its behavior with geometrical configurations and material characteristics were investigated using numerical modal analysis. The results showed that the eigenfrequency increased with the increasing pear Young＇s modulus, while decreased with increasing pear density, and decreased with increasing pear volume. The results of this study provided foundation for further investigations of the physical characteristics of fruits and vegetables by using finite element simulations.     

不同电极数目下电磁流量计权重函数的仿真

给出了不同电极数目下电磁流量计权重函数的仿真结果.权函数表示流动横截面上特定空间位置上的流体速度对流量计输出信号的相对作用.权函数的求解对根据电磁流量计测量的电位差来推断速度分布是很重要的.通过对比不同数目电极下权重函数的分布,可以得出,电极数目越多,对流型的敏感度越小,从而可以实现对非轴对称流流量准确的测量.     

薄壁铸件反重力充填的守恒标量法研究

1　Introduction　SofarthemainalgorithmsadaptedtothenumericalsimulationformouldfillingareMAC ,SMAC[1] ,SOLA VOF[2 ] ,SIMPLEconservativescalarmethod[3] ,etc .Tosimulatemoldfilling ,MACandSMACmethodswilloccupylargequantityofcomputermemory ,andcostmuchcomputationtime .Atpresent ,thealgorithmsusedtosimulatemouldfillingofcastingsmainlyfocusonSOLA VOFandSIMPLEconservativescalar .Com paringwithSOLA VOF ,SIMPLEconservativescalaralgorithmismuchmoresuperior .Inaspectofmomen tumequat…     

Detecting and tracking moving targets on omnidirectional vision

A method was presented to implement the detecting and tracking of moving targets through omnidirectional vision. The method combined optical flow with particle filter arithmetic, in which optical flow field was used to detect and locate moving targets and particle filter was used to track the detected moving objects. According to the circular image character of omnidirectional vision, the calculation equation of optical flow field and the tracking arithmetic of particle filter were improved based on the polar coordinates at the omnidirectional center. The edge of a randomly moving object could be detected by optical flow field and was surrounded by a reference region in the particle filter. A dynamic motion model was established to predict particle state. Histograms were used as the features in the reference region and candidate regions. The mutual information (MI) and Gaussian function were combined to calculate particle weights. Finally, the state of tracked object was computed by the total particle states with weights. Experiment results show that the proposed method could detect and track moving objects with better real-time performance and accuracy. Supported by Tianjin Higher Education Technology Development Foundation (No.20071308), Tianjin Natural Science Foundation (06YFJMJC03600) and National Natural Science Foundation of China (No.60773073). YANG Shuying, born in 1964, female, Dr, Prof.