Investigation of flow characteristics and energy dissipation over new shape of the trapezoidal labyrinth weirs

Labyrinth weirs are mainly used to increase the discharge capacity. The current study adds a new performance to labyrinth weirs as an energy dissipator. The labyrinth weirs' zigzag shape and flow behaviour could benefit energy dissipation. Therefore, the present study aims to investigate the hydraulic characteristics and energy dissipation of the compound labyrinth weir. Sixteen models were used for different sidewall angles (α°) of 6–35 and 90 (linear weir for comparison). The results demonstrated the highest values of the compound coefficient of discharge, C_dc, for a sidewall angle of 35°, and the lowest value of the compound coefficient of discharge for a sidewall angle of 6°. The C_dc increased initially at low H՛_t/P՛ values, and the C_dc showed a decreasing trend for higher values of H՛_t/P՛. For sidewall angles (α°) ranging from 6 to 35, the compound coefficient of discharge C_dc does not significantly change as it approaches a value of H՛_t/P՛ = 1.0. Furthermore, for the range of the relative critical head (y_c/P՛) between 0.07 and 0.95, the results showed that the compound labyrinth weirs could dissipate the energy of flow by 93%, 92%, 89%, 85%, 83%, 79%, and 75% for α° = 6, 8, 10, 12, 15, 20°, and 35, respectively. The amount of improvement in energy dissipation over a compound labyrinth weir was better than a linear weir by 17%, 15%, 14%, 12%, 11%, 10%, and 8% for α° = 6, 8, 10, 12, 15, 20, and 35, respectively. The residual energy (E₁/E_min) at the base of downstream compound labyrinth weirs was closer to the minimum potential amount of residual energy as y_c/P՛ increased. For a given value of y_c/P՛, the relative residual energy at the base of compound labyrinth weirs increased as the sidewall angle (α) increased. An empirical equation has been provided to predict the compound coefficient of discharge when relative energy dissipation data is available.     

Mechanism of energy dissipation in mechanical system with dry friction

Determination of the energy dissipative mechanism in a mechanical system composed of two elastic structures in dry contact is presented. The analysis is based on the measurement of displacement ratio of the contacting elastic structures as a function of frequency due to light impulse excitation at a single point on any of the two elastic structures. The theoretical analysis depends on a very simple model of a two-degree-of-freedom system where two solid friction models are adopted in the analysis of the mathematical model. Several experiments are presented to illustrate the dominant friction mechanism of contacting surfaces within the micro slip regime in a frequency range of oscillation up to 400 Hz. It was shown experimentally that the solid friction model behaves in a way that is described as structural (hysteretic) damping. In other words, the energy dissipated due to dry friction during micro slip regime does not depend on the relative velocity between the two contacting surfaces but it is proportional to their relative displacements. The determination of the contact stiffness and damping loss factor in addition to their variation with the applied normal load was also shown.     

Simulations on atomic-scale friction between self-assembled monolayers: Phononic energy dissipation

Atomic-scale friction between self-assembled monolayers (SAMs) on Au (1 1 1) has been studied through molecular dynamics simulations, with emphasis on the mechanism of energy dissipation. Results show that the shear stress and chain angle on commensurate SAMs exhibit a clean periodic pattern and atomic stick–slip friction, which manifests a gradual storage and sudden release of energy. Using a simple model of two atoms, analysis shows that the atomic stick–slip originates from the dynamic instability of molecule motion. Energy has been built up during the stick, followed by a sudden separation as the equilibrium becomes unstable, and most energy dissipates at the time of slip. Moreover, the simulations reveal that more energy is stored and released in commensurate sliding, resulting in much higher friction than that in incommensurate cases. The contradictory frictional behavior can be traced to the difference in the number and strength of the Van der Waals bonds, formed in the two types of contacts.     

Experimental study and numerical simulation of flow over the inclined airfoil-shaped weir in rectangular channel

In order to find an integrated flow measurement and control facility with stable outflow form, a series of experiments and numerical simulations were performed for the inclined airfoil-shaped weir. Based on the experimental and simulated data, the stage-discharge relationship was deduced by using dimensional analysis and incomplete self-similarity theory. And two flow measurement formulas with inclination angle of 0°–45° were obtained through different fitting schemes. The variation law of discharge ratio with inclination angle and the accuracy comparison of flow measurement formulas were analyzed. Moreover, the Froude number, critical submergence degree, head loss and velocity distribution of the flow field over the weir were studied. The results show that streamlined airfoil improves the stability of flow behind the weir. The average relative error of the free water surface line data between experiments and simulations is 5.05%, which indicates the reliability of the numerical simulation. The discharge ratio proposed in the fitting scheme I takes the 25° inclination angle as the limit, which increases slightly at first and then decreases gradually, and the flow measurement relative errors of the two fitting formulas are all within ±7%. The average Froude number of the upstream reference section at each inclination angle is less than 0.25, the critical submergence range is between 0.87 and 0.96, and the relative head loss ranges from 3.54% to 12.16%, so as to ensure the accuracy of flow measurement.     

Turbulent structures,integral length scale and turbulent kinetic energy (TKE) dissipation rate in compound channel flow

In the present study, high data rate measurements were obtained for the streamwise and vertical velocity components using 2D Laser Doppler Velocimeter. The turbulent field in a straight compound-channel flow was characterized for three different uniform flow water depths, corresponding to “deep flows”, “intermediate flows” and “shallow flows” conditions. Several methodologies were studied to process the data and to obtain autocorrelation functions, integral length scale and turbulence kinetic energy (TKE) dissipation rate. The Sample and Hold method was adopted to interpolate the unevenly spaced record and calculate the autocorrelation function; the integral-stop-value 1/e was used to estimate the integral length scale; and the TKE dissipation rate was estimated through the velocity energy spectrum. A double shear layer composed of two counter-rotating vertical oriented vortices, interacting with the secondary currents, is observed in the interface region for deep flow conditions. By decreasing the water depth, the interface region becomes dominated by a strong mixing layer of vertical oriented vortices with high TKE dissipation rate and large integral length scale, acting as a vertical wall to the weak secondary currents that develop at the main channel. The determination of the integral length scale permits to confirm the existence and the strength of these turbulence structures, unveiling the strong mixing layer as the origin of the largest integral length scales, even larger than the flow depth, and as the most efficient mechanism to redistribute turbulence generated at the bottom towards upper flow regions. Despite the high complexity of turbulence structures present in the flow, for all water depths, a linear dependence is depicted between integral length scale, TKE dissipation rate, and streamwise turbulence intensity.     

基于能量输入与耗散关系的机床一维钢轴热变形预报研究

通过对一维钢轴热传导差分模型的推导发现,钢轴能量输入、耗散与热变形之间存在着密切关系,进而建立起基于反馈原理的机床能量输入、输出的热变形预报模型,最后,进行一维钢轴热变形预报模型推导和实验.实验结果表明,该模型具有较高的预报精度.     

Prediction of hydraulic jump characteristics in a closed conduit using numerical and analytical methods

The hydraulic jump is an economical alternative to dissipate energy in the conduit and to reduce erosion at the culvert outlet. In the literature, very limited studies have been reported on the performance of hydraulic jump in a closed conduit. The innovation of this research is to employ a numerical method for the estimation of the hydraulic jump characteristics in a closed conduit with different positive slopes (S₀). The analytical method was used to develop several equations for hydraulic jump and the provided results were compared with the numerical method. The results indicate that the numerical method predicts the flow depth ratio after conduit with higher accuracy (error less than 5%) in comparison to the analytical method (error less than 10%). Furthermore, in the slope of 0.00, the energy loss increases by 16% with increasing the Froude number from 4.617 to 5.562 while this value is 23% and 22% for slopes of 0.01 and 0.02, respectively. Finally, several equations were developed for the prediction of hydraulic jump characteristics in terms of Fr₁, S₀, and conduit depth (D).     

Generation of defects in model lubricant monolayers and their contribution to energy dissipation in friction

The structural, mechanical (friction) and spectroscopic properties of model lubricant films made of self-assembled and Langmuir–Blodgett monolayers on quartz, mica and gold have been investigated with atomic force microscopy, the surface forces apparatus and sum-frequency generation. In these films, the molecules tend to form densely packed structures, with the alkane chains mostly vertical and parallel to each other. The SFG results suggest that under moderate pressures of a few tens of MPa, the methyl end group of the alkane chains is rotated to accommodate a terminal gauche distortion. The molecule, however, retains its upright close-packed structure with a lattice periodicity when ordered, which can be resolved by AFM. At pressures above 0.1 GPa, changes in the form of collective molecular tilts take place that lower the height of the monolayer. Only certain angles of tilt are allowed that are explained by the interlocking of methylene units in neighboring chains. The discrete angular tilts are accompanied by increases in friction. A model based on the van der Waals attractive energy between chains is used to explain the stability of the films and to estimate the cohesive energy changes during tilt and, from that, the increases in friction force.     

Evaluating phase-detection-based approaches for interfacial velocity and turbulence intensity estimation in a highly-aerated hydraulic jump

Estimation of turbulence intensity within a highly-aerated turbulent flow is challenging. A possible way is to record bubble arrival information using intrusive phase-detection probes, but the derivation of velocity variation is subject to the correlation and de-correlation of the signals, especially in highly-turbulent and rapidly-varied flows with complex bubble transport such as in hydraulic jumps. Although attempts have been devoted to the improvement of data processing, it is difficult to assess the existing approaches in strong hydraulic jumps due to the lack of alternative measurement techniques applicable to the internal air-water flow region. In this letter, a substantial amount of work is devoted to manual analysis of instantaneous interfacial velocity in strong hydraulic jumps, and the velocity variation results are compared with the results of full-signal cross-correlation and adaptive-window cross-correlation approaches, to evaluate their performance in approximating the velocity turbulence. The manual results are validated in terms of the time-averaged velocity. The interfacial turbulence intensity is suggested to be greater than the water-phase velocity turbulence, typically between 20% and 40% in the unidirectional jet-shear region. Overall the manual results agree better with the calculation of adaptive-window cross-correlation technique. The relevance of the signal decomposition processing is also discussed, and it is emphasized that the phase-detection-based approaches are subject to the limitations of one-dimensional measurements in a complex three-dimensional flow.     

汽车制动能量再生系统中的液压蓄能分析与计算

能源危机是全球面临的重大问题，也是现代科学技术亟待解决的课题。节约能源从广义上说应包括能量的再生。液压技术在能量的再生系统中，有着广泛的应用。该文拟在这方面进行探讨，并结合在汽车设计中的应用进行分析计算。结果表明，能量回收效率约为50％，对从事节能技术的科研与设计工作的工程技术人员有参考价值。     

基于DEM算法的多颗粒碰撞耗能机理分析与振动抑制研究

针对机械构件主系的封闭空间中填充微小颗粒,进行振动抑制问题,对填充颗粒的尺寸、数量以及材料特性因素对振动抑制效果的影响开展了研究。通过采用离散单元法(discrete element method,DEM),分析了颗粒与颗粒以及颗粒与主系统之间的运动学特性,建立了能够充分表达颗粒在相互碰撞摩擦过程中的受力、变形关系以及耗能计算模型,分析了多个颗粒之间的碰撞与耗能机理,确定了碰撞过程中颗粒的状态、受力及耗能大小的计算求解算法。在Matlab环境下,针对不同颗粒材质、数量及大小对系统振动抑制性能进行了仿真分析,得出了颗粒材质大小以及数量对减振性能的影响规律,并通过搭建的试验台,进行了试验数据采集和分析。试验结果与仿真结果相吻合,验证了算法的有效性,该算法为提高机械构件的减振性能设计提供了重要参考。     

Achieving constant speed of a hydrostatic drive using controlled operation of the pump and enhancing its energy efficiency

Novel ways of regulating the speed of a hydraulic drive through controlled operation of the pump either by loading/unloading the UPRV (unloading pressure relief valve) or by switching on/off of the prime mover driving the pump have been presented in this paper. The UPRV is used to operate the pump with a minimum load by diverting the pump delivery to the tank at low pressure. The energy efficiency of the said two methods adopted for controlling the hydro-motor speed is also compared. In this respect, the MATLAB^®/Simulink model of the system is made and validated experimentally. The static and dynamic performances of the system are investigated using the validated model. The results show that the UPRV controlled system demonstrates better dynamic performance and controllability in terms of decreased fluctuations in hydro-motor speed for the small load torque demand. However, by comparing the energy efficiency of the hydraulic system for the two control strategies, it is found that with the increase in the machine torque demand, the intermittent operation of the pump unit may be more preferable with respect to the amount of energy saved for the complete duty cycle of the system.     

某发动机液压皮带张紧器参数优化与分析

为了提高某液压皮带张紧器的动力学性能,以某公交车发动机附件轮系的实际运行特性为基础,在动力学软件AVL EXCITE Timing Drive中建立液压张紧器的仿真模型。以张紧器吸收的阻尼能为评价指标,通过加权平均法对张紧器泄漏间隙进行优化计算;最后,将体积的变化加以考虑,通过仿真计算得出了最优解随顶杆伸出长度的变化规律以及油液体积和频率对最优阻尼能的影响。为今后其他类型张紧器的参数优化提供了一种可参考的方法,同时为张紧器的参数选取提供了理论支撑。