基于CFD的滑靴副多场耦合特性研究

滑靴副作为斜盘式轴向柱塞泵的主要摩擦副之一,长期工作于高速、高压、高温等一些复杂极端环境中.本文建立了滑靴副中流场的基本控制方程,构建了滑靴副多物理场耦合的有限元模型,通过CFD技术对滑靴副流场进行模拟分析,并在此基础上对滑靴副流-固-热多物理场耦合进行仿真,得到不同工况下油液的压力场和速度场的变化规律以及流体的压力和温度对滑靴副的总变形量、应力的影响.结果表明,随着工作压力增大,油液在阻尼管路中压力损失增加,油腔内涡旋个数及尺寸增大.除此之外还发现滑靴副变形量和最大应力对油液温度较为敏感.本文工作为滑靴副的结构设计提供了一定的技术支持,并且对于提高滑靴副的整体性能有着重要的意义.     

Linear regression models of floor surface parameters on friction between Neolite and quarry tiles

For slips and falls, friction is widely used as an indicator of surface slipperiness. Surface parameters, including surface roughness and waviness, were shown to influence friction by correlating individual surface parameters with the measured friction. A collective input from multiple surface parameters as a predictor of friction, however, could provide a broader perspective on the contributions from all the surface parameters evaluated. The objective of this study was to develop regression models between the surface parameters and measured friction. The dynamic friction was measured using three different mixtures of glycerol and water as contaminants. Various surface roughness and waviness parameters were measured using three different cut-off lengths. The regression models indicate that the selected surface parameters can predict the measured friction coefficient reliably in most of the glycerol concentrations and cut-off lengths evaluated. The results of the regression models were, in general, consistent with those obtained from the correlation between individual surface parameters and the measured friction in eight out of nine conditions evaluated in this experiment. A hierarchical regression model was further developed to evaluate the cumulative contributions of the surface parameters in the final iteration by adding these parameters to the regression model one at a time from the easiest to measure to the most difficult to measure and evaluating their impacts on the adjusted R² values. For practical purposes, the surface parameter R_a alone would account for the majority of the measured friction even if it did not reach a statistically significant level in some of the regression models.     

Simulation and modeling of friction for honed and wave-cut cylinder bores of marine engines

A fluid solid interaction model (FSI) developed for two different structural cylinder patterns in order to simulate the frictional performance of piston ring–cylinder liner systems for marine engines. Computational fluid dynamics (CFD) analysis is used to solve the Navier Stokes equations. A fully flooded inlet and outlet conditions are assumed taking into account the piston ring elasticity. Honing and wave-cut cylinder geometries were compared while the pressure distribution along the ring thickness and the lubricant film is determined for each crank angle. The present results demonstrate that the honed cylinder geometry improves the friction results increasing the oil film thickness. Further, the artificially textured piston rings with the honed cylinder liner were investigated. As a result, we propose the operational function between the textured rings with the modified cylinder liners in marine engines.     

Proportional-integral based fuzzy sliding mode control of the milling head

A-axis (that is, the milling head) is an essential assembly in the five-axis CNC machine tools, positioning precision of which directly affects the machining accuracy and surface qualities of the processed parts. Considering the influence of nonlinear friction and uncertain cutting force on the control precision of the A-axis, a novel fuzzy sliding mode control (FSMC) based on the proportional-integral (PI) control is designed according to the parameters adaptation. Main idea of the control scheme is employing the fuzzy systems to approximate the unknown nonlinear functions and adopting the PI control to eliminate the input chattering. Simulation analyses and experimental results illustrate that the designed control strategy is robust to the uncertain load and the parameters perturbation.     

Neuro-fuzzy approach in estimating Hazen–Williams friction coefficient for small-diameter polyethylene pipes

Low-pressurized multiple outlet pipelines are extensively used to uniformly distribute irrigation water under different types of low-volume micro-irrigation systems. Polyethylene (PE) is the main pipe material for smooth pipes in sub-main unit of a micro-irrigation system due to its flexibility and resistibility to the sun. For computing friction loss in PE pipes, many practicing engineers hesitate to use the generalized Darcy–Weisbach equation since the friction coefficient varies at each section of the lateral. Although its non-dimensional homogeneity and limitations in applicability, the empirical Hazen–Williams equation is still commonly preferred, because of its simplicity in practice. In the current hydraulic computations for friction loss, some typical fixed values for the Hazen–Williams coefficient (C_HW) in PE pipes are still recommended regardless of pipe diameter. Experimental works have confirmed that there is a strong dependence of the C_HW on pipe diameter (D), therefore a single value of the C_HW cannot be used for all ranges of pipe diameters. The primary focus of this research is to investigate the accuracy of a fuzzy rule system approach to estimate the proper value of the C_HW coefficient for different pipe diameters because of the imprecise, insufficient, ambiguous and uncertain data available. A neuro-fuzzy approach was developed to relate the input (flow rate and pipe diameter) and output (C_HW and friction loss) variables. The application of the proposed approach was performed using the measured data for friction losses available from the recent experimental analysis, hence its performance was tested using some statistic parameters for error estimation. The examination results indicated that through fuzzy rules and membership functions the proposed model can be successfully used to identify the proper values of C_HW coefficient hence accurately estimate friction losses through smooth PE pipes.     

Methods for low-velocity friction compensation: Theory and experimental study

A study is conducted of different classes of controllers for mechanisms under the influence of low velocity friction. Many methods are proposed in the literature for friction compensation, but there has been no significant analysis of these methods with respect to each other. Also lacking in the literature is some form of categorization under which it is possible to describe and study their performance. This article provides an experimental and analytical study of controllers previously proposed for low velocity friction compensation. Because each controller is evaluated on the same experimental platform, the results can be quantified to provide an approach by which to evaluate the performance of the controllers relative to each other. Some simulations will also be performed to show the effect of certain system parameters on the performance of these controllers. © 1996 John Wiley & Sons, Inc.     

Comment: collision with friction; part B: Poisson’s and Stronge’s hypotheses

For planar oblique collision with friction that opposes sliding at the contact point, this comment compares calculations of terminal relative velocity and energy dissipation obtained using the kinematic (Newton), kinetic (Poisson) and energetic (Stronge) definitions for coefficient of restitution. The kinematic and kinetic definitions yield results which are energetically inconsistent for some initial velocities when sliding stops or changes direction before separation. If initial sliding does not come to a stop before separation, the three definitions are equivalent. In general, only the energetic coefficient of restitution e _S satisfies e _S≤1.     

Mechanical behavior of metallic glasses Mg–Cu–Y using nano-indentation

The mechanical properties of amorphous bulk metallic glassy (BMG) alloy, Mg₅₈Cu₃₁Y₁₁, are examined using nano-indentation scratching test. This study investigates the influences of different scratching conditions on the mechanical properties such as the friction force and the friction coefficient (μ) to understand the abrasive behavior of the BMG. The scratching conditions include applied normal load, depth of scratch, scratching velocity, and scratching temperature. The experimental results of the friction force, friction coefficient, hardness, and scratching morphology of BMG are characterized. The result shows that the friction force is nearly proportional to the normal load; and the friction force exhibits a slightly dependent on the scratching temperature. Then, regression analysis method is utilized to establish a formula to fit the scratching condition of BMGs. The regression analysis can be applied to model the mathematical relationship between the scratching parameters. The regression result shows a good agreement with experimental one.     

转台伺服系统滑模变结构控制器的设计与仿真研究

摩擦阻力是转台伺服系统低速运行状态下主要的非线性干扰，该文在介绍了非线性摩擦环节的动态，静态模型及其对转台伺服系统性能的影响的基础上，设计了一个补偿摩擦的滑模变结构控制器，在转台伺服系统的输入加入一个滑模变结构控制器，来补偿非线性摩擦带来的影响，保证了系统的鲁棒性能，仿真结果表明，效果良好。     

Underactuated fingers controlled by robust and adaptive trajectory following methods

Choosing an appropriate control scheme to alleviate nonlinearities and uncertainties is not a trivial task, especially when models are not easily available and practical evaluation provides the only means for actual performance assessment. Various factors can contribute to these nonlinearities and uncertainties, such as friction and stiction. Thus, this article investigates four different control schemes, namely PID, adaptive, conventional sliding mode control (SMC) and integral sliding mode control (ISMC) which are implemented in the Bristol Elumotion Robot Hand (BERUL) to analyse and overcome the aforementioned problems. The hand has five fingers with 16 joints and all fingers are underactuated. The implementation of the proposed control schemes are challenging since the BERUL fingers have significant friction, stiction and unknown parameters. The fingers are light in weight and fragile. Comparative performance characteristics have shown that the ISMC is the most suitable candidate to provide good experimental trajectory following and positioning control for underactuated BERUL fingers.     

Scaling effects on flow hydrodynamics of confined microdroplets induced by Rayleigh surface acoustic wave

This paper reports an experimental and numerical investigation on the scaling effects in the flow hydrodynamics for confined microdroplets induced by a surface acoustic wave (SAW). The characteristic parameters of the flow hydrodynamics were studied as a function of the separation height, H, between the LiNbO₃ substrate and a top glass plate, for various droplets volumes and radio-frequency powers. The ratio of the gap height to attenuation length of the SAW, H/l _SAW, is shown to be an important parameter affecting the streaming flow induced in this confined regime. The reported numerical and experimental results are in good agreement over the range examined in this study and demonstrate that, at a lower gap heights of H?≤?100?μm, a significant decrease in streaming velocity or Reynolds number is induced, with the velocity approaching zero when the gap height is decreased to ~50?μm. An increase in the gap height results in an increased streaming velocity; however, if the gap height exceeds 70?% of the SAW attenuation length, any further increase in the gap height induces a drop in the streaming velocity.     

Planar collision-type dependence on incident angle and on friction coefficient

This work addresses the ‘hard collision’ approach to the solution of planar, simple non-holonomic systems undergoing a one-point collision-with-friction problem, showing that (i) there are no coherent types of collision whereby forward sliding follows sticking, unless the initial relative tangential velocity of the colliding points vanishes; and (ii) the type of collision can be determined directly, given the collision angle of incidence \(\alpha\) and Coulomb’s coefficient of friction \(\mu\) between the colliding points. The classic hitting rod problem is used to illustrate the \(\alpha \)–\(\mu\) collision-type dependence. Finally, the relation between collision with friction and tangential impact problems in multibody systems is briefly discussed.     

Contact constraints and dynamical equations in Lagrangian systems

Constraints work in the simplest way among a variety of methods of modeling the mechanical behaviors on the interface between bodies. The constraints within a persistent point contact usually fall into the following three categories: geometry-dependent constraints due to non-penetration limitation between the two rigid bodies; velocity- or force-dependent constraints due to the vanishing of tangential velocity or sliding friction engaged in tangential interaction. Though those constraints may be intuitively obtained for some simple problems, they are essentially associated with the evolution of location parameters denoting the temporal position of the contact point. Focusing on a multibody system subject to a persistent point contact, we propose a uniform and programmable procedure to formulate the constraint equations. Kinematic analysis along the procedure can clearly expose the dependence of the constraint equations on the location parameters, unveil the reason why the velocity-dependent constraints may become nonholonomic, and exhibit the fulfillment of the Appell–Chetaev’s rule naturally. Furthermore, we employ d’Alembert–Lagrangian principle to yield the dynamical equations of the system via the method of Lagrange’s multipliers. The dynamical equations so obtained are then compared with those derived from a quite different method that characterizes the contact interplay as a pair of contact force vectors. Accordingly, the correlations between the Lagrange multipliers and the components of the real contact force can be clarified. The clarification enables us to correctly embed the force-dependent constraints into the dynamical equations. A classical example of a thin disk contacting a horizontal rough surface is provided to demonstrate the validation of the proposed theory and method.     

A mixed H2/H∞ adaptive tracking control for constrained non-holonomic systems

In this study, a PID type controller incorporating an adaptive control scheme for the mixed H₂/H_∞ tracking performance is developed for constrained non-holonomic mechanical systems under unknown or uncertain plant parameters and external disturbances. By virtue of the skew-symmetric property of the non-holonomic mechanical systems and an adequate choice of a state variable transformation, sufficient conditions are developed for the adaptive mixed H₂/H_∞ tracking control problems in terms of a pair of coupled algebraic equations instead of a pair of coupled non-linear differential equations. The coupled algebraic equations can be solved analytically.     

Oedometric test, Bauer's law and the micro-macro connection for a dry sand

What is the relationship between the macroscopic parameters of the constitutive equation for a granular soil and the microscopic forces between grains? In order to investigate this connection, we have simulated by molecular dynamics the oedometric compression of a granular soil (a dry and bad-graded sand) and computed the hypoplastic parameters hs (the granular skeleton hardness) and η (the exponent in the compression law) by following the same procedure than in experiments, that is by fitting the Bauer's law e/e₀=exp(−n(3p/hs)), where p is the pressure and e₀ and e are the initial and present void ratios. The micro-mechanical simulation includes elastic and dissipative normal forces plus slip, rolling and static friction between grains. By this way we have explored how the macroscopic parameters change by modifying the grains stiffness, V; the dissipation coefficient, γn; the static friction coefficient, μs; and the dynamic friction coefficient, μk. Cumulating all simulations, we obtained an unexpected result: the two macroscopic parameters seems to be related by a power law, hs=0.068(4)η9.88(3). Moreover, the experimental result for a Guamo sand with the same granulometry fits perfectly into this power law. Is this relation real? What is the final ground of the Bauer's Law? We conclude by exploring some hypothesis.     

Analysis and control of the near-wake flow over a square-back geometry

A 3D numerical simulation, based on the Lattice Boltzmann method is carried out on the near-wake flow behind a generic square-back blunt body to analyze and establish a method to control the near-wake flow. The flow topology is described by the velocity and the pressure fields. The influence of the wake vortices on the aerodynamic drag is clarified and quantified. In order to reduce this drag, an active open-loop flow control is applied by continuous blowing devices distributed around the base periphery. The blowing effect on the behind body flow is a reduction of the wake section and of the total pressure loss in the wake and an increase of the static pressure on the base of the square body. This control leads to a significant drag reduction of ΔC_x = −29% with a blowing velocity of 1.5V₀. The efficiency is then studied, and we found that the most efficient control is obtained for a blowing velocity of 0.5V₀ and a jet angle of 45°. In this case, a 20% drag reduction is obtained, and the energy needed to control the system is seven times lower than the energy saved by the control.     

一种提高RS422通信BIT可靠性的设计方法

电动静液作动器是飞机操纵系统的关键部件,要求有较好的速度平稳性。系统内存在泄漏非线性和摩擦非线性等影响速度平稳性的因素。滑模控制可以有效抑制系统内非线性因素的影响,但是由于抖振现象的存在限制了速度平稳性的进一步提升。针对固定切换增益的滑模控制方法的不足,提出一种基于变结构滤波器的自适应滑模控制方法。采用变结构滤波器估计系统状态信息,估计的系统状态信息用于构建滑模面,采用自适应切换增益来导出控制率,有效减小了抖振幅度。仿真结果证明了自适应滑模控制方法的有效性,采用这种方法提高了电动静液作动器的速度平稳性。     

Systematic profiling of substrate binding response to multidrug-resistant mutations in HIV-1 protease: Implication for combating drug resistance

Human immunodeficiency virus 1 (HIV-1) protease (PR) represents one of the primary targets for developing antiviral agents for the treatment of HIV-infected patients. However, a number of multidrug-resistant mutations in the enzyme have been observed over the past decades, largely limiting the application of PR inhibitors in antiviral therapy. A systematic investigation of the intermolecular interaction between the multidrug-resistant mutants of HIV-1 PR and its substrates would help to establish a complete profile of substrate response to PR mutations and to design new antiviral agents combating drug resistance. Here, we describe an integrative method to profile 6 clinical multidrug-resistant PR mutants against a panel of 16 substrate octapeptides that flank 12 distinct PR cleavage sites in viral precursor polyproteins. It is found that most multidrug-resistant mutations have only a modest or moderate effect on substrate peptide binding, although these mutations would cause a large free energy loss in PR inhibitor binding. Structural analysis reveals that the substrate peptides are loosely bound within PR active pocket to form a wide contact interface between them, and thus mutation of just single or few residues seems not to influence PR–substrate binding considerably. In addition, peptides derived from variable cleavage sites are generally more sensitive to the mutations as compared to those derived from conserved sites, supporting the co-evaluation mechanism of HIV-1 PR and its substrates under drug suppression. We also identify 12 functionally conserved key residues around the enzyme’s active site, which play crucial role in substrate recognition. In vitro fluorescence anisotropy assays confirm that wild-type PR can bind substrate peptides ARVL/AEAM and NLAF/PQGE with a moderately high affinity (K_D = 2 and 16 μM, respectively). In contrast, the key residue mutations N25D/D29N can completely eliminate (K_D = n.d.) or largely reduce (K_D = 32 and 120 μM, respectively) the binding capability of the two peptides, suggesting that these PR residues could be the potential target sites for developing resistance-free anti-HIV agents.     

Design theory and vibration characteristics of a contact head slider

In this study, the design theory of a previously proposed contact head slider was extended by considering a thermally protruding head slider and the intermolecular adhesive force between the head and disk surfaces. The waviness-excited vibration characteristics of the thermally protruding contact head slider were analyzed using a single-degree-of-freedom slider model, whose contact stiffness was calculated in accordance with the Johnson–Kendall–Roberts adhesive contact theory. It was found that, because of the adhesive force, the resonance frequency f _r of the contact slider changed from zero to a value higher than the original second-pitch-mode resonance frequency with an increase in the head-penetration depth. Because the waviness-excited vibration of the contact slider is amplified at f _r , the first- and second-pitch-mode vibrations of the thermally protruding slider can be excited when f _r approaches those resonance frequencies. Because the friction force varies with the vibration of the contact slider, vibration modes of the slider-suspension system often observed at the beginning of contact can be explained. It is suggested that the region of the head-penetration depth for perfect contact sliding can be widened by increasing the effective contact damping and decreasing the disk waviness.     

Collision with friction; Part A: Newton’s hypothesis

This paper deals with collision with friction. In Part A, equations governing a one-point collision of planar, simple nonholonomic systems are generated. Expressions for the normal and tangential impulses, the normal and tangential velocities of separation of the colliding points, and the change of the system mechanical energy are written for three types of collision (i.e., forward sliding, sticking, etc.). These together with Routh’s semigraphical method and Coulomb’s coefficient of friction are used to show that the algebraic signs of four, newly-defined, configuration-related parameters, not all independent, span five cases of system configuration. For each, the ratio between the tangential and normal components of the velocity of approach, called α, determine the type of collision, which once found, allows the evaluation of the associated normal and tangential impulses and ultimately the changes in the motion variables. The analysis of these cases indicates that the calculated mechanical energy may increase if sticking or reverse sliding occur. In Part B, theories based on Poisson’s and Stronge’s hypotheses are presented with more encouraging results.