Initial Gap Distance Research Articles

Magnetic circuit design for the performance experiment of shear yield stress enhanced by compression of magnetorheological fluids

The shear yield stress is an important parameter for the industrial application of magnetorheological (MR) fluids. A test equipment was designed and built to perform investigations on the behaviours of compression and shear after squeeze of MR fluids. Mathematical expression of magnetic flux density was further established. Furthermore, the magnetic field distribution of the test device based on two-coil mode and single-coil mode was simulated and compared using finite element analysis(ANSYS/Multiphysics). An experimental test system was fabricated and modified based on the final conditions and simulation results. The compression and shear after squeeze performances of MR fluids were tested. The results showed that a smaller initial gap distance or a larger compressive strain corresponds to a larger compressive stress under the same external magnetic field strength. The shear yield stress after the squeeze of MR fluids increases quickly with the increasing compression stress and the increasing magnetic flux density. This test equipment was thought to be suitable for studying the compression and shear after squeeze performances of MR fluids.

Dispersion characteristics of switching transients from large‐scale experiments in a 1100 kV gas insulated switchgear circuit

AbstractThe switching operation of gas insulated switchgear (GIS) disconnector will produce multiple random spark discharges between the gas gaps of the disconnector. Each spark discharge can be affected by various random factors, such as trapped charge on the load side, the initial operating phase, gap distance and operating speed of the disconnector, leading to dispersion characteristics. At present, some studies have mentioned this dispersion, but its impact is still unclear due to the lack of experimental data, resulting in a deviation between the simulation results and the measured results. In this study, thousands of switching operations were carried out based on the 1100 kV full‐scale GIS circuit in Wuhan ultra‐high voltage alternating current Test Base. The maximal information coefficient analysis, linear regression fitting and other statistical methods were used to discuss the dispersion characteristics of very fast transient overvoltage (VFTO) and transient enclosure voltage (TEV) from large‐scale experimental data. The results show that the dispersion of VFTO and TEV in amplitude aspects both increase gradually as the breakdown voltage increases, with the maximum difference of five times of TEV amplitude at normalised breakdown voltage. In terms of frequency characteristics, the dispersion of different frequency components in TEV is always greater than that of VFTO, especially for the high‐frequency components. Moreover, by comparing the frequency characteristics of TEV at different positions, we notice that the dispersion is almost independent of the spatial position but only determined by the randomness of spark discharges. These discoveries reveal the importance of the dispersion in switching transients and make up for the lack of theoretical understanding of the correlation among them.

An experimental study on mechanical properties of a magnetorheological fluid under slow compression

Mechanical properties of magnetorheological (MR) fluids have been investigated in slow compression under different magnetic fields. The compressive stress of the MR fluid has been deduced by assuming that it was a continuous shear flow in Bingham model and has been calculated. The compressive stress has also measured in different magnetic fields and initial gap distances. The compressive stress of the MR fluid in a high magnetic flux density and/or a small initial gap distance was much higher than that predicted by the traditional continuous media theory. Compressive experimental results were also compared with the continuous media theory by a normalized logarithmic form. The achieved experimental result seems to deviate from the prediction by the continuous media theory at a high magnetic flux density and a small initial gap distance. The MR fluid had a high compressive modulus when the compressive strain was lower than 0.042. The compressive modulus had an exponential relationship with the compressive strain higher than 0.042. Frictions between particles, which contribute to the high structure factor, were thought to play an important role in the large deviations in squeeze mode.

Squeeze Behaviors of Magnetorheological Fluids under Different Compressive Speeds.

The compression tests under the unidirection for magnetorheological (MR) fluids have been studied at different compressive speeds. The results indicated that curves of compressive stress under different compression speeds at the applied magnetic field of 0.15 T overlapped well and were shown to be an exponent of about 1 of the initial gap distance in the elastic deformation region and accorded well with the description of continuous media theory. The difference in compressive stress curves increases significantly with an increasing magnetic field. At this time, the continuous media theory description could not be accounted for the effect of compressive speed on the compression of MR fluid, which seems to deviate from the Deborah number prediction under the lower compressive speeds. An explanation based on the two-phase flow due to aggregations of particle chains resulting in much longer relaxation times at a lower compressive speed was proposed to explain this deviation. The results have guiding significance for the theoretical design and process parameter optimization for the squeeze-assisted MR devices such as MR dampers and MR clutches based on the compressive resistance.

The influence of the site of rupture and gap distance in acute Achilles tendon rupture treated with functional rehabilitation

AimTo determine the influence of gap distance and its location on clinical and radiological outcomes in patients with acute rupture of Tendo-Achilles (TA) treated non-operatively with functional rehabilitation. MethodsTwenty-six patients with acute complete TA rupture underwent ultrasound (US) scanning within a week of their injury. The measurements taken included the distance of the rupture from the enthesis and the gap distance between the tendon edges in varying degrees of ankle and knee positions. All patients were managed non-operatively in functional weight-bearing orthoses. Nineteen patients were then followed up at a mean of 6.1 years (range 5.8–6.5) to assess their clinical and radiological outcomes. The outcomes included ultrasound assessment of tendon healing, Achilles Tendon Rupture Score (ATRS) and modified Leppilahti Score (mLS). ResultsIn the initial ultrasound, the mean distance of the rupture from the enthesis was 52 mm (range: 40–76). The mean gap distance with the foot plantigrade was 11.4 mm, which reduced to 4.8 mm with the foot in equinus. At final follow up, no re-ruptures had occurred. The mean ATRS and mLS were 86 and 71 respectively. There was a significant correlation between the distance of the rupture from the enthesis with the MLS total score (p = 0.02), mLS Fatigue domain score (p = 0.03), and the ATRS domains of strength (p = 0.04) and fatigue (p = 0.02). There was no significant correlation between the measured gap distance with respect to the mLS, ATRS or individual ATRS domain scores. Also, there was no significant difference in ATRS and mLS outcomes when comparing tendons with respect to healed tissue appearance and fibre orientation on ultrasound. ConclusionThe initial gap distance had no relationship with any of the measured clinical outcomes at the final follow up. The distance of the gap from the enthesis, however, may carry a greater prognostic value following non-operative treatment of Achilles tendon injuries.

Elastic and viscous bumpers for seismic mitigation of base-isolated liquid storage tanks in near-fault zones

The use of friction pendulum system (FPS) as a base isolation system is one of the efficient and reliable techniques for seismic response reduction of liquid storage tanks. However, recent studies have shown that the FPS may experience substantial displacements under pulse-like near-fault earthquakes, which could lead to the collision of its articulated slider against the retainer ring. This internal impact in the FPS can make the isolation system less effective or even damaged. The best solution to this problem is to provide an appropriate displacement capacity of the FPS bearing to accommodate the resulting movements. However, due to any practical limitation or under extreme ground motions, the base displacement may exceed the displacement capacity provided and an internal impact may occur. This paper proposes and investigates the use of elastic and viscous bumpers in FPS-isolated tanks subjected to near-fault earthquakes to limit excessive bearing displacements and to prevent direct impact with the retainer. The material properties of the bumpers and their initial gap distance are used as variable parameters in the nonlinear time-history analyses of selected isolated tanks with different aspect ratios. The results, while showing the advantages of viscous bumpers over elastic ones, indicate the effectiveness of both shock absorbers in isolated tanks that are subject to large displacement demands. Based on the results of the parametric study, design recommendations are given for the use of such bumpers in FPS-isolated tanks.

Toward a Simplified Arc Impingement Model in a Direct-Current Electric Arc Furnace

A 2D axisymmetric two-phase model was developed to study the effect of an arc impingement on the liquid metal inside an electric arc furnace. In addition to the arc flow dynamics, the model covered the heat transfer and magneto hydrodynamics of the arc and the liquid metal. Through a parametric study, three different parameters were considered to predict the most important factors affecting the arc and overall behaviour of the process: the arc gap, the density of the gas, and the total electric current. Understanding the effect of these parameters can show the key factors altering the arc dynamics. The study showed that the total applied current was the most important parameter that influenced the impingement depth and mixing of the liquid metal. The depth of the impingement and strength of the mixing of the liquid bath were directly proportional to the current applied in the furnace. The initial arc gap distance was found to be crucial for sustaining a continuous and stable arc. The value of the gas density was very important for the velocity profile; however, it had no significant effect on the impingement depth. This showed that a constant density could be used instead of a varying gas density with temperature to increase the computational efficiency. The study assessed the effects of the aforementioned factors on the arc impingement depth, velocity magnitude, and arc stability. The conclusions acquired and challenges are also presented.

Compressions of magnetorheological fluids under instantaneous magnetic field and constant area

Compressions of magnetorheological (MR) fluids have been carried out under instantaneous magnetic fields. The yield strength of the MR fluid in compressive mode has been derived by assuming that it was a transformed shear flow in Bi-visous model. The compressive stresses have experimentally studied under different magnetic fields, different initial gap distances and different compressive velocities. The nominal yield shear stresses of the compressed MR fluid under different influential factors have been calculated. The compressive stress increased in a power law as the applied magnetic field increased, while it decreased as the initial gap distance and the compressive velocity increased. With the increase of magnetic field, the difference between the nominal yield shear stress curves increased, and the exponents of the power law increased with the increase of the magnetic field strengths. A larger initial gap distance and a lower compressive velocity resulted in a higher nominal yield shear stress under the same instantaneous magnetic field. The achieved results of the nominal yield shear stress with magnetic field seemed to deviate from the prediction of dipole model, and the chain structure aggregation effect, the sealing effect and the friction effect by compression should be considered.

Phase-mediated locomotion of two self-propelled flexible plates in a tandem arrangement

Unique propulsion systems have evolved in fish that interact with the effects on the surrounding fluid of upstream fish. The downstream fish utilize these complex interactions to swim efficiently. The immersed boundary method is used to explore the phase-mediated locomotion of two self-propelled flexible plates in a tandem arrangement. The interactions caused by the phase difference are elucidated, and the hydrodynamic benefits obtained from the phase-mediated interactions are scrutinized. The variations with the phase difference (Δϕ) and initial gap distance (Gx,0) in the average cruising speed (ŪC), the average input power (P¯), the swimming efficiency (η), and the equilibrium gap distance (G¯x) are determined. Three flapping modes are identified: a tandem flapping mode, a closely mediated flapping mode, and an interfered flapping mode. The propulsion mechanisms in these modes are analyzed in detail in terms of Δϕ and Gx,0. ŪC and η are increased by more than 45% in the closely mediated flapping mode. The vortical structures are visualized to characterize the three flapping modes qualitatively.

Study of a magnetorheological brake under compression-shear mode

A magnetorheological (MR) brake under compression-shear mode is designed, simulated and experimentally investigated in this paper. A MR brake under compression-shear mode was first designed considering compression enhanced shear yield stress of MR fluid. Then, the operating principle of the MR brake was illustrated and mathematical torque expressions, operating under compression-shear mode assuming Herschel–Bulkley model, was further established. Moreover, simulation analysis of the designed magnetic circuit was performed as well. An experimental prototype was fabricated and tested to evaluate the transmission performance of MR brake. The results showed that the large torque could be produced at high applied currents, high compressive stress, large compressive strain and small initial gap distances. The rotational speed and compressive speed had little effect on the torque. The characteristic rising time of the torque was greatly affected by the rotational speed, the compressive strain, and compressive speed. However, the current had little effect on the rising time. The time constant would became shorter when both the rotational speed and the compressive speed were faster. Through analyzing the compression of particle chains in MR fluids directly, it was found that the diameter and the length of the particles chains brought a strong influence on the essential property of MR fluids under compression. Thus, the compressive stress or compressive strain and the initial gap distance also played an important role in enhancing the torque. The results also showed that the proposed MR brake could generate a maximum torque of 241 Nm, about 17.9 times the magnitude of braking torque without compression, and achieve a high torque density of 125.6 kN m–2 and a time constant of 58 ms. This study provides a better understanding of MR brake under compression-shear mode and the implications for many high-power applications.

The impact of cathode surface roughness and multiple breakdown events on microscale gas breakdown at atmospheric pressure

Gas breakdown is typically driven by Townsend avalanche and predicted mathematically by Paschen’s law (PL). Gas breakdown deviates from PL at microscale due to field emission, which depends critically on electrode condition; however, understanding of the impact of initial electrode surface roughness and multiple breakdown events on breakdown voltage is incomplete. This paper assesses the variation of breakdown voltage for a pin-to-plate electrode setup in air at atmospheric pressure for gap distances of 1 ± 0.5 μm, 5 ± 0.5 μm, and 10 ± 0.5 μm with different surface roughnesses. Breakdown voltage generally increases with increasing gap distance and decreasing surface roughness for a single breakdown event; however, the breakdown voltage after ten breakdown events does not depend on initial gap distance. Atomic force microscopy and optical microscopy show that multiple discharges create circular craters on the flat cathode up to 40 μm deep, with more pronounced craters created at smaller gap sizes and greater cathode surface roughness. The resulting effective gap distances (deff, the sum of cathode placement distance and crater depth) for subsequent breakdown events are similar to those of the initially larger gap distances. Moreover, deff becomes sufficiently large to exceed the Meek criterion for streamer formation, indicating a potential for breakdown mechanisms to change from field emission to Townsend avalanche to streamer formation for a single electrode separation distance. The resulting impact of this change in the breakdown mechanism could have significant implications for ensuring consistent microdevice operation.

Shock response of electrostatically coupled microbeams under the squeeze-film damping effect

This paper presents a theoretical investigation of the dynamic response of electrostatically coupled microcantilever beams under the combined effect of squeeze-film damping and mechanical shock for MEMS applications. We consider two different microsystem designs, namely single and dual beams, operating at varying conditions to explore their possible use based on the application of interest. The single-beam system is actuated via a fixed electrode (uncoupled actuation), while the electric actuation of the dual-beam system, comprising two movable microbeams, is achieved by applying a DC voltage among them (coupled actuation). We develop a multi-physics model to simulate the dynamic response of single and dual microbeams while accounting for the fringing field effect, the squeeze-film damping resulting from the interactions between the vibrating microbeams and the surrounding fluid, and the mechanical shock. The squeeze-film damping is incorporated using a nonlinear analytical expression rather than solving the fully coupled fluid–structure problem. Numerous studies have shown the validity of this analytical approach under the assumption of long beams and neglecting the fluid compressibility. Some benchmark cases are considered to verify the predictive capability of the numerical model. The simulation results are in good qualitative and quantitative agreements with those reported in the literature. A parametric study is then conducted to investigate the effects of the initial gap distance, the fluid viscosity, and the beam geometry on the shock response of the microsystem. The squeeze-film damping enables the protection of the vibrating microbeams from hitting the substrate under mechanical shock and can be exploited to enhance the reliability of MEMS devices. However, this protection can be significantly reduced by increasing the gap distance or decreasing the operating pressure. The numerical study shows that the dual-beam systems withstand better to a mechanical shock. Breaking the symmetry of the dual-beam system in terms of the beams’ geometry is found to reduce significantly the resistance to shocks. On the other hand, given their high sensitivity to a mechanical shock, single-beam systems are observed to be more attractive for deployment as microswitches.

Simulation of Solenoid Valve Characteristics of Electronically Controlled Fuel System for Diesel Engine

To quantitatively analyze the influence of solenoid valve parameters on their dynamic response characteristics, mathematic models are built based on the electromagnetic theory. Research is carried out using mathematical models, whereby the dynamic response process of the solenoid valve driven by constant voltage is simulated. The four parameters, namely: driven voltage, initial air gap distance, spring preload, and the number of coil turns influence the dynamic characteristics of the solenoid valve. The solenoid valve displacement curves under different parameters are obtained. Theoretically determined relationships between the dynamic response characteristics and the structural and control parameters of the solenoid valve, provide the basis for the parameter matching of the solenoid valve, and the optimized design of electronic control fuel systems.

Study of a Hybrid Generator Based on Triboelectric and Electromagnetic Mechanisms

Energy harvesting technique has been identified as an innovative and environment-friendly way for powering micro sensors and electronic devices. In this paper, a hybrid generator based on triboelectric and electromagnetic mechanisms (TMHG), which provides two electrical power outputs at the same time, was proposed. The output performance in time and frequency domains are investigated at different initial gap distances and excitation accelerations. At an acceleration of 1 g and an initial gap distance of 5 mm, the maximum power outputs of electromagnetic and triboelectric parts can be obtained as 0.28 mW and 1.09 mW, respectively. The resonant frequency of the TMHG is as low as 23.5 Hz, making it more suitable to harvest energy from low frequency vibrations of less than 30 Hz. Besides, the operating bandwidth of TMHG can be adjusted by changing the initial gap distance according to the application requirements.

Numerical study on the sedimentation of single and multiple slippery particles in a Newtonian fluid

The dynamics of a single or a group of slippery spheres settling under gravity in a Newtonian fluid is studied numerically. We focus particularly on the effect of particle surface slip on the sedimentation behavior. The flows containing moving slippery spheres are solved by a three-dimensional lattice Boltzmann model where a kinetic boundary condition is used to handle the slip phenomenon at the curved particle surface. The method is first validated by simulating the slip flow in a cylindrical tube, and the no-slip flows around one and two spheres settling in a container. The hydrodynamic behaviors of one, two and multiple slippery spheres settling under gravity are then investigated. The results for a single sphere show that the surface slip makes the sphere fall faster than a no-slip particle and the wall correction factor decreases as the level of particle-surface slip is increased, indicating a drag reduction caused by the slip condition. For two settling spheres, when the no-slip particle is placed below the slip one, the two spheres will enter into the kissing phase earlier; on the contrary, deploying the no-slip particle above the slip one, the DKT process does not occur beyond a critical slip level and initial gap distance. If the two spheres are both slippery, the settling dynamics are similar to the no-slip case, but the time duration of the kissing phase decreases. As for the sedimentation of multiple spheres, it is found that the initial geometric arrangement has a significant impact on the sedimentation behavior. In general, slippery spheres in a cluster will experience larger fluctuations in the vertical velocity and position in the accelerated-falling stage, and smaller fluctuations in the decelerated-falling stage.

Vortex interaction between two tandem flexible propulsors with a paddling-based locomotion

Schooling behaviours among self-propelled animals can benefit propulsion. Inspired by the schooling behaviours of swimming jellyfish, flexible bodies that self-propel through a paddling-based motion were modelled in a tandem configuration. This present study explored the hydrodynamic patterns generated by the interactions between two flexible bodies and the surrounding fluid in the framework of the penalty immersed boundary method. The hydrodynamic patterns produced in the wake revealed flow-mediated interactions between two tandem propulsors, including vortex–vortex and vortex–body interactions. Two tandem flexible propulsors paddling with identical amplitude and frequency produced stable configurations as a result of the flow-mediated interactions. Both the upstream and downstream propulsors benefited from the tandem configuration in terms of the locomotion velocity and the cost, compared with an isolated propulsion system. The interactions were examined as a function of the initial gap distance and the phase difference in the paddling frequency. The equilibrium gap distance between two propulsors remained constant, regardless of the initial gap distance, although it did depend on the phase difference in the paddling frequency.

Casimir and Optical Forces Acting on a Silicon NOEMS Device Based on Slot-Waveguide Structure

We investigate the effect of Casimir force on a silicon nano-opto-electro-mechanical system (NOEMS) device, based on a slot-waveguide structure and driven by optical forces. Light is strongly confined in the slot-waveguide, which enhances the optical force and reduces the optical power necessary to control the device. However, we predict that the Casimir force affects the device's critical points of stability, by reducing the critical optical power for the quasi-TE fundamental eigenmode, as well as imposing a critical power for the quasi-TM one. Casimir force also changes the initial NOEMS equilibrium position, pre-deforming the two sides of the slot-waveguide, even in the absence of optical forces; this establishes a minimal initial gap distance for the slot-waveguide, thus affecting the device's design parameters.

A micromechanical model for magnetorheological fluids under slow compression

We propose a micromechanical model for the behavior of dilute magnetorheological fluids under unidirectional slow-compression, constant-volume squeeze flow mode. In the linear magnetization regime, the model predicts a power law scaling of the normal stress with the particle volume fraction and magnetic field strength squared at low fields. The predictions are satisfactorily compared with experimental measurements for different particle loadings, sample volume, surface roughness, and initial gap distance.

Squeeze behaviors of magnetic powders between two parallel plates

The dynamic compressive and static normal stresses of magnetic powders (MPs) with the constant volume squeezed between two parallel plates were experimentally studied. The compressive stress increased in a power law as the gap distance decreased, with an exponent range of −0.73 (0.04 T) to −2.63 (0.77 T). The values of the scale factor to normalize the compressive curves were mainly dominated by the applied magnetic field. The compressive behavior of the MPs showed a less significant velocity effect and initial gap distance effect than those of the magnetorheological (MR) and electrorheological (ER) fluids because of the absence of a host liquid. The compressive stress was generated by the serious particle aggregation, which was induced by a magnetic field and friction during compression. This study is designed to further the understanding of the behavior of smart ER/MR materials and the application of MP actuators in squeeze mode.

Bond formation of surface-tethered receptor-ligand pairs in relative separation

We theoretically and numerically investigate the interplay between diffusion of a surface-bound receptor and its reaction with an opposing ligand. Special attention has been paid to the mechanical regulation of bond association by varying the initial gap distance and relative separation speed between the protein-bearing surfaces. Such diffusion-reaction coupling effects can cause the apparent on-rate or reciprocal of the average waiting time for bond formation, to be not constant, but instead a function sensitive to the system parameters that affect the transport of proteins. The results provide a quantitative understanding of how significantly the transport mechanism can affect overall binding behavior of molecular interactions and call for a paradigm shift in modeling receptor-ligand bond association when the protein-bearing surfaces are in relative separation.