Lithium-rich manganese-based oxide(LRMO)cathode has emerged as a particularly promising candidate for achieving high energy densities in lithium-ion batteries due to its capability to access anion redox reactions at h...Lithium-rich manganese-based oxide(LRMO)cathode has emerged as a particularly promising candidate for achieving high energy densities in lithium-ion batteries due to its capability to access anion redox reactions at high voltage.The successful implementation of LRMO in energy storage systems is contingent upon the enhancement of their rate capabilities.However,the underlying relationship between high-rate cycling and electrode degradation for LRMO,particularly concerning structural evolution,still remains unclear.Benefiting from the high time resolution abilities of liquid-metal-jet operando twodimensional X-ray diffraction,it is observed that the Li_(2)MnO_(3)phase in LRMO is gradually activated accompanied by the emergence of oxygen vacancies during cycling at 1 C(1 C=250 mA/g).Consequently,the crystal lattice flexibility of LRMO is systematically enhanced,thereby preventing the collapse of the bulk structure.While,continuous release of oxygen during extended cycling results in deteriorations of the self-adjusting damping effect of the structure,ultimately leading to a decline in capacity.The findings of this study not only contribute to a more profound understanding of the structural changes of LRMOs at high rates,but also provide novel perspectives for the rational design of LRMOs with superior rate performances.展开更多
This paper studies the CoNbZr soft magnetic thin film by magnetic force microscopy （MFM）. By measuring in atmosphere circumstance, the magnetic force images display some clear dark dots which are corresponding to th...This paper studies the CoNbZr soft magnetic thin film by magnetic force microscopy （MFM）. By measuring in atmosphere circumstance, the magnetic force images display some clear dark dots which are corresponding to the clusters in the topography images welL Then the dark dots disappear in magnetic force images, scanning in high vacuum. This indicates that the dark dots are caused by air damping between the vibrating tip and the sample. An interpretation for the above observation is given.展开更多
This paper presents a quantitative assessment of the transient stability of grid-forming converters,considering current limitations,inertia,and damping effects.The contributions are summarized in two main aspects:Firs...This paper presents a quantitative assessment of the transient stability of grid-forming converters,considering current limitations,inertia,and damping effects.The contributions are summarized in two main aspects:First,the analysis delves into transient stability under a general voltage sag scenario for a converter subject to current limitations.When the voltage sag exceeds a critical threshold,transient instability arises,with its severity influenced by the inertia and damping coefficients within the swing equation.Second,a comprehensive evaluation of these inertia and damping effects is conducted using a model-based phase-portrait approach.This method allows for an accurate assessment of critical clearing time(CCT)and critical clearing angle(CCA)across varying inertia and damping coefficients.Leveraging data obtained from the phase portrait,an artificial neural network(ANN)method is presented to model CCT and CCA accurately.This precise estimation of CCT enables the extension of practical operation time under faults compared to conservative assessments based on equal-area criteria(EAC),thereby fully exploiting the system's low-voltage-ride-through(LVRT)and fault-ride-through(FRT)capabilities.The theoretical transient analysis and estimation method proposed in this paper are validated through PSCAD/EMTDC simulations.展开更多
Oscillating water columns(OWCs)are most widely used in coastal wave energy conversion.The air duct opens into the atmosphere through the air turbine,which is the power take-off device,and this results in a pressure dr...Oscillating water columns(OWCs)are most widely used in coastal wave energy conversion.The air duct opens into the atmosphere through the air turbine,which is the power take-off device,and this results in a pressure drop across the air chamber.However,because of the complex configure of the impulse turbine and its high rotation speed,it is difficult to install it in the experimental simulator and numerical model.Therefore,the turbine damping effects on the operation of the OWC air chamber are induced to predict its performance more accurately.Orifice plates are used as a substitute for the impulse turbine as it generates a similar pressure drop and power output;the experimental and numerical pressure drops and output powers are compared.A 3D numerical wave tank based on the two-phase VOF model is established using the commercial CFD code Fluent,which can predict air flow and pressure variations in the chamber and duct.Water surface elevations,air flow velocity and pressure variation inside the chamber with the orifice plate are studied numerically,and validated by the corresponding experimental data.The air chamber of the Yongsoo OWC pilot plant is used as the engineering project case.The operating performance of the air chamber installed with a 0.428D orifice plate as the substitute for the designed impulse turbine is computed and analyzed.It is found that the turbine damping effects will cause around 30%reduction in the peak values of the pneumatic energy output of the OWC air chamber in the resonant wave domain.展开更多
We investigate a continuous Heisenberg spin chain equation which models the local magnetization in ferromagnet with time-and site-dependent inhomogeneous bilinear interaction and timedependent spin-transfer torque.By ...We investigate a continuous Heisenberg spin chain equation which models the local magnetization in ferromagnet with time-and site-dependent inhomogeneous bilinear interaction and timedependent spin-transfer torque.By establishing the gauge equivalence between the spin chain equation and an integrable generalized nonlinear Schrödinger equation,we present explicitly a novel nonautonomous magnetic soliton solution for the spin chain equation.The results display how the dynamics of the magnetic soliton can be controlled by the bilinear interaction and spin-polarized current.Especially,we find that the site-dependent bilinear interaction may break some conserved quantity,and give rise to damping-like effect in the spin evolution.展开更多
This study examines the roles of soil-structure interaction （SSI）, higher modes, and damping in a base-isolated structure built on multiple layers of soil overlying a half space. Closed-form solutions for the entire...This study examines the roles of soil-structure interaction （SSI）, higher modes, and damping in a base-isolated structure built on multiple layers of soil overlying a half space. Closed-form solutions for the entire system, including a superstructure, seismic isolator, and numerous soil layers overlying a half-space, were obtained. The formulations obtained in this study simply in terms of well-known frequencies and mechanical impedance ratios can explicitly interpret the dynamic behavior of a base-isolated structure interacting with multiple soil layers overlying a half-space. The key factors influencing the performance of the isolation system are the damping ratio of the isolator and the ratio of the natural frequency of the fixed-base structure to that of the isolated structure by assuming that the superstructure moves as a rigid body. This study reveals that higher damping in the base isolator is unfavorable to higher mode responses that usually dominate the responses of the superstructure and that the damping mechanism plays an important role in transmitting energy in addition to absorbing energy. It is also concluded that it is possible to design a soft soil layer as an isolation system for isolating vibration energy.展开更多
To study the dynamics of mixed traffic flow consisting of motorized and non-motorized vehicles, a carfollowing model based on the principle of collision free and cautious driving is proposed. Lateral friction and over...To study the dynamics of mixed traffic flow consisting of motorized and non-motorized vehicles, a carfollowing model based on the principle of collision free and cautious driving is proposed. Lateral friction and overlapping driving are introduced to describe the interactions between motorized vehicles and non-motorized vehicles. By numerical simulations, the flux-density relation, the temporal-spatial dynamics, and the velocity evolution are investigated in detail The results indicate non-motorized vehicles have a significant impact on the motorized vehicle flow and cause the maximum flux to decline by about 13%. Non-motorized vehicles can decrease the motorized vehicle velocity and cause velocity oscillation when the motorized vehicle density is low. Moreover, non-motorized vehicles show a significant damping effect on the oscillating velocity when the density is medium and high, and such an effect weakens as motorized vehicle density increases. The results also stress the necessity for separating motorized vehicles from non-motorized vehicles.展开更多
Owing to the interactions among the complex terrain, bottom materials, and the complicate hydrodynam-ics, typhoon waves show special characteristics as big waves appeared at the high water level （HWL） and small wave...Owing to the interactions among the complex terrain, bottom materials, and the complicate hydrodynam-ics, typhoon waves show special characteristics as big waves appeared at the high water level （HWL） and small waves emerged at low and middle water levels （LWL and MWL） in radial sand ridges （RSR）. It is as-sumed that the mud damping, sandy bed friction and wave breaking effects have a great influence on the typhoon wave propagation in this area. Under the low wave energy, a mud layer will form and transport into the shallow area, thus the mud damping effects dominate at the LWL and the MWL. And high Collins coef-ficient （c around 1） can be applied to computing the damping effects at the LWL and the MWL. But under the high wave energy, the bottom sediment will be stirred and suspended, and then the damping effects disappear at the HWL. Thus the varying Collins coefficient with the water level method （VCWL） is imple-mented into the SWAN to model the typhoon wave process in the Lanshayang Channel （LSYC） of the RSR, the observed wave data under “Winnie” （“9711”） typhoon was used as validation. The results show that the typhoon wave in the RSR area is able to be simulated by the VCWL method concisely, and a constant wave breaking coefficient （γ） equaling 0.78 is better for the RSR where wide tidal flats and gentle bed slopes exist.展开更多
An analytical method to study the seismic response of a bridge pier supported on a rigid caisson foundation embedded in a deep soil stratum underlain by a homogeneous half space is developed. The method reproduces the...An analytical method to study the seismic response of a bridge pier supported on a rigid caisson foundation embedded in a deep soil stratum underlain by a homogeneous half space is developed. The method reproduces the kinematic and inertial responses, using translational and rotational distributed Winkler springs and dashpots to simulate the soil-caisson interaction. Closed-form solutions are given in the frequency domain for vertical harmonic S-wave excitation. Comparison with results from finite element （FE） analysis and other available solutions demonstrates the reliability of the model. Results from parametric studies are given for the kinematic and inertial responses. The modification of the fundamental period and damping ratio of the bridge due to soil-structure interaction is graphically illustrated.展开更多
Enhancing damping characteristic is one of the effective methods to solve the instability problem of the rotor system.The three-dimensional numerical analysis model of scallop damper seal was established,and the effec...Enhancing damping characteristic is one of the effective methods to solve the instability problem of the rotor system.The three-dimensional numerical analysis model of scallop damper seal was established,and the effects of inlet pressures,preswirl ratios,rotational speeds,interlaced angles and seal cavity depths on the rotordynamic characteristics of scallop damper seal were studied based on dynamic mesh method and multi-frequencies elliptic whirling model.Results show that the direct stiffness of the scallop damper seal increases with decreasing inlet pressure and increasing rotational speed and cavity depth.When the seal cavity is interlaced by a certain angle,which shows positive direct stiffness.The effective damping of the scallop damper seal increases with the increasing inlet pressure,the decreasing preswirl ratio and the rotational speed and cavity depth.There exists an optimal interlaced angle to maximize the effective damping and the system stability.The leakage of the scallop damper seal is significantly reduced with decreasing inlet pressure.The preswirl will reduce the leakage flowrate,and the rotational speed has a slight effect on the leakage performance.The leakage of the scallop damper seal decreases with increasing seal cavity depth.展开更多
Perforated structures are widely employed in MEMS devices for dissipation control,energy absorption,and performance optimization.Among these,the damping weakening effect is particularly intriguing,attracting considera...Perforated structures are widely employed in MEMS devices for dissipation control,energy absorption,and performance optimization.Among these,the damping weakening effect is particularly intriguing,attracting considerable attention and widespread application.Evaluating the impact of perforations on damping is crucial for enhancing the performance of MEMS devices.This paper investigates the damping tuning mechanisms of perforations and presents two theoretical models for accurately predicting viscous damping.The two models exhibit unique advantages under high and low perforation ratios,respectively.Both models account for complex boundary conditions and various hole geometries,including cylindrical,conical,prismatic,and trapezoidal holes.Modeling and simulations demonstrate the complementarity of the two models,enabling accurate viscous damping predictions across nearly all perforation ratios.Subsequently,the theoretical models are validated through a series of vibration tests on perforated oscillators,with errors consistently controlled within 10%.Experimental results demonstrate that perforations can easily achieve a damping reduction of more than one order of magnitude.Moreover,compared to normal cylindrical holes,trapezoidal holes are found to achieve superior damping reduction with a smaller sacrifice in surface area,which holds great potential for capacitive,acoustic,and optical MEMS devices.This work lays the foundation for viscous damping design and optimization of MEMS device dynamics,creating new applications.展开更多
With the rapid development of power electronic technology,many linear single-phase loads in low voltage systems are replaced by those equipped with a single-phase diode rectifier.Therefore,most of single-phase loads b...With the rapid development of power electronic technology,many linear single-phase loads in low voltage systems are replaced by those equipped with a single-phase diode rectifier.Therefore,most of single-phase loads become harmonic producers due to their nonlinear characteristics.The nonlinear loads may have different damping characteristics at harmonic frequencies,which could cause potential power quality problems.This paper studies the harmonic damping characteristics of such nonlinear loads.The harmonic damping index is defined to quantify the damping effect of nonlinear loads of harmonic frequencies by analyzing the harmonic model.Then,the harmonic damping index of different harmonic frequencies is compared through simulations and experiments.The results show that the nonlinear load has a constant damping effect for the fundamental frequency,but a stronger damping for the harmonic frequencies.The harmonic damping of such nonlinear loads varies with the harmonic phase angle of supply voltage,and becomes stronger with the increase of the harmonic order.It implies that this type of nonlinear load may be beneficial to the suppression of harmonic resonance.展开更多
This work extends the sediment-laden mixture model with consideration of the turbulence damping and particle wake effects under the framework of improved efficiency and accuracy.The mixture model consists of the conti...This work extends the sediment-laden mixture model with consideration of the turbulence damping and particle wake effects under the framework of improved efficiency and accuracy.The mixture model consists of the continuity and momentum equations for the sediment-laden mixture,and the continuity equation for the sediment.A theoretical formula is derived for the relative velocity between the water and sediment phases,with consideration of the effects of the pressure gradient,the shear stress and the lift force.A modified expression of the particle wake effect,inducing the local turbulence enhancement around the sediment particle,is employed to improve the turbulent diffusion of the coarse sediment.The k_(m)-ε_(m) model is proposed to close the mixture turbulence,with the turbulence damping effect due to the high sediment concentration expressed by the density-stratification term without an empirical parameter.The k_(m)-ε_(m) turbulence model requires smaller computational work and offers better results than an empirical density-stratification turbulence model in high sediment concentration cases.Consequently,with the proposed mixture model,the sediment transport in the open channel under a wide range of sediment sizes and concentrations can be revealed with the results in good agreement with experimental data for the velocity,the sediment concentration and the turbulent kinetic energy.展开更多
基金financial supports from the National Natural Science Foundation of China(52372211,52371225 and 92472115)the Guangdong Province Major Talent Introducing Program(2021QN020687)+1 种基金the Shenzhen Basic Research Foundation(JCYJ20230807112503007)the Guangdong Basic and Applied Basic Research Foundation(Grant No.2020A1515110176)。
文摘Lithium-rich manganese-based oxide(LRMO)cathode has emerged as a particularly promising candidate for achieving high energy densities in lithium-ion batteries due to its capability to access anion redox reactions at high voltage.The successful implementation of LRMO in energy storage systems is contingent upon the enhancement of their rate capabilities.However,the underlying relationship between high-rate cycling and electrode degradation for LRMO,particularly concerning structural evolution,still remains unclear.Benefiting from the high time resolution abilities of liquid-metal-jet operando twodimensional X-ray diffraction,it is observed that the Li_(2)MnO_(3)phase in LRMO is gradually activated accompanied by the emergence of oxygen vacancies during cycling at 1 C(1 C=250 mA/g).Consequently,the crystal lattice flexibility of LRMO is systematically enhanced,thereby preventing the collapse of the bulk structure.While,continuous release of oxygen during extended cycling results in deteriorations of the self-adjusting damping effect of the structure,ultimately leading to a decline in capacity.The findings of this study not only contribute to a more profound understanding of the structural changes of LRMOs at high rates,but also provide novel perspectives for the rational design of LRMOs with superior rate performances.
基金Supported by the National Key Fundamental Research Program of China (No. 51310Z07)
文摘This paper studies the CoNbZr soft magnetic thin film by magnetic force microscopy （MFM）. By measuring in atmosphere circumstance, the magnetic force images display some clear dark dots which are corresponding to the clusters in the topography images welL Then the dark dots disappear in magnetic force images, scanning in high vacuum. This indicates that the dark dots are caused by air damping between the vibrating tip and the sample. An interpretation for the above observation is given.
基金supported by the EPSRC project‘Sustainable Urban Power Supply through Intelligent Control and Enhanced Restoration of AC/DC Networks'under Grant EP/T021985/1.
文摘This paper presents a quantitative assessment of the transient stability of grid-forming converters,considering current limitations,inertia,and damping effects.The contributions are summarized in two main aspects:First,the analysis delves into transient stability under a general voltage sag scenario for a converter subject to current limitations.When the voltage sag exceeds a critical threshold,transient instability arises,with its severity influenced by the inertia and damping coefficients within the swing equation.Second,a comprehensive evaluation of these inertia and damping effects is conducted using a model-based phase-portrait approach.This method allows for an accurate assessment of critical clearing time(CCT)and critical clearing angle(CCA)across varying inertia and damping coefficients.Leveraging data obtained from the phase portrait,an artificial neural network(ANN)method is presented to model CCT and CCA accurately.This precise estimation of CCT enables the extension of practical operation time under faults compared to conservative assessments based on equal-area criteria(EAC),thereby fully exploiting the system's low-voltage-ride-through(LVRT)and fault-ride-through(FRT)capabilities.The theoretical transient analysis and estimation method proposed in this paper are validated through PSCAD/EMTDC simulations.
基金supported by the National Natural Science Foundation of China(Grant No.51279190&51311140259)Shandong Natural Science Funds for Distinguished Young Scholar(Grant No.JQ201314)+1 种基金"111"Project(Grant No.B14028)KRISO Endowment(Grant No.PES 2190)
文摘Oscillating water columns(OWCs)are most widely used in coastal wave energy conversion.The air duct opens into the atmosphere through the air turbine,which is the power take-off device,and this results in a pressure drop across the air chamber.However,because of the complex configure of the impulse turbine and its high rotation speed,it is difficult to install it in the experimental simulator and numerical model.Therefore,the turbine damping effects on the operation of the OWC air chamber are induced to predict its performance more accurately.Orifice plates are used as a substitute for the impulse turbine as it generates a similar pressure drop and power output;the experimental and numerical pressure drops and output powers are compared.A 3D numerical wave tank based on the two-phase VOF model is established using the commercial CFD code Fluent,which can predict air flow and pressure variations in the chamber and duct.Water surface elevations,air flow velocity and pressure variation inside the chamber with the orifice plate are studied numerically,and validated by the corresponding experimental data.The air chamber of the Yongsoo OWC pilot plant is used as the engineering project case.The operating performance of the air chamber installed with a 0.428D orifice plate as the substitute for the designed impulse turbine is computed and analyzed.It is found that the turbine damping effects will cause around 30%reduction in the peak values of the pneumatic energy output of the OWC air chamber in the resonant wave domain.
文摘We investigate a continuous Heisenberg spin chain equation which models the local magnetization in ferromagnet with time-and site-dependent inhomogeneous bilinear interaction and timedependent spin-transfer torque.By establishing the gauge equivalence between the spin chain equation and an integrable generalized nonlinear Schrödinger equation,we present explicitly a novel nonautonomous magnetic soliton solution for the spin chain equation.The results display how the dynamics of the magnetic soliton can be controlled by the bilinear interaction and spin-polarized current.Especially,we find that the site-dependent bilinear interaction may break some conserved quantity,and give rise to damping-like effect in the spin evolution.
文摘This study examines the roles of soil-structure interaction （SSI）, higher modes, and damping in a base-isolated structure built on multiple layers of soil overlying a half space. Closed-form solutions for the entire system, including a superstructure, seismic isolator, and numerous soil layers overlying a half-space, were obtained. The formulations obtained in this study simply in terms of well-known frequencies and mechanical impedance ratios can explicitly interpret the dynamic behavior of a base-isolated structure interacting with multiple soil layers overlying a half-space. The key factors influencing the performance of the isolation system are the damping ratio of the isolator and the ratio of the natural frequency of the fixed-base structure to that of the isolated structure by assuming that the superstructure moves as a rigid body. This study reveals that higher damping in the base isolator is unfavorable to higher mode responses that usually dominate the responses of the superstructure and that the damping mechanism plays an important role in transmitting energy in addition to absorbing energy. It is also concluded that it is possible to design a soft soil layer as an isolation system for isolating vibration energy.
基金Supported by the National Basic Research Program of China under Grant No.2006CB705500the National Natural Science Foundation of China under Grant Nos.70631001 and 70701004
文摘To study the dynamics of mixed traffic flow consisting of motorized and non-motorized vehicles, a carfollowing model based on the principle of collision free and cautious driving is proposed. Lateral friction and overlapping driving are introduced to describe the interactions between motorized vehicles and non-motorized vehicles. By numerical simulations, the flux-density relation, the temporal-spatial dynamics, and the velocity evolution are investigated in detail The results indicate non-motorized vehicles have a significant impact on the motorized vehicle flow and cause the maximum flux to decline by about 13%. Non-motorized vehicles can decrease the motorized vehicle velocity and cause velocity oscillation when the motorized vehicle density is low. Moreover, non-motorized vehicles show a significant damping effect on the oscillating velocity when the density is medium and high, and such an effect weakens as motorized vehicle density increases. The results also stress the necessity for separating motorized vehicles from non-motorized vehicles.
基金The National High Technology Research and Development Program(863 Program)of China under contract No.2012AA112509the National Natural Science Fundation of China under contract No.41373112the Open Research Foundation from the State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering,Nanjing Hydraulic Research Institute under contract No.2012491311
文摘Owing to the interactions among the complex terrain, bottom materials, and the complicate hydrodynam-ics, typhoon waves show special characteristics as big waves appeared at the high water level （HWL） and small waves emerged at low and middle water levels （LWL and MWL） in radial sand ridges （RSR）. It is as-sumed that the mud damping, sandy bed friction and wave breaking effects have a great influence on the typhoon wave propagation in this area. Under the low wave energy, a mud layer will form and transport into the shallow area, thus the mud damping effects dominate at the LWL and the MWL. And high Collins coef-ficient （c around 1） can be applied to computing the damping effects at the LWL and the MWL. But under the high wave energy, the bottom sediment will be stirred and suspended, and then the damping effects disappear at the HWL. Thus the varying Collins coefficient with the water level method （VCWL） is imple-mented into the SWAN to model the typhoon wave process in the Lanshayang Channel （LSYC） of the RSR, the observed wave data under “Winnie” （“9711”） typhoon was used as validation. The results show that the typhoon wave in the RSR area is able to be simulated by the VCWL method concisely, and a constant wave breaking coefficient （γ） equaling 0.78 is better for the RSR where wide tidal flats and gentle bed slopes exist.
基金U.S. Federal Highway Administration Under Grant No. DTFH61-98-C-00094U.S. National Science Foundation Under Grant No. EEC-9701471
文摘An analytical method to study the seismic response of a bridge pier supported on a rigid caisson foundation embedded in a deep soil stratum underlain by a homogeneous half space is developed. The method reproduces the kinematic and inertial responses, using translational and rotational distributed Winkler springs and dashpots to simulate the soil-caisson interaction. Closed-form solutions are given in the frequency domain for vertical harmonic S-wave excitation. Comparison with results from finite element （FE） analysis and other available solutions demonstrates the reliability of the model. Results from parametric studies are given for the kinematic and inertial responses. The modification of the fundamental period and damping ratio of the bridge due to soil-structure interaction is graphically illustrated.
基金co-supported by the National Natural Science Foundation of China(No.51875361)the Natural Science Foundation of Shanghai,China(No.20ZR1439200)。
文摘Enhancing damping characteristic is one of the effective methods to solve the instability problem of the rotor system.The three-dimensional numerical analysis model of scallop damper seal was established,and the effects of inlet pressures,preswirl ratios,rotational speeds,interlaced angles and seal cavity depths on the rotordynamic characteristics of scallop damper seal were studied based on dynamic mesh method and multi-frequencies elliptic whirling model.Results show that the direct stiffness of the scallop damper seal increases with decreasing inlet pressure and increasing rotational speed and cavity depth.When the seal cavity is interlaced by a certain angle,which shows positive direct stiffness.The effective damping of the scallop damper seal increases with the increasing inlet pressure,the decreasing preswirl ratio and the rotational speed and cavity depth.There exists an optimal interlaced angle to maximize the effective damping and the system stability.The leakage of the scallop damper seal is significantly reduced with decreasing inlet pressure.The preswirl will reduce the leakage flowrate,and the rotational speed has a slight effect on the leakage performance.The leakage of the scallop damper seal decreases with increasing seal cavity depth.
基金supported by National Natural Science Foundation of China(62004166)Natural Science Foundation of Ningbo(202003N4062)+1 种基金Natural Science Foundation of Zhejiang Province(LY23F040002)Aeronautical Science Foundation of China(20230008053003).
文摘Perforated structures are widely employed in MEMS devices for dissipation control,energy absorption,and performance optimization.Among these,the damping weakening effect is particularly intriguing,attracting considerable attention and widespread application.Evaluating the impact of perforations on damping is crucial for enhancing the performance of MEMS devices.This paper investigates the damping tuning mechanisms of perforations and presents two theoretical models for accurately predicting viscous damping.The two models exhibit unique advantages under high and low perforation ratios,respectively.Both models account for complex boundary conditions and various hole geometries,including cylindrical,conical,prismatic,and trapezoidal holes.Modeling and simulations demonstrate the complementarity of the two models,enabling accurate viscous damping predictions across nearly all perforation ratios.Subsequently,the theoretical models are validated through a series of vibration tests on perforated oscillators,with errors consistently controlled within 10%.Experimental results demonstrate that perforations can easily achieve a damping reduction of more than one order of magnitude.Moreover,compared to normal cylindrical holes,trapezoidal holes are found to achieve superior damping reduction with a smaller sacrifice in surface area,which holds great potential for capacitive,acoustic,and optical MEMS devices.This work lays the foundation for viscous damping design and optimization of MEMS device dynamics,creating new applications.
基金This work was supported by the National Natural Science Foundation of China(No.51777021).
文摘With the rapid development of power electronic technology,many linear single-phase loads in low voltage systems are replaced by those equipped with a single-phase diode rectifier.Therefore,most of single-phase loads become harmonic producers due to their nonlinear characteristics.The nonlinear loads may have different damping characteristics at harmonic frequencies,which could cause potential power quality problems.This paper studies the harmonic damping characteristics of such nonlinear loads.The harmonic damping index is defined to quantify the damping effect of nonlinear loads of harmonic frequencies by analyzing the harmonic model.Then,the harmonic damping index of different harmonic frequencies is compared through simulations and experiments.The results show that the nonlinear load has a constant damping effect for the fundamental frequency,but a stronger damping for the harmonic frequencies.The harmonic damping of such nonlinear loads varies with the harmonic phase angle of supply voltage,and becomes stronger with the increase of the harmonic order.It implies that this type of nonlinear load may be beneficial to the suppression of harmonic resonance.
基金supported by the National Natural Science Foundation of China(Grant Nos.41961144014,51836010)the Chinese Universities Scientific Fund(Grant No.2019TC133).
文摘This work extends the sediment-laden mixture model with consideration of the turbulence damping and particle wake effects under the framework of improved efficiency and accuracy.The mixture model consists of the continuity and momentum equations for the sediment-laden mixture,and the continuity equation for the sediment.A theoretical formula is derived for the relative velocity between the water and sediment phases,with consideration of the effects of the pressure gradient,the shear stress and the lift force.A modified expression of the particle wake effect,inducing the local turbulence enhancement around the sediment particle,is employed to improve the turbulent diffusion of the coarse sediment.The k_(m)-ε_(m) model is proposed to close the mixture turbulence,with the turbulence damping effect due to the high sediment concentration expressed by the density-stratification term without an empirical parameter.The k_(m)-ε_(m) turbulence model requires smaller computational work and offers better results than an empirical density-stratification turbulence model in high sediment concentration cases.Consequently,with the proposed mixture model,the sediment transport in the open channel under a wide range of sediment sizes and concentrations can be revealed with the results in good agreement with experimental data for the velocity,the sediment concentration and the turbulent kinetic energy.
