In-situ layerwise imaging measurement of laser powder bed fusion(LPBF)provides a wealth of forming and defect data which enables monitoring of components quality and powder bed homogeneity.Using high-resolution camera...In-situ layerwise imaging measurement of laser powder bed fusion(LPBF)provides a wealth of forming and defect data which enables monitoring of components quality and powder bed homogeneity.Using high-resolution camera layerwise imaging and image processing algorithms to monitor fusion area and powder bed geometric defects has been studied by many researchers,which successfully monitored the contours of components and evaluated their accuracy.However,research for the methods of in-situ 3D contour measurement or component edge warping identification is rare.In this study,a 3D contour mea-surement method combining gray intensity and phase difference is proposed,and its accuracy is verified by designed experiments.The results show that the high-precision of the 3D contours can be achieved by the constructed energy minimization function.This method can detect the deviations of common ge-ometric features as well as warpage at LPBF component edges,and provides fundamental data for in-situ quality monitoring tools.展开更多
An electromagnetic parametrically excited rolling pendulum energy harvester with self-tuning mechanisms subject to multi-frequency excitation is proposed and investigated in this paper.The system consists of two uncou...An electromagnetic parametrically excited rolling pendulum energy harvester with self-tuning mechanisms subject to multi-frequency excitation is proposed and investigated in this paper.The system consists of two uncoupled rolling pendulum.The resonance frequency of each the rolling pendulum can be automatically tuned by adjusting its geometric parameters to access parametric resonance.This harvester can be used to harvest the energy at low frequency.A prototype is developed and evaluated.Its mathematical model is derived.A cam with rolling follower mechanism is employed to generate multi-frequency excitation.An experimental study is conducted to validate the proposed concept.The experimental results are confirmed by the numerical results.The harvester is successfully tuned when the angular velocity of the cam is changed from 1.149 to 1.236 Hz.展开更多
The pore configuration in porous medium is assumed to be the randomly distributed cube-like particles which can overlap each other in the periodic cubic domain, and the impact of particle characteristics on the percol...The pore configuration in porous medium is assumed to be the randomly distributed cube-like particles which can overlap each other in the periodic cubic domain, and the impact of particle characteristics on the percolation property of these cube-like particle packing systems is analyzed.Firstly, by combining the percolation models and finite-size scaling analysis, three numerical parameters(i.e., percolation transition width △L, local percolation threshold ψ_c(L), and correlation length exponent v) for the cube-like particle systems with shape parameter s in[1.0, +∞] are derived successively. Then, based on the relation between the percolation thresholdψ_c in infinite space and the local percolation threshold ψ_c(L), the corresponding ψ_c with s in[1.0, +∞] are further determined. It is shown from the study that the characteristics of cube-like particles have significant influence on the global percolation threshold ψ_c of the particle packing systems. As the parameter s increases from 1.0 to +∞, the percolation threshold ψ_c will go down persistently. When the surface of cube-like particles is cubical and spherical, respectively, the minimum and maximum thresholds ψ_c,min and ψ_c,max are obtained.展开更多
The acoustic emission (AE) features in rock fracture are simulated numerically with discrete element model (DEM). The specimen is constructed by using spherical particles bonded via the parallel bond model. As a resul...The acoustic emission (AE) features in rock fracture are simulated numerically with discrete element model (DEM). The specimen is constructed by using spherical particles bonded via the parallel bond model. As a result of the heterogeneity in rock specimen, the failure criterion of bonded particle is coupled by the shear and tensile strengths, which follow a normal probability distribution. The Kaiser effect is simulated in the fracture process, for a cubic rock specimen under uniaxial compression with a constant rate. The AE number is estimated with breakages of bonded particles using a pair of parameters, in the temporal and spatial scale, respectively. It is found that the AE numbers and the elastic energy release curves coincide. The range for the Kaiser effect from the AE number and the elastic energy release are the same. Furthermore, the frequency-magnitude relation of the AE number shows that the value of B determined with DEM is consistent with the experimental data.展开更多
Intergranular slow crack growth in zirconia polycrystal is described with a cohesive zone model that simulate mechanically the reaction-rupture mechanism underlying stress and environmentally assisted failure.A 2D pol...Intergranular slow crack growth in zirconia polycrystal is described with a cohesive zone model that simulate mechanically the reaction-rupture mechanism underlying stress and environmentally assisted failure.A 2D polycrystal is considered with cohesive surfaces inserted along the grain boundaries.The anisotropic elastic modulus and grain-to-grain misorientation are accounted for together with an initial stress state related to the processing.A minimum load threshold is shown to originate from the onset of the reaction-rupture mechanism to proceed where a minimum traction is reached locally and from the magnitude of the initial compression stresses.This work aims at providing reliable predictions in long lasting applications of ceramics.展开更多
This paper considers the stable coordination problem of two vehicles equipping with internal moving mass actuators.The coordinating and stabilizing control are derived by energy shaping. The proposed method is physica...This paper considers the stable coordination problem of two vehicles equipping with internal moving mass actuators.The coordinating and stabilizing control are derived by energy shaping. The proposed method is physically motivated and avoids cancelation or domination of nonlinearities.展开更多
Surface effect,which is attributed to the different environment surrounding the surface or near-surface atoms from that embracing the bulk atoms,may become significant when the surface-to-volume ratio of a body is lar...Surface effect,which is attributed to the different environment surrounding the surface or near-surface atoms from that embracing the bulk atoms,may become significant when the surface-to-volume ratio of a body is large.This paper considers the effect of a plane boundary of a piezoelectric body modeled as a thin layer with specified material properties,for which a transfer relation between the state vectors at the top and bottom surfaces is derived based on the state-space formulations.The equations of surface piezoelectricity for different orders without any bias field are then presented by making use of the power series expression of the transfer matrix.The surface effect is demonstrated by considering the Bleustein-Gulyaev wave existing in a horizontally polarized piezoceramic half-space.展开更多
Pre-stack migration imaging (PMI) method,which is used in geophysical exploration by the performance of single side detection and visually display,can be used to identify the location,orientation,and severity of damag...Pre-stack migration imaging (PMI) method,which is used in geophysical exploration by the performance of single side detection and visually display,can be used to identify the location,orientation,and severity of damages in concrete structure.In particular,this letter focuses on the experimental study by using a finite number of sensors for further practical applications.A concrete structure with a surface-mounted linear PZT transducers array is illustrated.Three types of damages,horizontal,dipping and V-shaped crack damage,have been studied.A pre-stack reverse time migration technique is used to back-propagate the scattering waves and to image damages in concrete structure.The migration results from the scattering waves of an artificial damage are presented.It is shown that the existence of the damage in concrete structure is correctly revealed through migration process.展开更多
Two-dimensional time-resolved particle image velocimetry(TR-PIV)and stereographic particle image velocimetry(SPIV)techniques were used to investigate the effect of leading-edge tubercles on the flow over low-aspect-ra...Two-dimensional time-resolved particle image velocimetry(TR-PIV)and stereographic particle image velocimetry(SPIV)techniques were used to investigate the effect of leading-edge tubercles on the flow over low-aspect-ratio wing models.The angle of attack is fixed at 10°,and the Reynolds number based on chord length is 5.8×10^(3).It is shown that the leading-edge tubercles can effectively mitigate flow separation in the model and also reduce the contribution of wake vortex to the fluctuating energy of flow.Counter-rotating vortex pairs(CVPs)initiated from the peak of leading-edge tubercles can promote nearby momentum exchange,enhance mixing of the flow and increase the energy contained in the boundary layer,which results in resisting the larger adverse pressure gradient.Therefore,it is concluded that CVPs play an important role in mitigating the flow separation for wings with leading-edge tubercles.展开更多
U-shaped micro-nanochannels can generate significant flow disturbance as well as locally amplified electric field, which gives itself potential to be microfluidic mixers, electrokinetic pumps,and even cell lysis proce...U-shaped micro-nanochannels can generate significant flow disturbance as well as locally amplified electric field, which gives itself potential to be microfluidic mixers, electrokinetic pumps,and even cell lysis process. Numerical simulation is utilized in this work to study the hidden characteristics of the U-shaped micro-nanochannel system, and the effects of key controlling parameters(the external voltage and pressure) on the device output metrics(current, maximum values of electric field, shear stress and flow velocity) were evaluated. A large portion of current flowing through the whole system goes through the nanochannels, rather than the middle part of the microchannel, with its value increasing linearly with the increase of voltage. Due to the local ion depletion near micro-nanofluidic junction, significantly enhanced electric field(as much as 15 fold at V=1 V and P_0=0) as well as strong shear stress(leading to electrokinetic flow) is generated.With increasing external pressure, both electric field and shear stress can be increased initially(due to shortening of depletion region length), but are suppressed eventually at higher pressure due to the destruction of ion depletion layer. Insights gained from this study could be useful for designing nonlinear electrokinetic pumps and other systems.展开更多
The linear and nonlinear dynamic responses of a vibratory ring gyroscope are investigated in this study focusing on the response mechanism of such a vibratory gyroscope. It is found that the nonlinear equations govern...The linear and nonlinear dynamic responses of a vibratory ring gyroscope are investigated in this study focusing on the response mechanism of such a vibratory gyroscope. It is found that the nonlinear equations governing the drive and sense directions are coupled through both inertial linear and geometric nonlinear terms. Nonlinear responses are studied based on the full coupled nonlinear dynamic equations. The varying amplitude on the sense direction is analyzed for different input angular rates. The effect of nonlinearity on the ring gyroscope system is performed by comparing the results of nonlinear responses to those of linear responses. The contributions of some parameters to the amplitude responses and gyroscope sensitivity are analyzed, the conclusions of which provide guidelines to improve the sensitivity of the vibratory ring gyroscopes.展开更多
We review the previous attempts of rational subgrid-scale(SGS) modelling by employing the Kolmogorov equation of filtered quantities. Aiming at explaining and solving the underlying problems in these models, we also i...We review the previous attempts of rational subgrid-scale(SGS) modelling by employing the Kolmogorov equation of filtered quantities. Aiming at explaining and solving the underlying problems in these models, we also introduce the recent methodological investigations for the rational SGS modelling technique by defining the terms of assumption and restriction. These methodological works are expected to provide instructive criterions for not only the rational SGS modelling, but also other types of SGS modelling practices.展开更多
Biological materials such as bone, tooth, and nacre are load-bearing nanocomposites composed of mineral and protein. Since the mineral crystals often have slender geometry, the nanocomposites are susceptible to buckle...Biological materials such as bone, tooth, and nacre are load-bearing nanocomposites composed of mineral and protein. Since the mineral crystals often have slender geometry, the nanocomposites are susceptible to buckle under the compressive load. In this paper, we analyze the local buckling behaviors of the nanocomposite structure of the biological materials using a beam-spring model by which we can consider plenty of mineral crystals and their interaction in our analysis compared with existing studies. We show that there is a transition of the buckling behaviors from a local buckling mode to a global one when we continuously increase the aspect ratio of mineral, leading to an increase of the buckling strength which levels off to the strength of the composites reinforced with continuous crystals. We find that the contact condition at the mineral tips has a striking effect on the local buckling mode at small aspect ratio, but the effect diminishes when the aspect ratio is large. Our analyses also show that the staggered arrangement of mineral plays a central role in the stability of the biological nanocomposites.展开更多
The wave-seabed interaction issue is of great importance for the design of foundation around marine infrastructures.Most previous investigations for such a problem have been limited to uncoupled or oneway coupled meth...The wave-seabed interaction issue is of great importance for the design of foundation around marine infrastructures.Most previous investigations for such a problem have been limited to uncoupled or oneway coupled methods connecting two separated wave and seabed sub models with the continuity of pressures at the seabed surface.In this study,a strongly coupled model was proposed to realize both wave and seabed processes in a same program and to calculate the wave fields and seabed response simultaneously.The information between wave fields and seabed fields were strongly shared and thus results in a more profound investigation of the mechanism of the wave-seabed interaction.In this letter,the wave and seabed models were validated with previous experimental tests.Then,a set of application of present model were discussed in prediction of the wave-induced seabed response.Numerical results show the wave-induced liquefaction area of coupled model is smaller than that of uncoupled model.展开更多
Open-loop flow control method was used to affect the development of a turbulent wake behind a D-shaped bluff body.Loud speakers were embedded inside the bluff body to produce two zero-net-massflux jets through 2 mm-wi...Open-loop flow control method was used to affect the development of a turbulent wake behind a D-shaped bluff body.Loud speakers were embedded inside the bluff body to produce two zero-net-massflux jets through 2 mm-wide span-wise slots located along the upper and lower edges on the rear wall.The drag forces for different actuation amplitudes(C_μ the ratio between the momentum of the actuating jets and the moment deficit caused by the bluff body) and frequencies(Sr_A) were examined.The effects of the phase difference in the two jets(0 and π) were also studied.It was found that when C_μ was 0.1%,a drag reduction up to 5%was achieved when the velocities of the two jets varied in phase at a frequency of St_A = 0.16.When the velocities of the two jets varied rr out of phase,significant drag increase was observed.展开更多
Defects in kinematic joints can sometimes highly influence the simulation response of the whole multibody system within which these joints are included. For instance, the clearance, the friction, the lubrication and t...Defects in kinematic joints can sometimes highly influence the simulation response of the whole multibody system within which these joints are included. For instance, the clearance, the friction, the lubrication and the flexibility affect the transient behaviour, reduce the component life and produce noise and vibration for classical joints such as prismatics, cylindrics or universal joints. In this work, a new 3D cylindrical joint model which accounts for the clearance, the misalignment and the friction is presented. This formulation has been used to represent the link between the planetgears and the planet carrier in an automotive differential model.展开更多
Long waves generated by a moving atmospheric pressure distribution,associated with a storm,in coastal region are investigated numerically.For simplicity the moving atmospheric pressure is assumed to be moving only in ...Long waves generated by a moving atmospheric pressure distribution,associated with a storm,in coastal region are investigated numerically.For simplicity the moving atmospheric pressure is assumed to be moving only in the alongshore direction and the beach slope is assumed to be a constant in the on-offshore direction.By solving the linear shallow water equations we obtain numerical solutions for a wide range of physical parameters,including storm size(2a),storm speed(U),and beach slope(α).Based on the numerical results,it is determined that edge wave packets are generated if the storm speed is equal to or greater than the critical velocity,U cr,which is defined as the phase speed of the fundamental edge wave mode whose wavelength is scaled by the width of the storm size.The length and the location of the positively moving edge wave packet is roughly Ut/2 ≤ y ≤ Ut,where y is in the alongshore direction and t is the time.Once the edge wave packet is generated,the wavelength is the same as that of the fundamental edge wave mode corresponding to the storm speed and is independent of the storm size,which can,however,affect the wave amplitude.When the storm speed is less than the critical velocity,the primary surface signature is a depression directly correlated to the atmospheric pressure distribution.展开更多
This letter obtains a typhoon-like vortex solution by solving the Bragg-Hawthorne equation.The solution describes spiral paths of fluid material element on the Bernoulli surface,whereas some new exact solutions are ob...This letter obtains a typhoon-like vortex solution by solving the Bragg-Hawthorne equation.The solution describes spiral paths of fluid material element on the Bernoulli surface,whereas some new exact solutions are obtained which are bounded in the whole region.The first one is a continued umbrella vortex solution,which is a typhoon-like vortex.The second one is a multi-planar solution,which is periodic in z-coordinate.Within each layer,there is an umbrella vortex solution similar to the first one.The above explicit solutions can be applied to the study of radial structure of typhoon.Both the solutions and the approaches used in the present work can also be applied to other complex flows.展开更多
We consider a fluid stirred by the locomotions of squirmers through it and generalize the stochastic hydrodynamic model proposed by Thiffeault and Childress,Phys.Lett.A(2010)and Lin et al.,J.Fluid Mech.(2011)to the ca...We consider a fluid stirred by the locomotions of squirmers through it and generalize the stochastic hydrodynamic model proposed by Thiffeault and Childress,Phys.Lett.A(2010)and Lin et al.,J.Fluid Mech.(2011)to the case in which the swimmers move in anisotropically random directions.A non-diagonal effective diffusivity tensor is derived with which the diffusive preference of a passive particle along any given direction can be computed to provide more details of the phenomena beyond scalar statistics.We further identify a fraction from the orthogonal decomposition of the drift-induced particle displacement to distinguish the underlying nonlinear mixing mechanism for different types of swimmers.Numerical simulations verify the analytical results with explicit examples of prescribed,anisotropic stirring motions.We also connect our formulation to several measures used in clinical medical research such as diffusion tensor imaging where anisotropic diffusion has a significant consequence.展开更多
Trailing edge serrations(TESs)are capable of noticeably suppressing the turbulent trailing edge noise induced by rotating wind turbine blades and become an integral part of a blade.However,the challenges involved in t...Trailing edge serrations(TESs)are capable of noticeably suppressing the turbulent trailing edge noise induced by rotating wind turbine blades and become an integral part of a blade.However,the challenges involved in the dimensional design of serration height 2 h,wavelengthλand flap angleϕare yet to be dealt with in a satisfactory manner.To address the problem,a general model for simulating the effects of serrations on the hydrodynamic and aeroacoustic performance is proposed due to its ease of use and relatively low requirements for user input.The solid serrations are replicated by momentum sources calculated by its aerodynamic forces.Then,a case relevant to wind turbine airfoil is examined,a hybrid improved delay detached eddy simulation(IDDES)method coupled with FW-H integration is deployed to obtain the flow features and far-field sound pressure level.It is found that the modeling method could reproduce the flow field and noise as serrated airfoil.展开更多
基金This work was supported by the foundation of Key Research and Development Program of Hubei Province(2020BAB137)Shen-zhen Fundamental Research Program(JCYJ20210324142007022).
文摘In-situ layerwise imaging measurement of laser powder bed fusion(LPBF)provides a wealth of forming and defect data which enables monitoring of components quality and powder bed homogeneity.Using high-resolution camera layerwise imaging and image processing algorithms to monitor fusion area and powder bed geometric defects has been studied by many researchers,which successfully monitored the contours of components and evaluated their accuracy.However,research for the methods of in-situ 3D contour measurement or component edge warping identification is rare.In this study,a 3D contour mea-surement method combining gray intensity and phase difference is proposed,and its accuracy is verified by designed experiments.The results show that the high-precision of the 3D contours can be achieved by the constructed energy minimization function.This method can detect the deviations of common ge-ometric features as well as warpage at LPBF component edges,and provides fundamental data for in-situ quality monitoring tools.
文摘An electromagnetic parametrically excited rolling pendulum energy harvester with self-tuning mechanisms subject to multi-frequency excitation is proposed and investigated in this paper.The system consists of two uncoupled rolling pendulum.The resonance frequency of each the rolling pendulum can be automatically tuned by adjusting its geometric parameters to access parametric resonance.This harvester can be used to harvest the energy at low frequency.A prototype is developed and evaluated.Its mathematical model is derived.A cam with rolling follower mechanism is employed to generate multi-frequency excitation.An experimental study is conducted to validate the proposed concept.The experimental results are confirmed by the numerical results.The harvester is successfully tuned when the angular velocity of the cam is changed from 1.149 to 1.236 Hz.
基金financially supported by the National Natural Science Foundation of China (Grants 51878152 and 51461135001)the Ministry of Science and Technology of China "973 Project" (Grant 2015CB655102)
文摘The pore configuration in porous medium is assumed to be the randomly distributed cube-like particles which can overlap each other in the periodic cubic domain, and the impact of particle characteristics on the percolation property of these cube-like particle packing systems is analyzed.Firstly, by combining the percolation models and finite-size scaling analysis, three numerical parameters(i.e., percolation transition width △L, local percolation threshold ψ_c(L), and correlation length exponent v) for the cube-like particle systems with shape parameter s in[1.0, +∞] are derived successively. Then, based on the relation between the percolation thresholdψ_c in infinite space and the local percolation threshold ψ_c(L), the corresponding ψ_c with s in[1.0, +∞] are further determined. It is shown from the study that the characteristics of cube-like particles have significant influence on the global percolation threshold ψ_c of the particle packing systems. As the parameter s increases from 1.0 to +∞, the percolation threshold ψ_c will go down persistently. When the surface of cube-like particles is cubical and spherical, respectively, the minimum and maximum thresholds ψ_c,min and ψ_c,max are obtained.
基金supported by the National Basic Research Program of China (2010CB731502)
文摘The acoustic emission (AE) features in rock fracture are simulated numerically with discrete element model (DEM). The specimen is constructed by using spherical particles bonded via the parallel bond model. As a result of the heterogeneity in rock specimen, the failure criterion of bonded particle is coupled by the shear and tensile strengths, which follow a normal probability distribution. The Kaiser effect is simulated in the fracture process, for a cubic rock specimen under uniaxial compression with a constant rate. The AE number is estimated with breakages of bonded particles using a pair of parameters, in the temporal and spatial scale, respectively. It is found that the AE numbers and the elastic energy release curves coincide. The range for the Kaiser effect from the AE number and the elastic energy release are the same. Furthermore, the frequency-magnitude relation of the AE number shows that the value of B determined with DEM is consistent with the experimental data.
文摘Intergranular slow crack growth in zirconia polycrystal is described with a cohesive zone model that simulate mechanically the reaction-rupture mechanism underlying stress and environmentally assisted failure.A 2D polycrystal is considered with cohesive surfaces inserted along the grain boundaries.The anisotropic elastic modulus and grain-to-grain misorientation are accounted for together with an initial stress state related to the processing.A minimum load threshold is shown to originate from the onset of the reaction-rupture mechanism to proceed where a minimum traction is reached locally and from the magnitude of the initial compression stresses.This work aims at providing reliable predictions in long lasting applications of ceramics.
基金supported by the National Natural Science Foundation of China(11072002,10832006 )
文摘This paper considers the stable coordination problem of two vehicles equipping with internal moving mass actuators.The coordinating and stabilizing control are derived by energy shaping. The proposed method is physically motivated and avoids cancelation or domination of nonlinearities.
基金supported by the National Natural Science Foundation of China (11090333,10832009 and10725210)
文摘Surface effect,which is attributed to the different environment surrounding the surface or near-surface atoms from that embracing the bulk atoms,may become significant when the surface-to-volume ratio of a body is large.This paper considers the effect of a plane boundary of a piezoelectric body modeled as a thin layer with specified material properties,for which a transfer relation between the state vectors at the top and bottom surfaces is derived based on the state-space formulations.The equations of surface piezoelectricity for different orders without any bias field are then presented by making use of the power series expression of the transfer matrix.The surface effect is demonstrated by considering the Bleustein-Gulyaev wave existing in a horizontally polarized piezoceramic half-space.
基金supported by the National High-tech R&D Program (863 Program) (2009AA03Z107)NSFC Program (50875114)the Major Project of the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (10KJA140006)
文摘Pre-stack migration imaging (PMI) method,which is used in geophysical exploration by the performance of single side detection and visually display,can be used to identify the location,orientation,and severity of damages in concrete structure.In particular,this letter focuses on the experimental study by using a finite number of sensors for further practical applications.A concrete structure with a surface-mounted linear PZT transducers array is illustrated.Three types of damages,horizontal,dipping and V-shaped crack damage,have been studied.A pre-stack reverse time migration technique is used to back-propagate the scattering waves and to image damages in concrete structure.The migration results from the scattering waves of an artificial damage are presented.It is shown that the existence of the damage in concrete structure is correctly revealed through migration process.
基金financial support from the National Natural Science Foundation of China under grand No.11721202。
文摘Two-dimensional time-resolved particle image velocimetry(TR-PIV)and stereographic particle image velocimetry(SPIV)techniques were used to investigate the effect of leading-edge tubercles on the flow over low-aspect-ratio wing models.The angle of attack is fixed at 10°,and the Reynolds number based on chord length is 5.8×10^(3).It is shown that the leading-edge tubercles can effectively mitigate flow separation in the model and also reduce the contribution of wake vortex to the fluctuating energy of flow.Counter-rotating vortex pairs(CVPs)initiated from the peak of leading-edge tubercles can promote nearby momentum exchange,enhance mixing of the flow and increase the energy contained in the boundary layer,which results in resisting the larger adverse pressure gradient.Therefore,it is concluded that CVPs play an important role in mitigating the flow separation for wings with leading-edge tubercles.
基金supported by the Intergovernmental International Science,Technology and Innovation Cooperation Key Project of the National Key R&D Programme(2016YFE0105900)the National Natural Science Foundation of China(21576130and 11372229)Kuwait Foundation for the Advancement of Sciences(Kuwait-MIT signature project,Project code:P31475EC01)
文摘U-shaped micro-nanochannels can generate significant flow disturbance as well as locally amplified electric field, which gives itself potential to be microfluidic mixers, electrokinetic pumps,and even cell lysis process. Numerical simulation is utilized in this work to study the hidden characteristics of the U-shaped micro-nanochannel system, and the effects of key controlling parameters(the external voltage and pressure) on the device output metrics(current, maximum values of electric field, shear stress and flow velocity) were evaluated. A large portion of current flowing through the whole system goes through the nanochannels, rather than the middle part of the microchannel, with its value increasing linearly with the increase of voltage. Due to the local ion depletion near micro-nanofluidic junction, significantly enhanced electric field(as much as 15 fold at V=1 V and P_0=0) as well as strong shear stress(leading to electrokinetic flow) is generated.With increasing external pressure, both electric field and shear stress can be increased initially(due to shortening of depletion region length), but are suppressed eventually at higher pressure due to the destruction of ion depletion layer. Insights gained from this study could be useful for designing nonlinear electrokinetic pumps and other systems.
基金supported in part by the National Natural Science Foundation of China (11672007, 11672189, and 11290152)the Beijing Natural Science Foundation (3172003)
文摘The linear and nonlinear dynamic responses of a vibratory ring gyroscope are investigated in this study focusing on the response mechanism of such a vibratory gyroscope. It is found that the nonlinear equations governing the drive and sense directions are coupled through both inertial linear and geometric nonlinear terms. Nonlinear responses are studied based on the full coupled nonlinear dynamic equations. The varying amplitude on the sense direction is analyzed for different input angular rates. The effect of nonlinearity on the ring gyroscope system is performed by comparing the results of nonlinear responses to those of linear responses. The contributions of some parameters to the amplitude responses and gyroscope sensitivity are analyzed, the conclusions of which provide guidelines to improve the sensitivity of the vibratory ring gyroscopes.
基金supported by the National Natural Science Foundation of China (11772032, 11572025, and 51420105008)
文摘We review the previous attempts of rational subgrid-scale(SGS) modelling by employing the Kolmogorov equation of filtered quantities. Aiming at explaining and solving the underlying problems in these models, we also introduce the recent methodological investigations for the rational SGS modelling technique by defining the terms of assumption and restriction. These methodological works are expected to provide instructive criterions for not only the rational SGS modelling, but also other types of SGS modelling practices.
基金supported by the National Natural Science Foundation of China(11025208,11372042,and 11221202)
文摘Biological materials such as bone, tooth, and nacre are load-bearing nanocomposites composed of mineral and protein. Since the mineral crystals often have slender geometry, the nanocomposites are susceptible to buckle under the compressive load. In this paper, we analyze the local buckling behaviors of the nanocomposite structure of the biological materials using a beam-spring model by which we can consider plenty of mineral crystals and their interaction in our analysis compared with existing studies. We show that there is a transition of the buckling behaviors from a local buckling mode to a global one when we continuously increase the aspect ratio of mineral, leading to an increase of the buckling strength which levels off to the strength of the composites reinforced with continuous crystals. We find that the contact condition at the mineral tips has a striking effect on the local buckling mode at small aspect ratio, but the effect diminishes when the aspect ratio is large. Our analyses also show that the staggered arrangement of mineral plays a central role in the stability of the biological nanocomposites.
基金supported by the National Natural Science Foundation of China(41176073)
文摘The wave-seabed interaction issue is of great importance for the design of foundation around marine infrastructures.Most previous investigations for such a problem have been limited to uncoupled or oneway coupled methods connecting two separated wave and seabed sub models with the continuity of pressures at the seabed surface.In this study,a strongly coupled model was proposed to realize both wave and seabed processes in a same program and to calculate the wave fields and seabed response simultaneously.The information between wave fields and seabed fields were strongly shared and thus results in a more profound investigation of the mechanism of the wave-seabed interaction.In this letter,the wave and seabed models were validated with previous experimental tests.Then,a set of application of present model were discussed in prediction of the wave-induced seabed response.Numerical results show the wave-induced liquefaction area of coupled model is smaller than that of uncoupled model.
基金supported by the National Basic Research Program(2014CB744100)State Key Laboratory of Aerodynamics(SKLA20130102)Dalian University of Technology(DUT14LK07)
文摘Open-loop flow control method was used to affect the development of a turbulent wake behind a D-shaped bluff body.Loud speakers were embedded inside the bluff body to produce two zero-net-massflux jets through 2 mm-wide span-wise slots located along the upper and lower edges on the rear wall.The drag forces for different actuation amplitudes(C_μ the ratio between the momentum of the actuating jets and the moment deficit caused by the bluff body) and frequencies(Sr_A) were examined.The effects of the phase difference in the two jets(0 and π) were also studied.It was found that when C_μ was 0.1%,a drag reduction up to 5%was achieved when the velocities of the two jets varied in phase at a frequency of St_A = 0.16.When the velocities of the two jets varied rr out of phase,significant drag increase was observed.
基金the Belgian National Fund for Scientific research (FRIA) for its financial support
文摘Defects in kinematic joints can sometimes highly influence the simulation response of the whole multibody system within which these joints are included. For instance, the clearance, the friction, the lubrication and the flexibility affect the transient behaviour, reduce the component life and produce noise and vibration for classical joints such as prismatics, cylindrics or universal joints. In this work, a new 3D cylindrical joint model which accounts for the clearance, the misalignment and the friction is presented. This formulation has been used to represent the link between the planetgears and the planet carrier in an automotive differential model.
基金supported by an NSF grant to Cornell University,the China Scholarship Council and a Korean government MLTMA grant Development of Korea Operational Oceanographic System (KOOS) to KORDI
文摘Long waves generated by a moving atmospheric pressure distribution,associated with a storm,in coastal region are investigated numerically.For simplicity the moving atmospheric pressure is assumed to be moving only in the alongshore direction and the beach slope is assumed to be a constant in the on-offshore direction.By solving the linear shallow water equations we obtain numerical solutions for a wide range of physical parameters,including storm size(2a),storm speed(U),and beach slope(α).Based on the numerical results,it is determined that edge wave packets are generated if the storm speed is equal to or greater than the critical velocity,U cr,which is defined as the phase speed of the fundamental edge wave mode whose wavelength is scaled by the width of the storm size.The length and the location of the positively moving edge wave packet is roughly Ut/2 ≤ y ≤ Ut,where y is in the alongshore direction and t is the time.Once the edge wave packet is generated,the wavelength is the same as that of the fundamental edge wave mode corresponding to the storm speed and is independent of the storm size,which can,however,affect the wave amplitude.When the storm speed is less than the critical velocity,the primary surface signature is a depression directly correlated to the atmospheric pressure distribution.
基金supported by the National Basic Research Program of China(2007CB816004)Innovation Program of the Chinese Academy of Sciences (KZCX2-YW-QN514)
文摘This letter obtains a typhoon-like vortex solution by solving the Bragg-Hawthorne equation.The solution describes spiral paths of fluid material element on the Bernoulli surface,whereas some new exact solutions are obtained which are bounded in the whole region.The first one is a continued umbrella vortex solution,which is a typhoon-like vortex.The second one is a multi-planar solution,which is periodic in z-coordinate.Within each layer,there is an umbrella vortex solution similar to the first one.The above explicit solutions can be applied to the study of radial structure of typhoon.Both the solutions and the approaches used in the present work can also be applied to other complex flows.
基金supported by the National Natural Science Foundation of China(Grant No.12071429).
文摘We consider a fluid stirred by the locomotions of squirmers through it and generalize the stochastic hydrodynamic model proposed by Thiffeault and Childress,Phys.Lett.A(2010)and Lin et al.,J.Fluid Mech.(2011)to the case in which the swimmers move in anisotropically random directions.A non-diagonal effective diffusivity tensor is derived with which the diffusive preference of a passive particle along any given direction can be computed to provide more details of the phenomena beyond scalar statistics.We further identify a fraction from the orthogonal decomposition of the drift-induced particle displacement to distinguish the underlying nonlinear mixing mechanism for different types of swimmers.Numerical simulations verify the analytical results with explicit examples of prescribed,anisotropic stirring motions.We also connect our formulation to several measures used in clinical medical research such as diffusion tensor imaging where anisotropic diffusion has a significant consequence.
基金This work was supported by the National Natural Science Foundation of China(Grant No.51736008)“Transformational Technologies for Clean Energy and Demonstration”,Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDA21050303).
文摘Trailing edge serrations(TESs)are capable of noticeably suppressing the turbulent trailing edge noise induced by rotating wind turbine blades and become an integral part of a blade.However,the challenges involved in the dimensional design of serration height 2 h,wavelengthλand flap angleϕare yet to be dealt with in a satisfactory manner.To address the problem,a general model for simulating the effects of serrations on the hydrodynamic and aeroacoustic performance is proposed due to its ease of use and relatively low requirements for user input.The solid serrations are replicated by momentum sources calculated by its aerodynamic forces.Then,a case relevant to wind turbine airfoil is examined,a hybrid improved delay detached eddy simulation(IDDES)method coupled with FW-H integration is deployed to obtain the flow features and far-field sound pressure level.It is found that the modeling method could reproduce the flow field and noise as serrated airfoil.
