Theoretical and experimental studies associated with electric field effectson the stability and transport are briefly surveyed. The effects of radial electric field on thesuppression and/or enhancement of various micr...Theoretical and experimental studies associated with electric field effectson the stability and transport are briefly surveyed. The effects of radial electric field on thesuppression and/or enhancement of various microinstabilities such as drift waves, flute mode andtemperature gradient modes are discussed. The suppression of flow shear on the electron temperaturegradient mode in plasmas with slightly hollow density profiles is investigated by solving thegyrokinetic integral eigenvalue equation. Comparison between theoretical predictions andexperimental observations based on the HIBP measurements with high temporal and spatial resolutionsis made in bumpy tori and heliotron (CHS) devices.展开更多
An investigation is carried out for understanding the properties of ion–acoustic（IA） solitary waves in an inhomogeneous magnetized electron-ion plasma with field-aligned sheared flow under the impact of q-nonextens...An investigation is carried out for understanding the properties of ion–acoustic（IA） solitary waves in an inhomogeneous magnetized electron-ion plasma with field-aligned sheared flow under the impact of q-nonextensive trapped electrons. The Schamel equation and its stationary solution in the form of solitary waves are obtained for this inhomogeneous plasma. It is shown that the amplitude of IA solitary waves increases with higher trapping efficiency（β）, while the width remains almost the same. Further, it is found that the amplitude of the solitary waves decreases with enhanced normalized drift speed, shear flow parameter and the population of the energetic particles. The size of the nonlinear solitary structures is calculated to be a few hundred meters and it is pointed out that the present results are useful to understand the solar wind plasma.展开更多
This article presents a study we have made of one class of coherent structures of the tripolar vortex. Considering the sheared flow and sheared magnetic field which are common in the thermonuclear plasma and space pla...This article presents a study we have made of one class of coherent structures of the tripolar vortex. Considering the sheared flow and sheared magnetic field which are common in the thermonuclear plasma and space plasma, we have simulated the dynamics of the tripolar vortex. The results show that the tripolar vortex is largely stable in most cases, but a strongly sheared magnetic field will make the structure less stable, and lead it to decays into single vortices with the large space scale. These results are consistent with findings from former research about the dipolar vortex.展开更多
The position deviation of the underwater manipulator generated by vortex-induced vibration(VIV)in the shear flow increases relative to that in the uniform flow.Thus,this study established an experimental platform to i...The position deviation of the underwater manipulator generated by vortex-induced vibration(VIV)in the shear flow increases relative to that in the uniform flow.Thus,this study established an experimental platform to investigate the vibration characteristics of the underwater manipulator under shear flow.The vibration response along the manipulator was obtained and compared with that in the uniform flow.Results indicated that the velocity,test height,and flow field were the main factors affecting the VIV of the underwater manipulator.With the increase in the reduced velocity(U_(r)),the dimensionless amplitudes increased rapidly in the in-line(IL)direction with a maximum of 0.13D.The vibration responses in the cross-flow(CF)and IL directions were concentrated at positions 2,3 and positions 1,2,with peak values of 0.46 and 0.54 mm under U_(r)=1.54,respectively.In addition,the vibration frequency increased with the reduction of velocity.The dimensionless dominant frequency in the CF and IL directions varied from 0.39-0.80 and 0.35-0.64,respectively.Moreover,the ratio of the CF and IL directions was close to 1 at a lower U_(r).The standard deviation of displacement initially increased and then decreased as the height of the test location increased.The single peak value of the standard deviation showed that VIV presented a single mode.Compared with the uniform flow,the maximum and average values of VIV displacement increased by 104%and 110%under the shear flow,respectively.展开更多
Rock avalanches frequently lead to catastrophic consequences due to their unpredictably high mobility.Numerous researchers have studied the shear behavior of granular materials under various conditions,attributing the...Rock avalanches frequently lead to catastrophic consequences due to their unpredictably high mobility.Numerous researchers have studied the shear behavior of granular materials under various conditions,attributing the high mobility to ultralow resistance.However,the underlying physical mechanism of frictional weakening remains unclear.This study utilizes the discrete element method(DEM)incorporating the fragment replacement model to simulate plane shear flows under various normal stresses(0.2 e1.2 MPa)and shear velocities(0.01e2 m/s).The findings reveal a localized shear band characterized by a J-shaped velocity profile and high granular temperature,and a concentrated distribution of weak contact forces forms at a shear velocity exceeding 0.1 m/s and normal stress above 0.6 MPa.Moreover,frictional weakening is observed with increasing normal stress from 0.2 MPa to 1.2 MPa and increasing shear velocity from 0.1 m/s to 2 m/s.The evolution of the steady-state friction coefficient can be divided into two stages:an initial stage(I)and a weakening stage(II).During stage I,the steady-state friction coefficient slightly increases until reaching a peak value.However,upon entering stage II,it gradually decreases and approaches an ultimate value.The velocity-and normal stress-dependent frictional weakening can be attributed to shear localization and embedded packing structure induced by particle breakage,respectively.Finally,an optimized m(I)model is proposed to capture the full evolution of the friction coefficient with the shear strain rate,which can improve our understanding of rock avalanche dynamics.展开更多
This study conducted the experimental investigation of aerodynamic heating of Micro-scale Rotational Shearing Flow with Axial Limited-Length(MRSFALL).The temperature riseof the stator is captured by the high response ...This study conducted the experimental investigation of aerodynamic heating of Micro-scale Rotational Shearing Flow with Axial Limited-Length(MRSFALL).The temperature riseof the stator is captured by the high response thermocouples.The eccentricity ratio and clearanceheight are guaranteed by means of instantaneous trajectory and torsion monitoring of the rotator.The result shows that the maximum temperature rise takes place upstream of the minimum clear-ance height along circumferential direction.The distribution of temperature rise presents asymmet-ric curve along axial direction,and peak value occurs near the dimensionless axial position of-0.18.The effect of aerodynamic heating becomes notable as the rotational speed is larger than3×10^(4)r/min.The effect of end leakage and the viscous dissipation have great impact on temper-ature rise of MRSFALL.More specially,the peak value of temperature rise at dimensionless clear-ance height of 0.0080 is larger than the case at dimensionless clearance height of 0.0044.Furthermore,when the eccentricity ratio is too large,the viscous dissipation is induced,and theadditional temperature rise is achieved.The heat flux identification of shear flow has been realizedby Sequential Function Specification Method(SFSM)and its estimation of thermal load has been given.The heat flux induced by the aerodynamic heating in this study varies from 950 W/m^(2)to1330 W/m^(2).展开更多
Within the magnetohydrodynamics （MHD） frame, we analyse the effect of viscosity on magneto-Rayleigh Taylor （MRT） instability in a Z-pinch configuration by using an exact method and an approximate method separately...Within the magnetohydrodynamics （MHD） frame, we analyse the effect of viscosity on magneto-Rayleigh Taylor （MRT） instability in a Z-pinch configuration by using an exact method and an approximate method separately. It is demonstrated that the plasma viscosity indeed has a stabilization effect on the MRT mode in the whole wavenumber region, and its influence increases with the perturbation wavenumber increasing. After the characteristics and feasibility of the approximate method have been investigated, we apply it to the stability analysis of viscous plasma where a sheared axial flow （SAF） is involved, and we attain an analytical dispersion relation. It is suggested that the viscosity and the SAF are complemental with each other, and a wide wavenumber range of perturbation is possible to be restrained if the SAF and the viscosity are large enough. Finally, we calculate the possible value of viscosity parameter according to the current experimental conditions, and the results show that since the value of viscosity is much less than the threshold value, its mitigation effect is small enough to be neglected. The role of the viscosity in the stabilization becomes considerable only if special techniques are so developed that the Z-pinch plasma viscosity can be increased greatly.展开更多
Acoustic propagation problems in the sheared mean flow are numerically investigated using different acoustic propagation equations , including linearized Euler equations ( LEE ) and acoustic perturbation equations ( A...Acoustic propagation problems in the sheared mean flow are numerically investigated using different acoustic propagation equations , including linearized Euler equations ( LEE ) and acoustic perturbation equations ( APE ) .The resulted acoustic pressure is compared for the cases of uniform mean flow and sheared mean flow using both APE and LEE.Numerical results show that interactions between acoustics and mean flow should be properly considered to better understand noise propagation problems , and the suitable option of the different acoustic equations is indicated by the present comparisons.Moreover , the ability of APE to predict acoustic propagation is validated.APE can replace LEE when the 3-D flow-induced noise problem is solved , thus computational cost can decrease.展开更多
A magnetohydrodynamic (MHD) formulation is derived to investigate and compare the mitigation effects of both the sheared axial flow and finite Larmor radius (FLR) on the Rayleigh-Taylor (RT) instability in Z-pinch imp...A magnetohydrodynamic (MHD) formulation is derived to investigate and compare the mitigation effects of both the sheared axial flow and finite Larmor radius (FLR) on the Rayleigh-Taylor (RT) instability in Z-pinch implosions. The sheared axial flow is introduced into MHD equations in a conventional way and the FLR effect into the equations via /t → -i(w+ik⊥2pi2Ωi,), as proposed in our previous paper [Chin. Phys. Lett. 2002, 19:217] , where k⊥2 pi2 is referred to FLR effect from the general kinetic theory of magnetized plasma. Therefore the linearized continuity and momentum equations for the perturbed mass-density and velocity include both the sheared axial flow and the FLR effect. It is found that the effect of sheared axial flow with a lower peak velocity can mitigate RT instability in the whole wavenumber region and the effect of sheared axial flow with a higher one can mitigate RT instability only in the large wavenumber region (for normalized wavenumber k】2.4); The effect of FLR can mitigate RT instability in the whole wavenumber region and the mitigation effect is stronger than that of the sheared axial flow with a lower peak velocity in the almost whole wavenumber region.展开更多
Problem of the axisymmetric toroidal equilibrium with pure sheared toroidal flow isinvolved. For standard tokamak equilibrium, general approximate solutions are analytically pur-sued for arbitrary current profile and ...Problem of the axisymmetric toroidal equilibrium with pure sheared toroidal flow isinvolved. For standard tokamak equilibrium, general approximate solutions are analytically pur-sued for arbitrary current profile and non-circular cross-section. Equilibrium properties includingthe fiow-induced density asymmetry are analyzed.展开更多
The nonlinear stability of plane parallel shear flows with respect to tilted perturbations is studied by energy methods.Tilted perturbation refers to the fact that perturbations form an angleθ∈(0,π/2)with the direc...The nonlinear stability of plane parallel shear flows with respect to tilted perturbations is studied by energy methods.Tilted perturbation refers to the fact that perturbations form an angleθ∈(0,π/2)with the direction of the basic flows.By defining an energy functional,it is proven that plane parallel shear flows are unconditionally nonlinearly exponentially stable for tilted streamwise perturbation when the Reynolds number is below a certain critical value and the boundary conditions are either rigid or stress-free.In the case of stress-free boundaries,by taking advantage of the poloidal-toroidal decomposition of a solenoidal field to define energy functionals,it can be even shown that plane parallel shear flows are unconditionally nonlinearly exponentially stable for all Reynolds numbers,where the tilted perturbation can be either spanwise or streamwise.展开更多
Investigating the ignition response of nitrate ester plasticized polyether(NEPE) propellant under dynamic extrusion loading is of great significant at least for two cases. Firstly, it helps to understand the mechanism...Investigating the ignition response of nitrate ester plasticized polyether(NEPE) propellant under dynamic extrusion loading is of great significant at least for two cases. Firstly, it helps to understand the mechanism and conditions of unwanted ignition inside charged propellant under accident stimulus.Secondly, evaluates the risk of a shell crevice in a solid rocket motor(SRM) under a falling or overturning scene. In the present study, an innovative visual crevice extrusion experiment is designed using a dropweight apparatus. The dynamic responses of NEPE propellant during extrusion loading, including compaction and compression, rapid shear flow into the crevice, stress concentration, and ignition reaction, have been firstly observed using a high-performance high-speed camera. The ignition reaction is observed in the triangular region of the NEPE propellant sample above the crevice when the drop weight velocity was 1.90 m/s. Based on the user material subroutine interface UMAT provided by finite element software LS-DYNA, a viscoelastic-plastic model and dual ignition criterion related to plastic shear dissipation are developed and applied to the local ignition response analysis under crevice extrusion conditions. The stress concentration occurs in the crevice location of the propellant sample, the shear stress is relatively large, the effective plastic work is relatively large, and the ignition reaction is easy to occur. When the sample thickness decreases from 5 mm to 2.5 mm, the shear stress increases from 22.3 MPa to 28.6 MPa, the critical value of effective plastic work required for ignition is shortened from 1280 μs to 730 μs, and the triangular area is easily triggering an ignition reaction. The propellant sample with a small thickness is more likely to stress concentration, resulting in large shear stress and effective work, triggering an ignition reaction.展开更多
An experimental system for forming a rotating paraboloid shaped shallow water with a free surface was conducted to study coherent vortex structures in a differentially rotating quasi two dimensional zonal flow.Flow...An experimental system for forming a rotating paraboloid shaped shallow water with a free surface was conducted to study coherent vortex structures in a differentially rotating quasi two dimensional zonal flow.Flow visualization and laser light scattering techniques were used to obtain the information of spatial flow patterns.Experimental results show that the coexistence of Coriolis effect and strong shear in latitudinal zones may lead to formation of coherent vortices.Power spectra analysis and photographs which were taken in a reference frame rotating with the observed vortices also justified the emergence,drift and evolution of persistent vortices on the large scale.Locked vortex state manifests the cyclone and anticyclone asymmetry.展开更多
Data-driven reduced-order modeling opens new avenues of understanding,predicting,controlling,and optimizing system behavior.Simple systems may have state spaces in which sparse human-interpretable dynamical systems ca...Data-driven reduced-order modeling opens new avenues of understanding,predicting,controlling,and optimizing system behavior.Simple systems may have state spaces in which sparse human-interpretable dynamical systems can be identified.This approach has been pioneered by Brunton et al.(2016,PNAS)with sparse identification of nonlinear dynamics.Complex systems,however,cannot be expected to benefit from such simple analytical descriptions.Yet,smoothness may be exploited by analytical local descriptions.In this paper,we identify a clusterwise polynomial dynamics from time-resolved snapshot data.The full state space is partitioned into clusters with a reduced-order polynomial description for each cluster and a global patching strategy.The resulting clusterwise modeling is entirely data-driven and requires no prior knowledge of the system dynamics.We illustrate the approach on the well-known chaotic Lorenz and Rössler systems,on the more challenging chaotic fluid flow dynamics of higher state-space dimensions,on a noisy electrocardiogram signal,and finally on the time evolution of the monthly sunspot number.Clusterwise modeling offers a powerful and interpretable paradigm for dynamical modeling.Nonlinear dynamics can be approximated by assembling many simple local models of different resolutions,opening new paths to understand and control intricate nonlinearities.展开更多
Langevin molecular dynamics simulations reveal the impact of confinement strength on the structure and dynamics of a two-dimensional complex plasma under constant shear force.Structural analysis via Voronoi diagrams a...Langevin molecular dynamics simulations reveal the impact of confinement strength on the structure and dynamics of a two-dimensional complex plasma under constant shear force.Structural analysis via Voronoi diagrams and the local bond-order parameter|Ψ6|shows that stronger confinement enhances hexagonal order and mitigates shear-induced disorder.Dynamical properties,determined by mean-square displacement(MSD)and the velocity autocorrelation function(VACF),indicate that the shear-induced superdiffusion weakens with increasing confinement strength.The entropy change(?S)shows that strong confinement(ω>1)balances particle dynamics between shear and shear-free regions,thereby stabilizing the system.These findings highlight the interplay between confinement and shear force.展开更多
In this paper,the liquid–vapor phase separation under viscous shear is investigated by using a pseudopotential central moment lattice Boltzmann method.Physically,the multiphase shear flow is governed by two competing...In this paper,the liquid–vapor phase separation under viscous shear is investigated by using a pseudopotential central moment lattice Boltzmann method.Physically,the multiphase shear flow is governed by two competing mechanisms:surface tension and shear force.It is interesting to find that the liquid tends to form a droplet when the surface tension dominates under conditions of low temperature,shear velocity,and viscosity,and in larger domain size.Otherwise,the liquid tends to form a band if shear force dominates.Moreover,the average density gradient is used as a physical criterion to distinguish the spinodal decomposition and domain growth.Both spatial and temporal changes of density are studied during the phase separation under shear.展开更多
A series of fully three-dimensional(3 D) numerical simulations of flow past a free-to-oscillate curved flexible riser in shear flow were conducted at Reynolds number of 185–1015. The numerical results obtained by the...A series of fully three-dimensional(3 D) numerical simulations of flow past a free-to-oscillate curved flexible riser in shear flow were conducted at Reynolds number of 185–1015. The numerical results obtained by the two-way fluid–structure interaction(FSI) simulations are in good agreement with the experimental results reported in the earlier study. It is further found that the frequency transition is out of phase not only in the inline(IL) and crossflow(CF) directions but also along the span direction. The mode competition leads to the non-zero nodes of the rootmean-square(RMS) amplitude and the relatively chaotic trajectories. The fluid–structure interaction is to some extent reflected by the transverse velocity of the ambient fluid, which reaches the maximum value when the riser reaches the equilibrium position. Moreover, the local maximum transverse velocities occur at the peak CF amplitudes, and the values are relatively large when the vibration is in the resonance regions. The 3 D vortex columns are shed nearly parallel to the axis of the curved flexible riser. As the local Reynolds number increases from 0 at the bottom of the riser to the maximum value at the top, the wake undergoes a transition from a two-dimensional structure to a 3 D one. More irregular small-scale vortices appeared at the wake region of the riser, undergoing large amplitude responses.展开更多
We compare the space-time correlations calculated from direct numerical simulation （DNS） and large-eddy simulation （LES） of turbulent channel flows. It is found from the comparisons that the LES with an eddy-visco...We compare the space-time correlations calculated from direct numerical simulation （DNS） and large-eddy simulation （LES） of turbulent channel flows. It is found from the comparisons that the LES with an eddy-viscosity subgrid scale （SGS） model over-predicts the space-time corre- lations than the DNS. The overpredictions are further quantified by the integral scales of directional correlations and convection velocities. A physical argument for the overpre- diction is provided that the eddy-viscosity SGS model alone does not includes the backscatter effects although it correctly represents the energy dissipations of SGS motions. This argument is confirmed by the recently developed elliptic model for space-time correlations in turbulent shear flows. It suggests that enstrophy is crucial to the LES prediction of spacetime correlations. The random forcing models and stochastic SGS models are proposed to overcome the overpredictions on space-time correlations.展开更多
This work presents a numerical investigation on steady internal, external and surface flows of a liquid sphere immersed in a simple shear flow at low and intermediate Reynolds numbers. The control volume formulation i...This work presents a numerical investigation on steady internal, external and surface flows of a liquid sphere immersed in a simple shear flow at low and intermediate Reynolds numbers. The control volume formulation is adopted to solve the governing equations of two-phase flow in a 3-D spherical coordinate system. Numerical results show that the streamlines for Re = 0 are closed Jeffery orbits on the surface of the liquid sphere, and also closed curves outside and inside the liquid sphere. However, the streamlines have intricate and non-closed structures for Re ≠ 0. The flow structure is dependent on the values of Reynolds number and interior-to-exterior viscosity ratio.展开更多
A time domain model is presented to study the vibrations of long slender cylinders placed in shear flow. Long slender cylinders such as risers and tension legs are widely used in the field of ocean engineering. They a...A time domain model is presented to study the vibrations of long slender cylinders placed in shear flow. Long slender cylinders such as risers and tension legs are widely used in the field of ocean engineering. They are subjected to vortex-induced vibrations（VIV） when placed within a transverse incident flow. A three dimensional model coupled with wake oscillators is formulated to describe the response of the slender cylinder in cross-flow and in-line directions. The wake oscillators are distributed along the cylinder and the vortex-shedding frequency is derived from the local current velocity. A non-linear fiuid force model is accounted for the coupled effect between cross-flow and in-line vibrations. The comparisons with the published experimental data show that the dynamic features of VIV of long slender cylinder placed in shear flow can be obtained by the proposed model,such as the spanwise average displacement,vibration frequency,dominant mode and the combination of standing and traveling waves. The simulation in a uniform flow is also conducted and the result is compared with the case of nonuniform flow. It is concluded that the flow shear characteristic has significantly changed the cylinder vibration behavior.展开更多
文摘Theoretical and experimental studies associated with electric field effectson the stability and transport are briefly surveyed. The effects of radial electric field on thesuppression and/or enhancement of various microinstabilities such as drift waves, flute mode andtemperature gradient modes are discussed. The suppression of flow shear on the electron temperaturegradient mode in plasmas with slightly hollow density profiles is investigated by solving thegyrokinetic integral eigenvalue equation. Comparison between theoretical predictions andexperimental observations based on the HIBP measurements with high temporal and spatial resolutionsis made in bumpy tori and heliotron (CHS) devices.
文摘An investigation is carried out for understanding the properties of ion–acoustic（IA） solitary waves in an inhomogeneous magnetized electron-ion plasma with field-aligned sheared flow under the impact of q-nonextensive trapped electrons. The Schamel equation and its stationary solution in the form of solitary waves are obtained for this inhomogeneous plasma. It is shown that the amplitude of IA solitary waves increases with higher trapping efficiency（β）, while the width remains almost the same. Further, it is found that the amplitude of the solitary waves decreases with enhanced normalized drift speed, shear flow parameter and the population of the energetic particles. The size of the nonlinear solitary structures is calculated to be a few hundred meters and it is pointed out that the present results are useful to understand the solar wind plasma.
基金The project supported by the National Natural Science Foundation of China (Nos. 10075047, 40336052)
文摘This article presents a study we have made of one class of coherent structures of the tripolar vortex. Considering the sheared flow and sheared magnetic field which are common in the thermonuclear plasma and space plasma, we have simulated the dynamics of the tripolar vortex. The results show that the tripolar vortex is largely stable in most cases, but a strongly sheared magnetic field will make the structure less stable, and lead it to decays into single vortices with the large space scale. These results are consistent with findings from former research about the dipolar vortex.
基金Supported by the National Natural Science Foundation of China(No.51905211)the“20 Regulations for New Universities”of Jinan(No.202228116).
文摘The position deviation of the underwater manipulator generated by vortex-induced vibration(VIV)in the shear flow increases relative to that in the uniform flow.Thus,this study established an experimental platform to investigate the vibration characteristics of the underwater manipulator under shear flow.The vibration response along the manipulator was obtained and compared with that in the uniform flow.Results indicated that the velocity,test height,and flow field were the main factors affecting the VIV of the underwater manipulator.With the increase in the reduced velocity(U_(r)),the dimensionless amplitudes increased rapidly in the in-line(IL)direction with a maximum of 0.13D.The vibration responses in the cross-flow(CF)and IL directions were concentrated at positions 2,3 and positions 1,2,with peak values of 0.46 and 0.54 mm under U_(r)=1.54,respectively.In addition,the vibration frequency increased with the reduction of velocity.The dimensionless dominant frequency in the CF and IL directions varied from 0.39-0.80 and 0.35-0.64,respectively.Moreover,the ratio of the CF and IL directions was close to 1 at a lower U_(r).The standard deviation of displacement initially increased and then decreased as the height of the test location increased.The single peak value of the standard deviation showed that VIV presented a single mode.Compared with the uniform flow,the maximum and average values of VIV displacement increased by 104%and 110%under the shear flow,respectively.
基金supported by the National Key Research and Development Program of China(Grant Nos.2023YFC3008300 and 2023YFC3008302)the National Natural Science Foundation of China(Grant No.U22A20603).
文摘Rock avalanches frequently lead to catastrophic consequences due to their unpredictably high mobility.Numerous researchers have studied the shear behavior of granular materials under various conditions,attributing the high mobility to ultralow resistance.However,the underlying physical mechanism of frictional weakening remains unclear.This study utilizes the discrete element method(DEM)incorporating the fragment replacement model to simulate plane shear flows under various normal stresses(0.2 e1.2 MPa)and shear velocities(0.01e2 m/s).The findings reveal a localized shear band characterized by a J-shaped velocity profile and high granular temperature,and a concentrated distribution of weak contact forces forms at a shear velocity exceeding 0.1 m/s and normal stress above 0.6 MPa.Moreover,frictional weakening is observed with increasing normal stress from 0.2 MPa to 1.2 MPa and increasing shear velocity from 0.1 m/s to 2 m/s.The evolution of the steady-state friction coefficient can be divided into two stages:an initial stage(I)and a weakening stage(II).During stage I,the steady-state friction coefficient slightly increases until reaching a peak value.However,upon entering stage II,it gradually decreases and approaches an ultimate value.The velocity-and normal stress-dependent frictional weakening can be attributed to shear localization and embedded packing structure induced by particle breakage,respectively.Finally,an optimized m(I)model is proposed to capture the full evolution of the friction coefficient with the shear strain rate,which can improve our understanding of rock avalanche dynamics.
基金supports from the National Natural Science Foundation of China(No.52206091)the Aeronautical Science Foundation of China(No.201928052008)the Natural Science Foundation of Jiangsu Province,China(No.BK20210303)。
文摘This study conducted the experimental investigation of aerodynamic heating of Micro-scale Rotational Shearing Flow with Axial Limited-Length(MRSFALL).The temperature riseof the stator is captured by the high response thermocouples.The eccentricity ratio and clearanceheight are guaranteed by means of instantaneous trajectory and torsion monitoring of the rotator.The result shows that the maximum temperature rise takes place upstream of the minimum clear-ance height along circumferential direction.The distribution of temperature rise presents asymmet-ric curve along axial direction,and peak value occurs near the dimensionless axial position of-0.18.The effect of aerodynamic heating becomes notable as the rotational speed is larger than3×10^(4)r/min.The effect of end leakage and the viscous dissipation have great impact on temper-ature rise of MRSFALL.More specially,the peak value of temperature rise at dimensionless clear-ance height of 0.0080 is larger than the case at dimensionless clearance height of 0.0044.Furthermore,when the eccentricity ratio is too large,the viscous dissipation is induced,and theadditional temperature rise is achieved.The heat flux identification of shear flow has been realizedby Sequential Function Specification Method(SFSM)and its estimation of thermal load has been given.The heat flux induced by the aerodynamic heating in this study varies from 950 W/m^(2)to1330 W/m^(2).
基金Project supported by the National Science Foundation of China (Grant Nos 10575014 and 10635050)
文摘Within the magnetohydrodynamics （MHD） frame, we analyse the effect of viscosity on magneto-Rayleigh Taylor （MRT） instability in a Z-pinch configuration by using an exact method and an approximate method separately. It is demonstrated that the plasma viscosity indeed has a stabilization effect on the MRT mode in the whole wavenumber region, and its influence increases with the perturbation wavenumber increasing. After the characteristics and feasibility of the approximate method have been investigated, we apply it to the stability analysis of viscous plasma where a sheared axial flow （SAF） is involved, and we attain an analytical dispersion relation. It is suggested that the viscosity and the SAF are complemental with each other, and a wide wavenumber range of perturbation is possible to be restrained if the SAF and the viscosity are large enough. Finally, we calculate the possible value of viscosity parameter according to the current experimental conditions, and the results show that since the value of viscosity is much less than the threshold value, its mitigation effect is small enough to be neglected. The role of the viscosity in the stabilization becomes considerable only if special techniques are so developed that the Z-pinch plasma viscosity can be increased greatly.
基金Supported by the National Natural Science Foundation of China(10902050)the China Postdoctoral Science Foundation Funded Project(20100481138)the Aeronautical Science Foundation of China(20101452017)
文摘Acoustic propagation problems in the sheared mean flow are numerically investigated using different acoustic propagation equations , including linearized Euler equations ( LEE ) and acoustic perturbation equations ( APE ) .The resulted acoustic pressure is compared for the cases of uniform mean flow and sheared mean flow using both APE and LEE.Numerical results show that interactions between acoustics and mean flow should be properly considered to better understand noise propagation problems , and the suitable option of the different acoustic equations is indicated by the present comparisons.Moreover , the ability of APE to predict acoustic propagation is validated.APE can replace LEE when the 3-D flow-induced noise problem is solved , thus computational cost can decrease.
基金This work was supported by the National Natural Science Foundation of China No.10035020.
文摘A magnetohydrodynamic (MHD) formulation is derived to investigate and compare the mitigation effects of both the sheared axial flow and finite Larmor radius (FLR) on the Rayleigh-Taylor (RT) instability in Z-pinch implosions. The sheared axial flow is introduced into MHD equations in a conventional way and the FLR effect into the equations via /t → -i(w+ik⊥2pi2Ωi,), as proposed in our previous paper [Chin. Phys. Lett. 2002, 19:217] , where k⊥2 pi2 is referred to FLR effect from the general kinetic theory of magnetized plasma. Therefore the linearized continuity and momentum equations for the perturbed mass-density and velocity include both the sheared axial flow and the FLR effect. It is found that the effect of sheared axial flow with a lower peak velocity can mitigate RT instability in the whole wavenumber region and the effect of sheared axial flow with a higher one can mitigate RT instability only in the large wavenumber region (for normalized wavenumber k】2.4); The effect of FLR can mitigate RT instability in the whole wavenumber region and the mitigation effect is stronger than that of the sheared axial flow with a lower peak velocity in the almost whole wavenumber region.
基金National Science Foundation (19975015) and the China Nuclear Science Foundation(Y7l00C030l).
文摘Problem of the axisymmetric toroidal equilibrium with pure sheared toroidal flow isinvolved. For standard tokamak equilibrium, general approximate solutions are analytically pur-sued for arbitrary current profile and non-circular cross-section. Equilibrium properties includingthe fiow-induced density asymmetry are analyzed.
基金supported by the National Natural Science Foundation of China(21627813)。
文摘The nonlinear stability of plane parallel shear flows with respect to tilted perturbations is studied by energy methods.Tilted perturbation refers to the fact that perturbations form an angleθ∈(0,π/2)with the direction of the basic flows.By defining an energy functional,it is proven that plane parallel shear flows are unconditionally nonlinearly exponentially stable for tilted streamwise perturbation when the Reynolds number is below a certain critical value and the boundary conditions are either rigid or stress-free.In the case of stress-free boundaries,by taking advantage of the poloidal-toroidal decomposition of a solenoidal field to define energy functionals,it can be even shown that plane parallel shear flows are unconditionally nonlinearly exponentially stable for all Reynolds numbers,where the tilted perturbation can be either spanwise or streamwise.
基金National Natural Science Foundation of China(U22B20131)State Key Laboratory of Explosion Science and Technology(QNKT23-10)for supporting this project.
文摘Investigating the ignition response of nitrate ester plasticized polyether(NEPE) propellant under dynamic extrusion loading is of great significant at least for two cases. Firstly, it helps to understand the mechanism and conditions of unwanted ignition inside charged propellant under accident stimulus.Secondly, evaluates the risk of a shell crevice in a solid rocket motor(SRM) under a falling or overturning scene. In the present study, an innovative visual crevice extrusion experiment is designed using a dropweight apparatus. The dynamic responses of NEPE propellant during extrusion loading, including compaction and compression, rapid shear flow into the crevice, stress concentration, and ignition reaction, have been firstly observed using a high-performance high-speed camera. The ignition reaction is observed in the triangular region of the NEPE propellant sample above the crevice when the drop weight velocity was 1.90 m/s. Based on the user material subroutine interface UMAT provided by finite element software LS-DYNA, a viscoelastic-plastic model and dual ignition criterion related to plastic shear dissipation are developed and applied to the local ignition response analysis under crevice extrusion conditions. The stress concentration occurs in the crevice location of the propellant sample, the shear stress is relatively large, the effective plastic work is relatively large, and the ignition reaction is easy to occur. When the sample thickness decreases from 5 mm to 2.5 mm, the shear stress increases from 22.3 MPa to 28.6 MPa, the critical value of effective plastic work required for ignition is shortened from 1280 μs to 730 μs, and the triangular area is easily triggering an ignition reaction. The propellant sample with a small thickness is more likely to stress concentration, resulting in large shear stress and effective work, triggering an ignition reaction.
文摘An experimental system for forming a rotating paraboloid shaped shallow water with a free surface was conducted to study coherent vortex structures in a differentially rotating quasi two dimensional zonal flow.Flow visualization and laser light scattering techniques were used to obtain the information of spatial flow patterns.Experimental results show that the coexistence of Coriolis effect and strong shear in latitudinal zones may lead to formation of coherent vortices.Power spectra analysis and photographs which were taken in a reference frame rotating with the observed vortices also justified the emergence,drift and evolution of persistent vortices on the large scale.Locked vortex state manifests the cyclone and anticyclone asymmetry.
基金supported by the National Natural Science Foundation of China(Grant Nos.12172109,12202121,and 12302293)the China Postdoctoral Science Foundation(Grant Nos.2023M730866 and 2023T160166)+1 种基金the Guangdong Basic and Applied Basic Research Foundation(Grant No.2022A1515011492)the Shenzhen Science and Technology Program(Grant Nos.JCYJ20220531095605012,KJZD20230923115210021,and 29853MKCJ202300205).
文摘Data-driven reduced-order modeling opens new avenues of understanding,predicting,controlling,and optimizing system behavior.Simple systems may have state spaces in which sparse human-interpretable dynamical systems can be identified.This approach has been pioneered by Brunton et al.(2016,PNAS)with sparse identification of nonlinear dynamics.Complex systems,however,cannot be expected to benefit from such simple analytical descriptions.Yet,smoothness may be exploited by analytical local descriptions.In this paper,we identify a clusterwise polynomial dynamics from time-resolved snapshot data.The full state space is partitioned into clusters with a reduced-order polynomial description for each cluster and a global patching strategy.The resulting clusterwise modeling is entirely data-driven and requires no prior knowledge of the system dynamics.We illustrate the approach on the well-known chaotic Lorenz and Rössler systems,on the more challenging chaotic fluid flow dynamics of higher state-space dimensions,on a noisy electrocardiogram signal,and finally on the time evolution of the monthly sunspot number.Clusterwise modeling offers a powerful and interpretable paradigm for dynamical modeling.Nonlinear dynamics can be approximated by assembling many simple local models of different resolutions,opening new paths to understand and control intricate nonlinearities.
基金supported by the National Natural Science Foundation of China(Grant No.12075315)。
文摘Langevin molecular dynamics simulations reveal the impact of confinement strength on the structure and dynamics of a two-dimensional complex plasma under constant shear force.Structural analysis via Voronoi diagrams and the local bond-order parameter|Ψ6|shows that stronger confinement enhances hexagonal order and mitigates shear-induced disorder.Dynamical properties,determined by mean-square displacement(MSD)and the velocity autocorrelation function(VACF),indicate that the shear-induced superdiffusion weakens with increasing confinement strength.The entropy change(?S)shows that strong confinement(ω>1)balances particle dynamics between shear and shear-free regions,thereby stabilizing the system.These findings highlight the interplay between confinement and shear force.
基金supported by National Natural Science Foundation of China under Grant No.51806116Guangdong Basic and Applied Basic Research Foundation under Grant No.2024A1515010927+2 种基金China Scholarship Council under Grant No.202306380288Humanities and Social Science Foundation of the Ministry of Education in China under Grant No.24YJCZH163Fundamental Research Funds for the Central Universities,Sun Yat-sen University under Grant No.24qnpy044。
文摘In this paper,the liquid–vapor phase separation under viscous shear is investigated by using a pseudopotential central moment lattice Boltzmann method.Physically,the multiphase shear flow is governed by two competing mechanisms:surface tension and shear force.It is interesting to find that the liquid tends to form a droplet when the surface tension dominates under conditions of low temperature,shear velocity,and viscosity,and in larger domain size.Otherwise,the liquid tends to form a band if shear force dominates.Moreover,the average density gradient is used as a physical criterion to distinguish the spinodal decomposition and domain growth.Both spatial and temporal changes of density are studied during the phase separation under shear.
基金financially supported by the National Natural Science Foundation of China(Grant Nos.11502220 and51479126)the Youth Science and Technology Foundation of Sichuan Province(Grant No.2017JQ0055)the Youth Scientific and Technological Innovation Team of the Safety of Deep-Water Pipe Strings of Southwest Petroleum University(Grant No.2017CXTD06)
文摘A series of fully three-dimensional(3 D) numerical simulations of flow past a free-to-oscillate curved flexible riser in shear flow were conducted at Reynolds number of 185–1015. The numerical results obtained by the two-way fluid–structure interaction(FSI) simulations are in good agreement with the experimental results reported in the earlier study. It is further found that the frequency transition is out of phase not only in the inline(IL) and crossflow(CF) directions but also along the span direction. The mode competition leads to the non-zero nodes of the rootmean-square(RMS) amplitude and the relatively chaotic trajectories. The fluid–structure interaction is to some extent reflected by the transverse velocity of the ambient fluid, which reaches the maximum value when the riser reaches the equilibrium position. Moreover, the local maximum transverse velocities occur at the peak CF amplitudes, and the values are relatively large when the vibration is in the resonance regions. The 3 D vortex columns are shed nearly parallel to the axis of the curved flexible riser. As the local Reynolds number increases from 0 at the bottom of the riser to the maximum value at the top, the wake undergoes a transition from a two-dimensional structure to a 3 D one. More irregular small-scale vortices appeared at the wake region of the riser, undergoing large amplitude responses.
基金supported by the National Basic Research Program of China (973 Program) (2007CB814800)the National Natural Science Foundation of China (10325211 and 10628206)
文摘We compare the space-time correlations calculated from direct numerical simulation （DNS） and large-eddy simulation （LES） of turbulent channel flows. It is found from the comparisons that the LES with an eddy-viscosity subgrid scale （SGS） model over-predicts the space-time corre- lations than the DNS. The overpredictions are further quantified by the integral scales of directional correlations and convection velocities. A physical argument for the overpre- diction is provided that the eddy-viscosity SGS model alone does not includes the backscatter effects although it correctly represents the energy dissipations of SGS motions. This argument is confirmed by the recently developed elliptic model for space-time correlations in turbulent shear flows. It suggests that enstrophy is crucial to the LES prediction of spacetime correlations. The random forcing models and stochastic SGS models are proposed to overcome the overpredictions on space-time correlations.
基金Supported by the National Basic Research Program of China(2013CB632601)the National Science Fund for Distinguished Young Scholars(21025627)+1 种基金the National Natural Science Foundation of China(21276256,21106150)the National High Technology Research and Development Program of China(2012AA03A606)
文摘This work presents a numerical investigation on steady internal, external and surface flows of a liquid sphere immersed in a simple shear flow at low and intermediate Reynolds numbers. The control volume formulation is adopted to solve the governing equations of two-phase flow in a 3-D spherical coordinate system. Numerical results show that the streamlines for Re = 0 are closed Jeffery orbits on the surface of the liquid sphere, and also closed curves outside and inside the liquid sphere. However, the streamlines have intricate and non-closed structures for Re ≠ 0. The flow structure is dependent on the values of Reynolds number and interior-to-exterior viscosity ratio.
基金supported by the National Natural Science Foundation of China (10532070)the Knowledge Innovation Program of Chinese Academy of Sciences (KJCX2-YW-L07)the LNM Initial Funding for Young Investigators
文摘A time domain model is presented to study the vibrations of long slender cylinders placed in shear flow. Long slender cylinders such as risers and tension legs are widely used in the field of ocean engineering. They are subjected to vortex-induced vibrations（VIV） when placed within a transverse incident flow. A three dimensional model coupled with wake oscillators is formulated to describe the response of the slender cylinder in cross-flow and in-line directions. The wake oscillators are distributed along the cylinder and the vortex-shedding frequency is derived from the local current velocity. A non-linear fiuid force model is accounted for the coupled effect between cross-flow and in-line vibrations. The comparisons with the published experimental data show that the dynamic features of VIV of long slender cylinder placed in shear flow can be obtained by the proposed model,such as the spanwise average displacement,vibration frequency,dominant mode and the combination of standing and traveling waves. The simulation in a uniform flow is also conducted and the result is compared with the case of nonuniform flow. It is concluded that the flow shear characteristic has significantly changed the cylinder vibration behavior.
