Computational solid mechanics has become an indispensable approach in engineering,and numerical investigation of fracturing in composites is essential,as composites are widely used in structural applications.Crack evo...Computational solid mechanics has become an indispensable approach in engineering,and numerical investigation of fracturing in composites is essential,as composites are widely used in structural applications.Crack evolution in composites is the path to elucidating the relationship between microstructures and fracture performance,but crack-based finite-element methods are computationally expensive and time-consuming,which limits their application in computation-intensive scenarios.Consequently,this study proposes a deep learning framework called Crack-Net for instant prediction of the dynamic crack growth process,as well as its strain-stress curve.Specifically,Crack-Net introduces an implicit constraint technique,which incorporates the relationship between crack evolution and stress response into the network architecture.This technique substantially reduces data requirements while improving predictive accuracy.The transfer learning technique enables Crack-Net to handle composite materials with reinforcements of different strengths.Trained on high-accuracy fracture development datasets from phase field simulations,the proposed framework is capable of tackling intricate scenarios,involving materials with diverse interfaces,varying initial conditions,and the intricate elastoplastic fracture process.The proposed Crack-Net holds great promise for practical applications in engineering and materials science,in which accurate and efficient fracture prediction is crucial for optimizing material performance and microstructural design.展开更多
The relationship between stabilities of the buoyancy boundary layers along an inclined plate and a vertical plate immersed in a stratified medium is studied theoretically and numerically. The eigenvalue problem of ene...The relationship between stabilities of the buoyancy boundary layers along an inclined plate and a vertical plate immersed in a stratified medium is studied theoretically and numerically. The eigenvalue problem of energy stability is solved with the method of descending exponentials. The disturbance energy is found to be able to grow to 11.62 times as large as the initial disturbance energy for Pr = 0.72 when the Grashof number is between the critical Grashof numbers of the energy stability and the linear stability. We prove that, with a weighted energy method, the basic flow of the vertical buoyancy boundary layer is stable to finite-amplitude streamwise-independent disturbances.展开更多
In this work fingering double diffusive convection,i.e.the buoyancy-driven flow with fluid density being affected by two different scalar components,is investigated numerically with special efforts on the influences o...In this work fingering double diffusive convection,i.e.the buoyancy-driven flow with fluid density being affected by two different scalar components,is investigated numerically with special efforts on the influences of the physical properties of two scalar components.We show that different scalar properties can affect the global transport behaviors.The concentration flux exhibits different exponents in their power-law scalings for different combinations of scalar components.The scaling exponents of heat flux,however,depend mainly on the ratio of the diffusivities of two scalars.If one uses the local parameters of the finger layer in the bulk,the behaviors are very similar to those found in the fully periodic simulations.The horizontal width of the fingers is consistent with the wavelength of the fast growing mode.For one case we observe evidences of the thermohaline staircase,namely,the typical width of the flow structures changes significantly in different layers within the flow domain.展开更多
Lobes and clefts are characteristic structures at the front of sand storms.In this paper,their original formation mechanism and geometric features are studied experimentally and theoretically.A rotatable lock-exchange...Lobes and clefts are characteristic structures at the front of sand storms.In this paper,their original formation mechanism and geometric features are studied experimentally and theoretically.A rotatable lock-exchange tank is utilized to avoid the strong local disturbances existing in the conventional horizontal apparatus,and the original lobe size is selected as the dominant spanwise wavelength by a statistical method instead of the mean lobe size used in the literature.It is shown that at the initial formation stage,the measured lobe sizes for different tank geometries and density differences(or Atwood numbers)have a 1/3 scaling law with the Grashof number(Gr)as predicted by a Rayleigh-Taylor(RT)model,especially at moderate Gr range,and substantially depend on the diffusion effect represented by the Schmidt number.Furthermore,using turbulent Schmidt number and eddy viscosity for atmosphere in the RT model predicts large scale dominant lobes,whose sizes agree qualitatively with those observed in a real sand storm,revealing that the underlying control mechanism for these lobe and cleft structures is intrinsically related to the RT instability.展开更多
By direct numerical simulations of the plane Couette flow (PCF) in a large computational domain, it is shown that an isolated turbulent band decays monotonically at low Reynolds numbers but experiences transient growt...By direct numerical simulations of the plane Couette flow (PCF) in a large computational domain, it is shown that an isolated turbulent band decays monotonically at low Reynolds numbers but experiences transient growth before the eventual relaminarization at moderate Reynolds numbers. The lower bound Reynolds number of the transient-growth regime is determined as 286. The width, length, and tilt angle of the iso- lated band structure are defined based on the disturbance kinetic energy in the mid-plane, and the geometric characteristics of the band can be described with a tilted rectangle. It is illustrated that before its eventual fragmentation, the isolated turbulent band decays in a style of longitudinal contraction, where the center, width, and tilt angle of the band keep almost constant but the band length contracts with a statistically constant velocity.展开更多
In this study the effects of the actuation waveforms on the droplet generation in a drop-on-demand inkjet printing are studied systematically by numerical simulations.Two different types of waveforms,namely the unipol...In this study the effects of the actuation waveforms on the droplet generation in a drop-on-demand inkjet printing are studied systematically by numerical simulations.Two different types of waveforms,namely the unipolar and bipolar actuations,are investigated for three fluids with different physical properties.We focus on two key parameters,which are the dwell time and the velocity amplitude.For the unipolar driving,the ejection velocity and the ejected liquid volume are both increased as the velocity amplitude becomes larger.The dwell time only has minor effects on both the ejection velocity and the ejected liquid volume.The ejection velocity decreases significantly for large liquid viscosity,while the influences of viscosity on the ejected liquid volume are much weaker.Four different droplet morphologies and the corresponding parameter ranges are identified.The droplet radius can be successfully reduced to about 40%e of the nozzle exit radius.For the bipolar waveforms,same droplet morphologies are observed but with shifted boundaries in the phase space.The minimal radius of stable droplet produced by the bipolar waveforms is even smaller compared to the unipolar ones.展开更多
In shock tube experiments,the interaction between the reflected shock and boundary layer can induce shock bifurcation and weak ignition.The weak ignition can greatly affect the ignition delay time measurement in a sho...In shock tube experiments,the interaction between the reflected shock and boundary layer can induce shock bifurcation and weak ignition.The weak ignition can greatly affect the ignition delay time measurement in a shock tube experiment.In this work,two-dimensional simulations considering detailed chemistry and transport are conducted to investigate the shock bifurcation and non-uniform ignition behind a retlected shock.The objectives are to interpret the formation of shock bifurcation induced by the reflected shock and boundary layer interaction and to investigate the weak ignition and its transition to strong ignition for both hydrogen and dimethyl ether.It is found that the non-uniform reflection of the incident shock at the end wall produces a wedge-shaped oblique shock foot at the wall.The wedge-shaped structure results in strong interactions between reflected shock and boundary layer,which induces the shock bifurcation.It is demonstrated that the local high-temperature spots at the foot of the bifurcated shock is caused by viscous dissipation and pressure work.As the post-reflected shock temperature increases,the transition from weak ignition to strong ignition in a stoichiometric hydrogen/oxygen mixture is observed.The relative sensitivity of ignition delay time to the post-rellected shock temperature is introduced to characterize the appearance of weak ignition behind the reflected shock.Unlike in the hydrogen/oxygen mixture,weak ignition is not observed in the stoichiometric dimethyl-ether/oxygen mixture since it has a relatively longer ignition delay time and smaller relative sensitivity.展开更多
In this work,we numerically study the impact of a water droplet onto a deep oil pool.Two fluids are immiscible and the viscosity of the pool liquid is changed systematically.We focus on the cavity dynamics during the ...In this work,we numerically study the impact of a water droplet onto a deep oil pool.Two fluids are immiscible and the viscosity of the pool liquid is changed systematically.We focus on the cavity dynamics during the impact and especially the effects of the pool liquid viscosity and the impacting velocity.For the parameter range explored,we identify the regime where splashing occurs with corolla breaking into droplets,and the regime where no splashing is observed.Similarity is found for the time evolution of cavity depth for fixed impact velocity and different viscosity,if the cavity depth and time are nondimensionalized by the maximal depth and the time when the maximal depth is reached.Effective power-law scalings are also proposed to describe the dependence of the maximal cavity depth and the corresponding time on the impact velocity and pool liquid viscosity,in the term of Froude and Reynolds numbers.展开更多
In this study,we numerically investigate the droplet impact onto a thin liquid film deposited on a structured surface with square pillars and cavities.The time evolution of crown geometry is strongly affected by the s...In this study,we numerically investigate the droplet impact onto a thin liquid film deposited on a structured surface with square pillars and cavities.The time evolution of crown geometry is strongly affected by the surface structure.When the thickness of the liquid film is larger than the structure height,the expanding speed of the crown base radius is independent of the structure width.However,if the liquid film thickness is equal to the structure height,the crown base expands slower as the structure width increases.Surface structures have strong effects on the crown height and radius,and can prevent ejected filament from breaking into satellite droplets for certain cases.For the liquid film with the thickness equal to the pillar height,both the crown height and the radius exhibit non-monotonic behaviors as the pillar width increases.There exists one pillar width which produces the smallest crown height and the largest crown radius.展开更多
The instabilities of a pendent viscous thin film underneath two corrugated ceilings are studied numerically and theoretically in comparison with the case of a flat wall. With the same initial interface perturbations, ...The instabilities of a pendent viscous thin film underneath two corrugated ceilings are studied numerically and theoretically in comparison with the case of a flat wall. With the same initial interface perturbations, it is shown numerically that both the supercritical instability and the subcritical instability can be retarded by the in-phase corrugated ceilings. The lubrication approximation is used to explain the retardation effect of the corrugated ceiling on the supercritical instability of the pendant film, where the linear growth rate is revealed to be power three of the initial film thickness.展开更多
In this letter,the effect of slip boundary on the origin of subcritical transition in two-dimensional chan-nel flows is studied numerically and theoretically.It is shown that both the positive and the negative slip le...In this letter,the effect of slip boundary on the origin of subcritical transition in two-dimensional chan-nel flows is studied numerically and theoretically.It is shown that both the positive and the negative slip lengths will increase the critical Reynolds number of localized wave packet and hence postpone the transition.By applying a variable transformation and expanding the variables about a small slip length,it is illustrated that the slip boundary effect only exists in the second and higher order modulations of the no-slip solution,and hence explains the power law found in simulations,i.e.the relative increment of the critical Reynolds number due to the slip boundary is proportional to the square of the slip length.展开更多
The approximate but analytical solution of the viscous Rayleigh-Taylor insta- bility （RTI） has been widely used recently in theoretical and numerical investigations due to its clarity. In this paper, a modified anal...The approximate but analytical solution of the viscous Rayleigh-Taylor insta- bility （RTI） has been widely used recently in theoretical and numerical investigations due to its clarity. In this paper, a modified analytical solution of the growth rate for the viscous RTI of incompressible fluids is obtained based on an approximate method. Its accuracy is verified numerically to be significantly improved in comparison with the previous one in the whole wave number range for different viscosity ratios and Atwood numbers. Fur- thermore, this solution is expanded for viscous RTI including the concentration-diffusion effect.展开更多
Chaotic thermal convection in a rapidly rotating cylindrical annulus is investigated numerically and the relaxation oscillation state is obtained under the no-slip boundary condition. The dominant frequency of the osc...Chaotic thermal convection in a rapidly rotating cylindrical annulus is investigated numerically and the relaxation oscillation state is obtained under the no-slip boundary condition. The dominant frequency of the oscillation is inherited directly from a vacillating mode, whose nonlinear interaction with another high-frequency vacillating mode leads to the chaotic state at high Rayleigh numbers through an RTN-type route. Furthermore, the effects of Coriolis parameter and Rayleigh number on the quasi-periodic burst of kinetic energy are discussed as well.展开更多
Laminar flame speed is one of the most important intrinsic properties of a combustible mixture. Due to its importance, different methods have been developed to measure the laminar flame speed. This paper reviews the c...Laminar flame speed is one of the most important intrinsic properties of a combustible mixture. Due to its importance, different methods have been developed to measure the laminar flame speed. This paper reviews the constant-volume propagating spherical flame method for laminar flame speed measurement. This method can be used to measure laminar flame speed at high pressures and temperatures which are close to engine-relevant conditions. First, the propagating spherical flame method is introduced and the constant-volume method （CVM） and constant- pressure method （CPM） are compared. Then, main groups using the constant-volume propagating spherical flame method are introduced and large discrepancies in laminar flame speeds measured by different groups for the same mixture are identified. The sources of discrepancies in laminar flame speed measured by CVM are discussed and special attention is devoted to the error encountered in data processing. Different correlations among burned mass fraction, pressure, temperature and flame speed, which are used by different researchers to obtain laminar flame speed, are summarized. The performance of these correlations are examined, based on which recommendations are given. Finally, recommendations for future studies on the con- stant-volume propagating spherical flame method for laminar flame speed measurement are presented.展开更多
Recent experimental and numerical investigations reveal that the onset of turbulence in plane-Poiseuille flow and planeCouette flow has some similar stages separated with different threshold Reynolds numbers.Based on ...Recent experimental and numerical investigations reveal that the onset of turbulence in plane-Poiseuille flow and planeCouette flow has some similar stages separated with different threshold Reynolds numbers.Based on these observations and the energy equation of a disturbed fluid element,a local Reynolds number Re L is derived to represent the maximum ratio of the energy supplement to the energy dissipation in a cross section.It is shown that along the sequence of transition stages,which include transient localized turbulence,"equilibrium" localized turbulence,spatially intermittent but temporally persistent turbulence and uniform turbulence,the corresponding thresholds of Re L for plane-Couette flow,Hagen-Poiseuille flow and plane-Poiseuille flow are consistent,indicating that the critical(threshold) states during the laminar-turbulent transition are determined by the local properties of the base flow and are independent of global features,such as flow geometries(pipe or channel) and types of driving forces(shear driving or pressure driving).展开更多
The laminar flame speed is one of the most fundamental properties of a fuel/air mixture.It determines the fuel burning rate in combustion engines and is an important target used to validate chemical models.Due to its ...The laminar flame speed is one of the most fundamental properties of a fuel/air mixture.It determines the fuel burning rate in combustion engines and is an important target used to validate chemical models.Due to its importance,accurate measurement of the laminar flame speed receives great attention and different experimental methods have been developed.This review first introduces the laminar flame speed as well as its significance and dependence on different factors.Then,different experimental methods for the laminar flame speed measurement are described.Since the outwardly propagating spherical flame method is currently the most popular method for laminar flame speed measurement,its challenges and recent advances are reviewed.Both the constant pressure method and constant volume method using propagating spherical flames are discussed.展开更多
Data-driven discovery of partial differential equations(PDEs)has recently made tremendous progress,and many canonical PDEs have been discovered successfully for proof of concept.However,determining the most proper PDE...Data-driven discovery of partial differential equations(PDEs)has recently made tremendous progress,and many canonical PDEs have been discovered successfully for proof of concept.However,determining the most proper PDE without prior references remains challenging in terms of practical applications.In this work,a physics-informed information criterion(PIC)is proposed to measure the parsimony and precision of the discovered PDE synthetically.The proposed PIC achieves satisfactory robustness to highly noisy and sparse data on 7 canonical PDEs from different physical scenes,which confirms its ability to handle difficult situations.The PIC is also employed to discover unrevealed macroscale governing equations from microscopic simulation data in an actual physical scene.The results show that the discovered macroscale PDE is precise and parsimonious and satisfies underlying symmetries,which facilitates understanding and simulation of the physical process.The proposition of the PIC enables practical applications of PDE discovery in discovering unrevealed governing equations in broader physical scenes.展开更多
基金supported and partially funded by the National Natural Science Foundation of China(52288101)the China Postdoctoral Science Foundation(2024M761535)supported by the High Performance Computing Centers at Eastern Institute of Technology,Ningbo,and Ningbo Institute of Digital Twin.
文摘Computational solid mechanics has become an indispensable approach in engineering,and numerical investigation of fracturing in composites is essential,as composites are widely used in structural applications.Crack evolution in composites is the path to elucidating the relationship between microstructures and fracture performance,but crack-based finite-element methods are computationally expensive and time-consuming,which limits their application in computation-intensive scenarios.Consequently,this study proposes a deep learning framework called Crack-Net for instant prediction of the dynamic crack growth process,as well as its strain-stress curve.Specifically,Crack-Net introduces an implicit constraint technique,which incorporates the relationship between crack evolution and stress response into the network architecture.This technique substantially reduces data requirements while improving predictive accuracy.The transfer learning technique enables Crack-Net to handle composite materials with reinforcements of different strengths.Trained on high-accuracy fracture development datasets from phase field simulations,the proposed framework is capable of tackling intricate scenarios,involving materials with diverse interfaces,varying initial conditions,and the intricate elastoplastic fracture process.The proposed Crack-Net holds great promise for practical applications in engineering and materials science,in which accurate and efficient fracture prediction is crucial for optimizing material performance and microstructural design.
基金Project supported by the National Natural Science Foundation of China(Nos.11225209,11490553,and 11521091)
文摘The relationship between stabilities of the buoyancy boundary layers along an inclined plate and a vertical plate immersed in a stratified medium is studied theoretically and numerically. The eigenvalue problem of energy stability is solved with the method of descending exponentials. The disturbance energy is found to be able to grow to 11.62 times as large as the initial disturbance energy for Pr = 0.72 when the Grashof number is between the critical Grashof numbers of the energy stability and the linear stability. We prove that, with a weighted energy method, the basic flow of the vertical buoyancy boundary layer is stable to finite-amplitude streamwise-independent disturbances.
基金supported by the Major Research Plan of National Natural and Science Foundation of China for Turbulent Structures(Grants 91852107 and 91752202).
文摘In this work fingering double diffusive convection,i.e.the buoyancy-driven flow with fluid density being affected by two different scalar components,is investigated numerically with special efforts on the influences of the physical properties of two scalar components.We show that different scalar properties can affect the global transport behaviors.The concentration flux exhibits different exponents in their power-law scalings for different combinations of scalar components.The scaling exponents of heat flux,however,depend mainly on the ratio of the diffusivities of two scalars.If one uses the local parameters of the finger layer in the bulk,the behaviors are very similar to those found in the fully periodic simulations.The horizontal width of the fingers is consistent with the wavelength of the fast growing mode.For one case we observe evidences of the thermohaline staircase,namely,the typical width of the flow structures changes significantly in different layers within the flow domain.
基金support by the National Natural Science Foundation of China(Grants No.91752203,11490553)is acknowledged.
文摘Lobes and clefts are characteristic structures at the front of sand storms.In this paper,their original formation mechanism and geometric features are studied experimentally and theoretically.A rotatable lock-exchange tank is utilized to avoid the strong local disturbances existing in the conventional horizontal apparatus,and the original lobe size is selected as the dominant spanwise wavelength by a statistical method instead of the mean lobe size used in the literature.It is shown that at the initial formation stage,the measured lobe sizes for different tank geometries and density differences(or Atwood numbers)have a 1/3 scaling law with the Grashof number(Gr)as predicted by a Rayleigh-Taylor(RT)model,especially at moderate Gr range,and substantially depend on the diffusion effect represented by the Schmidt number.Furthermore,using turbulent Schmidt number and eddy viscosity for atmosphere in the RT model predicts large scale dominant lobes,whose sizes agree qualitatively with those observed in a real sand storm,revealing that the underlying control mechanism for these lobe and cleft structures is intrinsically related to the RT instability.
基金Project supported by the National Natural Science Foundation of China(Nos.91752203,11490553,and 11602148)
文摘By direct numerical simulations of the plane Couette flow (PCF) in a large computational domain, it is shown that an isolated turbulent band decays monotonically at low Reynolds numbers but experiences transient growth before the eventual relaminarization at moderate Reynolds numbers. The lower bound Reynolds number of the transient-growth regime is determined as 286. The width, length, and tilt angle of the iso- lated band structure are defined based on the disturbance kinetic energy in the mid-plane, and the geometric characteristics of the band can be described with a tilted rectangle. It is illustrated that before its eventual fragmentation, the isolated turbulent band decays in a style of longitudinal contraction, where the center, width, and tilt angle of the band keep almost constant but the band length contracts with a statistically constant velocity.
基金National Natural Science Foundation of China(Grants 91848201,11988102,11521202,11872004.11802004)The authors also acknowledge the partial support from the Beijing Natural Science Foundation(Grants L172002)+1 种基金A.B.Aqecl would like to thank the Chinese Scholarship Council(CSC)for providing Chinese Government Scholarship(CGs)The numericalsimulations were performed on the National Super Computing Centerin Tianjin.China.
文摘In this study the effects of the actuation waveforms on the droplet generation in a drop-on-demand inkjet printing are studied systematically by numerical simulations.Two different types of waveforms,namely the unipolar and bipolar actuations,are investigated for three fluids with different physical properties.We focus on two key parameters,which are the dwell time and the velocity amplitude.For the unipolar driving,the ejection velocity and the ejected liquid volume are both increased as the velocity amplitude becomes larger.The dwell time only has minor effects on both the ejection velocity and the ejected liquid volume.The ejection velocity decreases significantly for large liquid viscosity,while the influences of viscosity on the ejected liquid volume are much weaker.Four different droplet morphologies and the corresponding parameter ranges are identified.The droplet radius can be successfully reduced to about 40%e of the nozzle exit radius.For the bipolar waveforms,same droplet morphologies are observed but with shifted boundaries in the phase space.The minimal radius of stable droplet produced by the bipolar waveforms is even smaller compared to the unipolar ones.
基金supported by the National Natural Science Foundation of China(Grant Nos.52006001,and 52176096).
文摘In shock tube experiments,the interaction between the reflected shock and boundary layer can induce shock bifurcation and weak ignition.The weak ignition can greatly affect the ignition delay time measurement in a shock tube experiment.In this work,two-dimensional simulations considering detailed chemistry and transport are conducted to investigate the shock bifurcation and non-uniform ignition behind a retlected shock.The objectives are to interpret the formation of shock bifurcation induced by the reflected shock and boundary layer interaction and to investigate the weak ignition and its transition to strong ignition for both hydrogen and dimethyl ether.It is found that the non-uniform reflection of the incident shock at the end wall produces a wedge-shaped oblique shock foot at the wall.The wedge-shaped structure results in strong interactions between reflected shock and boundary layer,which induces the shock bifurcation.It is demonstrated that the local high-temperature spots at the foot of the bifurcated shock is caused by viscous dissipation and pressure work.As the post-reflected shock temperature increases,the transition from weak ignition to strong ignition in a stoichiometric hydrogen/oxygen mixture is observed.The relative sensitivity of ignition delay time to the post-rellected shock temperature is introduced to characterize the appearance of weak ignition behind the reflected shock.Unlike in the hydrogen/oxygen mixture,weak ignition is not observed in the stoichiometric dimethyl-ether/oxygen mixture since it has a relatively longer ignition delay time and smaller relative sensitivity.
基金This work was supported by the National Natural Science Foundation of China(Grants 91848201,11988102,11521202,11872004,and 11802004)The authors also acknowledge the partial support from the Beijing Natural Science Foundation(Grants L172002)The numerical simulations were performed on Tianhe-1 A,the National Super Computing Center in Tianjin,China.Muhammad Mohasan would like to thank Chinese Scholarship Council for providing Chinese Government Scholarship.
文摘In this work,we numerically study the impact of a water droplet onto a deep oil pool.Two fluids are immiscible and the viscosity of the pool liquid is changed systematically.We focus on the cavity dynamics during the impact and especially the effects of the pool liquid viscosity and the impacting velocity.For the parameter range explored,we identify the regime where splashing occurs with corolla breaking into droplets,and the regime where no splashing is observed.Similarity is found for the time evolution of cavity depth for fixed impact velocity and different viscosity,if the cavity depth and time are nondimensionalized by the maximal depth and the time when the maximal depth is reached.Effective power-law scalings are also proposed to describe the dependence of the maximal cavity depth and the corresponding time on the impact velocity and pool liquid viscosity,in the term of Froude and Reynolds numbers.
基金Project supported by the National Natural Science Foundation of China(Nos.11988102,91848201,11872004,and 11802004)
文摘In this study,we numerically investigate the droplet impact onto a thin liquid film deposited on a structured surface with square pillars and cavities.The time evolution of crown geometry is strongly affected by the surface structure.When the thickness of the liquid film is larger than the structure height,the expanding speed of the crown base radius is independent of the structure width.However,if the liquid film thickness is equal to the structure height,the crown base expands slower as the structure width increases.Surface structures have strong effects on the crown height and radius,and can prevent ejected filament from breaking into satellite droplets for certain cases.For the liquid film with the thickness equal to the pillar height,both the crown height and the radius exhibit non-monotonic behaviors as the pillar width increases.There exists one pillar width which produces the smallest crown height and the largest crown radius.
基金Project supported by the National Natural Science Foundation of China(Nos.91752203,11490553,and 11521091)
文摘The instabilities of a pendent viscous thin film underneath two corrugated ceilings are studied numerically and theoretically in comparison with the case of a flat wall. With the same initial interface perturbations, it is shown numerically that both the supercritical instability and the subcritical instability can be retarded by the in-phase corrugated ceilings. The lubrication approximation is used to explain the retardation effect of the corrugated ceiling on the supercritical instability of the pendant film, where the linear growth rate is revealed to be power three of the initial film thickness.
基金The present research is financially supported by the National Natural Science Foundation of China(Grants No.91752203).
文摘In this letter,the effect of slip boundary on the origin of subcritical transition in two-dimensional chan-nel flows is studied numerically and theoretically.It is shown that both the positive and the negative slip lengths will increase the critical Reynolds number of localized wave packet and hence postpone the transition.By applying a variable transformation and expanding the variables about a small slip length,it is illustrated that the slip boundary effect only exists in the second and higher order modulations of the no-slip solution,and hence explains the power law found in simulations,i.e.the relative increment of the critical Reynolds number due to the slip boundary is proportional to the square of the slip length.
基金Project supported by the National Natural Science Foundation of China(Nos.11225209,11490553,and 11221062)
文摘The approximate but analytical solution of the viscous Rayleigh-Taylor insta- bility （RTI） has been widely used recently in theoretical and numerical investigations due to its clarity. In this paper, a modified analytical solution of the growth rate for the viscous RTI of incompressible fluids is obtained based on an approximate method. Its accuracy is verified numerically to be significantly improved in comparison with the previous one in the whole wave number range for different viscosity ratios and Atwood numbers. Fur- thermore, this solution is expanded for viscous RTI including the concentration-diffusion effect.
基金Supported by the National Natural Science Foundation of China under Grant Nos 10672003 and 10972007.
文摘Chaotic thermal convection in a rapidly rotating cylindrical annulus is investigated numerically and the relaxation oscillation state is obtained under the no-slip boundary condition. The dominant frequency of the oscillation is inherited directly from a vacillating mode, whose nonlinear interaction with another high-frequency vacillating mode leads to the chaotic state at high Rayleigh numbers through an RTN-type route. Furthermore, the effects of Coriolis parameter and Rayleigh number on the quasi-periodic burst of kinetic energy are discussed as well.
基金supported by the National Natural Science Foundation of China(51322602)
文摘Laminar flame speed is one of the most important intrinsic properties of a combustible mixture. Due to its importance, different methods have been developed to measure the laminar flame speed. This paper reviews the constant-volume propagating spherical flame method for laminar flame speed measurement. This method can be used to measure laminar flame speed at high pressures and temperatures which are close to engine-relevant conditions. First, the propagating spherical flame method is introduced and the constant-volume method （CVM） and constant- pressure method （CPM） are compared. Then, main groups using the constant-volume propagating spherical flame method are introduced and large discrepancies in laminar flame speeds measured by different groups for the same mixture are identified. The sources of discrepancies in laminar flame speed measured by CVM are discussed and special attention is devoted to the error encountered in data processing. Different correlations among burned mass fraction, pressure, temperature and flame speed, which are used by different researchers to obtain laminar flame speed, are summarized. The performance of these correlations are examined, based on which recommendations are given. Finally, recommendations for future studies on the con- stant-volume propagating spherical flame method for laminar flame speed measurement are presented.
基金supported by the National Natural Science Foundation of China(Grant Nos. 10972007 and 10921202) and (Grant No.2009CB724100)
文摘Recent experimental and numerical investigations reveal that the onset of turbulence in plane-Poiseuille flow and planeCouette flow has some similar stages separated with different threshold Reynolds numbers.Based on these observations and the energy equation of a disturbed fluid element,a local Reynolds number Re L is derived to represent the maximum ratio of the energy supplement to the energy dissipation in a cross section.It is shown that along the sequence of transition stages,which include transient localized turbulence,"equilibrium" localized turbulence,spatially intermittent but temporally persistent turbulence and uniform turbulence,the corresponding thresholds of Re L for plane-Couette flow,Hagen-Poiseuille flow and plane-Poiseuille flow are consistent,indicating that the critical(threshold) states during the laminar-turbulent transition are determined by the local properties of the base flow and are independent of global features,such as flow geometries(pipe or channel) and types of driving forces(shear driving or pressure driving).
基金supported by the National Natural Science Foundation of China(Grant Nos.91741126,91541204and 51322602)
文摘The laminar flame speed is one of the most fundamental properties of a fuel/air mixture.It determines the fuel burning rate in combustion engines and is an important target used to validate chemical models.Due to its importance,accurate measurement of the laminar flame speed receives great attention and different experimental methods have been developed.This review first introduces the laminar flame speed as well as its significance and dependence on different factors.Then,different experimental methods for the laminar flame speed measurement are described.Since the outwardly propagating spherical flame method is currently the most popular method for laminar flame speed measurement,its challenges and recent advances are reviewed.Both the constant pressure method and constant volume method using propagating spherical flames are discussed.
基金the National Center for Applied Mathematics Shenzhen(NCAMS),the Shenzhen Key Laboratory of Natural Gas Hydrates(Grant No.ZDSYS20200421111201738)the SUSTech-Qingdao New Energy Technology Research Institute.
文摘Data-driven discovery of partial differential equations(PDEs)has recently made tremendous progress,and many canonical PDEs have been discovered successfully for proof of concept.However,determining the most proper PDE without prior references remains challenging in terms of practical applications.In this work,a physics-informed information criterion(PIC)is proposed to measure the parsimony and precision of the discovered PDE synthetically.The proposed PIC achieves satisfactory robustness to highly noisy and sparse data on 7 canonical PDEs from different physical scenes,which confirms its ability to handle difficult situations.The PIC is also employed to discover unrevealed macroscale governing equations from microscopic simulation data in an actual physical scene.The results show that the discovered macroscale PDE is precise and parsimonious and satisfies underlying symmetries,which facilitates understanding and simulation of the physical process.The proposition of the PIC enables practical applications of PDE discovery in discovering unrevealed governing equations in broader physical scenes.
