In-situ exploration of asteroid surfaces is of great scientific significance.Internally actuated rovers have been released to asteroid surfaces but without enough controllability.To investigate the attitude control ch...In-situ exploration of asteroid surfaces is of great scientific significance.Internally actuated rovers have been released to asteroid surfaces but without enough controllability.To investigate the attitude control characteristics of the cubic rover for asteroid surface exploration,a series of experiments are carried out using the self-designed rover and the low-gravity testbed.The experiments focus on two major themes:The minimum flywheel speed for cubic rover to produce a walking motion in different conditions,and the relationship between the rover’s rotation angle and the flywheel speed in twisting motion.The rover’s dynamical descriptions of the walking and twisting motions are first derived.The features and design of the low-gravity testbed are then summarized,including its dynamics,setup,and validation.A detailed comparison between the dynamic model and the experimental results is presented,which provides a basic reference of the cubic rover’s attitude control in low-gravity environments.展开更多
Under pitch excitation, the sloshing of liquid in circular cylindrical tank includes planar motion, rotary motion and rotary motion inside planar motion. The boundaries between stable motion and unstable motion depend...Under pitch excitation, the sloshing of liquid in circular cylindrical tank includes planar motion, rotary motion and rotary motion inside planar motion. The boundaries between stable motion and unstable motion depend on the radius of the tank, the liquid height, the gravitational intension, the surface tensor and the sloshing damping. In this article, the differential equations of nonlinear sloshing are built first. And by variational principle, the Lagrange function of liquid pressure is constructed in volume intergration form. Then the velocity potential function is expanded in series by wave height function at the free surface. The nonlinear equations with kinematics and dynamics free surface boundary conditions through variation are derived. At last, these equations are solved by multiple-scales method. The influence of Bond number on the global stable response of nonlinear liquid sloshing in circular cylinder tank is analyzed in detail. The result indicates that variation of amplitude frequency response characteristics of the system with Bond, jump, lag and other nonlinear phenomena of liquid sloshing are investigated.展开更多
In order to study natural convection effects on fluid flows under low-gravity in space, we have expanded variables into a power series of Grashof number by using perturbation theory to reduce the Navier-Stokes equatio...In order to study natural convection effects on fluid flows under low-gravity in space, we have expanded variables into a power series of Grashof number by using perturbation theory to reduce the Navier-Stokes equations to the Poisson equation for temperature T and biharmonic equation for stream function φ . Suppose that a square infinite closed cylinder horizontally imposes a specified temperature of linear distribution on the boundaries, we investigate the two dimensional steady flows in detail. The results for stream function φ, velocity u and temperature T are gained. The analysis of the influences of some parameters such as Grashof number G r and Prandtl number Pron the fluid motion lead to several interesting conclusions. At last, we make a comparison between two results, one from approximate equations, the other from the original version. It shows that the approximate theory correctly simplifies the physical problem, so that we can expect the theory will be applied to unsteady or three-dimensional cases in the future.展开更多
Under low gravity,the Lagrange equations in the form of volume integration of pressure of nonlinear liquid sloshing were built by variational principle. Based on this,the analytical solution of nonlinear liquid sloshi...Under low gravity,the Lagrange equations in the form of volume integration of pressure of nonlinear liquid sloshing were built by variational principle. Based on this,the analytical solution of nonlinear liquid sloshing in pitching tank could be investigated. Then the velocity potential function was expanded in series by wave height function at the free surface so that the nonlinear equations with kinematics and dynamics free surface boundary conditions were derived. Finally,these nonlinear equations were investigated analytically by the multiple scales method. The result indicates that the system's amplitude-frequency response changes from ‘soft-spring’ to ‘hard-spring’ in the planar motion with the decreasing of the Bond number,while it changes from ‘hard-spring’ to ‘soft-spring’ in the rotary motion.展开更多
Herein symmetrical four-legged suspension lunar lander was used as the research object, the six-degree-of-freedom dynamic model was built and the model of the lunar soil friction coefficient was improved. For the low-...Herein symmetrical four-legged suspension lunar lander was used as the research object, the six-degree-of-freedom dynamic model was built and the model of the lunar soil friction coefficient was improved. For the low-gravity simulation on objects outside earth for future work, the law of dynamic similarity for detectors was deduced. A new method was proposed for simulating the low-gravity field on the surface of objects outside earth, which was achieved by changing initial conditions of the landing by the probe and by subsequent treatment of experimental data. The prototype tested the limitation of this method was verified. It is shown that the prototypes of detectors can be used in detectors low-gravity simulation test with this method, and equipments are simple and operationally effective. This method can be used for later lunar exploration, and low-gravity simulations on extraterrestrial objects.展开更多
Rapid solidification of Cu-Pb monotectic alloys has been accomplished during free fall in a 3 m drop tube. Both macrosegregated and uniformly dispersed structures are observed in Cu-40 wt pct Pb alloy droplets, wherea...Rapid solidification of Cu-Pb monotectic alloys has been accomplished during free fall in a 3 m drop tube. Both macrosegregated and uniformly dispersed structures are observed in Cu-40 wt pct Pb alloy droplets, whereas droplets of composition Cu-64 wt pct Pb exhibit only macrosegregation morphologies. The microstructures are strongly dependent on droplet size. The higher undercooling tends to facilitate liquid phase separation and results in more extensive macrosegregation in smaller droplets. There exists a pronounced tendency for the Pb-rich liquid to occupy the surface of the droplets of both compositions, resulting from the quite lower surface tension of the Pb-rich phase and causing a Pb-rich layer at the surface of the solidified droplet. The nucleation of monotectic cells in the Cu-40 wt pct Pb droplets with dispersed structures preferentially occurs at the droplet surface. A single nucleation event takes place more frequently as droplet size is reduced.展开更多
We use the phase field method to track the gas-liquid interface based on the gas-liquid two-phase flow in the pool boiling process,and study the bubble nucleation,growth,deformation,departure and other dynamic behavio...We use the phase field method to track the gas-liquid interface based on the gas-liquid two-phase flow in the pool boiling process,and study the bubble nucleation,growth,deformation,departure and other dynamic behaviors on the heating surface under microgravity.By simulating the correlation between liquid undercooling and bubble dynamics,we find that the bubble growth time increases with the increase of liquid undercooling,but the effect of liquid undercooling on bubble height is not significant.Meanwhile,the gas-liquid-solid three-phase contact angle and the gravity level will also have an effect on the bubble growth time and bubble height.With the increase of the contact angle,the bubble growth time and bubble height when the bubble departs also increase.While the effect of gravity level is on the contrary,the smaller the gravity level is,the larger the bubble height and bubble growth time when the bubble separates.展开更多
To gain some insight into the behaviour of low-gravity flows in the material processingin space, an approximate theory has been developed for the convective motion of fluidswith a small Grashof number Gr. The expansio...To gain some insight into the behaviour of low-gravity flows in the material processingin space, an approximate theory has been developed for the convective motion of fluidswith a small Grashof number Gr. The expansion of the variables into a series of Gr reducesthe Boussinesq equation to a system of weakly coupled linearly inhomogeneous equations. Moreover, the analogy concept is proposed and utilized in the study of the plate bending problems in solid mechanics. Two examples are investigated in detail, i. e. the 2-dimensional steady flows in either circular or square infinite closed cylinder, which is horizontally imposed at a specified temperature of linear distribution on the boundaries. The results for stream function ψ, velocity u and temperature T are provided. The analysis of the influences of some parameters such as the Grashof number Gr and the Prandtl number Pr, on motions will lead to several interesting conclusions. The theory seems to be useful for seeking for an analytical solutions. At least, it will greatly simplify the complicated problems originally governed by the Navier-Stokes equation including buoyancy. It is our hope that the theory might be applicable to unsteady or 3-dimensional cases in future.展开更多
基金the National Natural Science Foundation of China(No.11972075)the Innovation Research Program of Beijing Institute of Technology(No.2021CX01029)。
文摘In-situ exploration of asteroid surfaces is of great scientific significance.Internally actuated rovers have been released to asteroid surfaces but without enough controllability.To investigate the attitude control characteristics of the cubic rover for asteroid surface exploration,a series of experiments are carried out using the self-designed rover and the low-gravity testbed.The experiments focus on two major themes:The minimum flywheel speed for cubic rover to produce a walking motion in different conditions,and the relationship between the rover’s rotation angle and the flywheel speed in twisting motion.The rover’s dynamical descriptions of the walking and twisting motions are first derived.The features and design of the low-gravity testbed are then summarized,including its dynamics,setup,and validation.A detailed comparison between the dynamic model and the experimental results is presented,which provides a basic reference of the cubic rover’s attitude control in low-gravity environments.
基金Project supported by the National Defense Pre-research Project of the Tenth Five-Year-Plan of China(No.41320020301)
文摘Under pitch excitation, the sloshing of liquid in circular cylindrical tank includes planar motion, rotary motion and rotary motion inside planar motion. The boundaries between stable motion and unstable motion depend on the radius of the tank, the liquid height, the gravitational intension, the surface tensor and the sloshing damping. In this article, the differential equations of nonlinear sloshing are built first. And by variational principle, the Lagrange function of liquid pressure is constructed in volume intergration form. Then the velocity potential function is expanded in series by wave height function at the free surface. The nonlinear equations with kinematics and dynamics free surface boundary conditions through variation are derived. At last, these equations are solved by multiple-scales method. The influence of Bond number on the global stable response of nonlinear liquid sloshing in circular cylinder tank is analyzed in detail. The result indicates that variation of amplitude frequency response characteristics of the system with Bond, jump, lag and other nonlinear phenomena of liquid sloshing are investigated.
基金The project supported by the National Natural Science Foundation of China
文摘In order to study natural convection effects on fluid flows under low-gravity in space, we have expanded variables into a power series of Grashof number by using perturbation theory to reduce the Navier-Stokes equations to the Poisson equation for temperature T and biharmonic equation for stream function φ . Suppose that a square infinite closed cylinder horizontally imposes a specified temperature of linear distribution on the boundaries, we investigate the two dimensional steady flows in detail. The results for stream function φ, velocity u and temperature T are gained. The analysis of the influences of some parameters such as Grashof number G r and Prandtl number Pron the fluid motion lead to several interesting conclusions. At last, we make a comparison between two results, one from approximate equations, the other from the original version. It shows that the approximate theory correctly simplifies the physical problem, so that we can expect the theory will be applied to unsteady or three-dimensional cases in the future.
基金the National Defense Foundation of China(Grant No.41320020301).
文摘Under low gravity,the Lagrange equations in the form of volume integration of pressure of nonlinear liquid sloshing were built by variational principle. Based on this,the analytical solution of nonlinear liquid sloshing in pitching tank could be investigated. Then the velocity potential function was expanded in series by wave height function at the free surface so that the nonlinear equations with kinematics and dynamics free surface boundary conditions were derived. Finally,these nonlinear equations were investigated analytically by the multiple scales method. The result indicates that the system's amplitude-frequency response changes from ‘soft-spring’ to ‘hard-spring’ in the planar motion with the decreasing of the Bond number,while it changes from ‘hard-spring’ to ‘soft-spring’ in the rotary motion.
基金supported by the National Natural Science Foundation of China(Grant No.51105196)Natural Science Foundation of Jiangsu Province(Grant No.BK2011733)
文摘Herein symmetrical four-legged suspension lunar lander was used as the research object, the six-degree-of-freedom dynamic model was built and the model of the lunar soil friction coefficient was improved. For the low-gravity simulation on objects outside earth for future work, the law of dynamic similarity for detectors was deduced. A new method was proposed for simulating the low-gravity field on the surface of objects outside earth, which was achieved by changing initial conditions of the landing by the probe and by subsequent treatment of experimental data. The prototype tested the limitation of this method was verified. It is shown that the prototypes of detectors can be used in detectors low-gravity simulation test with this method, and equipments are simple and operationally effective. This method can be used for later lunar exploration, and low-gravity simulations on extraterrestrial objects.
基金Financial support from the National Natural Science Founda tion of China(No.59871040 and 50071009)is gratefuly acknowledged.
文摘Rapid solidification of Cu-Pb monotectic alloys has been accomplished during free fall in a 3 m drop tube. Both macrosegregated and uniformly dispersed structures are observed in Cu-40 wt pct Pb alloy droplets, whereas droplets of composition Cu-64 wt pct Pb exhibit only macrosegregation morphologies. The microstructures are strongly dependent on droplet size. The higher undercooling tends to facilitate liquid phase separation and results in more extensive macrosegregation in smaller droplets. There exists a pronounced tendency for the Pb-rich liquid to occupy the surface of the droplets of both compositions, resulting from the quite lower surface tension of the Pb-rich phase and causing a Pb-rich layer at the surface of the solidified droplet. The nucleation of monotectic cells in the Cu-40 wt pct Pb droplets with dispersed structures preferentially occurs at the droplet surface. A single nucleation event takes place more frequently as droplet size is reduced.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.52161002,51661020,and11364024)the Postdoctoral Science Foundation of China(Grant No.2014M560371)the Funds for Distinguished Young Scientists of Lanzhou University of Technology of China(Grant No.J201304)。
文摘We use the phase field method to track the gas-liquid interface based on the gas-liquid two-phase flow in the pool boiling process,and study the bubble nucleation,growth,deformation,departure and other dynamic behaviors on the heating surface under microgravity.By simulating the correlation between liquid undercooling and bubble dynamics,we find that the bubble growth time increases with the increase of liquid undercooling,but the effect of liquid undercooling on bubble height is not significant.Meanwhile,the gas-liquid-solid three-phase contact angle and the gravity level will also have an effect on the bubble growth time and bubble height.With the increase of the contact angle,the bubble growth time and bubble height when the bubble departs also increase.While the effect of gravity level is on the contrary,the smaller the gravity level is,the larger the bubble height and bubble growth time when the bubble separates.
基金Project supported by the National Natural Science Foundation of China.
文摘To gain some insight into the behaviour of low-gravity flows in the material processingin space, an approximate theory has been developed for the convective motion of fluidswith a small Grashof number Gr. The expansion of the variables into a series of Gr reducesthe Boussinesq equation to a system of weakly coupled linearly inhomogeneous equations. Moreover, the analogy concept is proposed and utilized in the study of the plate bending problems in solid mechanics. Two examples are investigated in detail, i. e. the 2-dimensional steady flows in either circular or square infinite closed cylinder, which is horizontally imposed at a specified temperature of linear distribution on the boundaries. The results for stream function ψ, velocity u and temperature T are provided. The analysis of the influences of some parameters such as the Grashof number Gr and the Prandtl number Pr, on motions will lead to several interesting conclusions. The theory seems to be useful for seeking for an analytical solutions. At least, it will greatly simplify the complicated problems originally governed by the Navier-Stokes equation including buoyancy. It is our hope that the theory might be applicable to unsteady or 3-dimensional cases in future.
