Flying and marine animals often use flapping wings or tails to generate thrust. In this paper, we will use the simplest flapping model with a sinusoidal pitching mo- tion over a range of frequency and amplitude to inv...Flying and marine animals often use flapping wings or tails to generate thrust. In this paper, we will use the simplest flapping model with a sinusoidal pitching mo- tion over a range of frequency and amplitude to investigate the mechanism of thrust generation. Previous work focuses on the Karman vortex street and the reversed Karman vor- tex street but the transition between two states remains un- known. The present numerical simulation provides a com- plete scenario of flow patterns from the Karman vortex street to reversed Karman vortex street via aligned vortices and the ultimate state is the deflected Karman vortex street, as the parameters of flapping motions change. The results are in agreement with the previous experiment. We make further discussion on the relationship of the observed states with drag and thrust coefficients and explore the mechanism of enhanced thrust generation using flapping motions.展开更多
The performance of a single or the collection of microswimmers strongly depends on the hydrodynamic coupling among their constituents and themselves.We present a numerical study for a single and a pair of microswimmer...The performance of a single or the collection of microswimmers strongly depends on the hydrodynamic coupling among their constituents and themselves.We present a numerical study for a single and a pair of microswimmers based on lattice Boltzmann method(LBM)simulations.Our numerical algorithm consists of two separable parts.Lagrange polynomials provide a discretization of the microswimmers and the lattice Boltzmann method captures the dynamics of the surrounding fluid.The two components couple via an immersed boundary method.We present data for a single swimmer system and our data also show the onset of collective effects and,in particular,an overall velocity increment of clusters of swimmers.展开更多
The effect of mechanical properties of erythrocytes on the near-wall motion of platelets was numerically studied with the immersed boundary method. Cells were modeled as viscous-fluid-filled capsules surrounded by hyp...The effect of mechanical properties of erythrocytes on the near-wall motion of platelets was numerically studied with the immersed boundary method. Cells were modeled as viscous-fluid-filled capsules surrounded by hyper-elastic membranes with negligible thickness. The numerical results show that with the increase of hematocrit, the near-wall approaching of platelets is enhanced, with which platelets exhibit larger deformation and orientation angle of its near-wall tank-treading motion, and the lateral force pushing platelets to the wall is increased with larger fluctuation amplitude. Meanwhile the near-wall approaching is reduced by increasing the stiffness of erythrocytes.展开更多
The vortex formation and organization are the key to understand the intrinsic mechanism in flying and swimming in nature. The vortex wake of dual-flapping foils is numerically investigated using the immersed boundary ...The vortex formation and organization are the key to understand the intrinsic mechanism in flying and swimming in nature. The vortex wake of dual-flapping foils is numerically investigated using the immersed boundary method. Beside the deflection of the reversed von-Kármán vortex street, an interesting phenomenon, the deflection of the von-Kármán vortex street, is observed behind the dual-flapping foils. The deflected direction is not according to the initial direction of biplane’s flapping motion. And the deflection angle is related to the difference between upward and downward deflecting velocities.展开更多
We extend the immersed boundary(IB)method to simulate the dynamics of a 2D dry foam by including the topological changes of the bubble network.In the article[Y.Kim,M.-C.Lai,and C.S.Peskin,J.Comput.Phys.229:5194-5207,2...We extend the immersed boundary(IB)method to simulate the dynamics of a 2D dry foam by including the topological changes of the bubble network.In the article[Y.Kim,M.-C.Lai,and C.S.Peskin,J.Comput.Phys.229:5194-5207,2010],we implemented an IB method for the foam problem in the two-dimensional case,and tested it by verifying the von Neumann relation which governs the coarsening of a two-dimensional dry foam.However,the method implemented in that article had an important limitation;we did not allow for the resolution of quadruple or higher order junctions into triple junctions.A total shrinkage of a bubble with more than four edges generates a quadruple or higher order junction.In reality,a higher order junction is unstable and resolves itself into triple junctions.We here extend the methodology previously introduced by allowing topological changes,and we illustrate the significance of such topological changes by comparing the behaviors of foams in which topological changes are allowed to those in which they are not.展开更多
采用基于Volume of Fluid(VOF)方法和浸入边界法的黏性二相流模型对LNG液舱(棱形液舱)的晃荡问题进行数值模拟。黏性二相流模型采用一套控制方程进行处理。整个计算区域为矩形,自由液面的跟踪和更新采用VOF方法,斜边边界的处理考虑用到...采用基于Volume of Fluid(VOF)方法和浸入边界法的黏性二相流模型对LNG液舱(棱形液舱)的晃荡问题进行数值模拟。黏性二相流模型采用一套控制方程进行处理。整个计算区域为矩形,自由液面的跟踪和更新采用VOF方法,斜边边界的处理考虑用到浸入边界法,把边界以外的当成是固体,在对VOF中的体积分数F的处理时引入通度系数以适应斜边边界。最后模拟得到液舱横荡的运动过程,对研究这一类型的晃荡运动具有参考价值。展开更多
基金supported by the Natural Science Foundation of Jiangxi Province(2010GZC0162)
文摘Flying and marine animals often use flapping wings or tails to generate thrust. In this paper, we will use the simplest flapping model with a sinusoidal pitching mo- tion over a range of frequency and amplitude to investigate the mechanism of thrust generation. Previous work focuses on the Karman vortex street and the reversed Karman vor- tex street but the transition between two states remains un- known. The present numerical simulation provides a com- plete scenario of flow patterns from the Karman vortex street to reversed Karman vortex street via aligned vortices and the ultimate state is the deflected Karman vortex street, as the parameters of flapping motions change. The results are in agreement with the previous experiment. We make further discussion on the relationship of the observed states with drag and thrust coefficients and explore the mechanism of enhanced thrust generation using flapping motions.
基金The DFG Priority Programme SPP 1726“Microswimmers—From Single Particle Motion to Collective Behaviour”(HA 4382/5-1)and SFB 1411(Project-ID 416229255)supported this work.
文摘The performance of a single or the collection of microswimmers strongly depends on the hydrodynamic coupling among their constituents and themselves.We present a numerical study for a single and a pair of microswimmers based on lattice Boltzmann method(LBM)simulations.Our numerical algorithm consists of two separable parts.Lagrange polynomials provide a discretization of the microswimmers and the lattice Boltzmann method captures the dynamics of the surrounding fluid.The two components couple via an immersed boundary method.We present data for a single swimmer system and our data also show the onset of collective effects and,in particular,an overall velocity increment of clusters of swimmers.
基金supported by the National Natural Science Foundation of China(11072155 and 11232010)Doctoral Fund of Ministry of Education of China(20100073120009)
文摘The effect of mechanical properties of erythrocytes on the near-wall motion of platelets was numerically studied with the immersed boundary method. Cells were modeled as viscous-fluid-filled capsules surrounded by hyper-elastic membranes with negligible thickness. The numerical results show that with the increase of hematocrit, the near-wall approaching of platelets is enhanced, with which platelets exhibit larger deformation and orientation angle of its near-wall tank-treading motion, and the lateral force pushing platelets to the wall is increased with larger fluctuation amplitude. Meanwhile the near-wall approaching is reduced by increasing the stiffness of erythrocytes.
文摘The vortex formation and organization are the key to understand the intrinsic mechanism in flying and swimming in nature. The vortex wake of dual-flapping foils is numerically investigated using the immersed boundary method. Beside the deflection of the reversed von-Kármán vortex street, an interesting phenomenon, the deflection of the von-Kármán vortex street, is observed behind the dual-flapping foils. The deflected direction is not according to the initial direction of biplane’s flapping motion. And the deflection angle is related to the difference between upward and downward deflecting velocities.
基金supported by National Research Foundation of Korea Grant funded by the Korean Government(2010-0006165)The second author was supported by the Chung-Ang University Research Scholarship Grant in 2010The third author is supported in part by National Science Council of Taiwan under research grant NSC-97-2628-M-009-007-MY3,NSC-98-2115-M-009-014-MY3,and the support of NCTS in Taiwan.
文摘We extend the immersed boundary(IB)method to simulate the dynamics of a 2D dry foam by including the topological changes of the bubble network.In the article[Y.Kim,M.-C.Lai,and C.S.Peskin,J.Comput.Phys.229:5194-5207,2010],we implemented an IB method for the foam problem in the two-dimensional case,and tested it by verifying the von Neumann relation which governs the coarsening of a two-dimensional dry foam.However,the method implemented in that article had an important limitation;we did not allow for the resolution of quadruple or higher order junctions into triple junctions.A total shrinkage of a bubble with more than four edges generates a quadruple or higher order junction.In reality,a higher order junction is unstable and resolves itself into triple junctions.We here extend the methodology previously introduced by allowing topological changes,and we illustrate the significance of such topological changes by comparing the behaviors of foams in which topological changes are allowed to those in which they are not.
文摘采用基于Volume of Fluid(VOF)方法和浸入边界法的黏性二相流模型对LNG液舱(棱形液舱)的晃荡问题进行数值模拟。黏性二相流模型采用一套控制方程进行处理。整个计算区域为矩形,自由液面的跟踪和更新采用VOF方法,斜边边界的处理考虑用到浸入边界法,把边界以外的当成是固体,在对VOF中的体积分数F的处理时引入通度系数以适应斜边边界。最后模拟得到液舱横荡的运动过程,对研究这一类型的晃荡运动具有参考价值。
