该文采用VOF(Volume-of-Fluid)中的PLIC(Piecewise Linear Interface Calculation)界面重构方法模拟研究了不同放置方式气泡之间的相互作用,重点分析了外围流体黏性及气泡间距对其影响。结果表明,外围流体黏性及气泡间距对气泡的融合及上升速度都具有不同程度的影响,气泡水平放置时,泡间距较小时,气泡上升速度比单个要大。在气泡开始融合到此过程结束的同时,上升速度出现了先降低后反弹的现象,且随着雷诺数的增加速度降低开始时间滞后。
<正>Three dimensional numerical simulations of self-propelled swimming of bionic fish and fish school in a visc...
WU Chuijie~(*,2)) WANG Liang~(*,+) *(Institute of Fluid Mechanics,Hohai University,Nanjing 210098,China) +(Research Center for Fluid Dynamics,PLA University of Science and Technology,Nanjing 211101,China)
Numerical simulations and the control of self-propelled swimming of three-dimensional bionic fish in a viscous flow and the mechanism of fish swimming are carried out in this study,with a 3D computational fluid dynamics package,which includes the immersed boundary method and the volume of fluid method,the adaptive multi-grid finite volume method,and the control strategy of fish swimming.Firstly,the mechanism of 3D fish swimming was studied and the vorticity dynamics root was traced to the moving body surface by using the boundary vorticity-flux theory.With the change of swimming speed,the contributions of the fish body and caudal fin to thrust are analyzed quantitatively.The relationship between vortex structures of fish swimming and the forces exerted on the fish body are also given in this paper.Finally,the 3D wake structure of self-propelled swimming of 3D bionic fish is presented.The in-depth analysis of the 3D vortex structure in the role of 3D biomimetic fish swimming is also performed.
Numerical simulation and control of self- propelled swimming of two- and three-dimensional biomimetic fish school in a viscous flow are investigated. With a parallel computational fluid dynamics package for the two- and three-dimensional moving boundary problem, which combines the adaptive multi-grid finite volume method and the methods of immersed boundary and volume of fluid, it is found that due to the interactions of vortices in the wakes, without proper control, a fish school swim with a given flap- ping rule can not keep the fixed shape of a queue. In order to understand the secret of fish swimming, a new feedback con- trol strategy of fish motion is proposed for the first time, i,e., the locomotion speed is adjusted by the flapping frequency of the caudal, and the direction of swimming is controlled by the swinging of the head of a fish. Results show that with this feedback control strategy, a fish school can keep the good order of a queue in cruising, turning or swimming around circles. This new control strategy, which separates the speed control and direction control, is important in the construction of biomimetic robot fish, with which it greatly simplifies the control devices of a biomimetic robot fish.
An adaptive version of immersed boundary method for simulating flows with complex stationary and moving boundaries is presented.The method employs a ghost-cell methodology which allows for a sharp representation of the immersed boundary.To simplify the implementation of the methodology,a volume-of-fluid method is introduced to identify the immersed boundary.In addition,the domain is spatially discretized using a tree-based discretization which is relatively simple to implement a fully flexible adaptive refinement strategy.Finally,the methodology is validated by comparing it with numerical and experimental results on three cases:(1) the flow passing a circular cylinder at Re=40 and Re=100,(2) a periodic oscillation of a circular cylinder in fluid at rest and(3) the self-propelled fish-like swimming at Re=6400.
WANG Liang1 & WU ChuiJie2,3 1 School of Science,PLA University of Science and Technology,Nanjing 211101,China
Shape optimization of the caudal fin of the three-dimensional self-propelled swimming fish,to increase the swimming efficiency and the swimming speed and control the motion direction more easily,is investigated by combining optimization algorithms,unsteady computational fluid dynamics and dynamic control in this study.The 3D computational fluid dynamics package contains the immersed boundary method,volume of fluid method,the adaptive multi-grid finite volume method and the control strategy of fish swimming.Through shape optimizations of various swimming speeds,the results show that the optimal caudal fins of different swimming modes are not exactly the same shape.However,the optimal fish of high swimming speed,whose caudal fin shape is similar to the crescent,also have higher efficiency and better maneuverability than the other optimal bionic fish at low and moderate swimming speeds.Finally,the mechanisms of vorticity creation of different optimal bionic fish are studied by using boundary vorticity-flux theory,and three-dimensional wake structures of self-propelled swimming of these fish are comparatively analyzed.The study of vortex dynamics reveals the nature of efficient swimming of the 3D bionic fish with the lunate caudal fin.
采用VOF(volume-of-fluid)中的PLIC(piecewise linear interface calculation)界面重构方法模拟研究了三维气泡群的上升过程及其对周围流场的扰动.重点分析考察了大量气泡上升过程中的动力学特性,伴随着随机的碰撞与融合等现象,气泡分布经历了由规则到扇形体再到随机的过程;在大雷诺数时融合过程表现为整体行为.受到融合过程的影响,周围流场表现出明显的振荡现象,且气泡群中心受到的扰动高于外围流体.
