ON THE STABILITY OF DISTORTED LAMINAR FLOW(Ⅱ)——THE LINEAR STABILITY ANALYSIS OF DISTORTED PARALLEL SHEAR FLOW

Based on the hydrodynamic stability theory of distorted laminar flow and the kind ofdistortion profiles on the mean velocity in parallel shear flow given in paper[1],this paperinvestigates the linear stability behaviour of parallel shear flow.presents unstable results ofplane Couette flow and pipe Poiseuille flow to two-dimensional or axisymmetricdisturbances for the first time,and obtains neutral curves of these two motions under certaindefinition.     

Water structures inside and outside single-walled carbon nanotubes under perpendicular electric field

The structures of water inside and outside(6,6),(8,8), and(10,10) singlewalled carbon nanotubes(SWCNTs) under an electric field perpendicular to the tube axis are investigated by molecular dynamics simulations. The results show that dipole reorientation induced by electric field plays a significant role on the structures of confined water inside and outside SWCNTs. Inside SWCNTs, the average water occupancy and the average number of hydrogen bonds(H-bonds) per water molecule decrease as the electric intensity increases. Because the field intensity is sufciently strong, the initial water structures inside the SWCNTs are destroyed, and the isolated water clusters are found. Outside SWCNTs, the azimuthal distributions of the density and the average number of H-bonds per water molecule around the solid walls become more and more asymmetric as the electric intensity increases. The percentages of water molecules involved in 0–5 H-bonds for all the three types of SWCNTs under diferent field intensities are displayed. The results show that those water molecules involved with most H-bonds are the most important to hold the original structures. When the electric field direction is parallel with the original preferred orientation, the density and the H-bond connections in water will be increased; when the electric field direction is perpendicular to the original preferred orientation, the density and the H-bond connections in water will be decreased.     

第二届全国航天动力学与控制青年学者论坛暨第七届全国空间轨道设计竞赛研讨会致辞

正各位学者:早上好!我非常高兴能够代表中国力学学会出席第二届全国航天动力学与控制青年学者论坛暨第七届全国空间轨道设计竞赛研讨会.首先,欢迎来自全国各地的青年学者、学生,感谢你们的积极参与,看得出你们在航天动力学与控制、空间轨道设计研究方面一定取得了丰硕的成果.同时,也感谢本次会议的承办单位:南京航空航天大学航天学院、中国科学院太空应用重点实验室、南京航空航天大学青年教授学术联谊会.     

柱状纤维上超薄液膜的线性稳定性分析

运用线性理论分析了粘性超薄液膜沿柱状纤维垂直下落的稳定性特征,研究了厚度低于100 nm的薄膜在外力驱动下的流动以及van der Waals力的影响．结果表明随着薄膜相对厚度的下降,纤维表面的曲率将使得线性扰动的发展得到抑制,而van der Waals力促进扰动的增长,这一竞争机制导致了增长率随薄膜相对厚度非单调的变化．还得到了流动的绝对和对流不稳定分区．结果表明van der Waals力扩大绝对不稳定流动区域,表面张力也会有利于绝对不稳定的发展,而外驱动力正好起到相反的作用．     

Formation of radially expanding liquid sheet by impinging two round jets

A thin circular liquid sheet can be formed by impinging two identical round jets against each other. The liquid sheet expands to a certain critical radial distance and breaks. The unsteady process of the formation and breakup of the liquid sheet in the ambient gas is simulated numerically. Both liquid and gas are treated as incompressible Newtonian fluids. The flow considered is axisymmetric. The liquid-gas interface is modeled with a level set function. A finite difference scheme is used to solve the governing Navier-Stokes equations with physical boundary conditions. The numerical results show how a thin circular sheet can be formed and break at its circular edge in slow motion. The sheet continues to thin as it expands radially. Hence, the Weber number decreases radially. The Weber number is defined as ρu 2 h/σ, where ρ and σ are, respectively, the liquid density and the surface tension, and u and h are, respectively, the average velocity and the half sheet thickness at a local radial location in the liquid sheet. The numerical results show that the sheet indeed terminates at a radial location, where the Weber number reaches one as observed in experiments. The spatio-temporal linear theory predicts that the breakup is initiated by the sinuous mode at the critical Weber number We c =1, below which the absolute instability occurs. The other independent mode called the varicose mode grows more slowly than the sinuous mode according to the linear theory. However, our numerical results show that the varicose mode actually overtakes the sinuous mode during the nonlinear evolution, and is responsible for the final breakup. The linear theory predicts the nature of disturbance waves correctly only at the onset of the instability, but cannot predict the exact consequence of the instability.     

Floquet instability of a large density ratio liquid-gas coaxial jet with periodic fluctuation

By numerical simulation of basic flow, this paper extends Floquet stability analysis of interracial flow with periodic fluctuation into large density ratio range. Stability of a liquid aluminum jet in a coaxial nitrogen stream with velocity fluctuation is investigated by Chebyshev collocation method and the Floquet theory. Parametric resonance of the jet and the influences of different physical parameters on the instability are discussed. The results show qualitative agreement with the available experimental data.     

On the instability in gas atomization

ＩｎｔｒｏｄｕｃｔｉｏｎＧａｓａｔｏｍｉｚａｔｉｏｎｉｓａｐｒｏｃｅｓｓｉｎｗｈｉｃｈｔｈｅｈｉｇｈ＿ｖｅｌｏｃｉｔｙｇａｓｓｔｒｅａｍｉｍｐｉｎｇｅｓｏｎｔｈｅｍｏｌｔｅｎｍｅｔａｌｊｅｔａｎｄｂｒｅａｋｓｉｔｕｐｉｎｔｏｄｒｏｐｌｅｔｓ．Ｂｅｉｎｇａｂａｓｉｃｐｒｏｃｅｓｓ,ｇａｓａｔｏｍｉｚａｔｉｏｎｉｓｗｉｄｅｌｙｕｓｅｄ,ｉｎｃｌｕｄｉｎｇｐｏｗｄｅｒｍｅｔａｌｌｕｒｇｙ,ｓｐｒａｙｆｏｒｍｉｎｇ,ｅｔｃ．．Ｃａｓｔｌｅｍａｎ［１］ｐｏｓｔｕｌａｔｅｄｔｈａｔａｔｏｍｉｚａｔｉｏｎｉｓ…     

On the nonlinear stability behaviour of distorted plane Couette flow

This paper discusses the nonlinear stability behaviour of distorted plane Couette flowto 2-dimensional disturbances,and compares it with that of distorted plane Poiseuille flow.The results show that plane Couette flow is more unstable than plane Poiseuille flow tofinite-amplitude disturbances.     

The linear stability of plane Poiseuille flow under unsteady distortion

This paper investigates the linear stability behaviour of plane Poiseuille flow underunsteady distortion by multiscale perturbation method and discusses further the problemproposed by paper[1].The results show that in the initial period of disturbancedevelopment,the distortion profiles presented by paper[1]will make the disturbances growup,thus augmenting the possibility of instability.     

圆弧形波纹膜片的矩阵联乘解法

将圆弧形波纹膜片看作几段圆环壳和中心圆板的组合构件,本文利用钱伟长教授的圆环壳一般解[1]及圆薄板小参数振动理论[2],导出环壳和板的传递矩阵和连接矩阵,用矩阵联乘法求得线性精确解和非线性解,计算结果同W.A.Wildhack的实验[3]是吻合的.