Vibration analysis of fluid-conveying nanotubes embedded in an elastic medium considering surface effects

An analytical model is developed to study the surface effects on the vibration behavior including the natural frequency and the critical flow velocity of fluid-conveying nanotubes embedded in an elastic medium.The effects of surface elasticity and residual surface stress are accounted through the surface elasticity model and the Young-Laplace equation.A Winkler-type foundation is employed to model the interaction of nanotubes and the surrounding medium.The results show that the surface effects have more prominent influences on the nature frequency with smaller nanotube thickness,larger aspect ratio and larger elastic medium constants.Both surface layers and the elastic medium enhance the stability of nanotubes.This study might be helpful for designing the fluid-conveying nanotube devices in NEMS and MEMS systems.

Growth and Optical Spectra of Tb^(3+)/Eu^(3+) Co-doped Cubic NaYF_4 Single Crystal for White Light Emitting Diode

High quality Tb^(3+)/Eu^(3+) co-doped cubic NaYF_4 single crystal in the size of Φ1.0 cm×6.6 cm was grown by a modified Bridgman method using KF as assistant flux for NaF-YF_3 system under the condition of completely closed Pt crucible.A white light emission from the combination of the violet-blue,blue,green,orange,and red lights with chromaticity coordinates of x = 0.3107,y = 0.3274,correlated color temperature of T_c = 6637 K,color rendering index of R_a = 83,and color quality scale of Q_a = 82 could be obtained from 1.51 mol%Tb^(3+) and 1.42 mol%Eu^(3+) co-doped cubic NaYF_4 single crystal when being excited by a 369 nm light.This indicates that Tb^(3+)/Eu^(3+) co-doped cubic NaYF_4 single crystal has a potential application in white light emitting diode excited by ultraviolet light.

Local vibrations and lift performance of low Reynolds number airfoil

The 2D incompressible Navier-Stokes equations arc solved based on the finite Flexible structure;Airfoil;Lock-in phenomenon;Lift coefficient;volume method and dynamic mesh technique is used to carry out partial fluid structure interaction.The local flexible structure(hereinafter termed as flexible structure)vibrates in a single mode located on the upper surface of the airfoil.The Influence of vibration frequency and amplitude are examined and the corresponding fluid flow characteristics are investigated Computational fluid dynamics(CFD)which add complexity to the inherent problem in unsteady flow.The study is conducted for flow over NACA0012 airfoil at 600≤Re≤3000 at a low angle of attack.Vibration of flexible structure induces a secondary vortex which modifies the pressure distribution and lift performance of the airfoil.At some moderate vibration amplitude,frequency synchronization frequency of rigid airfoil.Evolution and shedding of vortices corresponding to the deformation of flexible structure depends on the Reynolds number.In the case of Re≤1000,the deformation of flexible structure is considered in-phase with the vortex shedding i.e.,increasing maximum lift is linked with the positive deformation of flexible structure.At Re=1500 a phase shift of about 1/π exists while they are out-of-phase at Re>1500.Moreover,the oscillation amplitude of lift coefficient increases with increasing vibration amplitude for Re£1500 while it decreases with increasing vibration amplitude for Re>1500.As a result of frequency lock-in,the average lift coefficient is increased with increasing vibration amplitude for all investigated Reynolds numbers(Re).The maximum increase in the average liftcoefficient is 19.72% within the range of investigated parameters.

An Improved Model Reduction Method on AIMs for N-S Equations Using Multilevel Finite Element Method and Hierarchical Basis

A numerical method is proposed to approach the Approximate Inertial Man-ifolds(AIMs)in unsteady incompressible Navier-Stokes equations,using multilevel fi-nite element method with hierarchical basis functions.Following AIMS,the unknown variables,velocity and pressure in the governing equations,are divided into two com-ponents,namely low modes and high modes.Then,the couplings between low modes and high modes,which are not accounted by standard Galerkin method,are consid-ered by AIMs,to improve the accuracy of the numerical results.Further,the multilevel finite element method with hierarchical basis functions is introduced to approach low modes and high modes in an efficient way.As an example,the flow around airfoil NACA0012 at different angles of attack has been simulated by the method presented,and the comparisons show that there is a good agreement between the present method and experimental results.In particular,the proposed method takes less computing time than the traditional method.As a conclusion,the present method is efficient in numer-ical analysis of fluid dynamics,especially in computing time.

Study on Mass Transports in Evolution of Separation Bubbles Using LCSs and Lobe Dynamics

The lobe dynamics andmass transport between separation bubble andmain flow in flow over airfoil are studied in detail,using Lagrangian coherent structures(LCSs),in order to understand the nature of evolution of the separation bubble.For this problem,the transient flow over NACA0012 airfoil with low Reynolds number is simulated numerically by characteristic based split(CBS)scheme,in combination with dual time stepping.Then,LCSs and lobe dynamics are introduced and developed to investigate themass transport between separation bubble andmain flow,from viewpoint of nonlinear dynamics.The results show that stablemanifolds and unstable manifolds could be tangledwith each other as time evolution,and the lobes are formed periodically to induce mass transport between main flow and separation bubble,with dynamic behaviors.Moreover,the evolution of the separation bubble depends essentially on themass transportwhich is induced by lobes,ensuing energy andmomentum transfers.As the results,it can be drawn that the dynamics of flow separation could be studied using LCSs and lobe dynamics,and could be controlled feasibly if an appropriate control is applied to the upstream boundary layer with high momentum.