Dynamical Behavior Of Micropipe Conveying Fluid

Pipe conveying fluid is a common fluid-structure interaction system in engineering and mechanical structure.With the development of science and technology,pipes in microscale are becoming the core components of micro/nano electro-mechancial systems.The characteristic sizes of micropipes conveying fluid are generally on the order of microns or nanometers.A large number of experimental results and theoretical analysis show that the size effect plays an important role in the analysis of mechanical behavior of micro-structures.In many cases,the size effect must be considered when studying the mechanical behavior of such microstructures.In this thesis,through theoretical and numerical analysis,on the basis of the existing results,a further research on the dynamic behavior of pipes conveying fluid at the microscale is carried out.Some related achievements have been made,which may be useful for the design and improvement of some types of microscale machinery.The research and results of this thesis are as follows:1.Based on the modified couple stress theory,a nonlinear theoretical model and nonlinear equation of motion of cantilever micropipes conveying fluid are obtained by considering the geometrical nonlinearity,gravity,size effect and loose supports at the downstream end.The influence of these factors on the dynamic behavior of the cantilever micropipe conveying fluid is studied and discussed.The results show that the stability of micropipes can be enhanced by the additional parameters in the presence of material length scale,but the size dependence of the post-flutter responses of the system is not pronounced.The gravity effect not only changes the stability of micropipes,but also has a significant influence on the nonlinear dynamic behavior of micropipes.For the "horizontal" or "hanging" micropipe conveying fluid,it is found that flutter instability occurs at a critical flow velocity,beyond which the micropipe would undergo a limit cycle motion.However,for a ‘standing' micropipe with relatively long length,both buckling and flutter instabilities could occur.When the axial internal flow velocity in the micropipe exceeds the critical velocity,the cantilever micropipe restrained by loose supports will show more abundant dynamic phenomena,including periodic,multi-periodic and chaotic motions.The bifurcation behavior of micropipes and the mechanism of routes to chaos under nonlinear conditions are revealed.2.The critical flow velocity and lowest mode for flutter instability of a flexible micropipe conveying fluid with pinned-free boundary conditions are analyzed.Results show that the flexible micropipe system is stable at low flow velocities,but at sufficiently high flow velocities the system becomes subject to flutter instability.The flutter instability may occur in the second or third mode first,mainly depending on the selected value of the dimensionless mass ratio(?).Further studies on the mass ratio parameter show that the mass ratio of micropipes not only affects the mode order of the first flutter instability of the system,but also leads to the existence of exchange of complex frequency trajectories between the second and third modes.It is also found that for certain ranges of mass ratio,the system would undergo the instabilityrestabilization-instability sequence.Finally,through the analysis of the gravity parameter,it is concluded that a positive value of gravity causes additional restoring force and enhances the stability of the micropipe system;however,it can generate the complexity of stability boundaries.3.A theoretical model for suspended microchannel resonators modeled as a cantilevered micropipe conveying fluid and nanoparticle is established.The dynamic behavior and stability of single-channel and U-shaped two-parallel-channel micorpipes are studied.For single-channel micropipes containing nanoparticles,the equation of motion can be obtained by modifying the simplest equation of micropipe conveying fluid.For two-channel micropipes(TCMPs),the governing equation is derived using the Newtonian approach by essentially accounting for the flow-induced tensile force due to the fact that the flow reverses direction near the free end of the micropipe.Results of our analysis show that the presence of a moving nanoparticle can make the originally stable micropipe system become unstable.The vibration characteristics and stability of both systems are strongly dependent on the instantaneous position of the moving nanoparticle.For the same TCMP system,of particular interest is that in the absence of external damping,flutter instability may concurrently occur in several modes even for infinitesimal flow velocity.With consideration of external damping,however,the TCMP can retain stability at low flow velocity.These results indicate that the fluidstructure interaction must be fully considered for suspension microchannel resonators containing internal flows and nanoparticles.For these contents listed above,the stability and dynamic behavior of micorpipes conveying fluid were studied further in this thesis,and the influence of different system parameters on the system was discussed.These results may have important reference significance for the design and analysis of micro-scale microtubule systems and devices,and for the improvement of the service efficiency and service life of MEMS equipments.