On the assumption that mass and heat transfer was neglected and instant thermal equilibrium was satisfied, different thermal state models for contents within a bubble were introduced when numerically studying a bubble axisymmetrically collapsing near a rigid wall. Fluid fields were controlled by full Navier-Stokes equations. Moving bubble walls were tracked by the volume of fluid method with piecewise linear interface construction and time unsplit propagation. Detailed dynamics information of bubble collapse, such as pressure pulses and high-speed micro fluid jets, was obtained. Among the thermal state models: isothermal, adiabatic and pseudo-adiabatic, adiabatic processes led to the most severe restraint to bubble collapse and substantially reduced cavitation erosion in some cases. In adiabatic processes and those similar, bubble rebounds appeared.
Carbon nanotubes have been proposed as one of the most promising materials in nanoelectromechanical systems (NEMS). In this paper, using the bistability of single-walled carbon nanotubes, we at first give a prototype design of nanoswitches which can serve as fundamental building blocks for the design of NEMS components or random access memory. The switch may be switched between ON and OFF states by applying electrostatic forces. We then focus on the mechanical behavior of the switch being turned on from off state, that is, a redial compressed single-walled carbon nanotube collapsing into a ribbon-like structure. The molecular dynamics simulation we used is based on second-generation-bond-order potential developed by Brenner. We show that, with the tube diameter increasing from 2 to 5 nm, the energy barrier between the two stable states of a nanotube increases firstly and then decreases. The peak value of energy barrier is reached at the diameter equal to about 3.8nm. This conclusion is very useful to the potential application of the bistability of carbon nanotubes in NEMS. We also investigate the radial elastic modulus of carbon nanotubes and the results show that it is approximately in linear relation with the tube diameter in a double logarithm coordinate system for the tubes with diameters ranging from 1 to 5 nm. Keywords: nanotube; molecular dynamics; bistability; nanoswitch
The electric field-strain and electric field- electric displacement responses of thePZT53 with a constant compressive or tensile pre-stress with perpendicular or parallel to thepoling direction have been investigated experimentally in this paper. Obtained results show thatthe pre-stress when applied along the poling direction has a significant effect on electromechanicalbehavior of PZT53 (e.g., the electric hysteresis loops and the butterfly loops) whilst the pre-stressin a direction transverse to the poling direction has negligible effect on the electric hysteresisloops and coercive electric field. The concept of two successive non-1800 domain switching isused to explain the observed experimental phenomenon. Obtained results shed insightful light intothe multi-axial constitutive modeling of the electromechanical behavior of ferroelectric ceramics.
The viscosity of material is considered at propagating crack-tip. Under the assumption that the artificial viscosity coefficient is in inverse proportion to power law of the plastic strain rate, an elastic-viscoplastic asymptotic analysis is carried out for moving crack-tip fields in power-hardening materials under plane-strain condition. A continuous solution is obtained containing no discontinuities. The variations of numerical solution are discussed for mode I crack according to each parameter. It is shown that stress and strain both possess exponential singularity. The elasticity, plasticity and viscosity of material at crack-tip only can be matched reasonably under linear-hardening condition. And the tip field contains no elastic unloading zone for mode I crack. The quasi-static solution is recovered when the crack moving speed approaches zero, which show that the quasi-static solution is a special case of a dynamic one. If the limit case of zero hardening coefficient is further considered, the solution can be transformed to the elastic-nonlinear-viscous one of Hui and Riedel.
Vented explosion tests in a 0.00814m3 steel cylindrical vessel connected to a duct, filled with stoichiometric ratio of CH4-Air, have been performed with central ignition in the end plane and the failure pressure 230±15KPa. Moreover it was simulated by using colocated grids SIMPLE schemes, based on the k- turbulent model and EBU combustion model. The pressure-time profiles at the four measurement points along the outside axis and the sequential shadowgraphs of the external flowfield both in test and simulation were clear to show the generation and development of the rupture shock and secondary explosion wave. The calculated results were in good agreement with the measured results in trend. At the early stage of venting, there existed rupture shock wave, low pressure area and suspended shock (high pressure area) about the axis in the external flowfield, which was formed for the convergence of the reflected rarefaction wave from the boundary layer. At the same time, the high pressure area was also the high density pre-heating area for the combustible gas compressed. When the jet flame came out and penetrated into the high pressure area, the secondary explosion was formed for the violent combustion with the interaction of the turbulence, barocline effect etc. In this paper,based on the test and numerical results, the main mechanisms of the occurrence of secondary explosions was elucidate systematically.
The nature of the fixed points of the compound Logistic Map is researched and the boundary equation of the first bifurcation of the map in the parameter space is given out. Using the quantitative criterion and rule of chaotic system, the paper reveal the general features of the compound Logistic Map transforming from regularity to chaos, the following conclusions are shown: ?chaotic patterns of the map may emerge out of double-periodic bifurcation; ? the chaotic crisis phenomena and the reverse bifurcation are found. At the same time, we analyze the orbit of critical point of the compound Logistic Map and put forward the definition of Mandelbrot-Julia set of compound Logistic Map. We generalize the Welstead and Cromer's periodic scanning technology and using this technology construct a series of Mandelbrot-Julia sets of compound Logistic Map. We investigate the symmetry of Mandelbrot-Julia set and study the topological inflexibility of distributing of period region in the Mandelbrot set, and finds that Mandelbrot set contain abundant information of structure of Julia sets by founding the whole portray of Julia sets based on Mandelbrot set qualitatively.