Stability Criteria to the Incompressible Inviscid Linear Fluid between Two Rotation Coaxial Cylinders

The stability and instability phenomenon coupled with the rotation effect and the thermal convection between two concentric cylinders was studied. By means of the Normal-Modes method, the stability or instability criteria for the linearized system in terms of the oscillation frequency, the axial wavelength and the background thermal gradient are proved. Besides, some numerical simulation for the axisymmetric perturbations is presented.

Processing Map of C71500 Copper-nickel Alloy and Application in Production Practice

The isothermal compression tests of C71500 copper-nickel alloy at different temperatures (1 073-1 273 K) and strain rates (0.01-10 s^-1) were carried out on Gleeble-3500 thermo-mechanical simulator.The real stress-strain data were obtained.On the basis of dynamic material model,the power dissipation was established.The peak efficiency of the power dissipation is 57%.At the same time,Prasad’s,Murty’s and Babu’s instability criteria based on Ziegler’s expectant rheology theory,and Gegel’s and Malas’s instability criteria based on Lyaponov’s function theory,were used to predict the unstable regions in the processing map.The maximum entropy generation rate and large plastic deformation principle are more in line with the hot deformation process of C71500 alloy,so the accuracy of Prasad’s instability criterion is much better.According to the obtained macro-crack and micro-metallographic structure morphologies,the temperature range of 1 098-1 156 K and the strain rate range of 2.91-10 s^-1,and the temperature range of 1 171-1 273 K and the strain rate range of 0.01-0.33 s^-1 are more suitable for the processing area of C71500 alloy.The accuracy of the above conclusions were verified by the forging of materials and the analysis of hot piercing tubes.The significance of this paper is to provide theoretical basis and technological conditions for hot-press processing of C71500 alloy.

Application of strength reduction method to dynamic anti-sliding stability analysis of high gravity dam with complex dam foundation

Considering that there are some limitations in analyzing the anti-sliding seismic stability of dam-foundation systems with the traditional pseudo-static method and response spectrum method, the dynamic strength reduction method was used to study the deep anti-sliding stability of a high gravity dam with a complex dam foundation in response to strong earthquake-induced ground action. Based on static anti-sliding stability analysis of the dam foundation undertaken by decreasing the shear strength parameters of the rock mass in equal proportion, the seismic time history analysis was carried out. The proposed instability criterion for the dynamic strength reduction method was that the peak values of dynamic displacements and plastic strain energy change suddenly with the increase of the strength reduction factor. The elasto-plastic behavior of the dam foundation was idealized using the Drucker-Prager yield criterion based on the associated flow rule assumption. The result of elasto-plastic time history analysis of an overflow dam monolith based on the dynamic strength reduction method was compared with that of the dynamic linear elastic analysis, and the reliability of elasto-plastic time history analysis was confirmed. The results also show that the safety factors of the dam-foundation system in the static and dynamic cases are 3.25 and 3.0, respectively, and that the F2 fault has a significant influence on the anti-sliding stability of the high gravity dam. It is also concluded that the proposed instability criterion for the dynamic strength reduction method is feasible.

On the Criteria of Instability for Electrochemical Systems

Both cyclic-voltammetry-based and impedance-based experimental criteria thathave been developed recently for the oscil-latory electrochemical systems are critically appraisedwith two typical categories of oscillators. Consistent conclusions can be drawn by the two criteriafor the category of oscillators that involve the coupling of charge transfer mainly with surfacesteps (e.g. ad- and desorption) such as in the electrooxidation of C_1 organic molecules. Whereas,impedance-based criterion is not applicable to the category of oscillators that involve the couplingof charge transfer mainly with mass transfer (e.g. diffusion and convection) such as in theFe(CN)_6^(3-) reduction accompanying periodic hydrogen evolution. The reason is that the negativeimpedance cannot include the feedback information of convection mass transfer induced by thehydrogen evolution. However, both positive and negative nonlinear feedbacks, i. e., thediffusion-limited depletion and convection-enhanced replenishment of the Fe(CN)_6^(3-) surfaceconcentration, that coexist between the bistability, i. e., Fe(CN)_6^(3-) reduction with and withouthydrogen evolution at lower and higher potential sides respectively, are all reflected in thecrossed cyclic voltammo-gram (CCV). It can be concluded that the voltammetry-based criterion (intime domain) is more intuitive, less time-consuming and has a wider range of applications than theimpedance-based one (in frequency domain).