Transition to turbulence in rotating-disk boundary layers—convective and absolute instabilities

This work is an experimental study of mechanisms for transition to turbulence in the boundary layer on a rotating disk. In one case, the focus was on a triad resonance between pairs of traveling cross-flow modes and a stationary cross-flow mode. The other was on the temporal growth of traveling modes through a linear absolute instability mechanism first discovered by Lingwood (1995, J Fluid Mech 314:373–405). Both research directions made use of methods for introducing controlled initial disturbances. One used a distributed array of ink dots placed on the disk surface to enhance a narrow band of azimuthal and radial wave numbers of both stationary and traveling modes. The size of the dots was small so that the disturbances they produce were linear. Another approach introduced temporal disturbances by a short-duration air pulse from a hypodermic tube located above the disk and outside the boundary layer. Hot-wire sensors primarily sensitive to the azimuthal velocity component, were positioned at different spatial (r,θ) locations on the disk to document the growth of disturbances. Spatial correlation measurements were used with two simultaneous sensors to obtain wavenumber vectors. Cross-bicoherence was used to identify three-frequency phase locking. Ensemble averages conditioned on the air pulses revealed wave packets that evolved in time and space. The space–time evolution of the leading and trailing edges of the wave packets were followed past the critical radius for the absolute instability, r _{c A} . With documented linear amplitudes, the spreading of the disturbance wave packets did not continue to grow in time as r _{c A} was approached. Rather, the spreading of the trailing edge of the wave packet decelerated and asymptotically approached a constant. This result supports the linear DNS simulations of Davies and Carpenter (2003, J Fluid Mech 486:287–329) who concluded that the absolute instability mechanism does not result in a global mode, and that linear-disturbance wave packets are dominated by the convective instability. In contrast, wave-number matching between traveling cross-flow modes confirmed a triad resonance that lead to the growth of a low azimuthal number (n = 4) stationary mode. At transition, this mode had the largest amplitude. Signs of this mechanism can be found in past flow visualization of transition to turbulence in rotating disk flows.     

An investigation of the unstable region for dense phase conveying in sliding bed flow

For fine powdered materials, capable of being conveyed in dense phase, a conveying region exists in which instability in flow and pipeline blockage can occur if the pressure gradient available is insufficient to maintain a minimum value of solids loading ratio. A reduction in material flow rate can also lead to pipeline blockage in this region. Conveying trials under-taken with cement are used to illustrate the nature of the problem.     

Dynamic elastic-plastic buckling phenomena in a rod due to axial impact

Dynamic elastic-plastic buckling phenomena which might develop in a rod from an axial impact loading are studied in order to identify the conditions for quasi-static behaviour. A discrete model for dynamic elastic-plastic buckling, which retains the axial and the lateral inertia forces, is proposed, and the relationship between the model parameters and the characteristics of an actual structure is given. Examples of different external loadings and boundary conditions are considered in order to clarify the influence of elastic-plastic axial wave propagation on the buckling process. The critical time for the initiation of buckling is obtained and the post-buckling behaviour of the model is analysed. Particular attention is paid to the role of the striking mass on the characteristics of the buckling process and on the development of the buckling shape. The numerical study reveals that the inertia of the striking mass affects considerably the development of the buckling shape causing different patterns of axial strain distributions at the initiation of buckling. The comparisons which are made between the model predictions and some previously published experimental data show that the buckling process is governed by the impact velocity as well as by the external loading history provided by the experimental technique.     

大断面交岔点失稳分析及治理

通过工程地质调查、室内围岩物化分析和力学试验 ,深入研究了围岩变形力学机制和破坏机理 .根据新奥法施工原理 ,采取了有针对性的综合治理措施 ,将围岩复杂的变形力学机制转化为单一的变形力学机制 ,使该工程得到了有效治理 .取得的技术经验推广应用于该矿井其它失稳破坏工程 ,均获得了圆满成功     

Surface Position-Resolved Thermophysical Properties for Metallic Alloys

Thermophysical properties are collective measures of a material to transport dynamical quantities of physical nature on its surface or through the bulk. As such, the exact nature of couplings between particles in a many-body assembly of building block atoms or molecules sensitively determines their values. The couplings between nearest neighbors are the product of the local elemental composition and the material phase. In this study, thermal cycling of a four-element Wood’s alloy specimen brings out cadmium-rich patches to the top surface of the specimen. An assembly of such patches leads to depth-dependent deviations of elemental composition from that of the bulk. Surface-layer atoms are driven to form a high temperature laser-produced plasma (LPP), and time-resolved spectroscopy of their emissions show the variability of elemental composition over surface positions as well as over depth from the surface. These thermal history-driven composition anomalies contribute to significant variability in the measured values of spectral emissivity and thermal diffusivity.     

Evaluation of Morphology—Stable Critical Size of KNbO3 Crystals

Skeletal form of KnbO3 crystals growing in Li2B4O7 solvent was in-situ observed at 900℃ and it was found that shallow depression started to develop on the surface of KnbO3 crystals when the crystal size exceeded several micron,typically 7 micron.Based on the quantitative criterion derived by Chernov,the estimated critical size of KNbO3 crystals was 1 micron,which was consistent with the experimental measurement.The kinetic coefficients,Kcorner and Kcr,in the criterion were experimentally obtained in the diffusive-convective and diffusive-advective flow states respectively.     

Numerical analyses of high‐beta spherical tokamaks with varied elongation

The effect of plasma elongation on the second‐stable spherical tokamak (ST) was numerically studied using the experimentally measured pressure and current profiles of ultrahigh‐beta STs. The maximum beta of ST over 50% was obtained in the TS‐3 ST/CT experiment by applying an external toroidal field to an FRC. It was found that the marginal beta for the ballooning instability increased with the plasma elongation κ of ST. The elongated STs with κ > 2 have the magnetic shear (S)–pressure gradient (α) profiles located in the second‐stable regime for the ballooning mode and the stability margin increased with κ. The close relation between the absolute minimum‐B profile and the second stability was documented. The effect of elongation on maximum beta was observed to saturate when κ exceed 3, indicating that the optimized elongation for high‐beta STs is located around 2 < κ < 3. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 155(4): 1–6, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20132     

Fast‐scale bifurcation in power‐factor‐correction buck‐boost converters and effects of incompatible periodicities

In this paper, we derive the discrete‐time model for the power‐factor‐correction (PFC) buck‐boost converter in terms of a stroboscopic switching map. Fast‐scale instability is analysed through a fold diagram, which exposes the periodicity of the operation as well as the locations of the critical phase angles of the line voltage at which instability begins to occur along a half‐line cycle. The asymmetrical locations of the critical phase angles along a half‐line cycle is explained in terms of ‘under‐developed’ bifurcation. Border collision bifurcations are observed and analysed in detail. Copyright © 2006 John Wiley & Sons, Ltd.     

Development of ultra light magnesium-lithium alloys

Magnesium-lithium alloys are among the lowest density metallic materials. Addition of lithium, with a relative density of 0·53, in magnesium reduces the density of the alloy significantly. Furthermore, addition of nearly 11 wt.% lithium converts hexagonal close packed structure of pure magnesium to a body centered cubic lattice, markedly improving formability of the alloy. The development of these alloys, however, had been hampered due to the high reactivity of lithium and magnesium in the molten state and also, due to poor creep resistance and instability of mechanical properties at room temperature. In an attempt to indigenize these ultra light alloys for possible applications in Indian satellite programme, detailed research work was initiated in DMRL. The difficulties associated with producing sound cast ingots have been overcome by controlling melting and casting parameters of these alloys. Extensive work has been done on structure-property correlation of alloys with varying lithium content and minor alloying additions. Based on these work, advanced magnesium-lithium alloys have been developed with improved tensile properties, room temperature stability and creep resistance. Wrought products (plates/sheets) of magnesium-lithium alloy have been supplied to ISAC, Bangalore and are being used in their INSAT-2 programme. This paper describes the systematic studies carried out in the laboratory to indigenize these ultra light alloys.     

Instability due to Viscosity Stratification Downstream of a Centerline Injector

A common injector geometry upstream of a static mixer is the centerline injector. A flow instability can arise due to viscosity differences between the injected core‐flow and the outer co‐flow. This instability can adversely affect the effectiveness of the mixing operation. An experimental investigation of miscible viscosity‐stratified flow in a circular geometry was performed using Laser Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV). The experimental results for the stable region agree with the analytical results. The unstable region exhibits different modes depending on the viscosity ratio, volume flux ratio, and Reynolds number. The modes include wavy core‐flow with fissures and wavy core‐flow with core breakup. The time‐averaged experiment velocity profiles for the unstable core indicate a broadening of the jet at the centerline, which is consistent with the LIF visualization.