Composite ceramics fabricated from “blotting paper” technology

The “blotting paper” technology is developed to fabriacte composite materials. It consists of heating the initial oxide of a given composition to a temperature interval where it decomposes intothe necessary solid oxide and a liquid infitrating a porous medium as the matrix. Laminated composites could be fabricated with relatively high critical temperature, T_c at which the material exhibits superconductive property.     

Expanding the limits of “equilibrium” second-moment turbulence closures

The paper discusses possibilities for refinements of conventional “equilibrium” second-moment turbulence closure models, aimed at improving model performances in predicting turbulent flows of greater complexity. In focus are the invariant modelling of the low-Re-number and wall proximity effects, as well as extra strain-rates and control of the turbulence length-scale. In addition to satisfying most of the basic physical constraints, the main criterion for model validation was the quality of reproduction of flow and turbulence details, particularly, in the vicinity of a solid wall, in a broad variety of non-equilibrium flows featured by different phenomena. It is demonstrated that the new model, which includes several new modifications, but also some proposed in the past, can satisfactorily reproduce a range of attached and separating flows with strong time- or space-variations or abrupt changes of boundary conditions. Cases considered cover a wide range of Re-numbers involving in some cases also the laminar-to-turbulent or reverse transition.     

Observations of “granular jump” in the pneumatic conveying system

This paper presents a preliminary study of a previously unreported phenomenon of the “gas driven granular jump”, observed in the gas–solids flow within the pneumatic conveying system. From the phenomenological point of view, it resembles the already known processes such as hydraulic jumps in shallow water or granular jumps in granular flows in chutes or avalanches (although it seems most appropriate to explain it by analogy to a propagating granular bore). Clearly, unlike in classical phenomena of this type, the flow itself is driven by the aerodynamic forces related to the gas flow and the behaviour of the front of the “jump” is modified significantly by their presence. A series of high-speed camera visualisations are presented, which focus on this unusual behaviour of the flow on the border-line between cluster and stratified flow regimes in a horizontal pipe. Some similarities are drawn between the observed phenomenon and the broader class of problems exhibiting transition between super- and sub-critical flows. The fluid dynamical aspects and possible mechanisms behind the new phenomenon are discussed and the results obtained are compared quantitatively with simple theoretical models.     

Turbulent pipe flow of a drag-reducing rigid “rod-like” polymer solution

Fully developed turbulent pipe flow of an aqueous solution of a rigid “rod-like” polymer, scleroglucan, at concentrations of 0.005% (w/w) and 0.01% (w/w) has been investigated experimentally. Fanning friction factors were determined from pressure-drop measurements for the Newtonian solvent (water) and the polymer solutions and so levels of drag reduction for the latter. Mean axial velocity u and complete Reynolds normal stress data, i.e. u′, v′ and w′, were measured by means of a laser Doppler anemometer at three different Reynolds numbers for each fluid. The measurements indicate that the effectiveness of scleroglucan as a drag-reducing agent is only mildly dependent on Reynolds number. The turbulence structure essentially resembles that of flexible polymer solutions which also lead to low levels of drag reduction.     

Restoration of charpy toughness for “in service” temper embrittled steels

This paper describes a detailed examination which could be conducted during a planned outage. It is concerned with assessing reverse temper embrittlement of CrMoV steel turbine bolts after 120 000 h of service. A small section of material was removed from a non-critical location of all the 92 IP and HP bolts. From this section, the chemical composition, average hardness and average prior austenite grain size were measured. The toughness of the bolts was measured by Charpy impact testing and/or Auger electron spectroscopy. From the various parameters investigated, it was established that grain size and phosphorus level were the only factors which consistently identified whether a bolt was embrittled or non-embrittled. Indeed, at a phosphorus level of 0.01%, bolts with grain sizes less than 20 μm were not embrittled while those with larger grain sizes suffered reverse temper embrittlement during service. An embrittlement estimative diagram was established by plotting grain size versus phosphorus level (%P). This portrayed two distinct regimes, an embrittled and non-embrittled regime which were separated by a critical embrittled-non-embrittled interface which could be described by d × (%P) = 0.18 where d is the grain size in μm. Such an embrittlement estimate diagram represent a cogent and practical route in the identification of in-service embrittled bolts.     

Report on a symposium on “Computational approaches to disperse multiphase flow”

Some themes and developments emerged in the course of an IUTAM Symposium on “Computational Approaches to Disperse Multiphase Flow” held at Argonne National Laboratory on October 4–7 2004 are briefly summarized.     

“Wheels vs. tracks” – A fundamental evaluation from the traction perspective

The issue of wheeled vehicles vs. tracked vehicles for off-road operations has been a subject of debate for a long period of time. Recent interest in the development of vehicles for the rapid deployment of armed forces has given a new impetus to this debate. While a number of experimental studies in comparing the performances of specific wheeled vehicles with those of tracked vehicles under selected operating environments have been performed, it appears that relatively little fundamental analysis on this subject has been published in the open literature, including the Journal of Terramechanics. This paper is aimed at evaluating the tractive performance of wheeled and tracked vehicles from the standpoint of the mechanics of vehicle–terrain interaction. The differences between a tire and a track in generating thrust are elucidated. The basic factors that affect the gross traction of wheeled and tracked vehicles are identified. A general comparison of the thrust developed by a multi-axle wheeled vehicle with that of a tracked vehicle is made, based on certain simplifying assumptions. As the interaction between an off-road vehicle and unprepared terrain is very complex, to compare the performance of a wheeled vehicle with that of a tracked vehicle realistically, comprehensive computer simulation models are required. Two computer simulation models, one for wheeled vehicles, known as NWVPM, and the other for tracked vehicles, known as NTVPM, are described. As an example of the applications of these two computer simulation models, the mobility of an 8 × 8 wheeled vehicle, similar to a light armoured vehicle (LAV), is compared with that of a tracked vehicle, similar to an armoured personnel carrier (APC). It is hoped that this study will illustrate the fundamental factors that limit the traction of wheeled vehicles in comparison with that of tracked vehicles, hence contributing to a better understanding of the issue of wheels vs. tracks.     

Low-density layer formation and “lifting force” effect at micro- and meso-scale levels

The processes at various scale levels in the contact area of interacting objects under high-energy action will be examined from the viewpoint of mesomechanics. Modeling of contact area at atomic- and meso-scale levels was carried out on the base of discrete computational approach (method of particles). Molecular dynamic method was used at the micro-scale level; movable cellular automata method—at the meso-scale level. The gradient of velocity in areas near the surface leads to formation of low density and fragmented areas. This effect is accompanied by the failure of crystal lattice stability and intensive mixing process at the atomic level. The mechanisms of mass transfer in contact area were discussed. The results allow us to explain a host of experimental data of mechanochemistry such as phase formation at friction surface, alloy formation due to contact interaction under “pressure + shear” loading conditions.     

“Variable mass braking system” un nouveau systeme de freinage de vehicule automobile

This paper proposes an alternative to the standard anti-lock braking system.We call it “variable mass braking system”.This new system of braking avoids the locking of the wheels mostly in the case of high speeds; that cannot do the first one.These two systems of braking, are not incompatible; and if they are coupled, they will be able to complete each other harmoniously; the first operating in the case of low speeds requiring no great braking forces for the immobilization of the vehicle without blockage of the wheels, the second operating rather in the case of the high speeds, that requires great braking forces for the instantaneous stop of the vehicle without blockage of the wheels.     

Analytical simulation as a support tool in the decision process “wheels or tracks”

In the development of a weapon system, the maximization of demands concerning features like fire power, armor and mobility often leads to incompatibilities within the overall systems, to an imbalanced design and excessively high cost. The consequence is a decrease in mission availability. Therefore, system analysis utilizing simulation techniques is being used more and more in the early stages of the development process. This paper describes the application of such simulation techniques to practical cases. First, the development of a system is described by which to select wheeled or tracked running gears for combat vehicles considering the compatibility of fire power, armor, mobility and cost. Secondly, the possible upper weight ranges for wheeled combat vehicles are discussed, using analytical modeling of vehicle mobility as a basis. The methods used in these cases can also be applied to non-military problems.     

Interaction and “apparent” reconnection of 3D vortex tubes via direct numerical simulations

We present evidence of: “binding” of anti-parallel vortex tube segments; strong noncircular core development; evolution of new secondary finger-like vortex structures: and finally “apparent” vortex reconnections due to entanglement. The latter three processes are not present in Biot-Savart filament simulations.     

A “quasi-elastic” affine formulation for the homogenised behaviour of nonlinear viscoelastic polycrystals and composites

The derivation of the overall behaviour of nonlinear viscoelastic (or rate-dependent elastoplastic) heterogeneous materials requires a linearisation of the constitutive equations around uniform per phase stress (or strain) histories. The resulting Linear Comparison Material (LCM) has to be linear thermoviscoelastic to fully retain the viscoelastic nature of phase interactions. Instead of the exact treatment of this LCM (i.e., correspondence principle and inverse Laplace transforms) as proposed by the “classical” affine formulation, an approximate treatment is proposed here. First considering Maxwellian behaviour, comparisons for a single phase as well as for two-phase materials (with “parallel” and disordered morphologies) show that the “direct inversion method” of Laplace transforms, initially proposed by Schapery (1962), has to be adapted to fit correctly exact responses to creep loading while a more general method is proposed for other loading paths. When applied to nonlinear viscoelastic heterogeneous materials, this approximate inversion method gives rise to a new formulation which is consistent with the classical affine one for the steady-state regimes. In the transient regime, it leads to a significantly more efficient numerical resolution, the LCM associated to the step by step procedure being no more thermoviscoelastic but thermoelastic. Various comparisons for nonlinear viscoelastic polycrystals responses to creep as well as relaxation loadings show that this “quasi-elastic” formulation yields results very close to classical affine ones, even for high contrasts.     

Stability analysis of annular flow structure, using a discrete form of the “two-fluid model”

The differential form of the “two-fluid model” for annular flow, neglecting surface tension, is ill-posed, and it is not suited for examining the stability of the steady-state solutions with respect to the average film thickness. It is shown here that a discrete (difference) representation of the two-fluid model may lead to an appropriate criterion for the stability of the steady-state solutions. Exactly the same criterion is obtained from the requirement that the kinematic waves will propagate in the downstream direction. The suggested discrete form of the “two-fluid model” is used to perform transient simulation and for examining the system response to finite disturbances.     

Influence of geometrical parameters on the mode of collapse of a “pinched” rigid-plastic cylindrical shell

The modes of collapse of a rigid-plastic circular cylindrical shell subjected to centrally applied, opposed point loads in which the deformation is not confined to the vicinity of the loaded points are analysed. It is shown that the solution is analogous to that of an equivalent beam-foundation problem and the analogy is discussed. The results are compared with observations made during a series of experiments concerned with the crushing of aluminium tubes by opposed indenters.     

Tests of a new numerical scheme on a “non-reflection and free-transmission” open-sea boundary for longwaves

A new numerical scheme of a “non-reflection and free-transmission” boundary for longwave equations proposed by Hino (1987) has been tested for a variety of cases. The test results verify the effectiveness of the method for (a) a single progressive wave train on a horizontal bottom, (b) two wave trains each propagating in opposite directions on a horizontal bottom, (c) a single wave train propagating on a sloping bottom with friction, (d) oscillatory flood waves in an open channel flow, (e) two-dimensional waves travelling obliquely to open boundaries and (f) water surface oscillation in a harbor by waves incident through an opening.