An insight into the mechanical behavior of binary granular soils

This paper investigates the participation of the fines fraction in the load-carrying structure of binary mixtures of granular soils. For this purpose, various fractions of two fine sands were added to two coarse sands with the same particle size distribution, but different particle shape characteristics. Based on the results of 144 direct shear tests, it was found that fines participation in the load-bearing structure increases with fines content. At the same fines content, the participation of the fines in the load-carrying structure of loose mixtures is greater than in samples that were initially compacted. In addition, it was observed that fines participation rises with the increase in the average size of the fines fraction.     

An insight into the mechanical behavior of binary granular soils

This paper investigates the participation of the fines fraction in the load-carrying structure of binary mixtures of granular soils.For this purpose,various fractions of two fine sands were added to two coarse sands with the same particle size distribution,but different particle shape characteristics.Based on the results of 144 direct shear tests,it was found that fines participation in the load-bearing structure increases with fines content.At the same fines content,the participation of the fines in the load-carrying structure of loose mixtures is greater than in samples that were initially compacted.In addition,it was observed that fines participation rises with the increase in the average size of the fines fraction.     

New insight on acoustoplasticity - Ultrasonic irradiation enhances subgrain formation during deformation

Many industrial applications make use of ultrasonic vibration to soften metals. The existing understanding of such an acoustoplastic effect is one in which the ultrasonic irradiation either imposes additional stress waves to augment the quasi-static applied load, or causes heating of the metal, whereas the metal’s intrinsic deformation resistance or mechanism is assumed to be unaltered by the ultrasound. In this study, indentation experiments performed on aluminum samples simultaneously excited by ultrasound reveal that the latter intrinsically alters the deformation characteristics of the metal. The deformation microstructures underneath the indents were investigated by a combination of cross-sectional microscopic techniques involving focused-ion-beam milling, transmission electron microscopy and crystal orientation mapping by electron backscattered diffraction. The softening effect of the ultrasound is found to constitute recovery associated with extensive enhancement of subgrain formation during deformation. By comparing the microstructures of samples deformed with and without simultaneous application of ultrasound, and those subsequently excited by ultrasound after deformation, the enhanced subgrain formation is proved to be one due to the combined application of the quasi-static loading and the ultrasound, but not a simple addition of the two. Similarly, by comparing with samples deformed while being simultaneously or subsequently heated up, the enhanced subgrain formation by the ultrasound is proved to be a lot greater than that due to the heat that it generates within the metal. Such effects of the ultrasound are interpreted by its ability to enhance dipole annihilation. The superimposed ultrasound causes dislocations to travel longer distances in a jerky manner, so that they can continuously explore until dipole annihilation.     

New insight into two-color LIF thermometry applied to temperature measurements of droplets

When laser-induced fluorescence of droplets is used for measurements such as droplet temperature, a new dependence of the droplet size on the spectral distribution of fluorescence has been highlighted. The two-color laser-induced fluorescence technique applied to droplet temperature measurement requires a single fluorescent tracer and two spectral bands of detection for which the temperature sensitivity is different. Generally, the ratio of the intensities measured on each of the spectral bands of detection is assumed to be only temperature dependent. However, droplet dependence on diameter is also likely to influence the intensities ratio. This study provides some illustrations of the phenomenon, first on sprays with different mean statistical diameters and secondly on single droplets, for two temperature-sensitive fluorescent tracers in their solvents: sulforhodamine B dissolved in water and pyrromethene 597-8C9 dissolved in n-decane.     

Rheology of crosslinking poly vinyl alcohol systems during film formation and gelation

Gelation of crosslinking polymers, with ionic groups, is of interest in membrane processing of these materials. The gelation of polyvinyl alcohol in the presence of an ionic crosslinker, sulfosuccinic acid, was monitored through rheological measurements. The evolution of rheological material functions during film formation, which involves solvent evaporation and crosslinking, was observed at different polymer and crosslinker concentrations. Relative effect of water evaporation leading to physical gelation and the chemical crosslinking was examined. To understand the effect of crosslinker type, the rheology of nonionic crosslinker, glutaraldehyde was also examined. The gel points as well as network parameters for these covalent networking systems were different and depended on crosslinker type and polymer/crosslinker concentrations. However, qualitative evolution of rheological behavior during film formation was largely similar, demonstrating the dominating effect of solvent evaporation. Crosslinking in the absence of solvent evaporation was examined at different temperatures. It can be concluded that similar mechanism was involved, independent of temperature, in the temperature range of interest. The effect of crosslinker concentration on evolution of rheological properties near gelation was insignificant for ionic crosslinker though marginally significant for nonionic crosslinker.     

Vortex formation process in gravity-driven starting jets

Experiments were conducted to investigate the vortex formation process in gravity-driven starting jets at three Reynolds numbers ( 3,528 and 4,716, where D is the nozzle diameter, the average discharging velocity and ν the kinematic viscosity). Planar laser induced fluorescence (PLIF) was used to visualize the flow while particle image velocimetry (PIV) was used to quantify the 2D velocity and vorticity fields. Vortex leapfrogging was found to appear at Re = 2,358, while pinch-off for the leading vortex was observed in the other two cases. Conditions for the vortex pinch-off appeared to be different from those found in the piston-driven starting jets. Although the leading vortex attained the maximum circulation and energy level very quickly after the jet was initiated, its detachment from the main stem may not be achieved necessarily. Attempt had been made to elaborate the different flow characteristics for respective cases from the related distribution of circulation and energy level via the measured velocity field by PIV.     

Mind the gap: a new insight into the tip leakage vortex using stereo-PIV

The tip leakage vortex (TLV), which develops in the clearance between the rotor and the stator of axial hydro turbines, has been studied for decades. Yet, many associated phenomena are still not understood. For instance, it remains unclear how the clearance size is related to the occurrence of cavitation in the vortex, which can lead to severe erosion. Experiments are here carried out on the influence of the clearance size on the tip vortex structure in a simplified case study. A NACA0009 hydrofoil is used as a generic blade in a water tunnel while the clearance between the blade tip and the wall is varied. The 3D velocity fields are measured using Stereo Particle Image Velocimetry (SPIV) in three planes located downstream of the hydrofoil for different values of the upstream velocity, the incidence angle and a large number of tip clearances. The influence of the flow conditions on the structure of the TLV is described through changes in the vortex intensity, core axial flow, vortex center position and wandering motion amplitude. Moreover, high-speed visualizations are used to highlight the vortex core trajectory and clearance flow alteration, turning into a wall jet as the tip clearance is reduced. The measurements clearly reveal the existence of a specific tip clearance for which the vortex strength is maximum and most prone to generating cavitation.     

The elastic behaviour of gels during the structure formation process

Summary A simple modification of the stress-strain relation for a normal viscoelastic material has been derived for a gel with temporary structural bonds which is subjected to small deformations during gelation. This shows that the storage modulus should separate into two parts, one dependent on frequency and the other on the time for which the gel has been allowed to set. The loss modulus should depend only on frequency. These theoretical results, and others concerning creep under constant stress are in general agreement with observations on a number of gels. Exceptions occur, however, in the period immediately after preparation (before continuous structure has formed) and also after long periods of time during which irreversible changes can have occurred. Certain anomalies appear when the basic assumption of the theory are applied to structure formation in gels with permanent bonds, and a possible resolution of these is suggested.

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Zusammenfassung Es wird eine einfache Modifikation der Spannungs-Dehnungs-Beziehungen normaler viskoelastischer Stoffe abgeleitet, durch die das Verhalten eines Gels mit temporärer Bindungsstruktur bei kleinen Deformationen während des Geliervorgangs beschrieben werden kann. Diese sagt voraus, daß der Speichermodul in zwei Teile aufspaltet, wobei der eine von der Frequenz und der andere von der Erstarrungszeit des Gels abhängt. Dagegen sollte der Verlustmodul nur von der Frequenz abhängig sein. Diese theoretischen Voraussagen, sowie die daraus abgeleiteten Folgerungen bezüglich des Kriechens unter konstanter Spannung, stimmen durchweg mit den an einer Anzahl von Gelen gemachten Beobachtungen überein. Abweichungen kommen dagegen in der Zeit unmittelbar nach der Herstellung vor (bevor sich eine durchgehende Struktur gebildet hat) und nach einer so langen Zeitspanne, daß irreversible Strukturänderungen entstanden sein können. Gewisse Anomalien treten in Erscheinung, wenn die Grundvoraussetzungen der Theorie auf die Strukturbildung bei Gelen mit permanenten Bindungen angewendet werden. Hierfür wird eine mögliche Deutung vorgeschlagen.

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With 3 figures     

Density and total pressure behavior in the process of forced and spontaneous reconnection

The specific features of the density and total pressure (the sum of magnetic and gas-kinetic pressures) behavior in the process of forced (by the example of propagation of a magnetic acoustic wave in the vicinity of the X-point) and spontaneous (by the example of development of a helical Thirring mode) reconnection are considered. It is shown that the total pressure distribution depends weakly on the initial value of the gas-kinetic pressure, the strength of the z-component of the magnetic field, and the thermal conductivity in a wide range of parameters. The character of density distribution is determined only by the thermal conductivity. It is also shown that the behavior of the total pressure and density in the case of spontaneous reconnection weakly depends on the thermal conductivity, in contrast to the forced case. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 4, pp. 11–15, July–August, 1999.     

In-cylinder swirl formation process in a four-valve diesel engine

An experimental study is presented on the steady flow in a four-valve diesel engine rig by using hot-wire anemometry. Through the analysis of the in-cylinder three-dimensional flow field of a four-valve diesel engine, the in-cylinder swirl formation process at various valve lift was investigated. A new criterion is proposed for predicting the emergence of a stable-swirl formation interface, based on swirl angular momentum flux. A stable-swirl formation interface exists when the main swirl angular momentum flux is nearly equal to the in-cylinder air total angular momentum flux. Received: 13 March 2000/Accepted: 7 February 2001     

On the horizontal distribution of algal-bloom in Chaohu Lake and its formation process

Based on the remote sensing images of algae, the present work analyzes the horizontal distribution characteristics of algal blooms in Chaohu Lake, China, which also reveals the frequency of algal blooms under different wind directions. Further, an unstructured-grid, three-dimensional finite-volume coastal ocean model （FVCOM） is applied to investigate the wind-induced currents and the transport pro- cess to explain the reason why algal blooms occur at the detected places. We first deduce the primary distribution of biomass from overlaid satellite images, and explain the formation mechanism by analyzing the pollution sources, and simulating the flow field and transportation process under prevailing wind over Chaohu Lake. And then, we consider the adjustment action of the wind on the corresponding day and develop a two-time scale approach to describe the whole formation process of algae horizontal distribution in Chaohu Lake. That is, on the longer time scale, i.e., during bloom season, prevailing wind determines the primary distribution of biomass by inducing the characteristic flow field; on the shorter time scale, i.e., on the day when bloom occurs, the wind force adjusts the primary distribution of biomass to form the final distribution of algal bloom.     

Robust numerical analysis of the dynamic bubble formation process in a viscous liquid

The dynamics of bubble formation from a submerged nozzle in a highly viscous liquid with relatively fast inflow gas velocity is studied numerically. The numerical simulations are carried out using a sharp interface coupled level set/volume-of-fluid (CLSVOF) method and the governing equations are solved through a hydrodynamic scheme with formal second-order accuracy. Numerical results agree well with experimental results and it is shown that the sharp interface CLSVOF method enables one to reproduce the bubble formation process for a wide range of inflow gas velocities. From numerical results, one can improve their understanding of the mechanisms regarding the dynamics of bubble formation. For example, it is found that for some sets of parameters that the bubble formation process reaches steady state after several bubbles are released from the nozzle. At steady state, bubbles uniformly rise freely in the viscous liquid. It is observed that the fluid flow around a formed bubble has a significant role in determining the overall dynamic process of bubble formation; e.g. the effect of the fluid flow from the preceding bubble can be seen on newly formed bubbles.     

Numerical study on the shear-flow behavior and transport process in rough rock fractures

This paper presents a systematic research for understanding mechanical shearing effects on the fluid flow and the solute transport behavior of rough fractures through a numerical simulation approach. The aperture fields were modeled based on a real rock fracture geometry and the normal displacement obtained from the shear-flow test. The fluid flow through the rough fracture under shear was simulated using a finite element code that solves the Reynolds equation, and the transport behavior through the rough fracture under shear was simulated calculating the advection–dispersion equation. The results show that the fracture apertures increase as the shear displacement increases, with a few major flow channels detected through the fracture. The shear-induced flow channels increase both flow connectivity and transport connectivity, which accelerate the movement of solutes in a particular direction and lead to early breakthrough of the contaminants. Adsorption, acting as a retardation term, has a decisive influence on the transport process. These results can give a basic knowledge of the hydromechanical and solute transport progress through fracture, and will be helpful to safety assessment for high-level radioactive waste disposal facilities.