Showing papers in "Doklady Physics in 2006"

Nonlinear dynamics for the solidification of binary melt with a nonequilibrium two-phase zone

Abstract: The processes of crystallization of melts and solutions are usually described by the Stefan frontal model. In application of this model, it is necessary to determine the impurity concentration and temperature of the substance in both the melt and the growing crystal. In addition, the phase-transition boundary is also unknown, and the phase-transition temperature (liquidus) in the two-component system depends on the local impurity concentration and is determined from the phase diagram [1]. As is well known, the solidification of binary melts is sufficiently often accompanied by arising additional supercooled zones, i.e., domains in the liquid phase ahead of the crystallization front, with a temperature in these zones being lower than the liquidus temperature. One of the mechanisms governing the appearance of the supercooling in a melt was indicated in [2] and is referred to as the concentration supercooling. Due to bulk nucleation in the supercooled domain, the spontaneous generation of solid-phase elements in the form of dendrites or particles can begin. Thus, in the concentration-supercooling zone, the substance can exist in both solid and liquid states. This domain is called the two-phase zone. Numerous papers are devoted to investigating the processes of solidification with supercooled domains (see, e.g., [1, 3‐5]). However, in many studies, the twophase zone is considered in the quasi-equilibrium approximation [6]. This implies that the supercooling is completely destroyed due to the release of the crystallization latent heat by growing solid-phase elements. Thus, the substance temperature in the zone is equalized attaining the liquidus temperature. This fact essentially simplifies solving the problem, however, the solution presents no information on the internal structure and the topology of the zone. In addition, in the general case, the supercooling disappears not entirely. Therefore, in constructing a theory of the solidification in the presence of the two-phase domain supercooling, it is necessary to include into the theoretical scheme kinetic factors responsible for the generation of new-phase elements in the zone. In this paper, we offer a method for the theoretical study of the directional solidification process in a binary melt with a two-phase zone in which the supercooling is noticeable. We consider a zone as a suspension of interacting crystalline particles arising and growing in a supercooled melt. The nucleation rate for solid particles is determined by the Frenkel’‐Zel’dovich formula [7‐9]

New effects in dynamics of rattlebacks

Abstract: 272 The dynamics of a rattleback simulated by a heavy balanced ellipsoid of revolution rolling without sliding on a fixed horizontal plane is studied. The central ellipsoid of inertia is also an ellipsoid of revolution. In the presence of an angular misalignment between the two ellipsoids (characterizing the dynamic asymmetry of the body), new dynamic effects similar to the reverse phenomena in rattleback dynamics can be observed. However, in contrast to the conventional rattleback model in the form of a truncated biaxial paraboloid, the considered formulation allows motions representing the superposition of reverse motion (reversal of the spinning direction) and turnover (reversal of the spin axes). At appropriate values of energy and mass distributions, the reverse and turnover effect can occur many times. Such motions as repeated turnover or repeated reverse are also possible.

Gas Hydrate Formation in a Gas-Liquid Mixture Behind a Shock Wave

Abstract: 621 In this work, a new shock-wave technique for intensifying the process of hydrate formation of gases in gas‐liquid mixtures is proposed. There exist different methods for increasing the rate of this process, namely, fine dispersion of a jet saturated by a gas in a gas atmosphere [1‐3], intense stirring of water saturated by a gas dissolved in it [2, 4], exposure of a gas-saturated liquid to vibration [5], and exposure of a medium to ultrasound [6]. The main disadvantage of these methods is the low rate of gas hydrate formation and, as a consequence, the low efficiency of setups based on these techniques. In the absence of pipelines, one of the promising methods for transportation of natural gas consists in converting it into the gas hydrate state and then transporting it in a solid state at atmospheric static pressure and a low temperature (‐10 ° C to ‐20 ° C). The estimates made by Japanese and Norwegian scientists show that the gas hydrate technique for transportation and storage of natural gas is most cost-effective for small gas fields and shelf natural gas fields. A possible way of using the method proposed is the crystal hydrate method for demineralization of mineralized water. Using freon hydrates for this purpose is simplest from the engineering standpoint and most cost-effective [7]. In [8‐10] and some other books, the properties of gas hydrates and the main conditions and features of their formation are described and the mechanisms of gas hydrate formation and the types of their crystallization are presented. Much attention is given to physical and chemical methods for studying both man-made and natural gas hydrates. In this study, we experimentally investigate the process of gas bubble fragmentation and dissolution with the formation of freon-12 hydrate behind a shock wave of a moderate amplitude in water with gas bubbles. We carry out a theoretical analysis of the process of hydrate formation behind a steplike shock front and compare the results to the experimental data. The experiments were conducted in a shock tube. The test section was a 1.5 m-long vertical thick-walled steel tube with an inner diameter of 53 mm, which was limited from below by a rigid wall. The test section was filled with water containing freon-12 bubbles and was thermostatically controlled. The bubble dimensions were determined by capturing them with a digital camera equipped with additional optics through optical windows made in the upper part of the test section with a required time lag behind the shock front. The steplike pressure waves were generated by the breakdown of a diaphragm separating a 2 m-long high-pressure chamber and the test section. The pressure wave profiles were recorded by pressure transducers located along the test section and mounted flush with its inner wall. The local profile of the variation of the gas content (by volume) behind the shock wave was measured by a conductivity transducer placed at the middle part of the test section. Signals from the transducers were applied to an analog-to-digital converter and were then com

On the determination of eigenfrequencies for nanometer-size objects

Abstract: At present, the problem of the experimental determination of mechanical characteristics for nanometersize objects (nanoobjects) is urgent. One of the most efficient methods for the determination of elastic-moduli is based on the measurement of the eigenfrequencies of objects under study. However, the measurement of nanoobject frequencies, in particular, on the basis of optical methods turns out to be problematic [1]. The main, though not exclusive, limitation upon the application of optical methods is the fact that the laser-beam cross section is not a point ...

Transition to chaos in dynamics of four point vortices on a plane

Abstract: We briefly dwell on the main forms of the equations and the first integrals of the dynamics of point vortices on a plane; for a more detailed presentation, see [1, 12], which also contain the hydrodynamic assumptions under which these equations are adequate. The equations of motion of n point vortices with Cartesian coordinates ( x i , y i ) and intensities Γ i can be written in the Hamiltonian form

The influence of riblets on the development of a Λ structure and its transformation into a turbulent spot

Abstract: 144 Riblets are passive elements mounted on a smooth wall surface in a turbulent boundary layer, which can reduce friction drag up to 10%. These elements have the form of streamwise grooves with triangular or hemispherical cross section whose dimensions are comparable with those of a viscous sublayer. The systematic investigations of riblets as a means of reducing turbulent friction began in the late 1970s at the NASA Research Center in Langley [1‐4]. These investigations showed that the friction drag decreases when the dimensionless parameter of riblets s + = is approximately equal to 15. However, the drag increases at s + = 30. For the maximum drag reduction, the riblets must be oriented in the direction of the local velocity of the viscous sublayer. The net drag reduction is almost linearly proportional to the coverage of the entire streamlined surface by the riblets. The near-wall turbulent structure of flows on riblets was extensively studied using physical [3, 5, 6] and numerical experiments (DNS) [7, 8]. According to one of the hypotheses put forward in order to explain the drag reduction in the turbulent boundary layer on riblets, they modify coherent structures in the viscous sublayer. It was found that riblets operate as an obstacle for the transversal (spanwise) oscillations of streamwise vortices, which results in drag reduction [9]. In other words, riblets reduce the friction drag in the turbulent boundary layer by changing the sequence of the nearwall vortex dynamics by means of a passive spanwise su* ν

Stress relaxation of elastic precursor for unalloyed uranium and some uranium-based alloys

Abstract: The new data are presented on relaxation of an elastic precursor in unalloyed depleted uranium and two its alloys. Results were obtained under low-intense explosive loading. Statistic thermofluctuational model was used for approximation of experimental data. The inversion of strength properties of the tested U-Mo and U-Fe-Ge alloys at their quasi-static and high-rate loading was revealed.