Vol 60, No 7 (2019)

Structural elements made of shape memory alloys (SMAs) undergo cooling/heating and stresses changing in value and direction during operation. As a result, phase and structural transformations, which are accompanied by the shape memory effect, cross hardening and martensitic inelasticity, occur in a material. Moreover, a change in the stresses causes a shift in the phase-transition temperatures, and forward and reverse phase transitions are possible during an isothermal increase or decrease in the load (superelasticity phenomenon). The purpose of this work is to develop a phenomenological model in terms of a general approach to take into account these phenomena, since they substantially affect the state of stress in a construction. This model is based on the relation between forward transformation and martensitic inelasticity diagrams, which implies a general description of the strains of phase and structural transformations. This approach seems to be useful, since both strain components are caused by the formation of oriented martensite. A set of series-connected martensitic structural elements, each of which has a specific structural transformation limit (initial stress), is introduced into consideration. This limit depends on the conditions of appearance of an element during a phase transition and the subsequent deformation history. This approach can take into account the influence of, first, phase deformation and structural deformation on each other and, second, the deformation history on the subsequent transformation. To demonstrate the possibilities of the model, we solve the problem of joint deformation of a stack of SMA rods, which illustrates the evolution of the state of stress in the system during the simultaneous phase and structural transformations induced by an external thermomechanical action.

A coarse-grained molecular dynamics method is used to investigate a. microferrogel (MFG); a. small polymer object containing nanoparticles of magnetoactive filler. Despite the fact that an applied magnetic field acts directly on only the nanoparticles, structural and mechanical changes occur throughout the entire composite system due to the coupling between these particles and the polymer. The study of these changes is important in view of widely discussed prospects of using MFGs as field-controlled microcontainers for delivering bioactive substances or drugs. In this work, a. numerical model is proposed and realized, which enables one to perform a. detailed analysis of stationary states of an MFG suspended in a. neutral solvent. The effect of concentration of particles and their magnetic characteristics (magnetic moment magnitude, degree of uniaxial magnetic anisotropy, and interparticle dipolar coupling parameter) on the structure formation in the absence of an external magnetic field and under its influence is investigated. It is shown that the chain-like clusters are in fact the only type of aggregates arising in MFG. However, their effect on the polymer subsystem depends strongly on the type of magnetic anisotropy and on the magnetic phase concentration. This, in turn, entails different scenarios of the mechanical response of MFG and, in particular, differently affects the change in the amount of solvent present in the sample.

Measurement systems that are based on the hydrostatic leveling method under ideal conditions allow one to determine vertical displacements with accuracy on the order of a micron. Heterogeneous and time-varying environmental conditions have a significant effect on the measurement error. One method to increase the accuracy of results is to equalize the temperature of the fluid in the hydrostatic level by mixing the liquid inside it before taking measurements. In this paper, the possibility to perform this operation by forced circulation of the fluid is estimated. For this purpose, a model problem of the circulation flow created by a pump in a simplified analog of the hydrostatic level with allowance for the heat transfer through the hose wall is solved. The fluid dynamics is described by the Reynolds averaged Navier-Stokes equations which are closed by the Menter shear stress transport model. The analytically obtained estimates of heat transfer coefficients on the lateral surface of the hose are refined based on experiments at two values of the flow rate of water flowing through the pipe. The evolution of the temperature field is found from the numerical solution of the coupled heat transfer problem by the finite volume method. In a test example, in which two parts of the hydrostatic level are located in areas with markedly different temperature, the spatial inhomogeneity of the temperature field at different times is calculated. The mixing time sufficient to achieve a temperature distribution close to the homogeneous distribution of the flowing fluid in the hose at different volumes of the mixer joint with the hydrostatic level is determined. The proposed approach can be used under real external conditions for the selection of optimal operation parameters: the pump flow rate, mixing time, and mixing tank volume. The temperature field obtained in the calculation can serve as a basis for estimating the achievable accuracy of the measurement system.

At present, a significant weakening of the intensity of transverse mixing at the confluence of large rivers, which is observed in a number of cases, is widely discussed. Since the observed features of the confluence of large watercourses are not only of research interest but also of significant economic importance associated with the characteristics of water management at these water bodies, a large number of works are devoted to their study. Water resources management requires measures for the organization of water use which can be rational only under the understanding of processes occurring in water basins. To explain the phenomenon of suppression of the transverse mixing, which is interesting and important from the point of view of ecology, a wide range of hypotheses is proposed, up to the negation of turbulence in rivers. One of the possible mechanisms for explaining the suppression of transversal mixing can be the presence of transverse circulation manifesting itself as Prandtl’s secondary flows of the second kind. The characteristic velocity of these circulation flows is very small and difficult to measure directly by instruments; however, in our opinion, they can significantly complicate the transverse mixing at the confluence. The proposed hypothesis is tested in computational experiments in the framework of the three-dimensional formulation for dimensions of a real water object at the mouth of the Vishera River where it meets the Kama. Calculations demonstrate that, at sufficiently large flow rates, the two waters practically do not mix in the horizontal direction throughout the depth over long distances from the confluence. It has been found that a two-vortex flow is formed downstream the confluence, which just attenuates the mixing; the fluid motion in the vortices is such that, near the free surface, the fluid moves from the banks to the middle of the riverbed.

Turbulent convection of liquid sodium (Prandtl number Pr = 0.0093) in a cylinder of unit aspect ratio, heated at one end face and cooled at the other, is studied numerically. The flow regimes with inclination angles β = 0°, 20°, 40°, 70° with respect to the vertical are considered. The Rayleigh number is 1.5 × 107 . Three-dimensional nonstationary simulations allow one to get instant and average characteristics of the process and to study temperature pulsation fields. A mathematical model is based on the Boussinesq equations for thermogravitational convection with use of the LES (large-eddy simulations) approach for small-scale turbulence modeling. Simulations were carried out with a nonuniform numerical grid consisting of 2.9 × 106 nodes. It is shown that the flow structure strongly depends on β. The large-scale circulation (LSC) exists in the cylinder at any β. Under moderate inclination (β = 20°), the strong oscillations of the LSC orientation angle with dominant frequency are observed. Increasing the inclination up to 40° leads to stabilization of the large-scale flow and there is no dominant frequency of oscillations in this case. It is shown that more intensive temperature pulsations occur at small cylinder inclinations. At any β the regions with intensive pulsations are concentrated in the areas along low and upper cylinder faces. The maximum values of pulsations occur in the area close to lateral walls, where hot and cold fluid flows collide. The intensity of temperature pulsations decreases with increasing distance from the lateral walls. The Reynolds number which characterizes the total energy of the flow reaches its maximum value at β = 20° and then decreases with increasing β. The mean flow has maximum intensity at β = 40°. Turbulent velocity pulsation energy decreases monotonically with increasing inclination angle. It is shown that the inclination leads to an increase in heat transfer along the cylinder axis. The Nusselt number at β = 40° is 26% higher than that in the vertical cylinder.

The surfactant diffusion through the vertical interface in a system of two immiscible liquids filling a horizontal channel has been studied in a two-dimensional formulation. The densities of the base liquids were initially set equal to the surfactant density. Therefore, all the subsequent density variations in the system are determined only by the contraction effect. Under nonunifrom diffusion the interfacial tension is a function of the local surfactant concentration, which gives rise to Marangoni convection. Since there are uncontrolled surface-active impurities in the system, the capillary motion is initiated in a threshold manner. It is shown that at the initial stage, despite the presence of gravity, the Marangoni convection is in the form of a series of periodically emerging paired vortices located symmetrically relative to the channel axis (as in weightlessness conditions). As the vertical density difference in the channel increases, the number of vortex pairs is reduced to one. A full-scale experiment, during which the structure of the flows and surfactant concentration fields near the interface was visualized, has been performed to verify the results of numerical simulations. The dynamics of the oscillatory mode of convection has been studied. The results of the numerical and full-scale experiments have been shown to be in qualitative agreement. The pattern of the surfactant concentration fields and stream functions in the channel as well as the time dependence of the maximum value of the stream function are presented for several values of the Marangoni and Grashof numbers. It has been found that at sufficiently large Marangoni numbers (Ma ≥ 50 000) the diffusion process gives rise to instability in the system of immiscible liquids and a soluble surfactant, provided that their densities are equal, even in the absence of contraction.