MATHEMATICAL MODELING OF THERMOGRAVITATIONAL AND THERMOCA-PILLARY CONVECTION IN GAS-LIQUID PROCESSES

. The interaction of gas and liquid phases in some cases is accompanied by the spontaneous occur-rence of convective flows and turbulent pulsations at the phase boundary and in adjacent areas. Hy-drodynamic instability allows to accelerate the interfacial transfer of matter and leads to an increase in mass transfer coefficients. Research in this field is not only theoretical, but also practical, since sur-face convection can be artificially created in apparatus for intensifying the mass exchange process.

Prandtl’s Secondary Flows of the Second Kind. Problems of Description, Prediction, and Simulation

Abstract— The occurrence of turbulent pulsations in straight pipes of noncircular cross-section leads to the situation, when the average velocity field includes not only the longitudinal component but also transverse components that form a secondary flow. This hydrodynamic phenomenon discovered at the twenties of the last century (J. Nikuradse, L. Prandtl) has been the object of active research to the present day. The intensity of the turbulent secondary flows is not high; usually, it is not greater than 2–3% of the characteristic flow velocity. Nevertheless, their contribution to the processes of transverse transfer of momentum and heat is comparable to that of turbulent pulsations. In this paper, a review of experimental, theoretical, and numerical studies of secondary flows in straight pipes and channels is given. Emphasis is placed on the issues of revealing the physical mechanisms of secondary flow formation and developing the models of the apriori assessment of their forms. The specific features of the secondary flow development in open channels and channels with inhomogeneously rough walls are touched upon. The approaches of semiempirical simulation of turbulent flows in the presence of secondary flows are discussed.

CONTRIBUTION OF NON-ISOTHERMAL JETS TO THE PROCESSES OF NOISE GENERATION OF ENERGY MACHINES WHEN INSTALLING SILENCERS

The method of James Lighthill is known and widely used, which allows determining the acoustic power of isothermal jets. A mathematical model for calculating the acoustic parameters (sound power, radiation pattern) of non-isothermal sound jets is proposed, taking into account the noise silencer installed in the gas exhaust tract. At the output, the equations of continuity, the amount of motion, energy, as well as the Lighthill wave equation are used. A statistical model is used as a turbulence model for calculations. A physical mechanism of noise generation by turbulent flows is proposed, which consists in considering "own" and "shear" noise. The " own " noise is caused by turbulent pulsations of the gas-dynamic flow, the "shift" noise is caused by the presence of a flow velocity gradient. Analytical dependences of the components of "own" and "shift" noise are obtained.

STATIONARY RESEARCH IN THE CENTRAL FOREST STATE NATURAL BIOSPHERE RESERVE

The article presents a brief history of the formation and development of long-term stationary research in the Central Forest State Natural Biosphere Reserve, starting from the first soil-geomorphological works in the 1930s and up to the creation in 2017 of the IPEE RAS ecological observatory "Okovsky Les". The main results of stationary research in two directions are shown: climatic (implemented at ecological and climatic stations of turbulent pulsations) and landscape-ecological (implemented on the basis of field measurements and processing of remote information). The long-term stationary observations on the territory of the reserve provide extensive material for the study of natural processes and can serve as a basis for creating a national network of environmental monitoring.

EFFECT OF TURBULENT PULSATIONS ON THE FRACTIONAL SEPARATION OF FINE POWDERS OF METAL NITRIDES

This paper presents a numerical study of a two-phase swirling turbulent flow in a separation element of a vortex chamber. The paper considers a process of fractional separation in the separation chamber with three particular features in its design. The main feature is the separation mobile element located at the outlet of the separation area. Changes in the position of this element can affect the boundary size and the sharpness of the separation. The second feature of the considered separation chamber is the presence of a rotating element along the vertical axis. This element helps to align the field of the circumferential velocity component. The third feature of this chamber is the presence of a branch pipe for the auxiliary gas flow supply intended to push the particles away from the chamber wall and to blow the separated particles in order to prevent them from agglomeration. In this work, the trajectories of the motion of fine nitride particles are calculated taking into account the turbulent diffusion effect. A possibility to control the boundary size of the particles, when moving the separation element located at the outlet of the vortex chamber, is demonstrated. Numerical study results show that the turbulent pulsations cause significant changes in the separation process and affect the boundary size and the sharpness of the separation.

Modified Algebraical Cebeci --- Smith Turbulent Viscosity Model for the Entire Surface of a Blunted Cone

In order to compute the intensity of laminar-turbulent heat transfer, algebraic or differential models are commonly used, which are designed to compute the contribution of turbulent pulsations to the transfer properties of the gas. This, in turn, dictates the necessity of validating these semi-empirical models against experimental data obtained under conditions simulating the gas dynamics inherent to the phenomenon as observed in practice. The gas dynamic patterns observed during gradient flow around fragments of aircraft structure (such as a sphere or a cylinder) differs qualitatively from the patterns revealed by the flow around the lateral surfaces of these fragments, which necessitates using various semi-empirical approaches in this case, followed by mandatory validation against the results of respective experimental studies. In recent years, there appeared scientific publications dealing with modifying one of the algebraic models designed to compute the contribution of turbulent pulsations in the boundary layer to the transfer properties of the gas; this was accomplished by making use of experimental data obtained for a hemisphere at extremely high Reynolds numbers. The paper proposes a similar modification of the same turbulence model, based on fitting a wide range of experimental data obtained for lateral surfaces of spherically blunted cones. As a result of the investigations conducted, we stated a method for computing laminar-to-turbulent heat transfer over the entire surface of a blunted cone; the accuracy of the method is acceptable in terms of most practical applications. We show that the computational method presented is characterised by minimum error as compared to the most widely spread methods for solving this problem

Mechanism of formation of Prandtl’s secondary flows of the second kind

Turbulent flow in a tube of square cross section is investigated numerically. The concentrated longitudinal vortices responsible for the occurrence of secondary flows are studied. It is shown that the longitudinal vortices are formed as a result of nonlinear interaction of turbulent pulsations in which the pulsations of the longitudinal component of vorticity are specially adjusted in phase with the pulsations of the longitudinalvelocity component. A physical interpretation of this mechanism is given.

The effect of increasing gas shear on wave structure of thin liquid films

Evolution of thin liquid films sheared by co-current gas stream in a vertical 11.7 mm pipe is studied experimentally using BBLIF technique. The main goal is to investigate the transition of wave patterns due to increase in the gas stream velocity, VG, from 0 to 24 m/s. Apart from relatively weak quantitative changes in the characteristics of the primary waves, replacement of capillary precursor by slow secondary waves is found. The transition is indentified in the range of VG = 8 - 16 m/s for all liquid flow rates. It is observed that the appearing secondary waves may be the main reason of the decay of the capillary precursor. The experimental results are compared to prediction of evolutionary theoretical model, showing qualitative agreement on secondary waves generation, but with no agreement on precursor's disappearance. Introducing artificial perturbations mimicking the action of turbulent pulsations in the gas phase is recommended to improve the model.

Role of Conservation Laws in the Development of Nonequilibrium and Emergence of Turbulence

It turns out that the equations of mathematical physics, which consist equations of the conservation laws for energy, linear momentum, angular momentum, and mass, possess additional, hidden, properties that enables one to describe not only a variation of physical quantities (such as energy, pressure, density) but also processes such as origination of waves, vortices, turbulent pulsations and other ones. It is caused by the conservation laws properties. In present paper the development of nonequilibrium in gasdynamic systems, which are described by the Euler and Navier-Stokes equations, will be investigated. Under studying the consistence of conservation laws equations, from the Euler and Navier-Stokes equations it can be obtained the evolutionary relation for entropy (as a state functional). The evolutionary relation possesses a certain peculiarity, namely, it turns out to be nonidentical. This fact points out to inconsistence of the conservation law equations and noncommutativity of conservation laws. Such a nonidentical relation discloses peculiarities of the solutions to the Navier-Stokes equations due to which the Euler and Navier-Stokes equations can describe the processes the development of nonequilibrium and emergence of vortices and turbulence. It has been shown that such processes can be described only with the help of two nonequivalent coordinate systems or by simultaneous using numerical and analytical methods.