Annotation of the results obtained in 2021
The analysis of the properties of the fundamental equations system is carried out, which makes it possible to describe the dynamics and structure of flows without invoking additional hypotheses and constants taking into account the distribution of media densities and the Gibbs potential as equations of state. When analyzing physical models of flows, four mechanisms of energy transfer are taken into account - with the flow velocity and the group velocity of waves, slow dissipative processes, and during the fast direct processes of rearrangement of the atomic-molecular structure of matter upon destruction of the free surface of merging liquids. An analysis of the system of constitutive equations taking into account the compatibility condition in the linear and weakly nonlinear approximations made it possible to distinguish a new class of thin flows - ligaments. The conditions for complete and partial laboratory and numerical simulation of fluid flows are determined. The parameters of the ligaments, the dissipative-diffusion components of flows, which determine the fine spatial structure of flows, have been calculated. Ligaments in the experiment have the form of thin high-gradient interlayers and fibers. They occupy an intermediate place between atomic-molecular and macroscopic processes and play an important role in the dynamics and structure of droplet flows. The processes of transformation of internal energy into other forms form the finest components of the flow structure The methodology of experimental studies carried out at part 3 of the project, based on the results of the fundamental system, analysis has been improved giving room to study the dynamics and fine structure at the initial stage of the formation of droplet flows in a unified formulation. Several stages containing characteristic structural elements have been identified as the basis for future classifications of flow regimes. The works in 2021st performed with a modified system of illumination of the observation area. The technique of coordinated optical and acoustic experiments has been improved, which makes it possible to synchronize video and acoustics of processes no worse than 1 μs. The ESP stand of the GFK IPMech RAN ESP was improved, a wave generator for gravitational-capillary waves was manufactured, for the first time the studies of the spreading patterns of droplets of a miscible liquid (the fusion of an ink solution with water) in the field of two-dimensional regular waves were carried out. The parameters of the liquids monitored using a TS-1 tensiometer and viscometers. At the end of 2021, the stands were supplemented with a new high-speed video camera i-Speed 717, the PIV LaVision remote microparticle speed meter and a new experiment control unit of home design, which helped to increase the information content and space-time resolution. For the first time, the process of disintegration of the boundary of the region of fluids confluence has been experimentally traced. In the vicinity of the contact line, annular capillary waves and fast thin radial jets are visualized. Trickles were observed when both transparent and colored liquids merged. They flow along the surface of the cavity, reach the edge of the crown, and form spikes (thorns) from the tops of which small droplets fly out. The evolution of a complex pattern of a system of splashes escaping from the tops of primary spikes at the boundary of the contact area of liquids and secondary ones at the edge of the crown is traced. The general flow pattern is preserved in the selected confluence mode. In the experiments, two types of droplet coalescence processes were distinguished: intrusive one with a delay in the formation of a cavity and spreading one with the formation of a hemispherical cavity. However, the details of the structure change from experiment to experiment under the same conditions. The variability of the pattern is due to the complexity of the unsteady patterns of Rayleigh oscillations of the drop and annular capillary waves traveling along the surface of the drop, which are randomly excited during the drop pinch-off. Surface variations change the shape of the droplet bottom at the initial contact with the target fluid, which may be convex, concave, and uneven. Accordingly, the scenarios of the merging of the droplet with the receiving liquid also change. The substance of the spreading droplet forms a complex lined and lattice pattern on the surface of the cavity and the walls of the crown. At the grid nodes, the surface of the cavity is deformed and small vortices containing fibers with liquid droplets invade the receiving fluid. Over time, the vortices transform into oblique fibrous loops that invade the liquid. In the course of the further evolution of the flows, the geometry of the loops changes, but the filamentous distribution of the droplet matter in the resting receiving fluid is retained at all subsequent stages of evolution up to the formation of a ring vortex and its disintegration into vortex cascades. The disintegration of the droplet into fibrous structures persists even in the case of chemical reactions. In the experiments, systems of inclined fibrous loops under the surface of the cavity were registered for the first time during the coalescence of drops of a solution of ferric chloride and a solution of ammonium thiocyanate. The reaction area, which is visualized by a brightly colored solution of iron thiocyanate, is located on the fibers on the surface of the cavity and crown and in inclined fibrous loops that invade the receiving fluid. The experiments carried out indicate the existence of a new scenario for the merging of miscible liquids, namely, the interpenetration of fibrous structures. The formed inhomogeneities in the distribution of matter are carried by flows in the liquid and are leveled by the processes of molecular diffusion. The mechanism of interpenetration is universal, it was observed when a lighter neutral liquid was immersed in a heavy one, or a heavy one in a lighter one, as well as during chemical reactions. The fine fiber interpenetration mode complements the traditionally studied modes of fluids stirring and mixing. The transfer of matter of a droplet falling on the surface of a liquid with traveling capillary-gravitational waves is traced. In a wide range of wave frequencies in the vicinity of the group velocity minimum, the general pattern of droplet spreading retains its structure. Further, the primary spot disintegrates into a slowly moving own remnant, a faster sinking vortex ring, and a near-surface jet with a dipole tip. All flow components retain a fine fibrous structure, which is destroyed by molecular diffusion processes. For the first time, the structure of droplet flows, which are formed by a drop of water falling on the surface of a molten metal (Rose alloy at a temperature of 200-250° C), has been studied. The evolution of the general geometry of a complex flow is traced, including a suspended water-vapor volume and individual splashes escaping in a wide range of angles. Four types of splashes have been identified: water, water-steam, multicomponent with a metal core and a water shell, and metallic, the proportion of which changes with the evolution of the flow. It was experimentally shown for the first time that all modes of changing the flow structure are accompanied by the generation of waves of two types: gravitational-capillary and acoustic. Gravitational-capillary waves are formed with a sharp change in shape and loss of flow continuity (coalescence of a primary drop, around the base of a growing burst, separation of a secondary droplet from a burst or streamer, at various stages of the burst immersion). Packets of high-frequency acoustic waves are formed during the initial contact of the drop with the receiving liquid and with a delay during the separation of gas bubbles with the rupture of the connecting bridge. The relationship between the frequency and phase of volumetric oscillations of axisymmetric gas bubbles with the amplitude-frequency parameters of acoustic signals has been experimentally established for the first time. The development of a technique for recording capillary waves in the central region of a droplet impact enables to clarify the details of the complex dynamics of the splash immersion, the formation of a system of secondary cavities, and the formation of complex gas cavities that transform into gas bubbles. The fact of the formation of groups of capillary waves on the walls of the conical cavity indicated the process of separation of the base of the plunging splash from the moving surface of the liquid and secondary contact when the phase of its motion was changed. For the first time, the evolution of the forms of secondary cavities is traced in detail. The phases of rapid change in their sizes are identified, due to the interaction of the splash and the secondary droplet ejected from the top of the burst with the walls of the cavity, accompanied by processes of rapid transformation of surface energy into other forms. An experimental study of the geometry of jets of an electrohydraulic nozzle with a spray hole diameter of 0.12, 0.13, and 0.135 mm has been carried out. The influence of injection pressure in the range from 30 to 150 MPa and back pressure from 0 to 16 MPa, as well as the shape of the nozzle channel with two outputs, on the geometric parameters and dynamics of the development of droplet jets of hydrocarbon fuel (diesel fuel) has been studied. An analysis of the geometry of the flows shows that both jets begin their motion almost simultaneously, but with different velocities, which differ by more than two times. At large times, the jet fronts move in almost the same way. Increasing the injection pressure helps to reduce the angle of the boundary layer of the jet, containing a mixture of air and fuel droplets, from 22 ... 27 ° at 50 MPa to 17 ... 18 ° at 250 MPa. During injection, the largest oscillations of the boundary layer of the jet relative to its axis were fixed at 50 MPa. Then, with an increase in the injection pressure, the vibrations of the core decrease and become almost imperceptible at 250 MPa. The fuel outflow mode was estimated by the number of cavitation - the relative pressure difference and back pressure. With a spray hole diameter of 0.13 mm and a pressure of 30 MPa, the flow rate increases from 0.716 to 0.738 in the range of cavitation numbers from 29 to 149. Analytical forecasting of the development of the research directions in the hydraulics of sprayers has been carried out. In the near future development of the transport industry, the diesel engine will remain the dominant power source. Its workflow needs further improvement to meet current and future environmental regulations. This is achievable by controlling the following parameters at each operating mode of the engine: vortex motion, pressure and temperature of the charge; mass flow rate of fuel in the initial phase of injection; crushing the cyclic fuel supply into separate portions supplied at intervals to the combustion chamber (up to three per cycle); an increase in injection pressure up to 300 MPa, and in the future - up to 500 MPa; the distribution of fuel over the volume of the combustion chamber, taking into account the thermal state of the surfaces limiting it, due to the redistribution of the fuel supplied to the cylinder through the nozzle channels with different hydraulic characteristics. Further development of the theory of droplet flows, which is based on a scaled and parametrically invariant system of fundamental equations, taking into account the mechanisms of energy transfer and experimental techniques that determine natural physical variables and the geometry of their fields, will allow the development of engineering mathematics for describing flows with a guaranteed estimate of errors without invoking additional hypotheses and constants.

 

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Annotation of the results obtained in 2019
The interest in studying the dynamics and energetics of flows arising in a collision of free-falling droplets with a liquid surface, accompanying gravitational, capillary and sound waves, the transfer of droplet matter into the target liquid, and vice versa, the substance of the target liquid into the atmosphere, is due to the scientific significance of the problem and a steady increase in the number of practical applications in various areas of environmental hydroaerodynamics, mechanics, including aerodynamics and rocket production, materials science, various technologies in the chemical and biochemical industry, ecology and medicine. The number of publications is quite large, reviews cover from 100 to 700 references. Traditional studies are based on the classical laws of hydromechanics - the continuity and Navier - Stokes equations under the assumption of a constant density of the medium. However, as shown by precision measurements, the dynamic state (natural oscillations and capillary waves on the surface) affects the nature of the interaction of the incident drop with the target liquid, the direction and intensity of the processes of the substances exchange between the atmosphere and the hydrosphere, the sound radiation in the water and air, and the generation of capillary waves on the immersed drop surface, crown and cavity, as well as on the surrounding fluid. Studies of recent years have shown that the nature of flows, the processes of sound generation, and the transfer of matter are actively influenced by energy transfer processes, which include both mechanical and atomic-molecular components that form internal energy. Internal energy is characterized by thermodynamic potentials, among which the Gibbs potential (free enthalpy), of which the derivatives determine the density and other thermodynamic parameters of the medium, is distinguished. It was experimentally established that the thermodynamic potentials in inhomogeneous liquid with a free surface are non-uniformly distributed - the density, dielectric constant, dipole moment in the liquid bulk differ from similar properties in a structurally isolated surface layer with a thickness of the order of the size of the molecular cluster. The purpose of the work is to develop a technique and carry out high-resolution precision observations of flow patterns and synchronous measurements of acoustic signals. Clarification of the mechanisms of formation of flows arising from the interaction of a drop with a liquid, and the sources of acoustic radiation associated with them, is necessary to identify the role of energy transfer and transformation processes, and to construct adequate mathematical models. The experiments were carried out at the stands included in the complex of unique installations "GFK IPMeh RAS", the main elements of which are rectangular transparent pools of various sizes with paired optical windows in the side walls. The flow pattern was recorded with "Optronis CR3000x2" high-speed video camera. Drops were created using a dispenser mounted at a height of 1–230 cm. The observation area was illuminated by several "MultiLed" spotlights. Acoustic signals were measured by a GI-54 hydrophone (frequency band 0.002-100 kHz) and a microphone (0.01-40 kHz). A falling drop triggered a synchronization device which starts video and photo recording. The synchronization of optical and acoustic measurements was provided no worse than 0.2 μs. The metrological parameters of the experiment (sensitivity, spatial-temporal resolution of the instruments) were selected taking into account their own spatial-temporal scales of the studied processes. The high spatial-temporal resolution of the equipment made it possible for the first time to record and determine the sizes of short-lived fine-structure components of surface disturbances during the formation of a splashcreated by a falling drop when identical liquids are in contact (in the experiments performed, water drops fell freely into water). The observed structures reflect the existence of fine flow structures arising from the rapid destruction of the free surface. In this case, the available potential surface energy of a thin layer is quickly transformed into other forms (perturbations of temperature, pressure, and mechanical motion). The concentration of energy in the layer provides the complexity of the distribution of the velocity field in the water bulk. Traditional methods for detecting droplet flows do not allow studying the geometry of rapidly evolving processes because of insufficient temporal or spatial resolution even at high-resolution ultra-fast photograph. The experiments performed allowed us to detect a number of new thin, but stably reproducible components of the flow, which were not previously noticed. These include: fibers in the distribution pattern of the substance of the falling drop in the target liquid, which persist at all stages of evolution until the flow is completely degenerate, the effect of recurrence — the recovery of the fine structure of the cavity surface and the growing splash after attenuation of small-scale perturbations caused by the dropping of the drop, and a complex multi-frequency structure of acoustic signals, both initial and delayed. In acoustic signals, the properties of two main groups are determined - a shock pulse and resonant packets, which radically differ in the degree of repeatability and stability of time parameters. The spectral content of the signals in both groups is characterized by variability in a wide range of frequencies. Under constant experimental conditions, both simple single-frequency decaying and complex signals with modulation and varying frequency are observed. A comparison of the patterns of underwater currents and acoustic signals indicates that the generation of resonant sound packets is synchronized with the separation of gas cavities from the cavity, which forms when the droplet is immersed or breaks into fragments. The duration of the sound depends on the degree of initial non-uniformity of the sounding cavity, gradually transforming into a smooth spheroidal. The methods of photo and video recording also investigated in detail the evolution of the flow pattern resulting from the drop of a drop of an aqueous ink solution in oil. While maintaining the common elements — caverns, crowns, capillary waves, and splashes — the dynamics of the flow pattern in immiscible liquids is complicated by the unsteady contact surface of interacting media. Depending on the experimental conditions and the properties of the media, the picture of the spatial distribution of the droplet substance is different. When a drop of a substance mixed with the receiving liquid is immersed, a net pattern is formed with several tiers, the number of which at different phases of the process depends on the speed of the drop at the moment of contact. The number of tiers decreases with increasing speed. The amount of substance concentrating in the nodes of the mesh structure increases with a simultaneous decrease in the concentration in the edges of the mesh. At the initial stage, the fastest element of the flow is a thin double layer - a continuous interface between two liquids, the unity of which is broken in the phase of the collapse of the crown. Droplets ejected from the teeth of the crown always contain both contacting liquids, as in the case of miscible media. Since the ink solution escapes faster than its upper edge sinks, voids form on the inner surface of the crown - water-free sections of oil separated by thin dyed fibers. When immersing viscous immiscible liquids (petroleum, oil), a central spot forms, covering almost the entire inner surface of the crown, teeth appear from the crown chevron, to which short trickles emitting secondary drops adjoin. The position of the liquid contact surface on the crown depends on the relative surface tension coefficient of the media. On the whole, the net picture of the distribution of the droplet substance over the deformed surface of the receiving liquid is observed in a wide range of Weber and Ohnesorge numbers. The physical model of the appearance of thin components is based on the analysis of energy transfer processes, the characteristic time of which differs significantly over diffusion, translational, wave and direct atomic-molecular processes simultaneously occurring. The anisotropy of the action of atomic-molecular forces in areas with large gradients of thermodynamic quantities (in particular, high-gradient concentration layers and near the free surface) is manifested in the existence of available potential surface and chemical types of energy that can transform into other forms - thermal, mechanical energy of the fluid flow, and also work to create a new free surface. Accounting for energy transfer extends the traditional approach to the description of droplet flows. Here, the dynamics of liquids is described by a complete system of equations for the transfer of matter, density, momentum and energy — analogues of conservation laws for closed systems — with physically justified kinematic and dynamic boundary conditions on contact surfaces. The existence of fine-structure components of periodic and unsteady flows is a consequence of the high rank of the system of fundamental equations for weakly dissipative media. In the full description, the governing system of equations includes an equation for internal energy in the form of the Gibbs potential and its derivatives - traditional thermodynamic quantities, with physically justified boundary and initial conditions. The rank of the system, the degree of the linearized version, and the order of the characteristic (dispersion) equation are found from the compatibility condition. The linear system solutions are the basis for the classification of the structural components of flows, including ligaments (thin interfaces or filaments), waves and vortices. The developed classification will be used to improve the experimental methodology and calculation codes, taking into account the own scale of the processes. The experiments conducted at the stands of the Hydrophysical Complex "GFK IPMech RAS" to study the dynamics and fine structure of fast processes showed the possibility of its further improvement and carrying out experiments with existing equipment. Of particular interest is the study of the flow pattern when a drop falls into a continuously stratified fluid, where perturbations of the natural density field (and the optical refractive index associated with the density by the linear relation) can show new thin-structure flow components that cannot be visualized in the traditional formulation with a homogeneous receiving fluid.

 

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Annotation of the results obtained in 2020
For the first time, four independent mechanisms of energy transfer in drop flows have been identified - a fast direct atomic-molecular mechanism on ligaments, generally known with a flow velocity, with a group wave velocity, and a slow dissipative (diffusion) one. For the first time, the action of a fast direct atomic-molecular mechanism in the formation of fast splashes, the speed of which is higher than the speed of the drop, is shown experimentally. For the first time, the formation of vortex loops on the surface of a cavity in a homogeneous and stratified fluid was observed experimentally. For the first time, the role of thin flows (trickles) accompanying capillary waves in the separation and excitation of oscillations of gas bubbles and in the emission of sound has been shown experimentally. For the first time, the fine fibrous structuring of the distribution of matter in the target fluid has been traced and the physical mechanism of its implementation has been indicated. For the first time, a new class of structures of the distribution of matter of a falling composite drop in in the target fluid - a "crossed lattice" consisting of separate systems of stripes separated by radial boundaries, has been experimentally registered. For the first time, the transformation of the acoustic signal spectrum of a droplet impact has been experimentally traced. All results were published in domestic and foreign (translations and original articles), discussed at international conferences.

 

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