V. E. Prokhorov

Dynamics of Separation of a Single Drop in Air

The processes of formation of alcohol and water drops, as well as formation of small fragments—satellites, are traced using the high-speed filming. The trajectory of a water drop satellite is nonmonotonic, at first the satellite moves upward against the gravity force, reaches the oscillating residual fluid at the nozzle exit, and then starts to move down. From the satellite, a microdroplet is ejected, which bounces off the residual fluid at the nozzle, returns back to the satellite and merges. In the case of an alcohol drop, no accompanying microdroplet is formed, and the satellite follows a nearballistic trajectory.

Sound Generation as a Drop Falls on a Water Surface

Impact of a drop on a water surface is accompanied by a series of sound pulses propagating in air and underwater. Depending on the falling mode (drop size and initial velocity), pulses substantially differ in amplitude, duration, and modulation frequency. We study falling modes in which in addition to conventional sound packets—the shock pulse and single resonance sound packets—several packets are observed. Experiments were conducted with simultaneous recording of sound in air and underwater and were accompanied by synchronous video depiction of currents in the drop impact region. Comparison of videograms and phonograms demonstrate that the sources of sound packets are gas cavities of arbitrary shape detached from the underwater cavern under the action of large accelerations (several km/s2) during a sharp change in its surface area, which gradually achieve equilibrial elliptical and spherical shapes.

The Fine Structure of a Splash Induced by a Droplet Falling on a Liquid Free Surface at Rest

The hydrodynamic, acoustical, and optical phenomena arising in the collision of a freely falling droplet with a liquid surface at rest have been investigatedexperimentally for more than 100 years since the pioneer studies of Thomson, Newall, and Worthington [1,2]. Nowadays, attention is given to studying the practically important finestructure components of arisingflows. The interest is related to the development ofmethods of remote sounding [3] and caused by thedependence of the scatteredsignal properties on thesizes of the watersurface inhomogeneities. For thefrequencies of about a few hundred megahertz, theprobing electromagnetic wavelength becomes comparable to the diameters of the cavity formed from theimpact of a rain droplet. With a further increase in frequency, it becomes comparable to the length of capillary waves diverging from the splash region, with thecumulativejet diameters, and with the secondarydroplets ejected from the liquid layer [4]. Pronouncedsymmetric splash patterns also are widely used in theforming design [5].The characteristics of the flow arising as a result ofthe droplet fall depend on the properties of collidingliquids (density, viscosity, and surfacetension coefficient), the droplet sizes and velocity (or the height ofits free fall), and the conditions of the experiment (theliquidlayer depth, the bottom slope, the liquid anddroplet temperature, and the atmospheric pressure).The theory of the processes proceeding in the dropletimpact is developed mainly in the perfectliquidapproximation when the Froude number Fr =

INVESTIGATION OF THE VARIABILITY OF THE STRUCTURE OF A STRATIFIED WAKE BEHIND A HORIZONTAL CYLINDER USING OPTICAL AND ACOUSTIC METHODS

The shadow flow pattern behind a horizontal cylinder uniformly towed in a stratified fluid with constant buoyancy frequency (in the imbedded vortex and turbulent wake regime) is recorded synchronously with acoustic echo sounding (basic frequency equal to 1 MHz) in a laboratory tank. Using computer processing, the illumination profiles in the schlieren pattern are constructed on scales comparable with the sounding acoustic ray width. Although the optical and acoustic profiles are not similar, nevertheless they enable the basic structural elements of the wake, including its high-gradient core to be identified, and their time variability traced. The features of the integral acoustic scattering characteristic, in particular, the volume scattering strength, allow this characteristic to be used, together with optical images of the flow pattern, to distinguish the flow regime identification criteria.

Micro-scale instability in a continuously stratified fluid

Experimental observations of small-scale structures caused by flow instabilities at layers of high density gradient in the wake behind a cylinder in a fluid with a continuous salt concentration stratification are reported. In the density wake it is possible to discern a number of structures such as wedge-like structures or cusps; small-scale instabilities (breakers) in the zones of interaction of attached internal waves and the high-gradient wake envelope; small-scale instability of the density boundary layer with a complicated density gradient pattern superimposed on a smooth velocity profile, and small-scale wake structures behind attached vortices in the case of a closed (central) wake envelope.

Emission of the sequence of sound packets during a drop falling onto the surface of water

Emission of sound signals accompanying the fall of a water drop onto a free liquid surface is investigated in a laboratory bath by high-speed video filming and underwater acoustics. Emission of high-frequency sound is detected during the initial contact of the drop with the surface. Low-frequency sound packets are observed after a prolonged silence interval with the formation of nonequilibrium-shaped gas cavities with sharpenings during their reconstruction into spherical bubbles. By virtue of complexity of the flow pattern at the stages of detachment and coalescence of gas cavities, the frequency and intensity of the emitted sound varied in wide ranges even under invariable initial experimental conditions.

Underwater and Air Sound Signals in the Case of a Droplet Falling onto a Liquid Surface

Among fast natural phenomena, exchange pro� cesses involving the atmosphere and the ocean are of particular interest. These processes include the inter� action of rainfall (first of all, rain) with the oceanic surface. The kinetic energy of a rain droplet and the surface energy that becomes free once the droplet has been absorbed result in a series of hydrodynamic phe� nomena. These are formation of a cavern, a cumula� tive jet, streamers, regular capillary waves, and irregu� lar waves. The formation and fillingin of the caverns are accompanied by the detachment of air cavities of both regular and irregular shape, with a part of them emitting sound. The acoustic rain noise generated by falling drop� lets forms the total sonic background. This is an important source of information on the intensity and localization of rain, which is a significant fraction of the total ocean acoustic noise (1). In records of under� water signals obtained within the rain period, the spec� tral peak between 10 and 15 kHz is surely localized (2). Modernization of experimental methods has opened the possibility to measure fluctuations of sound pressure in water, which arise as a result of the fall of both the set of (rain) droplets and an individual droplet with the simultaneous registration of the flow pattern (2). Among competing mechanisms of sound emission, the impact pulse, hydrodynamic impact, and aircavity resonances were analyzed. In experi� ments, along with the sound registration, air bubbles were always observed. Therefore, basic attention was paid to the resonance mechanism for which the soundemission frequency was connected to the bub� ble size by a simple calculation relationship (3). The available experimental data made it possible to obtain a diagram of flow modes in the plane of the dropletfall velocity as a function of the droplet diam� eter. In this plane, a region of stable registration is iso� lated for gas bubbles of the size corresponding to the spectralpeak frequency (4). However, for the reso� nance mechanism, the question remained open on the transformation of the fallingdroplet energy to that of bubble resonance vibrations accompanied by sound emission. In further experiments, it was noted that gas cavities emit underwater sound in the course of recon� structing their irregular shape (with tapers and folds) to the equilibrium (spherical) one. This process is accompanied by the liberation of the potential surface energy (5). The analysis of the processes allows for the fact that a falling droplet leads to accepting the substance liq� uid, the momentum, and the mechanical energy, which results in generating visible and latent flows of different scales. Caverns, coronas, cumulative jets, and streamers correspond to visible flows and manifest themselves in the changing the shape of the free sur� face (6). Largescale latent flows cause chaotic displace� ments of gas bubbles in the bulk of the liquid. The vir� tually unstudied finestructure latent flows manifest themselves in peculiarities of both the freesurface shape and the substancetransport pattern near the interface. They also determine the properties of emit� ted sound packets. In the present paper, for the first time, we thoroughly study the frequencytime param� eters of sound signals initiated in air and water by freely falling droplets.

Acoustic Sounding of Vortex Rings in a Continuously Stratified Fluid

The experimental simulation of solitary vortex rings in a stratified fluid performed using high-frequency echo-sounding and optical visualization methods shows that on the range from turbulent to laminar regimes the vortex is a volume inhomogeneity with a sound scattering cross-section mv∼U5, where U is the translational velocity. The absolute value of mv is determined by the microscale component of the vortex microstructure, which is commensurable with the sounding sonic wave length.

Acoustics and hydrodynamics of a drop impact on a water surface

Hydrodynamic and acoustic processes associated with a drop impact on a water surface were studied experimentally. Acoustic signals were detected underwater (with a hydrophone) and in air (with a microphone), the flow pattern was recorded with a high-speed camera, and the surface perturbation was monitored with a laser detector. The dimensionless parameters of flows (Reynolds, Froude, and Weber numbers) induced by the impact varied with fall height within the ranges of 5000 < Re < 20000, 20 < Fr < 350, and 70 < We < 1000. The sequence of acoustic signals incorporated an impact pulse at the moment of contact between a drop and the surface and a series of acoustic packets attributable to the resonance emission of gas cavities. The top of the impact pulse, which was detected clearly in the entire fall height range, had a complex structure with short high-frequency and longer low-frequency oscillations. The total number and the parameters of emitted acoustic packets depended to a considerable extent on the fall height. The cases of lacking, one-time, and repeated emission of packets were noted in a series of experiments performed at a constant fall height. The analysis of video data showed that the signal variability was induced by considerable differences in the scenarios of water entry of a drop, which assumed an ovoid shape at the end trajectory segment, in the mentioned experiments.

Primary acoustic signal structure during free falling drop collision with a water surface

Consistent optical and acoustic techniques have been used to study the structure of hydrodynamic disturbances and acoustic signals generated as a free falling drop penetrates water. The relationship between the structures of hydrodynamic and acoustic perturbations arising as a result of a falling drop contacting with the water surface and subsequent immersion into water is traced. The primary acoustic signal is characterized, in addition to stably reproduced features (steep leading edge followed by long decay with local pressure maxima), by irregular high-frequency packets, which are studied for the first time. Reproducible experimental data are used to recognize constant and variable components of the primary acoustic signal.