P. A. Kuibin

Vortex reconnection in a swirling flow

Processes of vortex reconnection on a helical vortex, which is formed in a swirling flow in a conical diffuser, have been studied experimentally. It has been shown that reconnection can result in the formation of both an isolated vortex ring and a vortex ring linked with the main helical vortex. A number of features of vortex reconnection, including the effects of asymmetry, generation of Kelvin waves, and formation of various bridges, have been described.

Weber numbers at various stages of water projectile transformation during free fall in air

The process of breakage of water projectiles (spherical “balls” with initial volumes ranging within 0.05–40.5 L) during free fall in air from a height of 3 m has been studied by means of high-speed (up to 105 fps) video recording. Four stages of projectile transformation are distinguished that precede the formation of a cloud of droplets. It is established that the transitions from one stage to another proceed within shorter fall times for greater initial volumes of water. Weber numbers at these stages are determined.

The effect of air injection on the parameters of swirling flow in a Turbine-99 draft tube model

Results of experimental modeling of a swirling flow in a Turbine-99 draft tube prototype are presented. The influence of gas phase injection into the flow has been studied. Experiments were performed on a closed hydrodynamic setup containing a working stage with the Turbine-99 draft tube geometry. It is established that the gas content affects the flow structure. Gas injection leads to a change in the frequency of precession of the vortex core formed in the draft tube cone, which is not related to an increase in the gas-liquid mixture flow rate.

Features of water droplet deformation during motion in a gaseous medium under conditions of moderate and high temperatures

Using cross-correlation video recording facilities, the features of deformation of water droplets with characteristic dimensions of 3–6 mm are studied experimentally during their motion in a high temperature (about 1100 K) gaseous medium (in combustion products) with velocities of up to 5 m/s. In addition, experiments for water droplets moving with identical initial velocities and dimensions in air at moderate (about 300 K) air temperatures are carried out. Typical shapes of droplets and cyclical character of their variation during motion in the considered gaseous media are established. The numerical values of the main characteristics of the isolated “deformation cycles” (amplitude, times, and durations) are determined, and their susceptibility to high temperatures of the gaseous medium, droplet dimensions, and droplet motion velocities is revealed.

Experimental observation of the precessing-vortex-core reconnection phenomenon in a combined-flow turbine

Some results of experimental studies of the vortex-core reconnection phenomenon in the flowthrough duct of a combined-flow turbine were presented. The studies were performed on a medium-scale hydrodynamic test bench, the design of which was maximally similar to real hydroturbines. Using high-speed recording in a regime with a 75% load in a suction-pipe diffuser, the reconnection of a precessing vortex-core with detachment of vortex rings was observed. Aperiodic pressure surges due to the reconnection of a vortex and the formation of vortex rings were experimentally detected.

Thermal-wave-induced vorticity in a liquid film

We have theoretically studied the influence of a moving local heat source on the structure of flow in a thin liquid layer on a horizontal substrate. A two-dimensional problem is considered in the boundary layer approximation and a stationary equation that describes deformation of the liquid layer is obtained. Results of numerical calculations for a preset temperature distribution on the free surface are presented that demonstrate the formation of a vortex structure. It is established that, even in the absence of a gravity-driven flow, the motion of a local heat source ensures a stationary flow regime without film breakage and the formation of dry spots. The results justify a new scheme of experiments for the investigation of phenomena in nonisothermal liquid films.

Kelvin waves on helical vortex tube in swirling flow

Kelvin waves formed on precessing helical vortex tube after vortex reconnection were experimentally visualized. The experiments were performed in a swirling flow of viscous fluid in optically transparent conical diffuser. Vortex visualization was facilitated by cavitation in the vortex core and carried out using high-speed video cameras. Kelvin waves of zero, first and second modes m = 0, 1, 2 were formed in packets as a result of reconnection due to vortex tube self-intersection. Kelvin waves were observed to form both on the helical vortex tube and the detached vortex ring and to propagate along the axis of the vortex tubes.

Exprimental Study of Liquid Drop Surface Transformation in Air Within a Group of Successive Deformation Cycles

An experimental study has been made of transformation of surface of drops of liquids used widely in chemical engineering (water, kerosene, and ethyl alcohol) during their motion in air with moderate velocities (up to 5 m/sec). The initial sizes (nominal diameters) of the drops varied in the 3–6 mm range. The typical “cycles of deformation” of liquid drops with specific duration, length, and amplitudes of size variation have been established. More than 10 successive deformations during 1-m passage of liquid drops through the air are investigated. The typical time of transition from one drop shape to another is determined. Two deformation conditions differing in drop shapes and time of the respective transition are distinguished.

The effect of gas and water droplet temperature on characteristics of water-droplet deformation at moderate velocities of droplet movement

An experimental study has been performed on the effect of the temperature of gas and water droplets (3–6 mm in size) on deformation of water droplets that move with a moderate velocity (up to 5 m/s). A high-speed videocamera (recording at up to 105 fps) and a cross-correlation videocomplex were used. The droplet temperature varied in the range of 285–360 K. For gaseous mediums, air at a temperature of 280–300 K and condensed substance combustion products at a temperature about 1100 K were used. Typical deformation cycles were found. Their length, duration, and amplitude were calculated. The time a shape of droplet transforms into a new shape and the time the droplets retain their shape as they move through a gas medium have been determined. Approximating correlations for the basic characteristic of deformation under various heating or cooling conditions in a water droplet–gas system were derived.

The ranges of the aerodynamic drag coefficient of water droplets moving through typical gas media

The integral characteristics of the deformation processes of liquid (water) droplets moving through a gas medium (air at a temperature of about 300 K, kerosene combustion products with a temperature of about 1100 K) were experimentally investigated. The initial sizes (characteristic radii) of the droplets varied from 50 μm to 2.5 mm, and the initial velocities, from 0.5 m/s to 5 m/s. Velocities of the gas counter (relative to the direction of droplets displacement) flow weremaintained about 1.5 m/s by means of a special-purpose pressure system. Characteristic “deformation cycles” of droplets, their number, durations, and lengths, and also maximal amplitudes of the deformation process were identified. The ranges of numerical values of the aerodynamic drag coefficients cd for the characteristic deformation cycles were determined. The influence of droplets velocities and sizes, and also of the gas medium temperature on these parameters was established. Characteristic times of preserving the corresponding droplet forms and cd values within the range of the most typical deformation cycles were found.