Konstantin S. Pervunin

Experimental investigation of turbulence modification in bubbly axisymmetric jets

The paper deals with an experimental study of bubbly free and impinging axisymmetric turbulent jets (Re = 12,000) by means of a combination of optical planar methods: a fluorescence-based technique for bubbles imaging (PFBI, its description is given in [1]) and PIV and PTV approaches for measurement of velocity distributions of both continuous and dispersed phases, respectively. The bubbly jets are investigated at different volume gas fractions: 0, 1.2, 2.4, and 4.2%. Basing on 10,000 simultaneously measured instantaneous fields of the local gas fraction and velocities of the both phases, statistics on the mean and fluctuating characteristics for the both phases is estimated up to the third-order moments, including mixed one-point correlations. The paper reports on opposite effects of the volume gas fraction on turbulence in the free and confined jet flows. At a nozzle edge, the bubbles increase the growth rate of turbulence fluctuations, whereas downstream (z/d > 0.3) they suppress the turbulence fluctuations compared to the single-phase flow. Close to the impingement surface, the bubbles, however, significantly intensify turbulence fluctuations due to an increase of the slip velocity between the phases.

High-speed imaging of cavitation regimes on a round-leading-edge flat plate and NACA0015 hydrofoil

Graphical abstract

Cavitation on NACA0015 hydrofoils with different wall roughness: high-speed visualization of the surface texture effects

Graphical Abstract

Tip-leakage cavitation in the clearance of a 2D hydrofoil with fillets: high-speed visualization and PIV/PTV measurements

Tip-clearance cavitation is one of the most aggressive forms of cavitation as it can cause surface erosion of hydraulic machinery elements and, as a result, their fatigue damage and disturb designed operating conditions. At present, the literature lacks for detailed experimental data on the inception and development of this type of cavitation at various flow conditions. In the paper, a tip-leakage cavitation occurring in the clearance between an end face of a 2D hydrofoil (a scaled-down model of guide vanes (GV) of a Francis turbine) and a transparent wall of the test section was studied. The experiments were carried out for different cavitating regimes on the cavitation number and two attack angles of 3? and 9?, with the gap size (tip clearance width) varied in the range from 0.4 to 0.8 mm. In order to determine the cavitation inception conditions and investigate the dynamics of the tip-leakage cavitation, a high-speed visualization was applied. A modified PIV/PTV technique with a diverging laser beam instead of a laser light sheet was used to measure the mean velocity distributions within the gap. It was shown that the cavitation pattern on the suction side of the GV model impacts the dynamics of the leakage flow in the gap but does not affect the sheet cavity formed close to the foil leading edge in the clearance as well as its size and dynamics. When the gap size is increased, the tip-leakage cavitation initiates at higher cavitation numbers or, in other words, conditions for the cavitation occurrence become more favorable.

Bubbly free and impinging jets under forced flow conditions: experimental study by means of PIV/PFBI

Jet flows are extensively used in various practical applications. Presently, the development of technical equipment where jets are employed is connected with the improvement and optimization of different methods of flow control. In the paper, an experimental investigation of the turbulent structure of forced bubbly free and impinging jets was carried out by means of PIV and PFBI techniques. PIV was applied to measure velocity distributions and turbulent characteristics in the continuous phase, while PFBI approach was applied to visualize bubbles in the flow and evaluate their sizes. The flow was studied at the Reynolds number of 12,500 and three void fractions β = 0, 1 and 2% for forced conditions St = 0.5. The mean air bubble diameter was estimated to be roughly 0.8 mm for all β. It was revealed that in the free jet the air bubbles and flow pulsations reduces substantially the longitudinal dimension of the jet core. In two-phase flow with forcing distribution of turbulence kinetic energy was similar to one-phase case but maximum value was two and a half times higher then for one-phase unforced jet. In the impinging jet flow, the bubbles produced a maximum of the turbulence kinetic energy near the wall, which increased two and a half times in forced conditions.

Cavitation on a semicircular leading-edge plate and NACA0015 hydrofoil: Visualization and velocity measurement

Using high-speed visualization and particle image velocimetry (PIV), cavitating flows near a plane plate with a rounded leading edge and NACA0015 hydrofoil at angles of attack from 0° to 9° are studied. In the experiments, several known types of cavitation, as well as some differences, were detected with variation of the cavitation number. In particular, at small angles of attack (up to 3°), cavitation on the plate appears in the form of a streak array; on the hydrofoil, it appears in the form of individual bubbles. For the NACA0015 hydrofoil, isolated and intermittent streaks are divided and grow in regimes with developed cavitation; then, however, they merge in bubble clouds and form an extremely regular cellular structure. With an increase in the angle of attack to 9°, the structure of the cavitation cavity on the hydrofoil is changed by the streak structure, like in the case with the plate. In this work, it is shown that PIV permits one to measure the velocity in cavitating flows, in particular, within the gas-vapor phase. It was established from the analysis of distributions of the average flow velocity and moments of velocity fluctuations that the cavitation generation is caused by the development of the carrier fluid flow near the leading edge of the hydrofoil. Down the stream, however, the flow structure strongly depends on the cavitation regime, which is seen from the comparison of the distributions with the case of a single-phase flow. The presented measurements qualitatively verify general trends and show some quantitative distinctions for the two considered flowpast bodies.