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Liquid cyclopentane is frequently used in hydrate formation studies as an analogue of natural gas because cyclopentane hydrates are stable above the ice melting point at ambient pressure. In this study, hydrate growth was established on a sessile water drop of 11 mm in diameter and 4.5 mm in height (volume of 0.25 mL) immersed in liquid cyclopentane. The hydrate formation mechanism and growth processes were observed optically over an extended range of subcooling temperatures from 5.1 to 15.2 °C, with the cyclopentane bulk temperature maintained in different experimental runs between 2.6 and −7.5 °C. Qualitative and quantitative comparisons were performed to confirm the absence of ice freezing during hydrate formation, and thus, the lack of contamination of the latter from the former in the experiments. Different transformations in the hydrate film morphology were registered from macroscopic observation over the considered range of subcooling temperatures, with the hydrate crystals composing the film taking the form of polyhedral, dendritic, or spherulitic structures. It was also found that the hydrate growth rate varied depending on the subcooling temperature, with the variation of the growth rate as a function of this temperature changing from a power to an (approximately) linear law with an increase in the degree of subcooling. We postulate that hydrate film growth can be governed by different mechanisms, whose roles change over the range of explored subcooling temperatures.

The thermo-hydraulic characteristics of heated supercritical CO2 (SCO2) flows are investigated numerically in a vertical pipe from first- and second-law perspectives, and the influence of the flow direction, mass flux and heat flux (both distribution and average value) are evaluated. Two mass flux (254 kg/(m2∙s) and 400 kg/(m2∙s)) and three average heat flux (30 kW/m2, 50 kW/m2 and 70 kW/m2) conditions are simulated at an inlet temperature of 288 K and a pressure of 8.0 MPa (corresponding pseudo-critical temperature of 308 K) in a 4-mm diameter pipe. The simulation results reveal that the heat transfer is enhanced and the irreversibility is reduced in downward flows relative to flows without gravity, whereas the heat transfer deteriorates and the irreversibility is increased in upward flows. Both higher heat fluxes and lower mass fluxes also further hinder heat transfer in the upward flows, and multiple peaks are observed in the axial wall temperature profile. Moreover, it is found that the heat-flux distribution has a significant effect on the heat transfer performance of upward flows; the heat transfer further deteriorates and the irreversibility is further increased when a linearly decreasing heat-flux distribution is applied to the wall, while the heat transfer deterioration is alleviated when a linearly increasing heat-flux distribution is used. An analysis of the heat transfer mechanism indicates that the turbulence production in the core region of the supercritical flow is suppressed, and the accumulation of gas-like fluid in the near-wall region is promoted by the buoyancy effect in upward flows, leading to severe heat transfer deterioration and a sharp increase in the wall temperature, which is similar to the critical heat-flux phenomenon in subcritical boiling. The present study provides insights into the heat transfer characteristics of SCO2 flows, as well as practical guidance on the design and optimisation of relevant components and equipment.

We present results from an experimental investigation on flow boiling, in the slug flow regime, of refrigerant R245fa through a 12.6-mm inner diameter horizontal plain pipe using particle image velocimetry (PIV) and an interface detection method. The study is supplemented by an overview of the state-of-the-art in experimental research of two-phase dispersed pipe flows and the development of modern optical and laser-based full-field non-intrusive measurement techniques as applied to these flows. We consider different flow conditions, with heat fluxes over the range 5.3 to 7.9 kW/m2 and mass fluxes from 300 to 460 kg/m2•s. Significant disturbances in the instantaneous velocity fields are revealed in both the noses and tails of slugs, with their values being two times higher behind vapour bubbles. The slug passage frequency is determined based on the results of the interface detection method. The vapour bubble velocity is found to increase linearly with the interfacial velocity of the two-phase mixture, while its gradient grows with the heat flux. Moreover, at increased heat fluxes the bubbles may move even faster than the mixture itself, which implies that they must significantly enhance local turbulence, thereby additionally intensifying heat transfer. In addition to the conclusions, we provide practical recommendations for possible future research in this particular field of fluid mechanics and the further development of sophisticated laser-based measurement techniques for boiling, and similar, flows.

Operating characteristics of aeronautic and marine vehicles, as well as hydraulic machines designed for various purposes are largely affected by flow separation. Therefore, control of separated flows is an extremely important problem for modern aviation and marine engineering. Based on dynamometric measurements of forces and torque acting on model hydrofoils and the ship rudder, jet control of flow separation in cavitation-free and cavitation regimes at low and high angles of attack is studied. It is shown that generation of a near-wall jet can ensure a separationless flow around test models at angles of attack greater than 30 degrees. In this case, the lift coefficient of the hydrofoil can increase approximately by two or three times. Pressure fluctuations near the body and in its wake vanish due to flow stabilization; as a result, oscillations of hydrodynamic loads on the body decrease.

The article presents the results of an experimental study of the turbulent structure of flow over a two-dimensional hydrofoil, which is a scaled-down model of guide vane of high-pressure hydraulic turbine. The study was carried out for four regimes of cavitating flow, covering the cases of cavitation-free flow, quasi-steady sheet-vortex cavity with detachments of small-scale cavitation clouds, and unsteady cloud cavitation with periodic separation of the whole attached cavity, formation of large-scale cavitation cloud, and its advection. Distributions of the average velocity, turbulent kinetic energy, and functional shear stresses are given. The transformation of the turbulent structure of the flow over the hydrofoil during transition from the cavitation-free flow to the regime of cloud cavitation is analyzed. Flow regions with non-local characteristics of turbulent transport are detected, where the known semi-empirical gradient-type models used in RANS approaches give qualitatively improper results.

The paper deals with an experimental study of tip-clearance cavitation inception and development and its vortical structure coupled with dynamics of the main attached cavity on the suction side of a two-dimensional symmetric hydrofoil equipped with a rotation axis. The gap is formed by the end face of the model and a transparent sidewall of the test channel. The experiments were performed for attack angles of 3° and 9° and 0.4-, 0.8-, 1.75- and 3.75-mm gaps (or 1.8%–17% relative thickness) under various flow conditions on the cavitation number. In order to observe the tip-clearance cavitation occurrence, high-speed imaging was applied. The leakage flow velocity was measured inside the clearance by a modified Particle Tracking Velocimetry technique. It is shown that the mean velocity field of the leaking flow is split into eight distinctive zones where the flow direction and velocity magnitude substantially differ. Positions and extents of these zones are practically independent of the primary flow regime but are affected by the attack angle. Local velocity values of the leakage flow are unexpectedly found to be about 20% higher in the region of a gap cavity than the mean bulk velocity of the incoming flow. Cavitating cores of various vortices manifest themselves in the recorded images, showing that the vortex structure of the leakage flow associated with the tip-clearance cavitation is very complicated and the hydrofoil axis makes it even more complex by inducing new vortices and cavities. For the gaps considered, an increase of its size causes the tip-clearance cavitation to be initiated at higher cavitation numbers, i.e., this is favorable for its occurrence, while the development of the main cavity is hindered. In unsteady flow regimes, dynamics of the primary cavity on the hydrofoil suction side significantly influences the leakage flow direction and the tip-clearance cavitation evolution. Periods of oscillation cycles of the main and gap cavities coincide but the maximum size of the gap cavity is reached with a phase lag relative to the main one. At the small incidence angle, thicker gaps and unsteady flow conditions, extremely transient pressure waves are registered in the clearance, with their velocities ranging from 41 to 81 m/s

We present a new method of extracting information on average vapor distribution in a cavitating flow based on statistical processing of PIV data for a liquid phase. For this, vectors on instantaneous velocity fields are analyzed over the entire statistical ensemble of instantaneous realizations considering their status: valid—vectors that passed validation procedures, outliers—the ones with incorrect values, out-of-flow—those calculated on insufficient number of seeding particles (tracers), masked—they correspond to unilluminated flow regions. The suggested approach is based on the two basic principles: absence of the tracers in the vapor phase and statistical independence of the successive measurements. The case study is performed for a cavitating 2D symmetric hydrofoil under unsteady cloud cavitation conditions with regular shedding of large-scale cloud cavities. Comparing statistical distribution laws in different flow regions makes it possible to recognize the stable sheet cavity and its pulsating part and determine the location of cloud cavity detachments. This approach for PIV data analysis is shown to be an effective tool to characterize time-averaged distribution of the dispersed phase in cavitating flow based merely on velocity measurements for the liquid phase. Using it allows one to substantially reduce consumption of computational resources and save time when investigating the structure of cavitating flows, limiting to standard PIV measurements in liquid. This method can be also applied to analyze the structure of other types of dispersed two-phase flows.

We present results of Large-eddy simulations (LES) modeling of steady sheet and unsteady cloud cavitation on a two-dimensional hydrofoil which are validated against Particle image velocimetry (PIV) data. The study is performed for the angle of attack of 9° and high Reynolds numbers ReC of the order of 106 providing a strong adverse pressure gradient along the surface. We employ the Schnerr–Sauer and Kunz cavitation models together with the adaptive mesh refinement in critical flow regions where intensive phase transitions occur. Comparison of the LES and visualization results confirms that the flow dynamics is adequately reproduced in the calculations. To correctly match averaged velocity distributions, we propose a new methodology based on conditional averaging of instantaneous velocity fields measured by PIV which only provides information on the liquid phase. This approach leads to an excellent overall agreement between the conditionally averaged fields of the mean velocity and turbulence intensity obtained experimentally and numerically. The benefits of second-order discretization schemes are highlighted as opposed to the lower-order TVD scheme.

In this paper, we performed the numerical and experimental study of unsteady cavitation surge around a semi-circular leading-edge flat plate using a passive flow control method. We mounted a miniature spanwise wedge-type vortex generator on the suction side of the model close to its leading edge. To mitigate the destructive impact of this type of cavitation on the hydrofoil performance, we analyzed the effects of the passive control on the dynamics of cavitation surge. First, we investigated experimentally the unsteady cavitating flow around the semi-circular leading-edge flat plate without passive control using high-speed visualization, acoustic measurements and particle image velocimetry method. Next, we simulated numerically the dynamics of unsteady flow under the cavitation surge conditions with an open source code and validated the numerical results using the experimental data. We used a proper interaction between turbulence and cavitation model to capture a highly unsteady behavior of cavitation surge. Finally, we considered the effects of the passive control device on the mechanism of the cavitation surge instability. Our results revealed that using the passive control method, it is possible to stabilize the attached cavity on the suction side of the flat plate, to hinder the development of the spanwise instability of the attached cavity and to mitigate large-scale cavity structures. Furthermore, high-pressure pulsations in the wake region induced by unsteady cavitation surge were considerably reduced.

A machine learning approach for prediction the characteristics of tonal noise formed in a foil flow is tested. Experimental data are used to construct and analyze the mathematical models of pressure amplitude regression and models of classification of regimes of high-level tonal noise coming from the dimensionless parameters of the flow. Different families of algorithms are considered: from linear models to artificial neural networks. It is shown that a gradient boosting model with a determination coefficient 95% is the most accurate for describing and predicting the spectral curves of acoustic pressure on the entire interval of values of amplitudes and characteristic frequencies.

The article presents results of numerical and experimental investigation of cavitation flow around two subscale models of guide vanes of hydroturbines, one of which has a simplified geometry, while the other has an approximately real geometry. The emphasis was on studying the fluid flow through the gap between the end surface of the hydrofoil and the channel wall, and also on the onset and development of gap cavitation. In the study we used high-speed imaging to analyze the spatial structure and dynamics of cavities. The two-dimensional flow velocity distributions above the hydrofoil surface were measured by the PIV method. In the numerical simulation of cavitation flows around the vanes we used computational fluid dynamics methods based on the solution of Reynolds equations for turbulent flows by means of the finite volume method on a three-dimensional grid of hexahedral cells. The vapor phase was accounted for by solving the equation of transport of vapor fraction. Turbulence was described by the DES method based on a two-parameter model, k-ω SST. As a result, we compared the calculated profiles of flow velocity in the boundary layer on the low-pressure side of the vane with the measurement results; they were in good agreement. In the experiment and in the numerical simulation cavitating vortex structures were observed along with gap cavitation as a vapor film while the fluid was flowing through a narrow gap. At a small angle of attack the vapor film in the gap forms in the vicinity of the trailing edge of the hydrofoil, and immediately behind its leading edge at large angles of attack. In unsteady flow regimes the gap cavitation dynamics substantially depends of the phase of development of the main cavity on the low-pressure side of the vane decompression.

We report on the experimental investigation of cavitating flow control over a 2D model of guide vanes of a Francis turbine by means of a continuous tangential injection of liquid along the foil surface. The generated wall jet, providing supplementary mass and momentum, issues from a nozzle chamber inside the hydrofoil through a spanwise slot channel on its upper surface. High-speed imaging was used to distinguish cavity flow regimes, study the spatial patterns and time dynamics of partial cavities, as well as to evaluate the characteristic integral parameters of cavitation. Time-resolved LIF visualization of the jet discharging from the nozzle was employed to check if the generated wall jet is stable and spanwise uniform. Hydroacoustic measurements were performed by a hydrophone to estimate how the amplitudes and frequencies of pressure pulsations associated with cavity oscillations change with the injection rate. A PIV technique was utilized to measure the mean velocity, its fluctuations and the dominant turbulent shear stress component, which were all compared for different flow conditions and with the results for the unmodified (standard) foil. The effect of injection rate on cavitation and flow dynamics was examined for three attack angles, 0, 3 and 9°, and a range of cavitation numbers corresponding to different regimes. The low-speed injection was shown to lead to an intensification of turbulent fluctuations in the boundary layer and shrinking of the attached cavity length by up to 25% compared to the case without injection. The injection with a high velocity, in turn, causes a rise of the local flow velocity and a reduction of turbulent fluctuations near the wall, which, consequently, increases the foil hydrodynamic quality at a relatively low energy consumption for generation of the wall jet. However, in this case the vapor cavity becomes longer. Thus, the low-speed injection turns out to be effective to mitigate cavitation but the injection at a high velocity is more preferable from the standpoint of the flow hydrodynamics. In the whole, the implemented control method showed to be quite an efficient tool to manipulate cavitation and hydrodynamic structure of the flow and, thereby, under certain conditions, to suppress the cavitation-caused instabilities.

Cavitation is a process of liquid evaporation, bubble or vapor sheet formation, and further collapse of vapor structures, which plays a destructive role in many industrial applications. In marine transport and hydraulic machinery, cavitation usually occurs nearby the surface of a ship propeller and rudder, impeller blades in a pump, and distributor vanes and runner blades in a hydroturbine and causes various undesirable effects such as vibrations of frameworks and/or moving parts, material erosion, and noise enhancement. Based on an extensive literature review, this research is aimed at an experimental investigation of a passive approach to control cavitation on a benchmark hydrofoil using a wedge-type vortex generator in different flow regimes with a high Reynolds number. In this study, we employed a high-speed imaging method to explore the spatial patterns and time evolutions of cavitation structures and utilized a hydroacoustic pressure transducer to record and analyze local pressure pulsations due to the collapse of the cavities in the hydrofoil wake region. The results show that the examined control technique is quite effective and capable of hindering the formation of cloud cavities and reducing the amplitude of pressure pulsations associated with unsteady cavitation dynamics. This study provides important experimental information, which can be useful for improving industrial technologies and for promoting new developments in this particular research field.

Transformation of flow turbulence structure with cavitation occurrence, determination of the flow conditions favorable for nucleation of cavitation bubbles, influence of the statistical structure of turbulence on this process and the inverse effect of cavitation on the flow dynamics are challenging problems in modern fluid mechanics. The paper reports on the results of statistical processing of the velocity fields measured by a PIV technique in cavitating flow over a 2D symmetric hydrofoil for four flow conditions, starting from a cavitation-free regime and finishing by unsteady cloud cavitation. We analyze basic information on the statistical structure of velocity fluctuations in the form of histograms and Q-Q diagrams along with profiles of the mean velocity and turbulent kinetic energy. The research reveals that the flow turbulence pattern and distributions of turbulent fluctuations change significantly with the cavitation development. Under unsteady cloud cavitation conditions, the probability density function of the fluctuating velocity has a two-mode distribution, which indicates switching of two alternating flow conditions in a region above the hydrofoil aft part due to periodic passing of cavitation clouds. Behaviors of the mean and most probable velocities unexpectedly appear to be different with a monotonous increase of the incoming flow velocity. This finding must be caused by modification of the skewness coefficient of the fluctuating velocity.

Gas-liquid two-phase bubbly flows are found in different areas of science and technology such as nuclear energy, chemical industry, or piping systems. Optical diagnostics of two-phase bubbly flows with modern panoramic techniques makes it possible to capture simultaneously instantaneous characteristics of both continuous and dispersed phases with a high spatial resolution. In this paper, we introduce a novel approach based on neural networks to recognize bubble patterns in images and identify their geometric parameters. The originality of the proposed method consists in training of a neural network ensemble using synthetic images that resemble real photographs gathered in experiment. The use of neural networks in combination with automatically generated data allowed us to detect overlapping, blurred, and non-spherical bubbles in a broad range of volume gas fractions. Experiments on a turbulent bubbly jet proved that the implemented method increases the identification accuracy, reducing errors of various kinds, and lowers the processing time compared to conventional recognition methods. Furthermore, utilizing the new method of bubbles recognition, the primary physical parameters of a dispersed phase, such as bubble size distribution and local gas content, were calculated in a near-to-nozzle region of the bubbly jet. The obtained results and integral experimental parameters, especially volume gas fraction, are in good agreement with each other.

The research is aimed at the study of a passive control method to control unsteady cloud cavitation that is characterized by regular shedding of large vapour structures from the solid surface of a cavitating immersible body. The unsteady cloud cavitation is an important subject of research because of its destructive impacts in various industrial applications, including ship propellers and rudders, pumping and hydraulic machinery systems. For this, we placed miniature vortex generators (MVGs) of a cylindrical type on the surface of a benchmark CAV2003 hydrofoil and investigated effects of these MVGs on the spatial structure of unsteady cavitation clouds. We analyzed the temporal and spatial cavity characteristics in comparison with those for the original hydrofoil (without MVGs) by means of high-speed imaging. In addition, we used a hydrophone to register the signal of pressure pulsations in time and thereby derive power spectra of the pressure pulsations. The results showed that the implemented cavitation control method is an effective tool to manage the unsteady behaviour of cloud cavitation and to mitigate the amplitude of pressure pulsations. It was revealed that, with this control approach, the large-scale cavitation clouds appear to be broken and only small-scale cavity structures are shed away from the hydrofoil surface. Moreover, a notable reduction in the cavitation-induced vibrations of the solid surface may be expected.

Cavitation often causes a destructive impact on the performance of hydraulic machinery, such as erosive wear, noise and vibrations of the framework and moving parts of marine propellers, pumps, hydraulic turbines and other equipments, which eventually leads to a degradation of overall system effectiveness. The paper reports on an experimental investigation of a passive method of flow control for different cavitation conditions: starting from the cavitation inception, including quasi-steady partial cavitation with shedding of small-scale vortical structures and finishing by unsteady cloud cavitation. The passive flow control was implemented using miniature vortex generators of a cylindrical type referred to as Cylindrical Cavitating-bubble Generators (CCGs) that were placed on the surface of a benchmark CAV2003 hydrofoil. First, we performed high-speed visualization of cavitation on the suction side of the original hydrofoil (without the control element) to find the cavitation inception point near the leading edge and to analyze the spatial structure and time evolution of partial cavities. In order to improve our understanding of the mechanism of cavitating flow unsteadiness and the effect of CCGs on the cavitation dynamics, we also applied a PIV technique to measure the mean flow velocity profiles and a hydroacoustic pressure transducer to record local pressure pulsations in the hydrofoil wake. As a result, this allowed us to determine the influence of CCGs on turbulent structure of the flow at different cavitation regimes and amplitude-frequency spectra of the pressure pulsations associated with attached cavity length oscillations for unsteady flow conditions. It was revealed that, in the case of unsteady cloud cavitation, CCGs were capable to mitigate large-scale cloud cavities. In addition, a substantial decrease in the amplitude of pressure pulsations was registered for the modified hydrofoil (with the control element). In general, CCGs appeared to be quite effective to hinder the cavitation development and to reduce the strength of side- and middle-entrant jets as the primary mechanisms of unsteady cloud cavitation.

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.

Cavitation is one of the main sources of flow instabilities arising during the operation of hydraulic equipment and is also a cause of erosion wear of its operating elements. In this regard, the elaboration and development of various methods of cavitating flow control is an urgent problem for hampering cavitation evolution and reducing its negative impact. One of these methods is modification of the surface of a hydrofoil. In the paper, the results of an experimental study of a cavitating flow around a grooved 2D hydrofoil are presented in comparison with the ones for the original profile. In order to analyze the spatial structure and time dynamics of partial cavities and evaluate their integral parameters, a high-speed imaging was applied. The flow velocity over the hydrofoils and behind them was measured by a PIV technique. On the modified section, cavitation initiates in form of individual bubbles travelling inside the hollows that transform into cavitating streaks, when the cavitation number is decreased. While the streaks are located in the grooves and do not interact with one another, the flow regime remains stable. However, when their size becomes larger than the groove diameter, they extend beyond these hollows, interact and form an entire cavity which immediately becomes unstable and starts to oscillate. At the transitional flow regime, when cavitating streaks are formed inside the grooves, the intensity of turbulent fluctuations over the modified hydrofoil surface is decreased in comparison with the regime of transient bubble cavitation. This occurs because isolated cavities inside the grooves are likely to restore the shape of the modified foil, making the geometry of its surface closer to the original profile. Thus, the grooved section takes on a streamlined body shape due to local cavitation in the hollows. Besides, the grooves on the GV2 surface cause local flow turbulization in the near-wall region, which seems to be the reason of the delay in cavitation evolution on the grooved hydrofoil. In general, the grooves on a hydrofoil surface are capable to hinder the cavitation development to some extent and prevent the transition to unsteady flow regimes.

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.

Cavitating flows around a flat plate with semi-circular leading edge and a NACA0015 hydrofoil at attack angles ranging from 0° to 9° and with varying cavitation number are investigated using high-speed-imaging visualization (HIV) and particle-imaging velocimetry (PIV). Several known types of cavitation common to both foils, but also some different patterns, were observed. At small angles of incidence (less than 3°), cavitation on the plate begins in the form of a streak array (bubble-band) whereas on the hydrofoil as traveling bubbles. For the regimes with developed cavitation on the NACA0015 hydrofoil, the scattered and discontinuous bubble streaks branch and grow but subsequently merge into bubble clouds forming a remarkably regular lattice pattern. Once the incidence angle increased to 9°, the cavitation on the hydrofoil changed to a streaky pattern like that on the plate at small attack angles, whereas the regime on the plate showed no significant changes. The PIV method proved to be usable for measuring the instantaneous velocity also in the gas–vapor phase, albeit with reduced accuracy that was evaluated and accounted for on the basis of the effective (validation-surviving) number of imaging samples. The time-averaged velocity and turbulence moments show that the incipience of cavitation is governed by the development of the carrier-fluid flow around the foil leading edges, but the subsequent flow pattern depends strongly on the cavitation regime displaying markedly different distributions compared to the non-cavitating case. The main cavitation parameters: the maximum cavity length, the cloud cavity streamwise dimensions and the cloud shedding Strouhal number are analyzed and presented in function of the cavitation number and the attack angle in different scaling. The measurements confirm qualitatively the trends reported in the literature, but show also some quantitative differences, notably between the two foils considered.

Tip-clearance cavitation is one of the most aggressive and widespread forms of cavitation in hydraulic machinery that occurs due to liquid leaking through narrow gaps between tips or end faces of blades/vanes and a stator wall. The research is aimed at the study of a passive control of tip-leakage flow and tip-clearance cavitation by modifying the gap geometry. The test object was a NACA0022-34 hydrofoil with a 100 mm chord that was equipped with a double-sided axis of rotation. The gap geometry was changed by mounting side plates with different end surfaces (flat and grooved) to the hydrofoil end face. We used high-speed imaging to analyze the temporal and spatial cavity evolution simultaneously on the foil suction side and inside the clearance using three cameras. The implemented control method was shown to allow an effective management of the tip-leakage flow and tip-clearance cavitation, especially at higher angles of attack. The modified end plate makes the tip-leakage flow less prone to cavitation as compared to the original one, i.e. the tip-clearance cavitation inception and development appear to be hampered.

A correlation-based processing algorithm for bubble identification by a planar fluorescence for bubble imaging (PFBI) technique is presented in this paper. The algorithm includes procedures to identify bubble positions and sizes, as well as to track bubbles and correct bubble displacement vectors. Moreover, several schemes for calculation time optimisation were realised to achieve a reliable calculation time. The developed algorithm identifies and tracks overlapping bubble images or images with non-uniform intensity distributions. The employed correlation and iterative passing approach provides sub-pixel accuracy of bubble displacement estimation. In addition, the presented algorithm for bubble ring detection can be easily applied to shadow photography images of bubbles, after the application of a derivative filter. The PFBI technique, combined with the particle image velocimetry and particle tracking velocimetry algorithms, was applied for the experimental study of bubbly free jet two-phase flows at Re = 12,000. Four cases of volumetric gas content in the jet core were studied: 0, 1.2, 2.4 and 4.2%, with the same mean bubble diameter—0.85 mm. The developed technique measures two-dimensional distributions of instantaneous void fractions, as well as both gaseous and liquid-phase velocities. Consequently, the mean void fraction and velocity fields and a set of second-order statistical moments were obtained, including correlations of void fraction and velocity pulsations. It was shown that the increase in volumetric gas content leads to the suppression of liquid-phase velocity fluctuations in the jet mixing layer.

In the paper, the possibility of active control of a cavitating flow over a 2D hydrofoil that replicates a scaled-down model of high-pressure hydroturbine guide vane (GV) was tested. The flow manipulation was implemented by a continuous tangential liquid injection at different flow rates through a spanwise slot in the foil surface. In experiments, the hydrofoil was placed in the test channel at the attack angle of 9°. Different cavitation conditions were reached by varying the cavitation number and injection velocity. In order to study time dynamics and spatial patterns of partial cavities, high-speed imaging was employed. A PIV method was used to measure the mean and fluctuating velocity fields over the hydrofoil. Hydroacoustic measurements were carried out by means of a pressure transducer to identify spectral characteristics of the cavitating flow. It was found that the present control technique is able to modify the partial cavity pattern (or even totally suppress cavitation) in case of stable sheet c...

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.

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.

Two-phase bubble flows have been used in many technological and energy processes as processing oil, chemical and nuclear reactors. This explains large interest to experimental and numerical studies of such flows last several decades. Exploiting of optical diagnostics for analysis of the bubble flows allows researchers obtaining of instantaneous velocity fields and gaseous phase distribution with the high spatial resolution non-intrusively. Behavior of light rays exhibits an intricate manner when they cross interphase boundaries of gaseous bubbles hence the identification of the bubbles images is a complicated problem. This work presents a method of bubbles images identification based on a modern technology of deep learning called convolutional neural networks (CNN). Neural networks are able to determine overlapping, blurred, and non-spherical bubble images. They can increase accuracy of the bubble image recognition, reduce the number of outliers, lower data processing time, and significantly decrease the number of settings for the identification in comparison with standard recognition methods developed before. In addition, usage of GPUs speeds up the learning process of CNN owning to the modern adaptive subgradient optimization techniques.

The present paper describes a technique based on planar fluorescence for round bubble imaging in turbulent two-phase flows. Theoretical principles of the approach are described in terms of geometrical optics, and the result of the method application to a test case on singly rising bubbles is presented. One of principal advantages of the technique, named Planar Fluorescence for Bubbles Imaging (PFBI), is that bubbles located far away from the measurement plane become defocused rapidly, which allows to retrieve experimental information in a certain rectangular measurement volume of a low thickness. Due to the high degree of locality, PFBI represents a rather convenient tool for identification of round bubbles located within and very close to the laser sheet exploited for PIV measurements in a liquid phase.

In the work, cavitating flows around a two-dimensional foil of NACA0015 series and a bluff symmetrical body were studied experimentally by means of PIV/LIF technique and high-speed imaging. The flows were investigated for different angles of incidence (from 0° up to 9°) and cavitation numbers (from 0,7 to 2,4), which allowed to register various patterns of partial cavities. Cavitation inception and development were of a particular interest in this study. As a result, instantaneous velocity and vorticity fields were measured and an analysis of instantaneous structure and evolution of partial cavities was performed. Finally, the full set of statistical moments of turbulent fluctuations (including the thirdorder ones) was calculated for cavitation and cavitation-free conditions. The paper reports the effects of body shape and cavitation pattern on the turbulent structure of the flows.

We studied cavitating flow over the suction side of a symmetric 2D foil – a scaled-down model of high-pressure hydroturbine guide vanes (GV) – in different cavitation regimes at the attack angle of 3°. High-speed imaging was used to analyze spatial patterns and time dynamics of the gas-vapour cavities. A hydroacoustic pressure transducer was employed to register time-spectra of pressure fluctuations nearby the hydrofoil. A PIV technique was applied to measure the velocity fields and its fluctuations. The active flow control was implemented by means of a continuous liquid supply with different flow rates through a slot channel located in the GV surface. It was found that the active mass injection does not influence the primary flow upstream of the slot channel position. For the cavitation-free and cavitation inception cases, the injection was shown to make the turbulent wake past the GV section more intense. However, at the developed cavitation regimes the active flow management made it possible to reduce substantially the amplitude or even totally suppress the periodic cavity length oscillations and pressure pulsations associated with them.

Cavitation on two symmetric foils, a NACA0015 hydrofoil and a scaled-down model of a Francis turbine guide vane (GV), was investigated by high-speed visualization and PIV. At small attack angles the differences between the profiles of the mean and fluctuating velocities for both hydrofoils were shown to be insignificant. However, at the higher angle of incidence, flow separation from the GV surface was discovered for quasi-steady regimes including cavitation-free and cavitation inception cases. The flow separation leads to the appearance of a second maximum in velocity fluctuations distributions downstream far from the GV surface. When the transition to unsteady regimes occurred, the velocity distributions became quite similar for both foils. Additionally, for the GV an unsteady regime characterized by asymmetric spanwise variations of the sheet cavity length along with alternating periodic detachments of clouds between the sidewalls of the test channel was for the first time visualized. This asymmetric behaviour is very likely to be governed by the cross instability that was recently described by Decaix and Goncalves [8]. Moreover, it was concluded that the existence of the cross instability is independent on the test body shape and its aspect ratio.

The thermohydraulic characteristics of supercritical CO2 flows in a vertical tube at cooling conditions are numerically investigated, and the influence of the heat-flux condition and of the flow direction are evaluated. Constant (i.e., uniform), linearly increasing and linearly decreasing heat-flux conditions are considered as three typical heat-flux distributions over the pipe length. The simulation results show that there exists a maximum heat transfer coefficient at all heat-flux conditions when the fluid bulk temperature is slightly higher than the pseudo-critical temperature, but also that the heat-flux condition has little effect on the peak value of the heat transfer coefficient. From the viewpoint of the second law of thermodynamics, the influence of the heat-flux condition on the local entropy generation can be attributed to the distributed matching between the heat flux and the difference between the wall temperature and the fluid bulk temperature, as a better matching is associated with a higher uniformity of the local entropy generation and reduced overall irreversibilities. Upward and downward flows are considered, along with flows without gravity as a baseline case for comparison purposes, with the field synergy principle employed to explain the different phenomena in these flows. The buoyancy effect laminarises the downward flows and raises the temperature gradient; hence, the heat transfer deteriorates and the irreversibility increases. In the upward flows, the buoyancy effect augments the turbulence and alleviates the variations in temperature and velocity in the core region, consequently reducing the irreversible loss and enhancing heat transfer. The present study provides insights into the mechanisms of supercritical CO2 heat transfer characteristics as well as practical guidance on the design and optimisation of relevant components.

Thermo-hydraulic characteristics of supercritical CO2 (SCO2) flows in horizontal tubes with half-wall heat-flux conditions are investigated numerically, which is a common practice such as applications in solar parabolic trough collectors, while the heat transfer performance and the underlying mechanisms have not been fully understood. In heated flows, buoyancy acts to inhibit heat transfer when the top half of the tube wall is heated, however, when the bottom half of the tube wall is heated, this inhibition is alleviated, and the synergy between the temperature gradient and velocity fields improves thanks to the secondary flow in the near-wall region at the bottom wall. As a result, the heat transfer coefficient is ∼95% higher (on average) than in the case when the top half of the tube wall is heated. When the bottom half of the tube wall is cooled, buoyancy is expected to enhance heat transfer, while the synergy between the temperature gradient and velocity fields is supressed by the secondary flow in the near-wall region at the bottom of the tube. Conversely, when the top half of the tube wall is cooled, the buoyancy effect inhibits heat transfer, while the synergy between the temperature gradient and velocity fields is improved by the secondary flow in the near-wall region at the top of the tube, which eventually leads to an increase of ∼21% (on average) in the heat transfer coefficient relative to the case when the bottom half of the tube wall is cooled. Finally, the heat transfer discrepancy due to different heat flux conditions revealed in this study are employed in a heat exchanger model, indicating that the thermal performance of this device can be increased by ∼6% through an appropriate arrangement of the hot and cold flows without additional costs.