High Cavitation Numbers Research Articles

Large eddy simulation of micro vortex generator-controlled cavitation across multiple stages

Micro vortex generators (mVGs) control cavitation by altering the boundary layer flow structure. This study employs the wall-adapting local eddy-viscosity large eddy simulation (WALE-LES) turbulence model combined with the Zwart–Gerber–Belamri cavitation model to conduct transient numerical simulations on the National Advisory Committee for Aeronautics 0015 baseline hydrofoil and the hydrofoil equipped with mVGs under various cavitation numbers. The proper orthogonal decomposition method and experiments verify the accuracy and consistency of these simulations regarding cavity scale. The study elucidates mechanisms by which mVGs suppress cloud cavitation at low cavitation numbers and induce vortex cavitation at high cavitation numbers. Results indicate that mVGs maintain sheet cavitation characteristics at low cavitation numbers, reducing wall pressure fluctuations and enhancing flow stability. During cavitation inception, mVG-induced vortex cavitation leads to early cavitation formation. In the sheet cavitation phase, modal energy distribution is more dispersed, while in the inception phase, energy is concentrated with significant dominant modes. Moreover, the counter-rotating vortices generated by mVGs mitigate flow separation, enhance leading-edge flow attachment stability, and reduce high-frequency vibrations caused by bubble shedding. This study significantly advances the understanding of cavitation control by accurately simulating and revealing the cavitation control mechanisms of mVGs across different stages using the WALE-LES model. The findings demonstrate that mVGs can effectively stabilize cavity structures at low cavitation numbers, reducing flow instabilities and enhancing overall hydrofoil performance. These insights will have a significant impact on the design of hydrofoils and the development of cavitation control strategies.

Experimental study on the unsteady behavior and frequency characteristics of high-speed submerged cavitating water jets

The frequency characteristics of cavitation fluctuations in high-speed cavitating jets are intricate due to the coupling mechanisms of unsteady behaviors. This study employs high-speed photography to experimentally investigate the relationship between frequency characteristics and the unsteady behavior of cavitating jets with various cavitation numbers. Temporal evolution patterns of the cavitating jets are analyzed through spatiotemporal (s-t) diagrams. The spatial distribution and temporal evolution of cavitation fluctuation frequencies are examined using fast Fourier transform (FFT) and continuous wavelet transform (CWT), respectively. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) are employed to identify coherent structures and their corresponding frequencies. In results, the s-t diagrams reveal the distinct regions influenced by cavitation shedding and collapse. FFT results indicate that upstream of the jet trajectory, spectral energy is concentrated in the shedding band, shifting toward lower frequencies with increasing axial distance. The CWT spectrum exhibits a single peak in the upstream, identifying it as the shedding frequency. POD modes associated with shedding dominate the energy contribution at higher cavitation numbers, while they become less prominent at lower cavitation numbers. DMD extracts and identifies coherent structures associated with shedding through frequency-specific decomposition.

Behavior of hydrofoil cavitation in a slit channel

The paper presents the results of a cavitation in a slit channel study and offers an analytical description of cavity development. Special emphasis was placed on examining partial cavitation near a NACA 0012 hydrofoil (NACA – the National Advisory Committee for Aeronautics) inside slit channels of different geometries. Experimental investigation was carried out via high-speed imaging and the laser Doppler anemometry (LDA) method. The experimental data showed that the local flow velocity in hydrofoil leading edge area increased abruptly under arising cavitation. In addition, occurrence of cavitation raised flow velocity pulsation by 20%. In the case of a shorter channel, cavity growth occurred at higher cavitation numbers than for a longer channel. The cavity growth velocity was higher for a shorter channel. We showed that the tendency of partial cavitation development in the slit channel can be described as follows: L/C ∼ σ−1, where L is the cavity length; C is the hydrofoil chord; σ is the cavitation number; and parameter A changes as the slit channel length is varied. Comparison of cavitation development near hydrofoil at different attack angles α inside the slit channel with a three-dimensional (3D) cavitation tunnel was conducted. Cavitation in the slit channel occurred at lower σ/2α values compared to 3D cavitation flow around the hydrofoil. To directly compare lengths of the attached cavities arising in slit channels and 3D cavitation tunnels, an additional parameter is proposed, taking into account friction of the slit channel: K = λ·l/D. This parameter allowed us to quantitatively compare the characteristics of cavitating hydrofoils in slit channels and 3D tunnels. The paper provides the governing criteria of the cavitation in the slit channel. Our results propose the physical foundations for the development of cavities in the slit channel.

Large eddy simulation investigation of micro-vortex generator control effect on early stage sheet cavitation

The wall-adapting local eddy-viscosity large-eddy simulation method is employed for the numerical simulation of a hydrofoil, with transient calculations conducted to compare and analyze the near-wall flow characteristics and cavity morphologies of both the baseline and micro-vortex generator (mVG) hydrofoil models under conditions of high cavitation numbers. High-speed photography combined with numerical analysis revealed that mVGs generate a pair of counter-rotating vortices, boosting the transfer of momentum between the boundary layer and the main flow while reducing flow separation. These vortices induce a new mixed cavity structure at the leading edge, combining vortex cavitation with attached sheet cavitation. During cavity evolution, the mVGs prevent overall tail shedding in the baseline hydrofoil, confining shedding to the sides, while the central vortex cavitation structure remains stable. It enhances hydrofoil stability by reducing pressure fluctuations and guiding cavitation toward more predictable dynamics without causing significant pressure impacts. This research elucidates the mechanism of mVGs in guiding fluid attachment, transforming the structure and shedding cycle of attached cavities, and emphasizing its effectiveness by controlling early-stage sheet cavitation.

Enhancing Efficiency and Reliability of Tidal Stream Energy Conversion through Swept-Blade Design

The current limited efficiency and reliability of tidal current turbines (TCTs) have posed significant challenges in effectively harnessing tidal stream energy. To address this issue, this paper undertakes both numerical and experimental studies to explore the advantages of swept blades over conventional straight blades in terms of energy capture efficiency and cavitation resistance. It is found that both the sweep length and sweep angle of the blade can influence the power generation efficiency of the TCT. For the particular swept blade investigated in this study, the highest power coefficient is achieved when the sweep length is 0.544 m and the sweep angle is 28.88°. The research also demonstrated that the swept-blade TCT shows a higher power generation efficiency than the straight-blade TCT across a broad range of rotor speeds. To be precise, with the swept blades, the power coefficient of the TCT can be improved by 5–17%, depending on the tip speed ratio. Additionally, swept blades exhibit a superior cavitation resistance. This is evidenced by their higher cavitation numbers across all tip speed ratios in comparison to conventional straight blades.

Exploring hydrodynamic cavitation for citrus waste valorisation in Malta: from beverage enhancement to potato sprouting suppression and water remediation.

Introduction: The endorsement of circular economy, zero-waste, and sustainable development by the EU and UN has promoted non-thermal technologies in agro-food and health industries. While northern European countries rapidly integrate these technologies, their implementation in Mediterranean food-supply chains remains uncertain. Aims: We evaluated the usefulness of hydrodynamic cavitation (HC) for valorizing orange peel waste in the fresh orange juice supply chain of the Maltese Islands. Method: We assessed: a) the effectiveness of HC in extracting bioactive compounds from orange peels (Citrus sinensis) in water (35°C) and 70% (v/v) ethanol (-10°C) over time, compared to conventional maceration, and b) the potato sprouting-suppression and biosorbent potential of the processed peel for copper, nitrate, and nitrite binding. Results: Prolonged HC-assisted extractions in water (high cavitation numbers), damaged and/or oxidized bioactive compounds, with flavonoids and ascorbic acid being more sensitive, whereas cold ethanolic extractions preserved the compounds involved in radical scavenging. HC-processing adequately modified the peel, enabling its use as a potato suppressant and biosorbent for copper, nitrate, and nitrite. Conclusion: Coupling HC-assisted bioactive compound extractions with using leftover peel for potato-sprouting prevention and as biosorbent for water pollutant removal offers a straightforward approach to promoting circular economic practices and sustainable agriculture in Malta.

Experimental characterization of cavitation zone and cavity oscillation mechanism transitions in planar cavitating venturis

Cavitating venturi is a passive flow rate anchoring device used in varied industrial applications. The dynamics of the cavitation zone can be of interest to ascertain the controlled operation of cavitating venturi under varying pressure ratios. In the current work, we present the results of the complete characterization of three planar cavitating venturis with different divergent angles. Quasi-steady experiments are conducted for a pressure ratio range of 0.39–0.95 and an inlet Reynolds number range of 7.3 × 104–1.28 × 105. Shadowgraphy and high-speed imaging are used to obtain the cavitation zone length and the oscillation frequencies. Spectral proper orthogonal decomposition and discrete Fourier transform are used to assess the dynamics of the cavitation zone. The cavitation zone behavior has been delineated into three specific zones (named R1, R2, and R3 in this work) during the operation when the cavitation is fully contained within the divergent section. Two Strouhal number ranges (based on the inlet dimensions), StD,in≥ 0.1 for large-scale cloud shedding and StD,in≤ 0.05 for small-scale oscillations of the attached cavity, are ascertained as a primary indicator of the dynamic behavior. The current work confirms that the dynamics is governed by re-entrant jet at high cavitation numbers in R1 and the combined action of the re-entrant jet and the bubbly shock wave (collapse-induced) at low cavitation numbers in R3. The transition in the cavitation zone behavior in R2 primarily causes a shift in the sensitivity of the cavitation zone and the dominant frequencies over the operating pressure ratios. In the present work, we show that the span of the transition region (R2) decreases with an increase in the divergent angle.

Effect of a textured surface on the occurrence and development of cavitation on the hydrofoil

An experimental study of the cavitation flow around the National Advisory Committee for Aeronautics 0012 hydrofoil with different surface morphology was carried out in this work. The surface morphology was set by modern laser ablation technology. The average values and intensity of vapor–gas cavities were determined. It has been revealed that laser texturing delays the emerging cavitation and somewhat decreases its intensity at higher cavitation numbers. A decrease in the cavitation number leads to an increase in its intensity for a smooth hydrofoil in comparison with a rough one, which is also expressed in an increase in the frequency of cavities. The paper presents a comparison of the flow regime with equal cavitation numbers, which clearly describes the features of the development of a vapor–gas cavity on the suction side of the foil with different surface morphologies. The paper provides an explanation of the reasons for the influence of surface morphology on the development of cavities.

Experimental investigation of three distinct mechanisms for the transition from sheet to cloud cavitation

The conventional high-speed images of cavitation with a set of X-ray phase contrast images reveal the presence of three different types of mechanisms responsible for large cloud shedding: re-entrant jet mechanism, condensation shock wave mechanism, and collapse-induced pressure wave mechanism. At higher cavitation numbers, the sheet cavity is relatively short and the cavity detachment is a consequence of a re-entrant jet pinching off the cavity from its leading edge. At lower cavitation numbers, the re-entrant jet plays a smaller role in the cavitation instabilities and the primary reason for periodic cloud shedding is the condensation shock mechanism where a void fraction discontinuity propagates upstream until collapsing the entire cavity. If the amount of shed vapour cloud reaches a certain extent, the collapse will emit a pressure wave strong enough to disturb the growing cavity, and subsequently make it detached from the wall. This is the third mechanism observed in the experiments. We point out the inappropriate classification of combining condensation shock and collapse-induced pressure wave mechanisms in the literature, since we identify pronounced differences between them: (i) the pressure increase across the condensation front is very weak (a few kPa) while the amplitude of collapse-induced pressure wave can be hundreds of kPa, (ii) the travelling velocity of the collapse-induced pressure wave within the cavity is much faster than the condensation shock, and (iii) the collapse-induced pressure wave does not result in an obvious discontinuity in void fraction when it propagates through the cavity, in contrary to the condensation shock.

Effect of an end-clearance width on the gap cavitation structure: Experiments on a wall-bounded axis-equipped hydrofoil

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.

Numerical study on the effects of installing an inducer on a pump in a turbopump

In a projectile, a turbopump is widely used to pressurize oxidizer and fuel to gain high thrust instead of using high pressure tank which has a disadvantage in total weight. Pumps in a turbopump employ an inducer upstream to prevent performance degradation by increasing pressure at the impeller inlet. However, cavitation and related instabilities take place frequently, so numerous analysis have been conducted for turbopumps. In this study, the pumps with and without an inducer were numerically studied using ANSYS CFX 13.0 to analyze the effects of installing an inducer, which was rarely treated before. By comparing these two pumps, the merits and demerits of installing an inducer can be analyzed in the aspects of hydraulic/suction performances and internal flow characteristics. Also, the role of an inducer can be understood in both of non-cavitating and cavitating conditions. As a result, head rise and efficiency of the pump were degraded by installing an inducer in non-cavitating conditions. The inducer could not play a role at high flow rate, and efficiency of the inducer was steeply declined in the off-design conditions. However, suction performance of the pump was greatly improved by adopting the inducer. Head of the pump with an inducer abruptly decreased at the low cavitation numbers, while head of the pump without an inducer started to decline gradually at the high cavitation numbers. In spite of high cavitation number, severe cavitation appeared in the case without an inducer, so it can be said that installing an inducer can prevent performance deterioration from the cavitation and satisfies stable operation which is more important for pumps rather than obtaining high hydraulic performance.

Thermodynamic effects on the cavitation flow of a liquid oxygen turbopump

Cavitation is a severe problem in rocket turbopumps. As for turbopumps with liquid oxygen, it exhibits notable thermodynamic effects during cavitation, leading to complex flow characteristic. In this study, to assess the influence of thermodynamic effects on the cavitation flow in a liquid oxygen turbopump, the entire flow passage was numerically simulated using the shear stress transport k–ω turbulence model and a thermodynamic model based on the Zwart–Gerber–Belamri cavitation model to couple the physical parameters with temperature. The numerical schemes were validated using available experimental data. Numerical results show that cavitation primarily occurs at the leading edge of the suction surface of the inducer inlet and at the head of the central blades of the impeller inlet. The thermodynamic effects notably decrease the vapor-phase volume fraction of the cavitation zone at the low cavitation number of σ=0.07, but increases it at high cavitation numbers. Moreover, the hydraulic loss in the turbopump was mainly distributed in the impeller-inducer and diffuser. A novel entropy analysis reveals that the thermodynamic effects reduced the dissipation losses generated by the velocity gradient and increased the hydraulic losses induced by the temperature gradient in the wall region. The present study can provide a theoretical reference for improving the hydraulic-instability induced cavitation flow in liquid oxygen rocket turbopumps.

Cavitation patterns in high-pressure homogenization nozzles with cylindrical orifices: Influence of mixing stream in Simultaneous Homogenization and Mixing

High-pressure homogenization is the state of the art to produce high-quality emulsions with droplet sizes in the submicron range. In simultaneous homogenization and mixing (SHM), an additional mixing stream is inserted into a modified homogenization nozzle in order to create synergies between the unit operation homogenization and mixing. In this work, the influence of the mixing stream on cavitation patterns after a cylindrical orifice is investigated. Shadow-graphic images of the cavitation patterns were taken using a high-speed camera and an optically accessible mixing chamber. Results show that adding the mixing stream can contribute to coalescence of cavitation bubbles. Choked cavitation was observed at higher cavitation numbers σ with increasing mixing stream. The influence of the mixing stream became more significant at a higher orifice to outlet ratio, where a hydraulic flip was also observed at higher σ. The decrease of cavitation intensity with increasing back-pressure was found to be identical with conventional high-pressure homogenization. In the future, the results can be taken into account in the SHM process design to improve the efficiency of droplet break-up by preventing cavitation or at least hydraulic flip.

Reducing energy consumption during bitumen separation from oil sand

Oil sands, found in Canada, Venezuela, USA and Russia, contain bitumen, water, clay and some metals. Two different methods of producing oil from oil sands exist: open-pit mining and in situ. Approximately 20% of oil sands are recoverable through open-pit mining.After oil sand is mined, bitumen is separated from solids and water within the bitumen production facilities. The process of bitumen separation consists of 3 main steps. (1) Hot/warm water is added to the oil sands producing a slurry that can be pumped to the processing plant through hydrotransport pipelines which connect ore preparation to the main extraction facility. These large-diameter pipelines can be several kilometers long, providing the additional time and shear required to break down the lumps of mined oil sands. (2) After hydrotransport pipeline oil sand slurry enters flotation facility where bitumen is gravity separated from the coarse solids producing an intermediate bitumen froth product. (3) Then solvent is added to the froth, to reduce the bitumen viscosity and to remove remaining water and fine solids.In addition to efficiency concerns, this separation process suffers from high operating cost due to a number of factors such as the energy needed for heating the water, the energy consumed by the cyclofeeder and the energy consumed by hydrotransport. The other problem of hydrotransport is extensive wear since the slurry contains quartz sand particles which are extremely abrasive.In the present study, an alternative technology based on cavitation jets is suggested. It is found that jets can separate bitumen at low temperatures (5–6 °C) and cavitation jet technologies have the potential to replace hydrotransport.A series of nozzles were designed for low intensity cavitation at high cavitation numbers (σ=0.37 - 0.46) to test different types of cavitation generators. It was shown that selfresonating nozzle shows significantly better results than conventional nozzles and conventional nozzles with passive control.

Experimental and numerical study of cavitating flow around a surface mounted semi-circular cylinder

In this paper, the cavitating flow around a bluff body is studied both experimentally and numerically. The bluff body has a finite length with semi-circular cross section and is mounted on a surface in the throat of a converging-diverging channel. This set-up creates various 3D flow structures around the body, from cavitation inception to super cavities, at high Reynolds numbers (Re=5.6×104−2.2×105) and low cavitation numbers (σ=0.56−1.69). Earlier studies have shown this flow to be erosive and the erosion pattern varies by changing the flow rate and w/o the cylinder; hence, this study is an attempt to understand different features of the cavitating flow due to the cylinder effect. In the experiments, high-speed imaging is used. Two of the test cases are investigated in more detail through numerical simulations using a homogeneous mixture model. Non-cavitating simulations have also been performed to study the effect of cavitation on the flow field. Based on the observed results, vortex shedding can have different patterns in cavitating flows. While at higher cavitation numbers the vortices are shed in a cyclic pattern, at very low cavitation numbers large fixed cavities are formed in the wake area. For mid-range cavitation numbers a transitional regime is seen in the shedding process. In addition, the vapour structures have a small effect on the flow behaviour for high cavitation numbers, while at lower cavitation numbers they have significant influence on the exerted forces on the bluff body as well as vortical structures and shedding mechanisms. Besides, at very low cavitation numbers, a reverse flow is observed that moves upstream and causes the detachment of the whole cavity from the cylinder. Such a disturbance is not seen in non-cavitating flows.

The gap size effect on leakage flow and cavitation in the clearance of a 2D hydrofoil: high-speed visualization and velocity measurements

In the study, tip-clearance cavitation inception and development was studied in the gap between an end face of a NACA0022-34 hydrofoil and a transparent sidewall of a test channel. The experiments were carried out for the attack angles of 3° and 9° and the gap width of 0.4 to 0.8 mm under various flow conditions on the cavitation number. In order to observe the tip-clearance cavitation inception and its dynamics, a high-speed visualization was applied. The leakage flow velocity was measured inside the clearance by a modified PTV technique. It was shown that unsteady dynamics of the primary cavity on the foil suction side affects the leakage flow direction and the gap cavitation evolution. An increase of the gap width causes the gap cavitation to be initiated at higher cavitation numbers.

Cavitating flow-induced unsteady pressure pulsations in a low specific speed centrifugal pump.

With the development of cavitation, the high-energy pressure wave from a cavitation bubble collapsing is detrimental to the stable operation of centrifugal pumps. The present paper concentrates on pressure pulsations under cavitation conditions, and pressure amplitudes at the blade-passing frequency (fBPF) and RMS values in the 0–500 Hz frequency band are combined to investigate cavitation-induced pressure pulsations. The results show that components at fBPF always dominate the pressure spectrum even at the full cavitation stage. For points P1–P7 on the volute side wall, with a decreasing cavitation number, the pressure energy first remains unchanged and then it rises rapidly after the critical point. For point In1 in a volute suction pipe located close to the cavitation region, the pressure energy changes slightly at high cavitation numbers; then for a particular cavitation number range, the pressure energy decreases, and finally increases again. For different flow rates, the pressure energy at the critical point is much lower than the initial amplitude at the non-cavitation condition for In1. This demonstrates that the cavitation cloud in the typical stage is partially compressible, and the emitted pressure wave from a collapsing cavitation bubble is absorbed and attenuated significantly. Finally, this leads to the pressure energy decreasing rapidly for the measuring point In1 near the cavitation region.

Study of surface wettability effect on cavitation inception by implementation of the lattice Boltzmann method

Cavitating flow through the orifice is numerically solved by implementation of the lattice Boltzmann method. The pseudo-potential single-component multiphase Shan-Chen model is used to resolve inter-particle interactions and phase change between the liquid and its vapor. The effect of surface wettability on the cavity formation and shape is studied by imposing an appropriate wall boundary condition for the contact angle between the liquid-vapor interface and the solid surface. Efficiency of the numerical approach presented is examined by computing the cavitation inception, growth, and collapse for internal cavitating flows over a sack-wall obstacle placed inside a channel and through a convergent-divergent nozzle section. The results obtained demonstrate that hydrophobic walls act as surface nuclei and contribute to the process of cavitation inception even at high cavitation numbers. In contrast, the solid wall with hydrophilic properties shows no contribution to the onset of cavitation in the geometries studied. High values for the flow velocity corresponding to low cavitation numbers are needed to observe the cavitation inception over the geometries studied with the hydrophilic solid wall. The study shows that the present computational technique based on the implementation of the lattice Boltzmann method with the Shan-Chen model employed is robust and efficient to predict the cavitation phenomena by considering surface wettability effects and also accurate enough for computing the cavitating flow properties at different conditions.

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.

Acoustic emission due to partial cavity shedding on a hydrofoil

The partial cavity shedding dynamics on a NACA 0015 hydrofoil were examined at angles of attack of 7 and 10 degrees over a range of cavitation numbers. Hydrophone measurements in conjunction with high-speed videos and time resolved X-ray densitometry were used to acquire time synchronous measurements of the void fraction field of the cavity and the emitted noise. Cavitation dynamics changed significantly with the reduction of the inlet cavitation number. At higher cavitation numbers, small stable partial cavities were observed. Upon reduction of cavitation number, the cavities grew in length and pinched off from the rear of the cavity, due to a liquid re-entrant jet. Upon further reduction of inlet cavitation number, the dynamics changed significantly with the cloud collapse of the shed vapor inhibiting the cavity growth. X-ray densitometry measurements revealed the presence of a propagation bubbly shockwave as a mechanism of shedding at the lowest cavitation numbers. This process caused complex, multi-st...