Low Cavitation Number Research Articles

Investigation on the Excitation Force and Cavitation Evolution of an Ice-Class Propeller in Ice Blockage

When an ice-class propeller is operating in an ice-covered environment, as some ice blocks slide along the ship hull in front of the propeller blades, the inflow ahead of the propeller will become non-uniform. Consequently, the excitation force applied to the blades will increase and massive cavitation bubbles will be generated. In this paper, a hybrid Reynolds-Averaged Navier–Stokes/Large Eddy Simulation method and Schnerr–Sauer cavitation model are used to investigate the hydrodynamics, excitation force, cavitation evolution and flow field characteristics of the propeller in ice blockage conditions. The results show that the numerical method adopted has a relatively high accuracy and the hydrodynamic error is controlled within 3.0%. At low cavitation numbers, although the blockage distance decreases, the cavitation phenomenon is still severe and the hydrodynamic coefficients hardly increase accordingly. Ice blockage causes a sharp increase in cavitation. When the distance is 0.15 times the diameter, the cavitation area amounts to 20% of the propeller blades. As the advance coefficient grows, the total cavitation area diminishes, while the cavitation area of the blade behind ice does not decrease, resulting in an increment in excitation force. Ice blockage also causes backflow in the wake. At this time, the largest backflow appears at the tip of the blade behind the ice. The higher the advance coefficient, the more significant the high-pressure area of the pressure side and the greater the pressure difference, causing the excitation force to rise sharply. This work offers a positive theoretical basis for the anti-cavitation design and excitation force suppression of propellers operating in icy regions.

An experimental study on the erosion of solid propellant by cavitation water jet in submerged environment

The submerged cavitation water jet (SCWJ) is a promising technology for removing solid propellant from old solid rocket engines and recycling them. In this study, the feasibility of using SCWJ to break solid propellant is investigated. The structure of the cavitation cloud was captured by a high-speed camera and then analyzed using the Proper Orthogonal Decomposition (POD) and Frame Difference Method (FDM) method for exploring the unsteady characteristic of SCWJ, the HTPB propellant erosion experiments were performed, and the relationship of the SCWJ unsteady characteristic and the erosion results on HTPB propellant was analyzed. The results show that the SCWJ develops in the flow field including inception, development, shedding, and collapse stages, the maximum density of cavitation cloud changes exponentially, and the cavitation cloud length grows as the cavitation numbers increase. Lower cavitation number is always beneficial to high penetration rate and low energy consumption when erosion on the HTPB propellant. When the erosion time of 120 s, in high cavitation number under the least amount of energy consumed, the unit energy consumption minimum. The mass loss curve is double peak, the first peak is caused by water jet effect, standoff distance is located on the stage of development, the formation of erosion pit diameter is small, the depth is larger, the second peak is caused by cavitation effect, and its standoff distance is located on the shedding phase, forming the erosion pit diameter is bigger, the depth is shallow. The optimum standoff distance for the cavitation effect for the cavitation numbers 0.0135, 0.0081, 0.0058 and 0.0045 is 35 mm, 50 mm, 60 mm and 70 mm in order, and the mass loss rate of HTPB propellant is 90 mg s−1, 200 mg s−1, 277 mg s−1 and 521 mg s−1, respectively. At best cavitation effect standoff distance, cavitation number by 2 times, mass loss rate can increase 3 times. The matrix brittle fractures occur when the SCWJ strikes the HTPB propellant at a high strain rate. Besides, the erosion mechanism of HTPB propellant is discussed.

Benchmark experimental study on cavitating flow around Clark-Y 11.7 % hydrofoil at various angles of attack under controlled levels of dissolved air

The present research aims to study cavitating flow around a CLARK-Y 11.7 % hydrofoil with variable angles of attack (α) while maintaining controlled levels of dissolved air in the operating fluid, which is water. A series of experiments were conducted using a water tunnel facility, where the cavitation characteristics were measured and observed using sensors and high-speed imaging techniques. The variables studied in the present work are cavitation number (1 ≤ σ ≤ 2.2) and angle of attack (α = 4°, 6°, 8°, 10°, and 12°), with dissolved air levels (DAL) in the range of 9.3 ppm to 13.1 ppm. The dimensionless cavity length decreases significantly with increasing values of σ/α, indicating an inverse relationship where higher cavitation numbers or lower angles of attack result in shorter cavities. The cavity length follows a power-law scaling relationship, with the empirical equation Lmax/C=4.78×σ/α−0.76.Increasing the angle of attack transitions the cavitation nature from stable (Mode I) to dynamic (Mode II) and highly oscillating (Mode III). Larger cavities result in lower Strouhal numbers, which indicates reduced vortex shedding activity. The relationship between the Strouhal number and the normalized cavitation number σ/α is characterized by the power-law equation St=0.041×σ/α0.3. The pressure coefficient at the leading-edge increases with the angle of attack at low cavitation numbers, while higher cavitation numbers lead to greater pressure coefficient differences between the leading and trailing edges.The present study offers an extensive dataset and empirical correlations that may serve as a benchmark framework, which facilitates the validation of computational and experimental models of cavitating flow under similar conditions.

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.

Modeling study of wake cavitation characteristics of DTMB propeller based on numerical computation

Based on the RANS method using unstructured grids, numerical simulations of cavitation on the DTMB propeller under uniform inflow conditions were conducted. Through numerical studies of pressure pulsations under different cavitation numbers, it was found that at low cavitation numbers, the pressure fluctuations become irregular with higher amplitude and increased noise. As the cavitation number increases, the pressure tends to stabilize and exhibits periodic fluctuations. Two linear regression models were constructed by the correlation analyzing between the spectral characteristics of the pressure pulsation signals and the operating conditions. The model for the rotational speed and shaft frequency amplitude of the 4381 propeller indicates that the amplitude of the second harmonic of the shaft frequency increases with the advance coefficient. The model for the skew angle and the fourth harmonic amplitude shows that an increase in the skew angle leads to a gradual increase in the amplitude of the fourth harmonic.

Experimental study on the mechanism of cavitation-induced ventilation

In this study, the cavitating flow and cavitation-induced ventilation flow around a surface-piercing hydrofoil were investigated to gain in-depth understanding of the interaction mechanism between the vaporous cavity and free surface at low cavitation numbers. Experiments were conducted in a constrained-launching water tank to visualize the cavity using a high-speed camera. Unsteady cloud cavitation and cavitation-induced ventilation at atmospheric pressure were observed and analyzed while piercing the free surface. The flow regime map was summarized at a fixed aspect ratio of ARh = 1.5. Subsequently, a physical model was proposed to predict the maximum depression depth of the water surface (H) at the trailing edge of the hydrofoil. Both the physical model and experimental results reveal that the non-dimensional depth H/c has a linear relation to Fn2 c × Rec × sin2α. Finally, a criterion for cavitation-induced ventilation based on the improved lifting-line theory and a physical model were proposed. A new relation H/Lc ∼ α0.5 was obtained, where Lc is the maximum cavity length. The results of this study can guide the design and application of hydrofoils for ventilation and cavitation processes.

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.

Numerical investigation of ventilated cavitating flow from high to low cavitation numbers

Cavitation occurs when an underwater vehicle moves at a high speed, causing the water to change from the liquid phase into the vapour phase in the low-pressure region. As one of the most efficient drag reduction technologies for high-speed underwater vehicles, the artificially ventilated cavitation flow needs to be studied. In this paper, the ventilated cavitating flow over a high-speed underwater vehicle is analysed with a multi-phase solver from high to low natural cavitation numbers. The large eddy simulation turbulence model, volume of fluid method and Kunz cavitation model are used in the numerical calculation to simulate and capture the three-phase (air, liquid water, and vapour) cavitation flow. The miscibility of the air and vapour phases inside the cavity is also considered in the simulation. To validate the solver, two benchmark cases with air ventilated into a cavitating flow are simulated. Then the characteristics of the ventilated cavitating flows from a high to low natural cavitation number are analysed to study the development of the ventilated cavity with miscible air and vapour inside it. The results show that air ventilation can prevent pressure fluctuations at the surface of an underwater vehicle caused by cloud cavitating flow at a moderate natural cavitation number. Compared to no ventilation, drag reduction of ventilated cavitating flow can reach up to about 85% at a low cavitation number.

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.

Experimental investigation on the effect of fluid–structure interaction on unsteady cavitating flows around flexible and stiff hydrofoils

We experimentally investigated the effect of fluid–structure interaction on unsteady cavitating flows around flexible and stiff National Advisory Committee for Aeronautics 0015 hydrofoils in a low-pressure cavitation tunnel. We analyzed the cavitating dynamics by capturing the cavitation dynamics using two high-speed cameras at different cavitating regimes on the surface of the hydrofoils, made of polyvinyl chloride, brass, and aluminum. We then measured the associated structural deformations in specific cavitation regime such as cloud and partial cavitation dynamics, using a digital image correlation technique. The hydrofoil's angle of attack was set to 10°, and the flow's Reynolds number was adjusted to 0.6 × 106. Results showed that the cavity's shedding frequency on the flexible hydrofoil shifted faster to a higher frequency than on the stiff hydrofoils under similar cavitating conditions. The flexible hydrofoil underwent strong structural oscillations at the low cavitation number for the cloud cavitation regime. The associated amplitudes of the vibration were about 20 times higher than those of the hydrofoil made of brass. It was observed that the fluid–structure interaction can significantly affect the cavitation-induced vibration of the flexible hydrofoil.

Experimental study on the boundary layer of the NACA16-012 hydrofoil during the cavitation disappearance phenomenon

The generated cavitation on the NACA16-012 hydrofoil suddenly disappears at a specific angle of attack, even at a low cavitation number, where cavitation normally develops. This was named the cavitation disappearance phenomenon by the authors. It has not been clear why, once cavitation occurs, it disappears. In this study, pressure distributions on a suction surface under noncavitation and disappearance conditions were measured in the cavitation tunnel at an angle of attack at which the cavitation disappearance phenomenon occurred. Moreover, the boundary layer and pressure distribution on a suction surface under noncavitation conditions were comprehensively measured in the wind tunnel at a wide range of the angle of attack. As a result of the wind tunnel experiment, the angle of attack where the cavitation disappearance phenomenon occurred corresponded to the angle of attack where the onset of the short bubble occurred under noncavitation conditions. As the result of the cavitation tunnel experiment, the pressure distribution was found to differ between noncavitation and disappearance conditions. Under disappearance conditions, the suction peak disappeared and the low-pressure region extended to the downstream region. Additionally, the pressure distribution under the disappearance condition seemed to correspond with the distribution in a burst state, which was measured in a wind tunnel. At the same time, the boundary layer in the sheet/cloud cavitation periodically repeated the short bubble (during cloud release) and burst (during sheet cavitation) states. The disappearance of cavitation may be caused by the situation in which the burst boundary layer cannot return to a short bubble after the cloud cavity release and sheet cavitation cannot occur because the pressure is high in the burst boundary layer.

Evolution Characteristics of Suction-Side-Perpendicular Cavitating Vortex in Axial Flow Pump under Low Flow Condition

In order to study the evolution characteristics of suction-side-perpendicular cavitating vortex in an axial-flow pump under low flow conditions, model tests, high-speed imaging, and an SST-CC turbulence model were used to simulate the external characteristics and cavitation morphology of the pump. The evolution law of suction-side-perpendicular cavitating vortex (SSPCV) was revealed by turbulent kinetic energy, liutex vortex identification, and vorticity transport equation. The results show that the evolution of suction-side-perpendicular cavitating vortex at low cavitation number can be divided into three stages: generation, development, and breaking stage. In the generation stage, the turbulent kinetic energy, velocity gradient and vortex kinetic energy continue to increase, reaching the maximum at the early stage of development. Afterwards, due to the viscosity of the water, the vortex slowly dissipates and enters the stage of development. Finally, it is affected by the next blade and enters the breaking stage, which accelerates the dissipation of the vortex. The vortex stretching term and vortex expansion term are the main contributors to the vorticity. During the development of the vortex, the vorticity is mainly caused by the deformation of the fluid micelle. The breaking stage mainly affects the stretching term, and the Coriolis force term cannot be ignored in the rotating coordinates.

Numerical study of cavitating flow over hydrofoil in the presence of air

PurposeThe presence of air in the water flow over the hydrofoil is investigated. The examined hydrofoil is ClarkY 11.7% with an angle of attack of 8 deg. The flow simulations are performed with the assumption of different models. The Singhal cavitation model and the models which resolve the non-condensable gas including 2phases and 3phases are implemented in the numerical model. The calculations are performed with the uRANS model with assumption of the constant temperature of the mixture. The two-phase flow is simulated with a mixture model. The dynamics and structures of cavities are compared with literature data and experimental results.Design/methodology/approachThe cavitation regime can be observed in some working conditions of turbomachines. The phase transition, which appears on the blades, is the source of high dynamic forces, noise and also can lead to the intensive erosion of the blade surfaces. The need to control this process and to prevent or reduce the undesirable effects can be fulfilled by the application of non-condensable gases to the liquid.FindingsThe results show that the Singhal cavitation model predicts the cavity structure and related characteristics differently with 2phases and 3phases models at low cavitation number where the cavitating flow is highly dynamic. On the other hand, the impact of dissolved air on the cloud structure and dynamic characteristic of cavitating flow is gently observable.Originality/valueThe originality of this paper is the evaluation of different numerical cavitation models for the prediction of dynamic characteristics of cavitating flow in the presence of air.

Numerical Research of the Submerged High-Pressure Cavitation Water Jet Based on the RANS-LES Hybrid Model

The submerged high-pressure water jet has the characteristics of high velocity, strong turbulence, and severe cavitation. In order to reveal the formation mechanism of shear cavitation in the submerged high-pressure water jet and to grasp the turbulent structure and velocity distribution characteristics in the jet, the prediction ability of different turbulence models is studied first. The models represent the RANS model and RANS-LES hybrid model which are used to simulate the same cavitation jet, and the results are compared with the experimental results. The most reasonable model is then used to investigate the submerged high-pressure cavitation jet with different cavitation numbers. It is found that the calculation accuracy for small-scale vortexes has a great influence on the prediction accuracy of cavitation in the submerged jet. Both the DDES model and the SBES model can effectively capture the vortexes in the shear layer, and the SBES model can obtain more turbulence details. The result of the simulation under different cavitation numbers using the SBES model agrees well with the experimental result. Under the condition with low cavitation number, an intensive shear layer is formed at the exit of the nozzle, and small-scale vortexes are distributed along the shear layer. Mass transfer rate is relatively high in the region with a stronger vortex, which confirms that the low pressure in the vortex center is the main reason for the generation of cavitation in the shear layer. With the decrease of the cavitation number, the cavitation intensity increases obviously, while the nondimensional velocity along the radial direction changes little, which follows an exponential function.

Experimental study on unsteady characteristics of the transient cavitation flow

The characteristics of unsteady cavitation flow under different cavitation numbers are investigated experimentally. An optical test rig is designed to visualize the cavitation flow and the cavity length is defined based on high-speed camera technology. The development of cavitation flow can be divided into inception and fusion region, expansion region and collapse region. Due to the interaction between the re-entrant jet and the cavity interface, the cavitation bubbles near the solid wall collapse earlier than them in the downstream. The cavitation number is also the important factor determining the cavity length pulsation. Further analysis shows that the smaller the cavitation number, the smaller the frequency of cavity length fluctuations and the larger the mean cavity length. The cavitation intensity is stronger at lower cavitation number. Meanwhile, the cycle collapsing frequency of cavitation is around 220–320 Hz. With the decrease of cavitation number, the amplitude of cavitation pulsation increases while the corresponding collapsing frequency decreases.

A Turbulence Model Assessment for Deep Part Load Conditions of a Francis Turbine

Today, hydropower plants received increased attention as they have the ability to stabilize the electrical grid. However, this is accompanied by more start-stop sequences as well as operation in off-design conditions. One big task is to develop improved runner designs that can resist the increased pressure fluctuations. In the scope of this study, a Francis turbine is simulated for a deep part load operating point with the use of the SAS model and the more recently developed SBES model with ANSYS CFX. Both turbulence models are hybrid RANS-LES models. The simulation results are compared against model test data from a measurement at high cavitation number. For the evaluation of the pressure fluctuations comparison is furthermore made against experimental data at low cavitation number. All in all, SAS and SBES model predict similar flow characteristics. Both, the velocity profiles in the PIV measurement plane and the predicted pressure fluctuations, show only minor differences. The comparison against experimental data shows that the amplitude of the broadband frequency spectrum, which results from the occurrence of vortices, is overestimated at the investigated locations on the runner blades.

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.

Experimental investigation of cavitation-induced erosion around a surface-mounted bluff body

The objective of this study is to investigate the collapsing behavior of cavitation, which leads to the erosion of material. An experimental examination was conducted in a channel with a semi-circular cylinder obstacle, which serves as a “vortex cavity” generator. Cavitation was achieved by employing a range of pressure differences over the test section and a high-speed camera was used to observe the cavitation behavior. The flow field behind the semi-circular cylinder was investigated as a characteristic example of bluff bodies that exhibit a distinct, separated vortex flow in their wake. The cases with the bluff body were also compared to the ones without the bluff body. Erosion tests were performed using paint (stencil ink). The intensity of cavitation is characterized by the cavitation number (σ); the lower the cavitation number, the higher the cavitation intensity. The erosion (removal of paint) after 40 min of operation revealed distinct and repeatable results. For a high cavitation number, a large number of von Karman-vortex-like cavities are shed downstream of the obstacle. This results in a higher number of collapse events and, ultimately, more erosion. On the other hand, at lower cavitation numbers, the erosion took place at the cavity's closure line. It was seen that with the increase in cavitation intensity, the erosion area increases. Moreover, the bluff body obstacle promotes and localizes cavitation-induced erosion on the sample plate compared to the cases without the bluff body. This ultimately means that in the cases with the bluff body, less power is required in the system to cause erosion. The erosion patterns caused by the bluff body cavitation are more repeatable compared to the cases without the bluff body due to the localized cavitation load. The erosion pattern from the paint test is also compared with a material loss test (30 h of operation). A very good qualitative agreement is found between the two tests, with the paint test requiring approximately two orders of magnitude less running time of the facility. We demonstrate that paint tests, combined with this geometry, provide an efficient and economical way to investigate erosion patterns compared to expensive material loss tests.

Experimental Study of Break-off Frequency in Cavitation Disappearance Phenomenon on NACA16-012 Hydrofoil

Under the specific angle of attack on NACA16-012 hydrofoil, generated cavitation suddenly disappears even in the low cavitation number condition in which cavitation normally develops. This cavitation disappearance phenomenon was confirmed to be unique to NACA16-012 hydrofoil in our previous research. It was experimentally confirmed that cavitation disappearance phenomenon occurs in a transient cavitation condition where the sheet cavity is periodically broken off and the cloud cavity is released. In the previous study, it was predicted that there is the frequency band in which sheet/cloud cavitation cannot exist and cavitation disappearance phenomenon occurs. In this study, the break-off frequency of NACA16-012 hydrofoil was experimentally investigated and relationship between the break-off frequency and cavitation disappearance phenomenon was considered. The pressure fluctuation was measured with the pressure transducer downstream reagion and the dominant frequency was estimated by the frequency analysis. As the results, cavitation disappearance phenomenon was confirmed at angles of attack of 6° and 7°. There was the region increasing the break-off frequency according to decreasing of cavitation number at angles of attack 6° to 12°. It means that the break-off frequency of NACA16-012 hydrofoil has unusual characteristics. When the break-off frequency was expressed by the Strouhal number, it was found that the Strouhal number took each same value immediately just before and after cavitation disappearance phenomenon in decompression experiment. It was suggested that the break-off frequency was related to cavitation disappearance phenomenon.