Traditional underwater acoustic jammers are usually large and complex, which causes frogman troops to be exposed in communication-jamming missions. In this paper, a new claw apparatus of bionic ...snapping shrimp was designed and manufactured based on the characteristics of cavitation noise generated by the high-speed closure of the claw of snapping shrimp. The cavitation erosion effect and cavitation noise of the apparatus were investigated experimentally. The results of the study are as follows: A new bionic apparatus was designed and manufactured on the basis of simplifying the structure of the claw of snapping shrimp. The cavitation erosion effect of the bionic apparatus at different target distances was obtained by using the 50 μm aluminum foil as the target. By analyzing the cavitation image of aluminum foil, it is found that the cavitation effect increases first and then decreases with the increase of target distance. And the target distance corresponding to the maximum cavitation diameter is 43 mm. The binarization processing was carried out on cavitation erosion images. By calculating its cavitation erosion area, it was found that the largest area appeared at 41 mm and the corresponding cavitation erosion area reached 6379.1px. Then the cavitation noise generated by the bionic apparatus was measured with hydrophones at different rotation angles of dactyl and target distances. And the cavitation noise was detected successfully by importing wavelet analysis into noise analysis and processing. Based on the three-layer wavelet decomposition, the energy value of the wavelet packet was calculated, and the PSD analysis based on the hydrophone sensitivity modification was carried out. Finally, the frequency distribution of the cavitation noise signal and the sound pressure level was studied.
•In order to realize large-scale production and manufacturing of bionic snapping-claw apparatus through CNC machining based on the CT scanning characteristics, a new bionic snapping-claw submerged cavitation jet apparatus was designed.•Based on the equivalence principle and the law of energy conservation, the cavitation efficiency of the bionic apparatus was calculated theoretically to prove its superiority.•The cavitation erosion effect of bionic apparatus was measured by using image graying and binary processing technology, and its corresponding law was obtained.•The numerical analysis of the wavelet packet was carried into the noise signal analysis generated by the bionic apparatus. Furthermore, the distribution rules of the frequency domain of the cavitation noise was studied.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The cavitating flow-induced vibration associated with the cloud cavitation behaviors is analyzed.•A hybrid coupled FSI model is applied to simulate the cavitating flow around a flexible hydrofoil.•A ...simplified one-dimensional model is adopted and calibrated against numerical results.•The physical mechanism of the cavitation-excited pressure pulsation is clarified.
The objective of this paper is to investigate the two-phase flow structure of cloud cavitation over a flexible hydrofoil with particular emphasis on cavitating flow-induced vibration and the cavity shedding dynamics via combined experimental and numerical studies. The experimental studies are conducted in a closed-loop cavitation tunnel using a simultaneous sampling technique and the numerical investigations are performed using a hybrid coupled fluid structure interaction model. Results are presented for a modified NACA66 hydrofoil made of POM Polyacetate and good agreement can be obtained between the numerical and experimental results. The predicted cavitation behaviors, including the cavity growth, break-off and collapse, agree fairly well with the experiment. The vibration velocity goes up with the growth of the attached cavity and the break off of the cloud cavity induces some fluctuations of the vibration velocity. For the cavity shedding dynamics, the cavity partially collapses with the vibration and breaks into several medium-scale cloud cavities accompanied with complex interaction between the counter-rotating vortexes. Good agreement can be obtained between the predicted and measured vibration amplitudes, with the dominant flow-induced vibration frequency in accordance with the cavity shedding frequency and other two vibration frequencies corresponding to the natural frequencies of the first bending and twisting mode respectively. The one-dimensional analytical model is applied to better illustrate the physical mechanism of the cavitating flow induced pressure pulsation. The results show that the transient cavitating behaviors lead to the periodic pressure fluctuation on the hydrofoil. The one-dimensional model can track the main features of the pressure fluctuations, which are mainly derived from the cavity volume acceleration.
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Erosion through synergetic effects between cavitation erosion and particle abrasion in silt-laden flow seriously affects the safe operations of hydroturbines. In this review, recent advances of ...cavitation inception on particles and microscopic interactions between bubbles and particles are reviewed and discussed. For cavitation inception, influences of several paramount parameters (e.g. types, sizes, shape and surface structure of particles, pressurization and memory effects) have been revealed and discussed. The interaction mechanisms between cavitation bubbles and particles are demonstrated using experimental data obtained with a single particle. Through the microscopic interactions, the particles can be accelerated by the collapsing bubbles up to 40m/s and also be possibly split up by the cavitation, leading to deagglomeration of particle clusters.
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•Noises induced by sheet and tip leakage vortex (TLV) cavitating flows are studied with the FW-H equation.•Relationship between cavitation noise and the evolution of cavities is ...analyzed.•Pseudo-thickness and pseudo-loading terms of cavitation noise are analyzed and compared.
In the present paper, cavitation noise induced by sheet cavity and tip leakage vortex cavity (TLVC) together generated by a NACA0009 hydrofoil are numerically investigated with Ffowcs Williams-Hawkings (FW-H) equations. Large eddy simulation (LES) combined with Schnerr-Sauer cavitation model is utilized and a satisfactory result is obtained as compared with the referenced experimental measurements. It is found that even though the volume of TLV cavity is much smaller than that of sheet cavity, their acoustic levels are comparable to each other. A close relationship is indicated between cavity evolutions and their radiated noises, which vary periodically for both sheet and TLV cavities. Moreover, intense sound pulses of pseudo-thickness and pseudo-loading noise are observed especially during the collapse process. It is indicated that the rise in far field pressure and action of the dilatation term inside the cavities may trigger the collapse of both kinds of cavities in the flow, resulting in sound impulses. It is worth mentioning that in agreement with previous investigations, high pressure distribution and jets at the ends of sub-TLVCs are noted to have a close relationship with the collapse of sub-TLVCs. Noise generated by sheet cavity is higher than that generated by TLV cavity at the dominant Strouhal number of 0.74, with relatively high values of both pseudo-thickness and pseudo-loading noises at the dominant Strouhal number. While noise of TLV cavitating flow has acoustic energy concentrated at either low Strouhal numbers or high Strouhal numbers for the two terms, respectively.
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The corrosion and cavitation corrosion resistance of a nickel-aluminum bronze was improved after quenching and quenching/aging at 450 °C, which can be attributed to the refined microstructure, ...enhanced corrosion resistance, improved hardness, and weakened synergy between corrosion and mechanical attack.
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•The microstructure of as-cast NAB was refined after quenching and ageing.•Selective phase corrosion was eliminated for the refined microstructure.•Static corrosion rate reduced about 50% through microstructure design.•Improved corrosion resistance was attributed to rapid formation of a protective film.
Microstructure evolution of the nickel-aluminum bronze alloy was studied by heat treatment, including annealing, normalizing, quenching and aging. The microstructure was refined and homogenized after quenching or quenching/aging at 450 ℃, which can eliminate selective phase corrosion effectively. Compared with the current production process, static corrosion rate reduced about 50%, due to the rapid formation of a protective film. In addition, cavitation corrosion rate reduced by a factor of 4.9 and 7.9 for the quenched and quenched/aged at 450 ℃ samples, respectively. This can be attributed to the improved hardness and weakened synergy between corrosion and mechanical attack.
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•We present a method to control various destructive effects of cloud cavitation such as vibration and high-pressure peaks.•Unsteady cloud cavitating flow around a hydrofoil was predicted using ...Partially-averaged Navier–Stokes (PANS) method.•Addition of the PANS Model in the OpenFOAM package.•Combination of the PANS method with Schnerr–Sauer cavitation model for obtaining the most accurate results.•Using a proper cavitation control a significant reduction in high wall-pressure peaks and vibration has been achieved.
We present an efficient method to control the evolution of unsteady cloud cavitation around the CAV2003 benchmark hydrofoil using passive cavitation controllers so called cavitation-bubble generators (CGs). Cavitation control may be used in many engineering applications, particularly in the marine and turbo machinery field. We first simulated the unsteady cavitating flow around the hydrofoil without CGs using a Partially-averaged Navier–Stokes (PANS) method, and validated the acquired results against experimental data. We coupled the turbulence model with a mass transfer model and successfully implemented it in the open source toolbox OpenFOAM. Next, we studied the effect of different CGs on the qualitative parameters, such as the cavitation structure and the cavity shape. We varied size and location of the CGs to find the proper control of the cloud cavitation. We also analyzed in detail the effect of CGs on various destructive mechanisms of cavitation, such as highly unsteady cloud cavitation, turbulent velocity fluctuations, wall-pressure peaks, and degrading hydrodynamic performances. Our results revealed that CGs can substantially reduce instantaneous high-pressure pulsations on the hydrofoil surface. We observed that the cyclic behavior of unsteady cloud cavitation was suppressed, and the hydrodynamic efficiency of the hydrofoil was increased. The local boundary layer on the hydrofoil surface was altered, and the turbulent velocity fluctuation was reduced significantly, confirming that the vortex structures on the suction side and the wake region of the hydrofoil were changed remarkably.
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► The cavitation bubbles can be characterized by the dynamics of oscillations and the maximum temperatures and pressures reached when they collapse. ► These aspects can be studied both experimentally ...and theoretically for a single bubble system. ► However, in a multibubble system, the formation of bubble streamers and clusters makes it difficult to characterize the cumulative properties of these bubbles. ► In this overview, some recently developed experimental procedures for the characterization of acoustic cavitation bubbles have been discussed.
Acoustic cavitation, in simple terms, is the growth and collapse of preexisting microbubbles under the influence of an ultrasonic field in liquids. The cavitation bubbles can be characterized by the dynamics of oscillations and the maximum temperatures and pressures reached when they collapse. These aspects can be studied both experimentally and theoretically for a single bubble system. However, in a multibubble system, the formation of bubble streamers and clusters makes it difficult to characterize the cumulative properties of these bubbles. In this overview, some recently developed experimental procedures for the characterization of acoustic cavitation bubbles have been discussed.
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•The cavitation in the u-notch is caused by vortex flow.•The cavity length could be normalized by using the local cavitation number dividing incipient cavitation number as the abscissa.•The starting ...position of cavitation is determined by the notch configuration and the height tends to rise with the increase of opening.•The classical flow equation was revised for the cavitation choked flow problem with a prediction error below 2%.
The u-shape notch of spool valve is usually combined with the other typical throttling notches to achieve proportional flowrate adjustments in practical hydraulic control applications. Due to the vacuum-suction nozzle-structural effect, u-shape notch is more likely for cavitation arising. Traditionally, notch throttling is based on the continuity and Bernoulli equation to explain the Harvey nuclei bubble flow from development to collapse, which is not entirely agreed with present proliferated large vapor vortex cavitation researches. In this paper, experimental and numerical analysis discuss the morphological characteristics of u-shape notch vortex cavitation and the choked flow issues caused by vortex cavitation. The large vapor vortex cavitation morphological reproduced by the Large Eddy Simulation (LES) model associated with the multi-phase cavitation model is very close to experimental observations. Further, based on the clear cavity entity obtained by simulation, it could be found that, as notch opening increases, the cavity starting position moves back and the cavity moves upward. And as pressure difference increases, the cavity length increases smoothly until critical pressure value, after which the length increases sharply and maintains in long length level. In addition, it seems that the normalized measurement manner of cavitation length could be proposed, if using local cavitation number dividing incipient cavitation as abscissa to measure the cavity length. Moreover, the cavitating flowrate equation is proposed considering the cavitation-induced serious choked flow problem. The vortex cavitation, especially its length, is assumed to represent intense enough notch vortex flow friction loss. Based on the added flow loss term, the classical flowrate equation is revised, according to which the cavitating flowrate could be predicted within the accuracy of 0.5–2.0%.
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•A two-way coupling Euler-Lagrange method is presented to investigate the multi-scale characteristics of unsteady cavitating flow.•A microbubble generation model is proposed to explain the formation ...mechanism of cloud cavitation.•The breaking ocean wave theory is introduced to statistical analysis the inner structure of cloud cavitation.
The inner structure and the formation mechanism of cloud cavitation around a twisted hydrofoil was numerically investigated using a multi-scale method. A homogeneous mixture model was used to capture the macroscopic vapor structures, while the microbubble behavior was tracked by Lagrangian model. The spatial and temporal evolution of the multi-scale cloud cavitation is analyzed with special emphasis on the microscopic bubble dynamics. A microbubble generation model was proposed to explain the source of the numerous bubbles inside the cloud cavity. The shear layer at the liquid-vapor interface was found to be an important cause of the bubble formation. Furthermore, a statistical analysis of the large number of bubbles inside the cloud cavity showed two distinct bubble size spectra, N(D), which is similar to the theory for breaking ocean waves. Two mechanisms were established to explain this phenomenon. The first is when the attached cavity pinches off and rolls up which creates small bubbles conforming to Ns(D) ∼ D-1.26 with the bubble generation controlled by shearing effects. The second is when the vortex structures absorb bubbles into their core, where the low local pressure allows them to grow resulting in larger bubble size satisfying Nb(D) ∼ D-2.8. When the U-shaped cloud collapses, it breaks into a mass of tiny bubbles with a bubble size spectrum conforming to Ns(D) ∼ D-1.26 as well as a significantly greater number of larger bubbles that conform to Nb(D) ∼ D-2.2, which is thought to be due to both vortex and turbulent fragmentation.
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