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In order to improve the corrosion resistance and conductivity of 316L stainless steel bipolar plates used for proton exchange membrane fuel cells, three Cr-containing nitride coatings were deposited on 316L stainless steel by multiarc ion plating. First, the microstructure, composition, and contact angle of the three coatings were systematically investigated. Then, electrochemical impedance spectroscopy, potentiodynamic polarization, potentiostatic polarization (PSP), and interfacial contact resistance (ICR) of the three coatings were also fully examined. The results revealed that CrN coating has the highest contact angle of 98.26°, indicating its superior hydrophobicity. Additionally, CrN coating performed the best corrosion resistance with the highest corrosion potential of 0.31 V, the lowest corrosion current density of 2.28 × 10−7 A cm−2, and the largest resistance. Furthermore, CrN coating showed the lowest current density during PSP tests and the smallest ICR value after corrosion. The superior corrosion resistance of CrN coating is mainly attributed to its decreased pore density caused by vacancylike defects and its uniform structure. This article provided evidence for the potential application of CrN coating to bipolar plates. [ABSTRACT FROM AUTHOR]

The objective of this paper is to investigate the flow characteristics of different cavitation flow patterns around a NACA (National Advisory Committee for Aeronautics) 66 hydrofoil by applying the BDIM (boundary data immersion method) and ILES (implicit large eddy simulation) with an artificial code. Meanwhile, an artificial compressibility method is also employed to consider the effects of compressibility on cavitating flow. The results present that the numerical method can effectively capture different cavitation patterns, which agrees well with the previous experimental data. Subsequently, the detailed analysis of vortex structures and dynamics for the non-cavitation (σ = 3.0), sheet cavitation (σ = 2.0), and cloud cavitation (σ = 1.6) cases with the Liutex method and the vortex enstrophy transport equation have been investigated. When cavitation occurs, the degree of turbulence and the enstrophy in the flow field have been enhanced, due to the disturbance of the velocity field. For sheet cavitation, complex vortex structures appear in the attached cavity region with high-intensity enstrophy causing by the highly intense velocity and density gradient. As the cavitation pattern transits from the sheet cavitation to the cloud cavitation, more complex vortex structures can be observed in the cavitation region. Furthermore, the value and the fluctuation amplitude of enstrophy intensity increase significantly under the effect of reentrant jet. Analysis of the enstrophy transport equation indicates that the vortex stretching term and dilatation term for cloud cavitation increase relatively significantly with the movement of the reentrant flow and are highly dependent on the cavitation evolution. In addition, the region affected by the baroclinic torque also increases. [ABSTRACT FROM AUTHOR]

The objective of this paper is to investigate the dynamic characteristics and thermal effects of the single cavitation bubble in liquid nitrogen. A fully enclosed experimental platform for the single cavitation bubble in free field is established. To analyze the impact of the strong thermal effects of cryogenic fluids on the evolution process of single cavitation bubble, the room-temperature water and the liquid nitrogen in the same ambient pressure are set for comparison. According to the experimental results, the evolutions of single cavitation bubble in the room-temperature water and liquid nitrogen both experience the expansion stage, shrinkage stage, and oscillation stage, respectively. To further analyze the unsteady dynamics, a theoretical model of single cavitation bubble considering the compressibility, temperature, and phase change is introduced. The results show that the bubble radius predicted by this theoretical model is in good agreement with the experimental data. During the expansion stage, the dynamic bubble behaviors in both the room-temperature water and liquid nitrogen are governed by the liquid inertia. During the shrinkage stage, the interphase mass transfer increases the shrinkage velocity of bubble. Compared to the room-temperature water bubble, the initial pressure difference and vapor mass transfer rate of the liquid nitrogen bubble are significantly smaller. Thus, the shrinkage velocity of the liquid nitrogen is small, corresponding to weaker liquid inertia. And the bubble behaviors in liquid nitrogen are dominated by the thermal effects. For the liquid nitrogen bubble, the minimum shrinkage radius is more than 3 times that of the bubble in room-temperature water; the maximum Mach number is about 0.2 times that of the room-temperature water bubble, and the influence of compressibility on the dynamic behaviors is weaker. Besides, the maximum pressure and temperature during the shrinkage stage of liquid nitrogen bubble are significantly smaller due to the weaker shrinkage of bubble. And the oscillation cycle and overall size of the liquid nitrogen bubble are significantly larger during the oscillation stage compared to the room-temperature water bubble. [ABSTRACT FROM AUTHOR]

Lock-in is of great importance in many engineering applications due to its practical implications for structural safety. The influence of composite bend-twist coupling on the wake dynamics and vortex-induced vibration around a carbon fiber composite hydrofoil is investigated and compared to a similar stainless-steel hydrofoil. Experiments are conducted by varying linearly and slowly the upstream velocity back and forth between 5 and 15 m/s, which allows reaching lock-in conditions for both hydrofoils. Due to the blunt truncation of the trailing edge, both hydrofoils produce strong and alternate vortices in their wake, whose effect is visible on vibration spectrograms. The steel hydrofoil produces a classical lock-in onto its first torsion mode, while the composite hydrofoil exhibits two lock-in phenomena onto both torsion and second bending modes. Interestingly, for the second bending mode, the vibration spectrogram reveals the existence of two frequencies: (i) the resonance frequency, which remains almost constant during the lock-in phase, and (ii) the Strouhal frequency, which increases linearly with the upstream velocity. Using flow visualization, we found that this peculiar behavior is the result of the twist-bending coupling, which leads to the co-existence of two different vortex-shedding mechanisms. Close to the hydrofoil tip, the large vibration amplitude dictates the shedding frequency while the shedding follows the Strouhal law elsewhere. This partial lock-in gradually fades away as the velocity is increased. This result provides guidance for the safe design of composite structures. [ABSTRACT FROM AUTHOR]

The objective is to analyze the errors and uncertainty in the simulation results of the complex, unsteady turbulent flow and vortices. The implicitly filtered large eddy simulation (LES) with the boundary data immersion method is used to investigate the transient turbulent flow around a fully appended underwater entity model (SUBOFF) proposed by Groves et al. [Geometric Characteristics of DARPA Suboff Models: (DTRC Model Nos. 5470 and 5471) (David Taylor Research Center, 1989)] with emphasis on verification and validation. As for the verification, the five-equation method is used to assess the LES error, including the modeling error and numerical error in the transient flow. They offset each other, leading to a small total error. As for the validation, it has been achieved in the most area when the experimental result is located at the band of ysim ± UV (ysim is the simulation result, and UV is the validation uncertainty). There is a strong relationship between the validation uncertainty and the complex vortex interaction. The validation uncertainty becomes significant, which means less accuracy of the simulation result, within the tip flow region, adverse pressure gradient region, tip vortex interacting with the boundary layer region and shear layer region. Most all of these regions occur behind the appendage and at the shape changing position of the SUBOFF. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation of the flow pattern regime and unsteady characteristics of ventilated cavities with different headform shapes. The test model consists of a removable headform with three different forebodies (the conical, the blunt, and the hemispherical) and a common cylinder rear body. Experiments are conducted in a closed-loop cavitation tunnel. First, the flow pattern regimes on the ventilated cavity for different headforms are discussed in detail, and the dimensions of several flow patterns are measured. The results show that the cavity dimension and the regime are strongly dependent on the headform shape, and all typical flow patterns are introduced by schematic illustrations. Second, the ventilation hysteresis that happened during the flow pattern transition is pointed out. A quantitative gas leakage model is employed to explain the cause of hysteresis and flow pattern transition, and the results show the Strouhal number for different headforms is approximately ∼0.21. However, the blunt presented stronger gas leakage with a large constant parameter, K = 0.80. Finally, the unsteady characteristics of the ventilated cavity around different tested headforms are involved through descriptions of developments of the recirculating vortex and transparent cavity. In addition, an estimated cavitation number is applied to investigate the unsteady characteristics, and the maximum cavitation number and the strongest characteristic are obtained by the blunt headform due to the large drag coefficient and strong flow separation. [ABSTRACT FROM AUTHOR]

Numerical investigation of the cavity dynamics around a composite hydrofoil with a blunt trailing edge in the cloud cavitating flow is carried out using a tightly coupled fluid–structure interaction method. The hydrofoil is made of a carbon-fiber-reinforced polymers with a ply angle of − 4 5 ∘ (CFRP −45). The results of a stainless-steel hydrofoil with the same geometry and conditions are used as a reference. Simulation results have been validated carefully against experimental data. Several fundamental mechanisms are dictated through simulation results and mode decomposition, including the multistage shedding process, the influence of the bend–twist coupling effect on cavity behaviors, cavitation–vortex interaction, and kinematics of coherent structures. The main reason for the generation of a secondary re-entrant jet is that the primary cloud cavity collapse leads to high pressure, which spreads to the residual sheet cavity closure and then induces a high-pressure gradient. The negative bend–twist coupling effect causes the CFRP −45 hydrofoil to exhibit a smaller cloud cavity scale and non-uniform re-entrant jet strength in the spanwise direction compared to the stainless-steel hydrofoil. Modal decomposition via proper orthogonal decomposition and dynamic mode decomposition indicates that the dominant coherent structures in the cloud cavitating flow include the large-scale cloud cavity, rotating structures due to the re-entrant jet, attached cavity, and small-scale vortex in the wake. The results obtained in this study provide physical insight into the understanding of the mechanisms relevant to complex cloud cavitating flow around a composite hydrofoil. [ABSTRACT FROM AUTHOR]

Sheet/cloud cavitation usually leads to a wide range of length scales in both turbulence and phase distribution from microbubbles to cavity advection. In the present work, the Eulerian–Lagrangian multiscale cavitation model with two-way coupling is utilized to simulate the cavitating flow around a (National Advisory Committee for Aeronautics) NACA66 hydrofoil at an incidence angle of 8° and a cavitation number of σ = 1.4. The model can simultaneously capture the large-scale cavities and the microscale bubbles. The cavitating flow features are in good agreement with the experimental observations containing not only the periodical formation, growth, detachment, and advection of large-scale cavities, but also thousands of microbubbles around the large-scale cavities. The results show that the overall evolution frequency in the flow is about 45 Hz. Meanwhile, the dynamic mode decomposition method is utilized to identify the large-scale coherent spatial and temporal features of the sheet/cloud cavitating flow, which indicates that complex vortices in various scales dominate the evolution of cavities in the corresponding scale, and the evolution frequency of large-scale vapor structure decreases with increasing the length scale of cavities. Under the effect of turbulence, the large-scale cavities break into microbubbles, causing the size and number of discrete bubbles to increase rapidly in the re-entrant jet and cloud shedding regions. Additionally, the bubble-size spectrum of the time-averaged distribution of a period in sheet/cloud cavitating flow has two size regimes. For larger bubbles, the bubble density is proportional to the bubble radius to the power of −10/3. The bubble size spectrum of smaller microbubbles exhibits a −4/3 power-law scaling. [ABSTRACT FROM AUTHOR]

This paper investigates the processes of droplet condensation and self-propelled jumping on microcolumn-structured superhydrophobic surfaces with various size parameters. Using a three-dimensional (3D) multiphase lattice Boltzmann method, a novel phenomenon of secondary coalescence jumping is identified, and the underlying mechanisms are analyzed in detail. The simulation results show that wettability has a significant influence on droplet jumping. As the hydrophobicity of the surface increases, the droplets tend to jump from the substrate. However, structure parameters, such as the microcolumn spacing and height, have non-monotonic effects on droplet jumping. The structure parameters determine whether droplet coalescence occurs under the bottom–bottom droplet coalescence mode or the bottom–top droplet coalescence mode. Bottom–bottom droplet coalescence is shown to promote droplet jumping. Based on the simulation results and kinetic analysis, the optimal spacing-to-width and height-to-width ratios of the microcolumns for droplet jumping are found to be approximately 0.6 and 1.0, respectively. We believe the results of this work will provide valuable guidance in the design of self-cleaning surfaces and enhancing heat transfer efficiency. [ABSTRACT FROM AUTHOR]

The objective of this study was to investigate the coupled dynamics of a collapsing bubble and the motion of a nearby elastic plate at different initial distances. This was achieved using a combination of experimental and computational models. In the experiments, high-speed photography was used to record the temporal and spatial evolution of the collapse of a single bubble near an elastic boundary under normalized standoff distances γ ranging from 1.0 to 3.3. Digital image correlation was used to synchronously record the motion of the elastic plate. For the numerical simulations, taking the fluid compressibility and boundary motion into account, the immersed-boundary method was introduced to simulate the interaction between the elastic plate and bubble collapse. The results show that, with different initial distances, the dynamic behaviors of the bubble, including oscillation time, impact mode, and energy conversion, are different, and this is caused by the elastic rebound of the plate. In addition, the direction and amplitude of the deformation of the elastic plate are also influenced by the impact effects during bubble oscillation and rebound. The combined form of these impact behaviors changes with initial distance, and there are three typical impact patterns: the shock-wave effect, jet-effect, and hybrid shock-wave and jet-effect modes. In particular, when γ < 1.5, the jet effect and hybrid impact forms, which are dominated by the high-speed jet, can result in asymmetric deformation and cause greater local damage to the elastic plate. Finally, we summarize the combined mechanisms that govern the impact of a collapsing bubble on an elastic plate. [ABSTRACT FROM AUTHOR]

The multiscale effect of cavitation is a complicated multiphase phenomenon involving macroscale cavities and microscale bubbles. The cavitating flows at four different patterns around a (National Advisory Committee for Aeronautics) NACA66 hydrofoil are simulated based on the multiscale model under the Eulerian–Lagrangian framework. The volume-of-fluid method is used to capture the transportation of large-scale cavities in the Eulerian framework, while small-scale bubbles smaller than the threshold value of computational cells are solved using the Lagrangian method and the simplified Rayleigh–Plesset equation. The turbulent flow is solved using the large-eddy simulation approach, and the two-way coupling source for momentum is calculated by integrating interacting forces of discrete bubbles. This work proposes a multiscale model to better investigate the vapor structure with an extensive range of length scales, and analyzes the evolution mechanism of vapor morphology and scale in different cavitation patterns first. The simulation results are compared with the experimental observations to verify the accuracy of the numerical method. Meanwhile, the results illustrate that the turbulence has a significant influence on the bubble behavior. With a decrease in cavitation number, the number and size of discrete bubbles increase significantly, and the probability density function of discrete bubble diameter similarly conforms to Gamma distribution at all cavitation patterns. For inception cavitation, sheet cavitation, and supercavitation, the shape of large-scale cavity is relatively stable, and the standard deviation of the number and Sauter mean diameter of microscale bubbles are much smaller than cloud cavitating flow. In contrast, the large-scale cavity sheds periodically in the cloud cavitating flow leading to the periodical variation of the number and the Sauter mean diameter of microscale bubbles as well. Additionally, the discrete bubbles are mainly distributed in the region with strong turbulence intensity and high vorticity. [ABSTRACT FROM AUTHOR]

This paper through the in-house code numerically examines the cavitation–vortex–turbulence interaction mechanism. The high grid resolution can obtain a more detailed flow field structure, which is helpful to reveal the relationship between cavitation occurrence and development and local turbulent flow field. Results are presented for a three-dimensional NACA66 hydrofoil fixed at an 8° angle of attack under a moderate Reynolds number of 1 × 106 and sheet/cloud cavitating conditions. Numerical simulations are performed via the boundary data immersion method coupled with the artificial compressibility method through a Fortran-based code. The results show that the numerical predictions are capable of capturing the unsteady cavitation characteristics, in accordance with the quantitative features observed in high-speed cavitation tunnel experiments. The evolution of the transient cavitating flow can be divided into three stages: growth of the attached sheet cavity, development of a re-entrant jet, and cloud shedding downstream. The Liutex method is applied to capture the vortex structure. Further analysis of the process of enstrophy transport reveals that cavitation promotes vortex production and increases the enstrophy as the cavity becomes more unstable. Moreover, the structure of the vortex gradually evolves from a vortex tube to a U-type vortex, Ω-type vortex, and streamwise vortex. Finally, the interaction between cavitation and turbulence is expounded using the turbulent energy transport equation, which demonstrates that cavitation promotes the production, diffusion, and dissipation of turbulent kinetic energy, while the viscous transport term only acts during the process of cloud cavity shedding. [ABSTRACT FROM AUTHOR]

We investigate the evolution and interaction mechanism of different vortex structures for the jet in crossflow by a high precision numerical method. To verify the accuracy of the numerical method, the numerical and experimental results are compared. Numerical results show a reasonable agreement with the experimental data. The typical vortex structures can be clearly identified in the flow field, including shear layer vortices, horseshoe vortices, counter rotating vortices pairs, and wake vortices. Through the analysis of spatial distribution of different vortex structures, the formation and interaction mechanisms of different vortices are discussed in detail. The results show that the shear layer rolling up appears due to the strong rotation, inducing the formation of the shear layer vortices. The influences of velocity ratios on the vortex structures are further investigated. At low velocity ratios, the rotation is weak along the windward of the jet. With the increase in the velocity ratios, the stronger rotation is formed near the jet exit hole, inducing the instability of interface and formation of the shear layer vortices to occur earlier. In the far flow field, as the shear layer vortices gradually break up into the fine-scale vortices, both the rotation and shear tend to become weaker at different velocity ratios. [ABSTRACT FROM AUTHOR]

The objective of this paper is to identify the dominant coherent structures within cavitating flow around a Clark-Y hydrofoil using two data-driven modal decomposition methods, proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). A snapshot data sequence is obtained using a large eddy simulation and the interaction between cavitation and the vortex during cloud cavity shedding evolution is investigated. Modal decomposition via POD and DMD indicates that the dominant coherent structures include the large-scale cavity–vortex, re-entrant jet, shear layer, and small-scale vortex in the wake. In addition, the flow field can be reconstructed from the most energetic POD or DMD modes. The errors in the flow reconstructions produced using the first four POD modes, first eight POD modes, and first eight DMD modes are 3.884%, 3.240%, and 3.889%, respectively. Furthermore, transient cavitating flow can be predicted via the DMD method with an error of 8.081%. The largest errors in the reconstructed and predicted results occur mostly in the shear layer, trailing edge, and near wake. POD and DMD provide accurate and practically beneficial techniques for understanding cavitating flow, although substantial challenges remain with regard to predicting this intense nonlinear system. [ABSTRACT FROM AUTHOR]

The objective of the present paper is to investigate the influence of the micro vortex generator (mVG) on the inception cavitation number and mode around a National Advisory Committee for Aeronautics 66 hydrofoil. Two different sets of mVG with varying position are employed in this paper, i.e., the mVG-1 (located upstream of the laminar separation point of the baseline hydrofoil) and the mVG-2 (located in the laminar separation zone of the baseline hydrofoil). A high-speed camera is applied to visualize the inception cavitating structures, and numerical simulation is assisted to the effect of mVG. The results indicate that compared to the baseline hydrofoil, the mVG-1 can promote the earlier inception cavitation while the mVG-2 delays the inception, especially for the cases with smaller angle of attack (α = 4°–8°). For the mVG-1 hydrofoil, there are two reasons to be responsible for this phenomenon. One is that the fingerlike vortex at the rear of mVG-1 induces the fingerlike vortex cavitation earlier. The other is that the mVG-1 increases the length of the laminar separation bubble (LSB) by comparison with the baseline hydrofoil, thus causing a cavitation due to the laminar boundary layer separation. For the mVG-2 hydrofoil, it is located at the high-pressure zone of leading edge and reduces the length of the LSB. More precisely, the fingerlike vortex in the high-pressure zone is not enough to induce a fingerlike vortex cavitation, and the smaller length of the LSB than that of the baseline hydrofoil suppressing the cavitation at some angles of attack. [ABSTRACT FROM AUTHOR]

The objective of this paper is to study flow-induced vibration in cavitating flow around a steel stainless hydrofoil with the experimental and dynamic mode decomposition (DMD) method. A synchronized measured system consisting of a high-speed camera, lift measurement, and laser Doppler vibrometer is used to observe the cavitation structures, survey the vibrations, and measure the force. The hydrodynamic force and vibration velocity fluctuate most sharply in the cloud cavitation stage among all the cavitation regimes due to the unsteady cloud cavity, corresponding to the largest square root of normal distribution in the cloud cavitation in the analysis of statistic nature. Cavity shedding frequency, system-related frequency, and trailing edge vortex shedding frequency are observed in the vibration velocity frequency spectrum for the whole range of cavitation numbers. Cavity frequency dominates the structure response in the cloud cavitation regime. Two primary shedding mechanisms, re-entrant jet, and shockwave mechanism are identified for the cloud cavitation. The vibration velocity induced by the re-entrant jet mechanism is lower amplitude and larger frequency, while that induced by the shockwave mechanism is higher amplitude and smaller frequency due to the intense collapse of cloud cavity and rapid collapse of the attached cavity. Two peak bands related to the re-entrant jet development frequency and small-scale cavity shedding frequency are identified for the shockwave mechanism with DMD method. [ABSTRACT FROM AUTHOR]

Compared to the drawbacks of traditional experimental and numerical methods for predicting bubble migration, such as high experimental costs and complex simulation operations, the data-driven approach of using deep neural network algorithms can provide an alternative method. The objective of this paper is to construct a two-branch deep neural network (TBDNN) model in order to improve the high-fidelity bubble migration results and further reduce dependence on the quantity of experimental data. A TBDNN model is obtained by embedding the features of the Kelvin impulse into a basic deep neural network (BDNN) system. The results show that compared to the original BDNN model, TBDNN performs much better in accurately predicting bubble migration based on the same amount of training data. Using the TBDNN model, the critical condition of bubble oscillation at a fixed location can be detected under the influence of boundary properties (normalized stiffness and mass) and bubble standoff. Furthermore, the initial position of the bubble and normalized stiffness of boundaries have a positive correlation with bubble migration, whereas normalized mass has a negative impact. It was found that the normalized mass of boundaries plays the most important role in affecting bubble migration compared to the standoff and stiffness when using the method of variable sensitivity analysis. [ABSTRACT FROM AUTHOR]

The objective of this paper is to investigate cavitating flows around a pitching hydrofoil via combined physical and numerical studies. The aims are to (1) improve the understanding of the interplay between unsteady cavitating flow, hydrofoil motion, and hydrodynamic performance, (2) quantify the influence of pitching rate on subcavitating and cavitating responses, and (3) quantify the influence of cavitation on the hydrodynamic load coefficients and surrounding flow structures. Results are presented for a NACA66 hydrofoil undergoing controlled, slow (α=6°/s) and fast (α=63°/s) pitching motions from α = 0° to α = 15° and back to α = 0° for both subcavitating and cavitating conditions at a moderate Reynolds number of Re = 750 000. The experimental studies were conducted in a cavitation tunnel at the French Naval Academy, France. The numerical simulations are performed by solving the incompressible, multiphase Unsteady Reynolds-Averaged Navier-Stokes Equations via the commercial code CFX using a transport equation-based cavitation model; a modified k-ω SST turbulence model is used to account for the effect of local compressibility on the turbulent eddy viscosity. The results showed that increases in the pitching rate suppressed laminar to turbulent transition, delayed stall, and significantly modified post-stall behavior. Cavitation inception at the leading edge modified the pressure distribution, which in turn significantly changed the interaction between leading edge and trailing edge vortices, and hence the magnitude as well as the frequency of the load fluctuations. For a fixed cavitation number, increases in pitching rate lead to increase in cavitation volume, which in turn changed the cavity shedding frequencies and significantly modified the hydrodynamic loads. Inversely, the leading edge cavitation observed for the low pitching velocity case tends to stabilize the stall because of the decrease of the pressure gradient due to the formation of the cavity. The results showed strong correlation between the cavity and vorticity structures, which suggest that the inception, growth, collapse and shedding of sheet/cloud cavities are important mechanisms for vorticity production and modification. [ABSTRACT FROM AUTHOR]