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In short-fiber composites (SFCs), fiber length distribution (FLD) is complicated and has a considerable impact on the mechanical properties of SFCs. This work proposes a fractal FLD in SFCs on the basis of the fractal theory, and develops a multi-step mean-field homogenization (MSMFH) method to accurately and efficiently predict the mechanical properties of SFCs with fractal FLD. In the developed MSMFH method, SFCs are first decomposed into virtual pseudo-grains (PGs) according to fiber orientation distribution (FOD), followed the further division of the PGs into virtual sub-pseudo-grains (SPGs) according to FLD. The Mori–Tanaka or Double-Inclusion model is adopted to homogenize the mechanical properties of each SPG in the first step, and the Voigt model is implemented to homogenize the mechanical properties of all the SPGs and the PGs, respectively, in the sequential steps. Fiber length and orientation averaging algorithms for the developed MSMFH method are detailed. The developed MSMFH method and the proposed fractal FLD are validated to accurately predict the mechanical properties of SFCs by the means of the comparison with the FE method and the available experimental tests. [ABSTRACT FROM AUTHOR]

• Two unified numerical algorithms of periodic boundary condition are proposed. • The proposed algorithms predict accurate thermo-mechanical properties of composites. • The proposed algorithms are available for RVEs with periodic and nonperiodic meshes. • The proposed algorithms can be used for simple and complicated nonradial loadings. This work emphasizes on the Periodic Boundary Condition (PBC) utilized in the Finite Element Homogenization (FEH) method to numerically determine the Thermo-mechanical (T-M) properties of composites. The unified numerical implementation algorithms of the PBC used for accurately predicting the elastic properties, coefficients of thermal expansion and elasto-plastic behaviors of composites and available for representative volume elements with conformal and non-conformal surface meshes, respectively, are proposed and detailed. Regarding not only the thermo-elastic properties of composites, but also the elasto-plastic behaviors of composites under both simple uniaxial tensile, shear, uniaxial cycle loading paths and complex non-radial loading paths, the unified numerical implementation algorithms are verified to accurately predict the T-M properties of composites, through comparison with the results of the analytical models, the DIGIMAT-FE method, the experimental test and the results from the literature. The unified numerical implementation algorithms of the PBC can be extended to the predictions of other properties of various composites. This work provides a comprehensive insight into the PBC used for predicting the T-M properties of composites and benefits development of future FE codes implemented in commercial software packages. [ABSTRACT FROM AUTHOR]

As the prevalence of cloud storage increases, many individuals and companies prefer outsourcing their data for backup and management. However, this has led to a significant increase in redundancy, decreasing storage utilization and wasting network bandwidth. While conventional resemblance detection methods remove redundancy among similar data by comparing the features extracted from each chunk's content. However, we observed that small changes between similar data chunks may cause false dissimilarity detection by conventional resemblance detection techniques. This is because features derived solely from the chunk content are highly susceptible to various modification patterns. Fortunately, we have discovered that two chunks are likely to be similar if their surrounding chunks are also similar, a concept we refer to as "chunk-context". Therefore, we propose a novel chunk-context aware resemblance detection method, called CARD, which includes a network-based chunk-context aware model and an N-sub-chunk shingles-based initial feature extraction strategy. By leveraging the Neural network, it can discover the complex patterns between the chunk-context and chunk content itself. A high-level understanding of the contextual information with chunk content can be synthesized into the representation of a chunk. The primary difference compared with others is that our design can significantly improves the accuracy or efficiency of resemblance detection by considering the chunk-context with chunk content itself. Furthermore, we implemented a CARD prototype and conducted extensive experiments using real workload, demonstrating that CARD can detect up to 75.03% more redundant data and accelerate the resemblance detection operations by 5.6 × to 86.7 × faster than state-of-the-art work. • Traditional schemes overlook chunk-context, limiting effectiveness in resemblance detection. • Proposing a chunk-context aware approach to improve the accuracy and efficiency in resemblance detection. • N-sub-chunk shingles and BP-neural network enabling qualified chunk representation. • Achieving up to 75.03% more data removal and 5.6x–86.7x faster detection than state-of-the-art work. [ABSTRACT FROM AUTHOR]

Combining the innovative concept of quasi-active control method, a numerical simulation is conducted to investigate the vortex-induced vibration (VIV) responses. The objects of this study include a main cylinder (MC) and a cylindrical system outfitted with a pair of rotors of different types (with Banki rotors, Darrieus rotors, and Savonius rotors, denoted as WBR, WDR, and WSR), and the VIV responses are analyzed within the Reynolds number range of 0.8 × 103≤ Re ≤ 5.6 × 103. The findings indicate that all three kinds of rotors show excellent vibration suppression effect in downstream and cross-flow directions by adjusting the VIV response (amplitude, frequency, hydrodynamic coefficient) and vortex pattern (vortex mode, wake shape). The maximum amplitude suppression rate can reach 99% for the Banki and Savonius cases at high reduced velocity. Within the whole velocity range, the maximum amplitudes of WBR, WDR, and WSR cases in the downstream and cross-flow directions were 0.32 and 0.45 times, 0.73 and 0.65 times, and 0.28 and 0.33 times, respectively, compared to the maximum amplitude of MC. The time-averaged drag coefficient of the controlled cylinder in the WBR case was minimized to about 0.015. The Banki rotor and Savonius rotor significantly reduced the vibration frequency of the VIV system compared to the Darrieus rotor. Especially, the Banki rotor could keep the VIV system vibration frequency within a narrow range at different reduced velocity. Moreover, the vorticity contour of the VIV system with assembled rotors was significantly improved, with a narrower flow wake width and more regular vortex shedding at high reduced velocity. Considering both amplitude suppression and drag coefficient, Banki rotors were recommended. • The quasi-active control method was used to investigate VIV response. • The VIV system with three types of passive rotors was proposed. • The maximum amplitude suppression rate can reach 99% for the Banki and Savonius cases. • The vibration frequency is reduced and locked at a certain value for the Banki case. [ABSTRACT FROM AUTHOR]

In July 2021, an extreme precipitation event occurred in Henan, China, causing tremendous damage and deaths; so, it is very important to study the observation technology of extreme precipitation. Surface rain gauge precipitation observations have high accuracy but low resolution and coverage. Satellite remote sensing has high spatial resolution and wide coverage, but has large precipitation accuracy and distribution errors. Therefore, how to merge the above two kinds of precipitation observations effectively to obtain heavy precipitation products with more accurate geographic distributions has become an important but difficult scientific problem. In this paper, a new information fusion method for improving the position accuracy of satellite precipitation estimations is used based on the idea of registration and warping in image processing. The key point is constructing a loss function that includes a term for measuring two information field differences and a term for a warping field constraint. By minimizing the loss function, the purpose of position error correction of quantitative precipitation estimation from FY-4A and Integrated Multisatellite Retrievals of GPM are achieved, respectively, using observations from surface rain gauge stations. The errors of different satellite precipitation products relative to ground stations are compared and analyzed before and after position correction, using the '720' extreme precipitation in Henan, China, as an example. The experimental results show that the final run has the best performance and FY-4A has the worse performance. After position corrections, the precipitation products of the three satellites are improved, among which FY-4A has the largest improvement, IMERG final run has the smallest improvement, and IMERG late run has the best performance and the smallest error. Their mean absolute errors are reduced by 23%, 14%, and 16%, respectively, and their correlation coefficients with rain gauge stations are improved by 63%, 9%, and 16%, respectively. The error decomposition model is used to examine the contributions of each error component to the total error. The results show that the new method improves the precipitation products of GPM primarily in terms of hit bias. However, it does not significantly reduce the hit bias of precipitation products of FY-4A while it reduces the total error by reducing the number of false alarms. [ABSTRACT FROM AUTHOR]

The cross-chain identity authentication method based on relay chains provides a promising solution to the issues brought by the centralized notary mechanism. Nonetheless, it continues to encounter numerous challenges regarding data privacy, security, and issues of heterogeneity. For example, there is a concern regarding the protection of identity information during the cross-chain authentication process, and the incompatibility of cryptographic components across different blockchains during cross-chain transactions. We design and propose a cross-chain identity privacy protection method based on relay chains to address these issues. In this method, the decentralized nature of relay chains ensures that the cross-chain authentication process is not subject to subjective manipulation, guaranteeing the authenticity and reliability of the data. Regarding the compatibility issue, we unify the user keys according to the identity manager organization, storing them on the relay chain and eliminating the need for users to configure identical key systems. Additionally, to comply with General Data Protection Regulation (GDPR) principles, we store the user keys from the relay chain in distributed servers using the InterPlanetary File System (IPFS). To address privacy concerns, we enable pseudonym updates based on the user's public key during cross-chain transactions. This method ensures full compatibility while protecting user privacy. Moreover, we introduce Zero-Knowledge Proof (ZKP) technology, ensuring that audit nodes cannot trace the user's identity information with malicious intent. Our method offers compatibility while ensuring unlinkability and anonymity through thorough security analysis. More importantly, comparative analysis and experimental results show that our proposed method achieves lower computational cost, reduced storage cost, lower latency, and higher throughput. Therefore, our method demonstrates superior security and performance in cross-chain privacy protection. [ABSTRACT FROM AUTHOR]

This work proposes a novel multi-step dynamic FE compression method to generate Representative Volume Elements (RVEs) of composites containing a variety of inclusions. This method is actualized through a sequential and four-stage procedure: (1) Sparse and periodic inclusions exhibiting a predefined orientation distribution are generated by implementing a modified random sequential adsorption algorithm, (2) Sparse inclusions undergo biaxial compression into the region of the targeted RVE via a dynamic FE analysis, (3) Periodic inclusions are compressed into the region utilizing a dynamic FE analysis with periodic boundary conditions, and (4) Positions and orientations of the compressed inclusions are extracted and the RVE in computer-aided design format is then generated. The proposed method confers advantages from four distinct perspectives: (1) applicability to various inclusion geometries, including ellipses, lobules, polygons, kidneys and stars, (2) ability to generate periodic RVEs of composites with high inclusion volume fractions (up to 80.0% for circular inclusions), (3) simple and straightforward numerical implementation without explicitly considering inclusion intersection check and (4) capacity to predefine inclusion orientation distribution. Statistical analyses utilizing multiple spatial descriptors, i.e., inclusion orientation angle, local volume fraction, Voronoi cell area, nearest-neighbor distance and orientation, second order intensity function, radial distribution function and two-point probability function, confirm randomness of inclusion distribution in the generated RVEs. The elastic properties and damage behaviors of composites via the FE homogenization method are predicted based on the generated RVEs and are compared with those of available experimental data, the literature and the mean-field homogenization models to demonstrate effectiveness of the proposed multi-step dynamic FE compression method. • A novel method to generate composite RVE with high inclusion volume fraction. • The method are available for composite RVEs with diverse inclusions. • The method has better computational efficiency and simpler implementation. • The method can predefine inclusion orientation distribution. [ABSTRACT FROM AUTHOR]

It is known that surface waves have significant influence on the hydrodynamic performance of ocean current turbines which locate near the water surface. In order to quantitatively analyze the wave influence and reveal the interaction mechanism between the wave and the turbine flow, this paper proposes a three-dimensional transient computational fluid dynamics (CFD) model which can accurately predict the hydrodynamic performance of ocean current turbines under current-wave interaction flow conditions. The influences of two key wave parameters, the wave height and the submerged depth of the turbine, on the hydrodynamic forces and flow structures of a three-bladed horizontal axis ocean current turbine are discussed in depth. It is found that the both the average value and the oscillation amplitude of the torque on the turbine increase with the increased wave height, but decrease with the increase of the submerged depth. It is also found that in the cases of shallow submerged depth, the wake structures of the turbine are affected by the surface wave. • A CFD model for HAOCT in coupled wave-current flows is proposed. • Influence of the wave height and submerged depth are studied. • Hydrodynamic forces are evaluated at different flow conditions. • Mechanism of interaction between the wave and the turbine is explained. [ABSTRACT FROM AUTHOR]

A dispersion-managed femtosecond optical parametric oscillator (OPO) with stable power is demonstrated in this article. The OPO, based on LiB3O5 (LBO) crystal, was synchronously pumped by a frequency-doubled, Kerr-lens mode-locked Yb: CGYA laser. The output signal had a wavelength tuning range of 680 ~ 740 nm, a maximum average power of 1.1 W, and a minimum pulse duration of 105 fs. By inserting ultraviolet fused silica (UVFS) windows into the cavity to introduce additional normal dispersion, the dual-signal-wavelength output was suppressed, and the power stability was significantly enhanced. The RMS of the one-hour power fluctuation decreased to one seventh of its original value, from 2.47% to 0.38%. This work is of particular significance for promoting the engineering application of femtosecond OPOs. [ABSTRACT FROM AUTHOR]

The influence of surface waves on the hydrodynamic performance of an autonomous underwater vehicle (AUV) could be significant when the AUV sails near the water surface. In this paper, the hydrodynamic performance of an axisymmetric AUV in coupled wave-current flows is numerically studied using a two-phase computational fluid dynamics (CFD) method. The CFD method is validated with existing experimental data. The influences of waves are quantitatively analyzed for different wave heights, Reynolds numbers and submerged depths of the AUV. The flow structures are characterized to better understand the interaction mechanism between the AUV and the waves. It is found that the variation of the lift of the AUV is significantly affected by the wave and that the average drag increases with the increased wave height and decreased submerged depth. [ABSTRACT FROM AUTHOR]

Highlights • Periodic BCs are used to predict the effective thermal conductivity of composites. • Numerical implementation algorithm of the periodic BCs is presented. • Thermal flux continuity of the RVE boundary is maintained by the periodic BCs. • Thermal conductivity of spherical inclusions reinforced composites is isotropic. • Thermal conductivity of short random fibers reinforced composites is isotropic. Abstract Boundary condition plays an important role in prediction of the effective thermal conductivity of composites. In this research, the periodic boundary condition and the representative volume element (RVE) based finite element (FE) homogenization method are adopted to evaluate the effective thermal conductivities of the composites reinforced by the spherical, ellipsoidal and cylindrical inclusions, and the emphases are on the numerical implementation algorithm and validation of the periodic boundary condition. The heat flux continuity of the node pairs on the opposite surfaces of the RVEs of the composites is analyzed and the effective thermal conductivity of the composites are homogenized. The results show that the heat flux continuity of the node pairs on the opposite surfaces of the RVEs of the composites can be guaranteed by the proposed numerical implementation algorithm for the periodic boundary condition, and that the predicted effective thermal conductivities of the composites agree well with those determined by the Lewis-Nielsen model and the experimental tests. Therefore, the RVE based FE homogenization method with the periodic boundary condition can accurately evaluate the effective thermal conductivity of the composites with discontinuous inclusions. [ABSTRACT FROM AUTHOR]

Abstract To evaluate the effective mechanical properties of the composites with complicated micro-structures, the RVE based FE homogenization method with the periodic boundary condition is introduced and implemented in this paper, and the emphasis is on the periodic boundary condition and its numerical implementation algorithm. The pre-processing (such as the generation of geometry model and application of periodic boundary condition), FE analysis and post-processing (such as the average of stress and strain and stress contouring of the surface nodes) concerning the evaluation of the effective mechanical properties of the composites with complicated micro-structures are conducted in the FE package ABAQUS through the Python Interface. Numerical results show that the proposed numerical implementation algorithm of the periodic boundary condition guarantees the stress and strain continuities and uniaxial deformation constraint of the RVEs for the composites with complicated micro-structures. Compared with the Halpin-Tsai model and two-step M-T/Voigt mean-field homogenization method, the RVE based FE homogenization method with the periodic boundary condition is verified to accurately predict the effective elastic properties and elasto-plastic responses of the composites with the complicated micro-structures. [ABSTRACT FROM AUTHOR]

Pores are common defects generated during fabrication, which restrict the application of carbon/carbon (C/C) composites. To quantitatively understand the effects of pores on mechanical strength, this paper proposes a representative volume element model of unidirectional (UD) C/C composites based on the finite element method. The Hashin criterion and exponential degraded rule are used as the failure initiation and evolution of pyrolytic carbon matrices, respectively. Interfacial zones are characterized using the cohesive constitutive. At the same time, periodic boundary conditions are employed to study transverse tensile, compressive, and shear deformations of UD C/C composites. Predicted results are compared with the experimental results, which shows that the proposed model can effectively simulate the transverse mechanical behaviors of UD C/C composites. Based on this model, the effects of microstructural parameters including porosity, pore locations, the distance between two pores, pore clustering, and pore shapes on the mechanical strength are investigated. The results show that porosity markedly reduces the strength as porosity increases. When the porosity increases from 4.59% to 12.5%, the transverse tensile, compressive, and shear strengths decrease by 35.91%, 37.52%, and 30.76%, respectively. Pore locations, the distance between two pores, and pore clustering have little effect on the shear strength of UD C/C composites. For pore shapes, irregular pores more easily lead to stress concentration and matrix failure, which greatly depresses the bearing capacity of UD C/C composites. [ABSTRACT FROM AUTHOR]

Highlights • A novel passive-pitch shield for the efficiency improvement of VAWTs is designed. • Influence of the shield on the turbine performance is studied numerically. • The passive-pitch capability of the shield is verified at different wind directions. • The efficiency of the VAWT is increased by 46.32% with the shield. Abstract The efficiency of a vertical axis wind turbine (VAWT) can be improved by placing a shield or deflector upstream of the turbine to reduce the negative torque on the blade. However, previous studies mainly focused on fixed shields or deflectors, and the performance of the VAWT was significantly affected by the wind direction. Considering this, this paper proposed a novel passive-pitch shield (PPS) which consists of two simple plates and could passively adjust its pitch angle according to the wind direction, so that the VAWT could operate with high efficiency under any wind directions. Two-dimensional transient CFD simulations were carried out to quantitatively analyze the influence of the PPS on the performance of the VAWT. The influence of PPS on the VAWT was explained in depth from the perspective of flow structures. Simulations were also performed to verify the passive pitch capability of the PPS and good results were obtained. To further improve the performance of the VAWT, several types of PPSs were compared. It was found that the power coefficient of the VAWT was increased by 46.32%. [ABSTRACT FROM AUTHOR]

Abstract To accurately predict the effective elasto-plastic responses of two-phase composites, an interpolative M-T/D-I mean-field homogenization model with the tangent formulation is proposed, and the numerical implementation algorithm of the proposed M-T/D-I model with the tangent formulation is presented. The comparisons between the effective elastic properties and elasto-plastic responses of two-phase composites predicted by the M-T/D-I model with the tangent formulation and the RVE based FE homogenization method show that the M-T/D-I model with the tangent formulations can accurately evaluate the effective mechanical properties of two-phase composites. The simulation results demonstrate that the elastic properties of two-phase composites predicted by the M-T/D-I model are more accurate than those predicted by the D-I and M-T models, and the effective elasto-plastic responses of two-phase composites predicted by the M-T/D-I model vary in the range of those predicted by the D-I and M-T models. It is revealed that the M-T/D-I model with the tangent formulation allows to simulate the tensile/compression and biaxial tensile/shear elasto-plastic responses of two-phase composites. Lastly, the effects of the inclusion volume fraction and elastic properties on the effective elasto-plastic responses of two-phase composites are discussed for showing the capability of the proposed interpolative M-T/D-I model with the tangent formulation. [ABSTRACT FROM AUTHOR]

2-D Cf/Mg composites consisting of AZ91D magnesium alloy and carbon non-woven fabrics are prepared by using the LSEVI process and their mechanical properties are experimentally tested. The experimental results show that the 2-D Cf/Mg composites have the better mechanical properties: elastic modulus of 71.0 GPa and ultimate tensile strength of 366.0 MPa, compared to AZ91D magnesium alloy matrix with the elastic modulus of 45.0 GPa and ultimate tensile strength of 250.0 MPa. The effective elastic properties of the 2-D Cf/Mg composites are numerically predicted by using a multi-scale homogenization method: the RVE based FE homogenization method at the macro-scale coupling the D-I mean-field homogenization model at the micro-scale. The agreement between the results of the experimental tests and the multi-scale homogenization method illustrates the validation of the multi-scale homogenization method to evaluate the effective mechanical properties of the 2-D Cf/Mg composites. For improving the computational efficiency, at the macro-scale the laminated composites theory is coupled with the RVE based FE homogenization method to predict the effective mechanical properties of the 2-D Cf/Mg composites. [ABSTRACT FROM AUTHOR]

This paper introduces an optimization procedure of a modified Savonius rotor with different convex and concave sides to maximize the power efficiency. A series of transient computational fluid dynamics (CFD) simulations are performed to find the peak coefficients of power ( C p ) of each blade geometry. Then, a global response surface model is created according to the Kriging Method, which defines the relationship between optimization objective C p and the design parameters. A Particle Swarm Optimization (PSO) algorithm is applied to finding the optimal design based on the response surface model. The optimal C p is 0.2580 and is 4.41% higher than the traditional design. Comprehensive comparisons of torque, power and flow structures between the optimal and the traditional designs are performed to illustrate the mechanism of how the blade shapes improve the rotor performance. It is find found that the optimal blade shape has stronger tip vortices and recovery flows, which contributes to an increase in the performance of the rotor. [ABSTRACT FROM AUTHOR]

The wind energy restored in the wakes of high-speed moving vehicles on highways has considerable potential but has yet to be used. In this study, a vertical axis wind turbine (VAWT) is used to recover energy from the wake of vehicles on highways. The VAWT is designed to be placed on the medians of the highway and produce power from the wakes of vehicles on both sides. To evaluate the performance of the VAWT and to determine the mechanism of interactions between the moving vehicle and the turbine, three-dimensional computational fluid dynamics simulations based on the Reynolds-Averaged Navier–Stokes equations are performed. Five typical situations, including one car on the passing lane, one bus on the passing lane, two opposite moving cars on the passing lane, one car on the fast main lane, and one bus on the fast main lane, are considered and studied. Results show that the VAWT could generate power from the wakes of vehicles on the passing lane. The maximum average power coefficient is 0.00464, which corresponds to an average power of 139.60 W. [ABSTRACT FROM AUTHOR]

This work proposes a novel method, entitled minimum potential method, to generate Representative Volume Elements (RVEs) of unidirectional fibers composites with high fiber volume fractions (FVFs, generally ≥ 70. 0 %). In the proposed method, a given VF of random and periodic fibers are initially created within the matrix and the fiber overlap is allowed, and then the fiber overlap is gradually removed by adjusting the fiber centroids based on the principle of the minimum overall potential. In the process of fiber overlap removal, the fiber periodicity and the fibers completely or partly contained in the matrix are preserved. The overall potential of the fibers consists of fiber overlap potential, fiber position potential and fiber periodicity potential, and is minimized using the gradient descent method. The numerical implementation of the proposed method is detailed, and the RVEs of the composites with various FVFs are generated. Through the probability distribution function of the nearest neighbor distance, the cumulative probability distribution function of the nearest neighbor orientation angle, the second-order intensity and pair correlation functions, the fiber distribution in the generated RVEs is analyzed, and the results indicate a completely random distribution of the fibers in the generated RVEs. Comparing with the available experimental test, the literature and the Double-Inclusion model, the proposed method to generate RVEs of the composites with high FVFs is validated. • A novel minimum potential method is proposed to generate RVEs of composites. • The method can generate the RVEs of composites with high fiber volume fractions. • The method owns good computational efficiency, even for high fiber volume fraction. • The numerical implementation of the method is relatively simple. [ABSTRACT FROM AUTHOR]

In this paper, a mean-field homogenization based approach implemented in the two steps sequentially is proposed to evaluate the macroscopic coefficients of thermal expansion (CTEs) of multi-phase composites. The adopted mean-field homogenization models are the quadratic interpolative M-T/D-I mean-field homogenization model in the first-step homogenization procedure and the simple interpolative Voigt/Reuss mean-field homogenization model in the second-step homogenization procedure, respectively. The proposed mean-field homogenization based approach is validated by comparing with the RVE based FE homogenization method. The simulation results indicate that with the increase of the inclusion volume fraction, the macroscopic CTEs of multi-phase composites generally decrease because that the inclusions possess the smaller CTEs than the matrices. However, in the case of multi-phase composites with the planarly distributed inclusions, the through-thickness CTEs firstly increase with the increase of the inclusion volume fraction and then decrease with the increase of the inclusion volume fraction. The effect of the inclusion aspect ratio on the macroscopic CTEs of multi-phase composites can be neglected, while the effect of the inclusion transversely isotropic material properties on the macroscopic CTEs of multi-phase composites must be considered. [ABSTRACT FROM AUTHOR]

Three-dimensional transient CFD (Computational Fluid Dynamics) simulations are performed to study the hydrodynamic performance of an ocean current turbine with a 3.0 m diameter 3-bladed rotor. Simulations are based on the RANS (Reynolds Averaged Navier–Stokes) equations and the shear stress transport k-ω turbulent model is utilized. The influence of yaw angle and upstream TI (turbulence intensity) on the turbine performance is studied. The CFD method is first validated using existing experimental data and good agreement is obtained. The performance of the turbine, including power, thrust and wake characteristics are then studied at different TSR (tip speed ratios). The turbine obtains a maximum coefficient of power ( C p ) of 0.4642 at TSR = 6 and the coefficient of thrust ( C t ) increases over the entire evaluated TSR range to a value of 0.8788 at a TSR = 10. Simulations are also performed at four different yaw angles, 0°, 5°, 10° and 15° which show that both C p and C t decrease as yaw angle increases. Finally simulations of three different TIs, 3%, 6% and 9%, are performed and analyzed. Results show that TI minimally affects C p and C t for the considered TI range, but greatly influences the downstream wake structure. [ABSTRACT FROM AUTHOR]

The effective thermal conductivity of metal matrix composites (MMCs) reinforced by the randomly distributed transversely isotropic fibers is evaluated using the representative volume element (RVE) based finite element (FE) homogenization method. The modified random sequential absorption (RSA) algorithm is proposed to generate the periodic RVEs of MMCs and thus the periodic boundary conditions are introduced. The transversely isotropic thermal conductivity of fiber is considered and its numerical implementation in the FE analysis is detailed. The RVE based FE homogenization method to predict the effective thermal conductivity of MMCs reinforced by the randomly distributed fibers is verified by comparing against the experiment and the Hashin–Shtrikman bounds. The simulation results show that the effect of the transverse isotropy of fiber thermal conductivity on the effective thermal conductivity of MMCs reinforced by the randomly distributed fibers can be neglected owing to the large matrix/fiber thermal conductivity ratio. The effective thermal conductivity of MMCs reinforced by the randomly distributed fibers decreases with the increase of fiber volume fraction, while the effect of fiber aspect ratio on the effective thermal conductivity of MMCs reinforced by the randomly distributed fibers is not significant. [ABSTRACT FROM AUTHOR]

To investigate the effect of fiber orientation on the effective elastic properties of short-fiber-reinforced metal matrix composites, the two-step mean-field homogenization procedures including the D-I/Reuss, M-T/Reuss, D-I/Reuss, M-T/Voigt and D-M/V-R models are introduced and the corresponding numerical implementations are detailed. Compared with the RVE based FE homogenization method, the two-step mean-field homogenization procedures: D-I/Reuss, M-T/Reuss, D-I/Reuss, M-T/Voigt and D-M/V-R models to predict the effective elastic properties of short-fiber-reinforced metal matrix composites are validated. The simulation results show that the effective elastic moduli E 22 and E 33 of short-fiber-reinforced metal matrix composites with the aligned fibers reach the minimums around θ = 60 ° and 30 ° , respectively, but not 90 ° or 0 ° , while the effective shear modulus G 23 reaches a maximum at θ = 45 ° and shows a symmetrical distribution. For short-fiber reinforced metal matrix composites with the planarly randomly distributed fibers, the effective elastic moduli E 22 and E 33 are close to those of short-fiber reinforced metal matrix composites with the aligned fibers presenting the orientation angle θ of 35 ° and 55 ° , respectively, but not 45 ° , while the effective shear modulus G 23 is close to those of short-fiber reinforced metal matrix composites reinforced by the aligned fibers with the orientation angles of both 20 ° and 70 ° . [ABSTRACT FROM AUTHOR]

The generation of 72 fs hyperbolic secant pulses centered at 1050 nm with 17.8 nm bandwidth from a diode pumped Kerr-lens mode-locked Yb: GSO laser is demonstrated. With the help of a semiconductor saturable absorber mirror, stable mode-locking with an average output power of 85 mW at a repetition rate of 113 MHz is realized. To the best of our knowledge, this is the first demonstration of Kerr-lens mode-locking in Yb: GSO laser. [ABSTRACT FROM AUTHOR]

In this paper, the two-step mean-field homogenization procedures including the Mori–Tanaka (M–T) and interpolative double inclusion (D-I) models in the first-step homogenization procedure and the Voigt and Reuss models in the second-step homogenization procedure, are introduced and implemented to predict the effective elastic properties of short-fiber-reinforced metal matrix composites. To take into account the effect of fiber orientation, the detailed implementation of the fiber orientation transformation and fiber orientation averaging procedure is described. Compared with the effective elastic properties of short-fiber-reinforced metal matrix composites predicted by the RVE based FE homogenization method and measured from the uniaxial tensile experiments, the two-step mean-field homogenization procedures: D-I/Reuss, M–T/Reuss, D-I/Voigt and M–T/Voigt would provide the acceptable predictions on the effective elastic properties of short-fiber-reinforced metal matrix composites, and the D-I/Reuss and M–T/Voigt two-step mean-field homogenization procedures provide more tight bounds (D-I/Reuss as the lower bound and M–T/Vogit as the upper bound). For more accurate estimate of the effective elastic properties of short-fiber-reinforced metal matrix composites, a modified two-step mean-field homogenization procedure is proposed and validated. [ABSTRACT FROM AUTHOR]

In recent years, there were some studies on arrays of multiple Darrieus turbines that have shown the potential benefits of interaction between Darrieus turbines in an array configuration to improve performance compared to isolated turbines. However, all these studies were for two separate turbines, whose spacing cannot be smaller than the diameter. This paper uses the 2D CFD method to propose a Darrieus turbine layout similar to the gear meshing principle, where the adjacent turbine gap can be smaller than the diameter. A comparative analysis of the new layout with 2 rotational directions, 5 turbine gaps and different numbers of turbines in the multi-turbine layout form is carried out. It is found that in the two-turbine layout, there can be an efficiency improvement of 28.92%; while in the multi-turbine layout, its efficiency gradually increases with the number of turbines at TSR = 3.0 and finally stabilizes at a Cp-corr of 0.588. • The efficiency of Darrieus turbines is improved through a gear-like turbine layout. • The two different layouts of turbine arrays are compared at different TSRs. • Influence of the gaps between the turbines and the numbers of turbines is studied. [ABSTRACT FROM AUTHOR]

The modified two-step mean-field homogenization procedure including the quadratic interpolative model on the basis of the Mori-Tanaka (M-T) and interpolative double inclusion (D-I) mean-field homogenization models in the first-step homogenization procedure and the simple interpolative model in the second-step homogenization procedure on the basis of the Voigt and Reuss mean-field homogenization models, and the direct finite element (FE) homogenization method based on the concept of the representative volume element (RVE) and the periodic boundary conditions (the RVE based FE homogenization method), are implemented to predict the effective elastic properties of metal matrix composites with the randomly distributed fibers. Compared with the results measured from the uniaxial tensile experiments, the modified two-step mean-field homogenization procedure and the RVE based FE homogenization method provide the accurate predictions on the effective elastic properties of metal matrix composites with the randomly distributed fibers. However, in the case of neglecting the detailed stress and strain fields in the metal matrix composites with the randomly distributed fibers, the modified two-step mean-field homogenization procedure gives the far better computational efficiency than the RVE based FE homogenization method. [ABSTRACT FROM AUTHOR]

In this article, the fiber orientation evolution in the Csf/Mg composites with a large deformation is investigated. The principal strain-induced fiber orientation evolution mechanism, which states that the fiber orientation evolution in the Csf/Mg composites with a large deformation is determined by the principal strains, is proposed. The fiber orientation distribution factors Fi(i=1, 2and3) taking the form of the principal strains are proposed to quantitatively characterize the fiber orientation distribution in the Csf/Mg composites. The fiber orientation factors Fi s predicted by the finite element simulation based on the principal strain-induced fiber orientation evolution mechanism are compared against those Fi m measured from the micrographs of the extrusion experiments of Csf/Mg composites. The results demonstrate that the principal strain-induced fiber orientation evolution mechanism is valid and convenient to predict the fiber orientation evolution in the Csf/Mg composites with a large deformation. In the extruded Csf/Mg composites, the fibers are reoriented toward the direction of the maximum principal strain and deviated from the direction of the minimum principal strain. [ABSTRACT FROM AUTHOR]

Combining with the random sequential absorption (RSA) algorithms, the method entitled Fiber Growth Method is presented for generating the representative volume element (RVE) with the spatially randomly distributed discontinuous fibers and with a relatively high fiber volume fraction ( ∼ 25%). The formulation of the Axes Distance Method for checking the overlapping of fibers in the RVEs is discussed. Within the framework of elasticity, the generated RVEs are analyzed to obtain the effective mechanical properties of composites reinforced by spatially randomly distributed discontinuous fibers using the numerical homogenization technique based on the finite element analysis (FEA). The validation is performed by comparing the Bulk modulus and Shear modulus predicted by the FEA against those obtained from the traditional equations based on the Halpin–Tsai estimation. FEA results show that due to the spatial random distribution of fibers, composites behaves homogeneously isotropic at the macroscopic scale. Bulk moduli and Shear moduli of composites increase with the fiber volume fraction increasing while the Poisson’s ratios of composites decrease. [ABSTRACT FROM AUTHOR]

The effective elastic properties of composites reinforced by spatially randomly distributed short cylindrical fibers with certain aspect ratio are investigated using the numerical homogenization method. A modified RSA algorithm is proposed to generate the periodic RVEs. The periodic boundary conditions are introduced and the periodic RVEs thus created are analyzed to obtain the mechanical properties of composites by using the FE package ABAQUS. The ABAQUS Python Interface is used to introduce the periodic boundary conditions and to obtain the average stresses and strains of RVEs. The simulation results show that the periodic boundary conditions guarantee the continuity of strain and stress fields on the boundaries of RVEs. In the case of the fiber aspect ratio of 15 and fiber volume fraction of 10%, it is sufficient to consider the size of RVE as L / l = 2.5 , in which an approximately random fiber orientation exists. The effective elastic properties of composites obtained by the numerical homogenization agreeing well with those obtained from the traditional equations for composites based on the Halpin–Tsai estimation and with those measured from the uniaxial tensile experiments shows the validation of the numerical homogenization for effective elastic properties of composites reinforced by spatially randomly distributed short fibers. [ABSTRACT FROM AUTHOR]

Numerical homogenization is implemented to predict the transverse elastic properties of Cf/Mg composites at an elevated temperature and containing a small fraction of liquid phase, using the XFEM with the level set method. The material microstructure is generated in a repeated 2D RVE, in which the fibers, liquid matrix and solid matrix coexist, based on the RSA algorithm. The evolving material interfaces between the fibers and matrix and between the liquid matrix and solid matrix are represented as the zero value of a level set function. The local enrichment functions for these material interfaces are constructed based on the level set functions and the shape functions of finite elements. A set of the statistical analyses is performed and the simulation results demonstrate that it is sufficient to consider the size of RVE as the matrix/fiber aspect ratio L / d = 20 in the case of the fiber volume fraction of 35%. The effective transverse elastic properties of Cf/Mg composites would decrease with increasing the liquid volume fraction and with increasing the liquid volume concentration factor c l , while increase with increasing the liquid size. [ABSTRACT FROM AUTHOR]

Based on the principal strain driven fiber orientation evolution mechanism, the fiber orientation variation factor V , which is a function of the principal strains, is proposed to quantitatively characterize the variation of fiber orientation angles in the extruded Csf/Mg composites. The FE analyses are implemented to numerically simulate the extrusion forming process of Csf/Mg composites. The fiber orientation variation factors predicted by the FE simulations are compared against those measured from the micrographs of the extrusion experiments of Csf/Mg composites. The results show that the fiber orientation variation factor V of the FE simulations and of the extrusion experiments match well such that the fiber orientation variation factor V is valid and convenient to characterize the fiber orientation variation in the extruded Csf/Mg composites. The distribution of the fiber orientation variation factors illustrates that the fibers in the extruded Csf/Mg composites are reoriented towards the direction of the maximum principal strain, and deviated from the directions of the medium and minimum principal strains. [ABSTRACT FROM AUTHOR]

Highlights: [•] Effect of fiber orientation on the tensile strength of composites is simulated. [•] Effect of fiber aspect ratio on the tensile strength of composites is simulated. [•] Composites get the worst properties in the case of fiber orientation angle of 60. [•] Tensile strength was improved with the fiber aspect ratio increasing until 20. [ABSTRACT FROM AUTHOR]

To accurately predict the Effective Thermal Conductivities (ETCs) of Three-Dimensional (3D) braided composites, this work proposes a multi-scale and multi-step Mean-Field Homogenization (MFH) method. The composites are considered as the composites with 3D multi-directional braiding yarns as inclusions at the meso-scale, and the braiding yarns are viewed as unidirectional fibers reinforced composites at the micro-scale. The ETCs of the braiding yarns are firstly determined using the MFH method at the micro-scale. According to the yarn orientation, the composites are virtually decomposed into several pseudo-grains, each of which consists of the oriented braiding yarns and matrix, and the multi-step MFH method is then developed to predict the ETCs of the pseudo-grains and the composites sequentially at the meso-scale, i.e. the MFH prediction of the ETCs of each pseudo-grain individually and the composites consisting of all the pseudo-grains in two sequential steps. The proposed multi-scale and multi-step MFH method is validated by comparing with the multi-scale FE homogenization method and the available experimental test. The modeling results show that the through-thickness and in-plane ETCs of the composites increase with the increase of fiber volume fraction and transverse ETC, and decrease and increase with the increase of interior braiding angle, respectively. The proposed multi-scale and multi-step MFH method possesses the advantages of better computational efficiency and simpler implementation compared with the multi-scale FE homogenization method. [Display omitted] • A multi-scale/step MFH method is proposed to predict ETCs of 3D braided composites. • The proposed method is validated at both the micro-scale and the meso-scale. • The method has better computational efficiency and simpler implementation. • Effects of structure parameters on the ETCs of 3D braided composites are studied. [ABSTRACT FROM AUTHOR]

• A new VAWT with overlapped side-by-side Savonius rotors is designed. • The two rotors of the new VAWT are evaluated and compared at different TSRs. • Influence of the distances between the rotors and the numbers of rotors is studied. A new vertical axis wind turbine (VAWT) with overlapped side-by-side Savonius rotors is designed and analyzed in this paper. The new VAWT consists of several Savonius rotors with the distance between adjacent rotor axes being smaller than the rotor diameter, and the rotation of the rotors is similar to meshing gears. Transient computational fluid dynamics (CFD) simulations are adopted to evaluate the aerodynamic performance of the VAWT. Comparisons between the new VAWT and a traditional Savonius rotor are performed to illustrate the differences in torque, power, pressure distributions, and flow structures. It is found that the optimal distance between the rotors is 0.7 times the rotor diameter. The VAWT with two rotors has a maximum coefficient of power of 0.323 for TSR = 0.8, which is 38.72% higher than the traditional Savonius rotor. Especially, the new VAWT has an amazing efficiency improvement of 46.95% at TSR = 0.4. Furtherly, simulations are carried out on VAWTs with different numbers of rotors. It is found that the VAWT with 4 rotors has the highest coefficients of power of 0.416 at TSR = 0.6 and is 100.55% higher at TSR = 0.4 than the traditional Savonius rotor. [ABSTRACT FROM AUTHOR]

In this work, a simple but effective fractal model is proposed to predict the mechanical properties of complex porous composites. By extending the Mori-Tanaka and Double Inclusion models, we develop a computationally efficient algorithm based on the mean-field homogenization, to quickly determine the macroscopic elastic properties of the random porous media by the fractal theory. The random pore sizes and locations are properly characterized by a single fractal dimension, and the fractal model agrees well with the numerical results obtained by the finite element homogenization technique and the existing experimental data from literature. Moreover, the effect of pore size distribution has been investigated, indicating that the elastic properties decrease with the increase in the fractal dimension. In particular, the computation time can be reduced from 4 h for the finite element analysis to 15 s based on the fractal model. Based on the mathematically analogous governing equations, the present fractal modeling has great potential to determine the other significant physical properties such as thermal conductivity and coefficient of thermal expansion. [Display omitted] [ABSTRACT FROM AUTHOR]

This work proposes a new algorithm entitled "sequential absorption algorithm" to generate Representative Volume Elements (RVEs) of randomly distributed spherical particles reinforced composites by combining the Random Sequential Absorption (RSA) algorithm and the molecular dynamics based method. The proposed algorithm overcomes limitations of the RSA algorithm and is capable of efficiently generating the RVEs with the high Particle Volume Fractions (PVFs) (≥ 50.0 %). The proposed algorithm comprises of the modified RSA algorithm, the detection algorithms of the collisions between particles and between a particle and matrix surface(s), the post-collision particle velocity update algorithm, the particle periodic image generation algorithm and the acceleration algorithm. Particle distribution in the generated RVEs is analyzed through several statistical functions and consistent with the completely spatial random pattern. Regarding the elastic properties of composites, the critical sizes of RVEs and meshed elements are determined. The agreement of the elastic properties of composites with the different PVFs acquired using the finite element homogenization method, the experimental tests and the analytical models exhibits the validation of the proposed algorithm to generate RVEs of composites. [Display omitted] • A new sequential absorption algorithm to generate RVE of composites is proposed. • The proposed algorithm can generate RVEs with high particle volume fractions. • The proposed algorithm has good computational efficiency for small particle number. • Particle distribution in the RVEs is consistent with completely spatial random pattern. [ABSTRACT FROM AUTHOR]

In this work, an improved version of the Random Sequential Absorption (RSA) algorithm, comprising of the particle re-orientation algorithm, the particle intersection checking algorithm, the particle periodicity constraint algorithm and the acceleration algorithm, is proposed to efficiently generate Representative Volume Elements (RVEs) of spheroidal particles reinforced composites with specified particle orientations. The re-orientation of the particles is realized using the gradient descent based optimization method, and the RSA algorithm is accelerated by combining with the RVE subcell method and the bounding sphere concept. Several statistical functions are introduced to analyze the distributions of the orientation and centroids of the particles in the RVEs generated by the improved algorithm. The results show that the particle orientations of the generated RVEs match well with the specified particle orientations, and the centroids of the particles in the generated RVEs are not completely randomly distributed. Based on the generated RVEs, the elastic properties and coefficients of thermal expansion of spheroidal particles reinforced composites are predicted by using the FE homogenization method, and the predicted thermo-elastic properties of the composites agree well with those of the analytical models. The advantage of the improved algorithm lies in two aspects: (1) much better computational efficiency and (2) capability of generating the RVEs with specified particle orientations. Image 1 [ABSTRACT FROM AUTHOR]

In this paper, a new algorithm for the periodic boundary condition used for numerically predicting the coefficients of thermal expansion (CTEs) of different composite systems based on the finite element homogenization method is proposed. The results demonstrate that the proposed algorithm guarantees stress and strain continuities on the opposite surfaces of the representative volume elements (RVEs) for composites with spherical particles and plain woven fabrics but not for composites with cylindrical fibers and three-dimensional four-directional braided yarns. Meanwhile, the proposed algorithm ensures the micro–macro energy balance (Hill's lemma) and the zero macro-stress constraint of the RVEs for all composite systems. Through the comparison with experimental tests and other numerical methods, the proposed algorithm is validated to be capable of accurately predicting the CTEs of composites. • A new numerical implementation algorithm of periodic BC is proposed. • The proposed algorithm guarantees thermal flux continuity on RVE boundaries. • The proposed algorithm ensures the balance of RVE micro-/macro-strain energy. • ETCs of four composite systems are accurately predicted by the proposed algorithm. [ABSTRACT FROM AUTHOR]

• An experiment is carried out to study the droplet-wall impinging phenomenon. • The curve of β max with changing We d , T w and α is analyzed. • A model was used to analyzed the effects of We d , T w and α on the spreading behavior. Droplet-wall impaction exists in many nuclear installations and severe accidents. In this study, we experimentally investigate a single droplet impinging on a heated and inclined wall at the early impaction stage. Especially, the droplet spreading behavior is analyzed by changing the wall temperature (T w = 40 °C to 262 °C), the droplet Weber number (We d = 0.66 to 589), the droplet Reynolds number (Re d = 189 to 14,046) and the wall inclined angle (α = 0° to 45.6°). The results show that the increase of We d and Re d is beneficial to the droplet spreading, but not to the shrinking. The droplet front and back spread factors are quite different due to the gravity. Further, the maximum spread factor (β max) with T w at different We d is obtained and analyzed. These findings are helpful to fundamentally understand the droplet spreading on the heated and inclined wall at the early impaction stage. [ABSTRACT FROM AUTHOR]

• Two-step homogenization method is developed to predict the ETCs of MOIRCs. • The two-step homogenization method is validated by comparing with the FE method. • The WC and HC interfaces decrease and increase the ETCs of MOIRCs respectively. • The ETCs of the composites with imperfect interfaces show an asymptotic behavior. For the accurate prediction of the Effective Thermal Conductivities (ETCs) of the Multi-Oriented Inclusions Reinforced Composites (MOIRCs) with imperfect interfaces, i.e., Weakly Conducting (WC) or Highly Conducting (HC) interfaces, this work develops a two-step mean-field homogenization method, in which the MOIRCs is virtually decomposed into a set of pseudo-grains, each of which is equivalent as a two-phase composite with imperfect interfaces and firstly homogenized by using the Mori-Tanaka (M-T) or Double-Inclusion (D-I) model, followed by the homogenization of all the pseudo-grains based on the assumption of the uniform intensity or heat flux and on the inclusion orientation distribution. The M-T and D-I models are derived by extending the solution of the Eshelby's single inclusion problem to the heat transfer behaviors of the two-phase composites with imperfect interfaces. Through the comparison with the Finite Element (FE) homogenization method, the developed two-step mean-field homogenization method and the corresponding models are validated to accurately and efficiently predict the ETCs of the MOIRCs with imperfect interfaces. The simulation results show that the presence of WC interface decreases the ETCs of the MOIRCs, while the existence of HC interface has an opposite effect on the ETCs of the MOIRCs. In addition, the ETCs of the MOIRCs with imperfect interfaces show an asymptotic behavior with the variation of interfacial thermal conductance. [ABSTRACT FROM AUTHOR]

• An interphase model is proposed to model imperfect interface in the composites. • The FE homogenization method is employed to predict the ETCs of composites. • The ETCs of composites maintain constant with the interphase thickness variation. • The ETCs of the composites with imperfect interfaces are inclusion size dependent. • The ETCs of the composites with imperfect interfaces show an asymptotic behavior. This paper addresses the Finite Element (FE) homogenization of the Effective Thermal Conductivities (ETCs) of the Composites with Imperfect Interfaces (CIIs). To model the imperfect interfaces between the matrix and inclusions in the composites, the thin interphases between the matrix and inclusions are introduced, which are combined with the FE homogenization method to predict the ETCs of the CIIs. The Representative Volume Elements (RVEs) containing the interphases are adopted to characterize the micro-structures of the CIIs and generated by the modified Random Sequential Absorption (RSA) algorithm. Compared with the micro-mechanical models, the proposed interphase model with the FE homogenization method is validated to be able to accurately predict the ETCs of the CIIs. The simulation results demonstrate that the ETCs of the CIIs are size-dependent, and the interphase thickness in the range of 50.0–100.0 nm has few impact on the ETCs of the composites. In addition, the ETCs of the CIIs show an asymptotic behavior so that a transition zone and two plateaus zones can be identified for the curves of the ETCs of the CIIs. This work provides a new and simple approach for predicting the ETCs of the CIIs. [ABSTRACT FROM AUTHOR]

We report a broadly tunable mid-infrared femtosecond KTiOPO4 optical parametric oscillator (OPO) pumped by a mode-locked Yb:KGW oscillator. Under pump power of 7 W, up to 2.05 W average power was generated in the 1.41–1.71 μm region at a repetition rate of 151 MHz, corresponding to a pump-to-signal conversion efficiency of 29.3%. After compensating the intracavity dispersion, the pulse width of the signal wave was measured to be 200 fs at 1.6 μm, close to the transform-limited pulse duration. For the idler pulse, during the tuning range from 2.61 to 3.84 μm, the average powers were greater than 1 W, with a passive stability better than 0.7% rms during 3 h. [ABSTRACT FROM AUTHOR]

Composite damage behavior was simulated based on the Hashin failure criterion and stiffness degradation scheme, and the damage behavior of the interlayer was modeled using the cohesive element. A carbon-fiber-reinforced magnesium (Cf/Mg) composite was prepared by the liquid–solid-extrusion-following-vacuum-infiltration technique. Its damage and failure behavior were characterized and the fracture surface was analyzed. A good agreement was obtained between the finite-element methods and experimental results. The strength of the notched Cf/Mg composite decreased with an increase in notch size. Damage occurred first in the interface layer, which then expanded within or between layers under load and caused the final failure of the Cf/Mg composite. Fiber failure dominated the carrying capacity of the Cf/Mg composite, and the major fracture behavior of the composite layer was matrix compression failure in a 90° layer. [ABSTRACT FROM AUTHOR]

Composite damage behavior was simulated based on the Hashin failure criterion and stiffness degradation scheme, and the damage behavior of the interlayer was modeled using the cohesive element. A carbon-fiber-reinforced magnesium (Cf/Mg) composite was prepared by the liquid–solid-extrusion-following-vacuum-infiltration technique. Its damage and failure behavior were characterized and the fracture surface was analyzed. A good agreement was obtained between the finite-element methods and experimental results. The strength of the notched Cf/Mg composite decreased with an increase in notch size. Damage occurred first in the interface layer, which then expanded within or between layers under load and caused the final failure of the Cf/Mg composite. Fiber failure dominated the carrying capacity of the Cf/Mg composite, and the major fracture behavior of the composite layer was matrix compression failure in a 90° layer. [ABSTRACT FROM AUTHOR]

• An original ICVI algorithm reproducing the pyrolytic carbon deposition process is proposed. • The relationship between the process parameters and mechanical properties of unidirectional carbon/carbon composites is investigated. • The transverse modulus and tensile strength increase with an increase of the infiltration time, pressure, and temperature. • Interfacial debonding and propagation of multiple cracks result in the complete failure of unidirectional carbon/carbon composites. This paper proposes an innovative algorithm derived from the isothermal chemical infiltration process to generate quasi-real microstructures of unidirectional carbon/carbon composites. This algorithm, which can predict the relationship between the composite density and infiltration time, is termed the isothermal chemical infiltration algorithm. To further evaluate the relationship between the process parameters and the mechanical properties, a mesoscale finite element model of the unidirectional carbon/carbon composite is established based on the isothermal chemical infiltration algorithm. After validation, the developed numerical model is employed to study the effects of the infiltration time, pressure, and temperature on the effective mechanical properties. The results show that the transverse modulus and tensile strength increase with an increase in the infiltration time, pressure, and temperature, with the effect of temperature being the most significant. In parallel, it is revealed that the complete failure of unidirectional carbon/carbon composites results from interfacial debonding and propagation of multiple cracks during transverse tension. The proposed model has potential applications in the process design and optimization of high-performance carbon/carbon composites. [Display omitted] [ABSTRACT FROM AUTHOR]

Recently, oblivious random access machine (ORAM) has been widely used to prevent privacy leakage from user's access pattern. However, in multi-user scenarios, the obliviousness property of ORAM facilitates the malicious data modification by unauthorized users, which brings a new security challenge of user accountability to ORAM applications. Moreover, based on our observations, existing user accountability schemes for multi-user ORAM induce the extremely unacceptable overhead in both time and storage. What is worse, it is still inherent the traditional cloud accountability problem that the untrusted cloud server may have misbehavior on storing the outsourced data. In this paper, we focus on the issue that how to do accountability for both malicious users and untrusted cloud server without the independent trusted third party server. To address the above problem, we design and implement a Traceable Oblivious RAM, or T-ORAM for short, a cryptographic system that protects the privacy of users and the integrity of outsourced data based on group signatures. It can detect malicious users quickly by utilizing the traceability property of group signatures, and cost less storage overhead comparing with the existing solutions. Then, we further propose a more secure solution of Blockchain-based Traceable Oblivious RAM (BT-ORAM). Specifically, by introducing the blockchain technology, BT-ORAM can detect the malicious behavior from both malicious users and untrusted cloud server. BT-ORAM is the first accountability work for multi-user ORAM that deal with both malicious users and the untrusted cloud server. Finally, security analysis and experimental results show that our method outperforms the state-of-the-art accountability work for oblivious RAM, S-GORAM, in both security and performance. • We summarize the two critical problems in data integrity protection by applying ORAM technology into the cloud storage. • We present a Traceable Oblivious RAM (T-ORAM) with malicious user accountability for multi-user ORAM. • To achieve a high-level secure ORAM system, we further propose a Blockchain-based Traceable Oblivious RAM (BT-ORAM) with a comprehensive accountability scheme. • Experimental results show that T-ORAM has a better performance than existing related solutions, and BT-ORAM can achieve a higher level security than previous work. [ABSTRACT FROM AUTHOR]

Abstract FIM (Flow Induced Motion) is a kind of widespread and high-energy phenomenon. In application, FIM could be used to harvest hydrokinetic energy from ocean/river currents. In this study, a spring-mounted rectangular cylinder were numerically investigated in a flow domain for the ranges of 7500 < Reynolds number <187500 (0.1 m/s < flow speed <2.5 m/s) to examine the effect of its cross-sectional aspect ratio on the FIM responses and hydrokinetic energy conversion. Results indicate that in general, high aspect ratio has a negative effect on FIM. When the aspect ratio increases from 1/6 to 1.5, the FIM amplitude is gradually suppressed. When the aspect ratio reaches up to 2.0, both VIV (Vortex Induced Vibration) and galloping won't occur at any flow speed. For energy harvesting, when the aspect ratio decreases from 2 to 1/4, the converted power shows the increasing trend. The maximum FIM energy conversion efficiency also accordingly increases to 15.5%. It should be noted that when the aspect ratio reduces to 1/6, the maximum power keeps the increasing trend; however, the total energy conversion efficiency is not further enhanced, which implies the optimal aspect ratio for energy harvesting is around 1/4. Highlights • High cross-sectional aspect ratio has a negative effect on the FIM response. • VIV and galloping won't occur at any flow speed when the aspect ratio increases to 2. • The optimal aspect ratio of a rectangular cylinder for energy harvesting is 1/4. • The maximum FIM energy conversion efficiency reaches up to 15.5%. • The converted power increases without upper limit as the flow speed increases. [ABSTRACT FROM AUTHOR]

In this study, the flow-induced motion (FIM) of three rigidly coupled cylinders arranged in an equilateral-triangular pattern is numerically investigated. The spacing ratio is set to four times the cylinder diameter, and simulations are conducted for flow incidence angles of α = 0°, 15°, 30°, 45°, and 60°. The reduced velocity Ur is varied from 1 to 30, with corresponding Reynolds numbers of ∼1000–30 000. The two-dimensional Reynolds-averaged Navier–Stokes equations are used in conjunction with the shear stress transport k-ω turbulence model to simulate the turbulent flow, and the vibration equations are solved using the Newmark-beta algorithm. The numerical results indicate that the flow incidence angles and reduced velocity have a significant effect on the FIM response and vortex shedding mode. The widest lock-in range (4 < Ur < 13) occurs when α = 45°, and the narrowest range (3 < Ur < 7) is observed when α = 60°. The optimal arrangement for suppressing the vibration of the cylinders is found to be α = 30°. The vibration and frequency responses for α = 0° and α = 15° are not the same as those of a single cylinder; in particular, galloping phenomena occur for α = 0° when Ur > 12 and become fully developed when Ur > 15. [ABSTRACT FROM AUTHOR]

For investigating the effect of the irregular pores on transverse elastic properties of the unidirectional Carbon/Carbon (C/C) composites, a new strategy is proposed to generate the representative volume element (RVE) on the basis of scanning electron microscope (SEM) images, in which these pores are approximated as the polygons and the identical fibers are generated by using the RSA algorithm. A good agreement is achieved by comparing the effective elastic properties obtained from the FEM-homogenization techniques to those predicted by experimental tests and two-step Mori-Tanaka method. FEM results indicate that the convergence of the results is seen when the number of vertices of the polygon reaches 40 and the RVE edge length is eight times of the maximum pores size ( L/l max  = 8). Meanwhile, the average departure from isotropy is below 5% and the UD composites can be considered as transverse isotropic. The effective transverse elastic properties would decrease with increase of the porosity and with increase of the pores clustering η p . [ABSTRACT FROM AUTHOR]