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Due to the structural design requirements, tip leakage flow is a common phenomenon in the field of axial flow rotating machinery. The tip leakage flow interacts with the mainstream and induce complex tip leakage vortex. When the local pressure drops to the saturation pressure, cavitation may occur in flow passage and the vortex core. The tip leakage cavitating flow has a huge effect on the flow stability, which may reduce the efficiency of the waterjet pump. In the present study, Zwart cavitation model and SST k-ω turbulence model are employed to simulate the cavitating flow. The new proposed Liutex criterion has been used to capture the vortex structures in the flow passage and the tip gap. The vorticity transport equation in cylindrical coordinates is also used to discuss the variation tendency of the vorticity in the vicinity of the gap region under cavitation condition. The evolution of the cavitation is proved to be vital important for the formation and the breakdown of the tip leakage vortex. By analyzing the vorticity transport equation, the stretching term is shown to be the decisive factor.

The authors of the article entitled 'Super Deformable Behavior in Layered Structure InSe Single Crystals' found that the strain calculation is incorrect in the Mechanical Analysis section. Consequently, this paper has been withdrawn from publication.

Indium selenide, a III–V group semiconductor with layered structure, attracts intense attention in various photoelectric applications, due to its outstanding properties. Here, we report super deformability and thermoelectricity of γ-InSe single crystals grown by modified Bridgeman method. The crystal structure of InSe is studied systematically by transmission electron microscopy methods combined with x-ray diffraction and Raman spectroscopy. The predominate phase of γ-InSe with dense stacking faults and local multiphases is directly demonstrated at atomic scale. The bulk γ-InSe crystals demonstrate surprisingly high intrinsic super deformative ability which is highly pliable with bending strains exceeding 12.5% and 264% extension by rolling. At the meantime, InSe also possesses graphite-like features which is printable, writable, and erasable. Finally, the thermoelectric properties of γ-InSe bulk single crystals are preliminary studied and thermal conductivity can be further reduced via bending-induced defects. These findings will enrich the knowledge of structural and mechanical properties’ flexibility of InSe and shed lights on the intrinsic and unique mechanical properties of InSe polytypes.

Thermoelectric material SnSe has aroused world-wide interests in the past years, and its inherent strong lattice anharmonicity is regarded as a crucial factor for its outstanding thermoelectric performance. However, the understanding of lattice anharmonicity in SnSe system remains inadequate, especially regarding how phonon dynamics are affected by this behavior. In this work, we present a comprehensive study of lattice dynamics on Na0.003Sn0.997Se0.9S0.1 by means of neutron total scattering, inelastic neutron scattering, Raman spectroscopy as well as frozen-phonon calculations. Lattice anharmonicity is evidenced by pair distribution function, inelastic neutron scattering and Raman measurements. By separating the effects of thermal expansion and multi-phonon scattering, we found that the latter is very significant in high-energy optical phonon modes. The strong temperature-dependence of these phonon modes indicate the anharmonicity in this system. Moreover, our data reveals that the linewidths of high-energy optical phonons become broadened with mild doping of sulfur. Our studies suggest that the thermoelectric performance of SnSe could be further enhanced by reducing the contributions of high-energy optical phonon modes to the lattice thermal conductivity via phonon engineering.

The object of this paper is to numerically investigate the fluid-structure interaction of composite hydrofoil. The aim is to validate 3D numerical simulation model with fluid-structure interaction of composite materials comparing to experimental data, and study the fluid-structure interaction of composite hydrofoil. Numerical results are presented for a composite hydrofoil. The hydrofoil has unswept trapezoidal planform of aspect ratio 3.33. The numerical model has been validated and showed reasonable agreement with the experiment measurements. Then the forces and deformations of composite hydrofoil with different plying angles have been investigated at Re=1x106 and initial angle of attack α=6°. The results show that lift and drag coefficients and tip twist angle decrease and get minimum at about plying angle θ=35°. According to Classical Laminate Theory, the intrinsic bending-twisting coupling of composite structures leads to the variation of twist angle for different plying angles. The relation expression on elements of matrices D has been derived to estimate twist angle at tip qualitatively.

Binary Cu-based chalcogenide thermoelectric materials have attracted a great deal of attention due to their outstanding physical properties and fascinating phase sequence. However, the relatively low figure of merit zT restricts their practical applications in power generation. A general approach to enhancing zT value is to produce nanostructured grains, while one disadvantage of such a method is the expansion of grain size in heating-up process. Here, we report a prominent improvement of zT in Cu2Te0.2Se0.8, which is several times larger than that of the matrix. This significant enhancement in thermoelectric performance is attributed to the formation of abundant porosity via cold press. These pores with nano- to micrometer size can manipulate phonon transport simultaneously, resulting in an apparent suppression of thermal conductivity. Moreover, the Se substitution triggers a rapid promotion of power factor, which compensates for the reduction of electrical properties due to carriers scattering by pores. Our strategy of porosity engineering by phonon scattering can also be highly applicable in enhancing the performances of other thermoelectric systems.

We have systematically studied the thermoelectric properties in Zn-doped SnTe. Strikingly, band convergence and embedded precipitates arising from Zn doping, can trigger a prominent improvement of thermoelectric performance. In particular, the value of dimensionless figure of merit zT has increased by 100% and up to ~0.5 at 775 K for the optimal sample with 2% Zn content. Present findings demonstrate that carrier concentration and effective mass play crucial roles on the Seebeck coefficient and power factor. The obvious deviation from the Pisarenko line (Seebeck coefficient versus carrier concentration) due to Zn-doping reveals the convergence of valence bands. When the doping concentration exceeds the solubility, precipitates occur and lead to a reduction of lattice thermal conductivity. In addition, bipolar conduction is suppressed, indicating an enlargement of band gap. The Zn-doped SnTe is shown to be a promising candidate for thermoelectric applications.

The objective of this paper is to investigate the correlation between fluid induced vibration and unsteady cavitation behaviours. Experimental results are presented for a modified NACA66 hydrofoil, which is fixed at α=8°. The high-speed camera is synchronized with a single point Laser Doppler Vibrometer to analyze the transient cavitating flow structures and the corresponding structural vibration characteristics. The results showed that, with the decreasing of the cavitation number, the cavitating flows in a water tunnel display several types of cavitation patterns, such as incipient cavitation, sheet cavitation and cloud cavitation. The cavity shedding frequency reduces with the decrease of the cavitation number. As for the cloud cavitation regime, the trend of the vibration velocity goes up with the growth of the attached cavity, accompanied with small amplitude fluctuations. Then the collapse and shedding of the large-scale cloud cavities leads to substantial increase of the vibration velocity fluctuations.

The main purpose of this study is to shed light on the cloud cavitating flow and associated characteristic of pressure fluctuation near wall. A simultaneous sampling technique is used to synchronize the observations of cavitation instantaneous behaviour and the measurements of pressure signals near wall in a convergent-divergent channel. The results show that, a typical quasi-periodical sheet/cloud cavitation can be categorized into three stages: (1) the growth of attached cavity; (2) the shedding of the attached cavity; (3) the development and collapse of the detached cavities. At the stage one, the magnitudes of pressure fluctuation under the attached cavity are limited. However, they become significant in the closure region of attached cavity, especially, when attached cavity reaches its maximum length. At the stage two, the attached cavity begins to shed small detached cavity, leading to the generation of small local pressure fluctuations with higher frequency. At the stage three, a large detached cavity is gradually formed in the rear of the channel. When it collapses rapidly in the downstream, pressure pulses with the magnitudes of the order of several atmospheres are detected. The propagation speeds of pressure pulses in different region are found to be related with the bubble density in the flow field.

A Curvature Correction model (CCM) based on the original k- model is proposed to simulate unsteady cavitating flows. The objective of this study is to validate the CCM model and further investigate the unsteady vortex behaviors of cavitating flows around a Clark-Y hydrofoil. Compared with the original k- model, predicted results are improved in terms of the cavity detachment and hydrofoil fluctuations. Results show that streamline curvature correction of CCM model overcomes the over-predictions of turbulence kinetic energy and eddy viscosity in cavitating vertical region with the original k- model, which leads to better simulation abilities for the unsteady cavitating flow computations. Based on computations, it is proved that the vortex structure is significantly modified by the transient cavitation, especially with respect to the cavity shedding behaviors. Complex vortex interactions and corresponding cavity shedding process near hydrofoil trailing edge lead to various load frequencies.

In this paper, an evaluation of inhomogeneous model for computations of gas-liquid two-phase flow is presented, and the mechanism of gas-liquid two-phase flow in a bubble column is studied based on Finite-Time Lyapunov Exponents (FTLE) and Lagrangian Coherent Structures (LCS). The simulation is conducted with the homogeneous and inhomogeneous models respectively, and the numerical results are compared with the experimental data. It is shown that the inhomogeneous model can calculate the force of the gas more accurately and simulates the details of transient flows well due to the consideration of the interaction between the two phases. With inhomogeneous model, the periodic fluctuation of the bubble hose is captured and the velocity distribution coincides exactly with the experimental data. For the gas-liquid two-phase flow in the bubble column, the process of gaseous flow injected into water can be divided into two stages: the gas rising and gas fluctuation. The Lagrangian Coherent Structures (LCS) which consist of the ridges of the FTLE field can capture the boundary of vortex and the interface between the forward and backward flows in the liquid region, and the LCS have unique value for representing the divergence extent of neighboring particles in regions with different dynamics characteristics.

The numerical simulations for a NACA66 hydrofoil are performed by using the standard k-ω SST turbulence model and revised γ-Reθ transition model respectively. The simulation results are compared with the experimental results, and the hydrodynamic property and the fluid structure during the pitching process is studied. It is revealed that, compared with the standard k-ω SST turbulence model, the revised γ-Reθ transition model is able to present the hydrodynamic property and the fluid structure of the transient flow around a pitching hydrofoil more accurately, and better predict the separation and transition process in the boundary layer. The transient flow process around a pitching hydrofoil can be divided into 5 parts. At small angle of attack, transition is observed at the leading edge of the foil, resulting in the inflection of dynamic property curves. As the angle of attack increases, a clockwise trailing edge vortex expands toward the leading edge of the foil. At high angles of attack, large-scale load fluctuations are observed due to the stall caused by separation of the leading edge vortex. The flow transitions back to laminar during the downward pitching process.

Tandem pump, compared with multistage pump, goes without guide vanes between impellers. Better cavitation performance and significant reduction of the axial geometry scale is important for high-speed propulsion. This study presents a global design optimization method based on surrogated method for an axial-flow tandem pump to enhance trade-off performances: energy and cavitation performances. At the same time, interactions between impellers and impacts on the performances are analyzed. Fixed angle of blades in impellers and phase angle are performed as design variables. Efficiency and minimum average pressure coefficient (MAPC) on axial sectional surface in front impeller are the objective function, which can represent energy and cavitation performances well. Different surrogate models are constructed, and Global Sensitivity Analysis and Pareto Front method are used. The results show that, 1) Influence from phase angle on performances can be neglected compared with other two design variables, 2) Impact ratio of fixed angle of blades in two impellers on efficiency are the same as their designed loading distributions, which is 4:6, 3) The optimization results can enhance the trade-off performances well: efficiency is improved by 0.6%, and the MAPC is improved by 4.5%.

Ultrathin Pr0.5Sr0.5MnO3 films 4?12?nm thick were grown on (LaAlO3)0.3(Sr2AlTaO6)0.7(1?1?0) substrates and their thickness-dependent properties were probed. Using high-resolution off-specular x-ray reciprocal space mapping on (1?3?0), (3?1?0), (2?2?2) and reflections, we clearly show that these films have a single-crystalline and a single-domain-like structure, suffering a monoclinic distortion (MA) with the unique axis bm along [?1?1?0] due to the in-plane anisotropic strain. With increasing film thickness, while the paramagnetic?ferromagnetic transition temperature increases, the antiferromagnetic charge-orbital ordering (COO) temperature decreases. More strikingly, the ultrathin films show a much more robust COO ground state than in the bulk or thick films, with the melting field higher than 14?T for the 4?nm films.