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To effectively analyses the flame propagation of premixed hydrogen-air explosion, this paper carries out a numerical study on the dynamics of flame propagation during hydrogen explosions in a closed duct under different blocking rates. The study shows that flame structure is roughly the same when the flame passes through an obstacle under different blocking rates. The difference in blocking rates only shows a slight difference in the degree of flame deformation. When the flame passes through the obstacle, Rayleigh -Taylor (R-T) instability accompanies the entire flame propagation process and corresponds to each stage flame acceleration. Kelvin-Helmholtz(K-H) instability has a more prominent influence on the tip flame propagation. When the explosion flame propagates, instabilities lead to difference in density gradient and pressure gradient in the duct. Interaction between density gradient and pressure gradient leads to formation of baroclinic torque, which is the main cause of the vorticity. During the flame propagation, the vorticity at the front of the flame is roughly zero, whereas the vorticity formed at the obstacle or in the burned gases is more apparent. The larger the blocking rate, the more prominent the turbulence intensity during the flame propagation. • Flame structure is roughly same under the different blocking rates obstacles. • Rayleigh -Taylor (R-T) instability accompanies the entire flame propagation. • Kelvin-Helmholtz(K-H) instability influence on the tip flame propagation. • Baroclinic torque is the main cause of the vorticity. [ABSTRACT FROM AUTHOR]

The runaway condition is unsteady and is of significant importance in pump turbines. In this research, the shear-stress transport (SST) k - ω turbulence model is used to simulate the unsteady flow in the vaneless region and the runner of a prototype pump turbine. The characteristics of pressure fluctuations, vortex identification based on Q criterion and local entropy generation rate (LEGR) in the pump turbine under runaway condition are studied, and the relationships of these physical quantities in the vaneless region are determined. Internal flow research reveals the motion and transmission characteristics of vortices in the vaneless region and the runner. The results show that the vortices that move clockwise in the vaneless region are responsible for the flow separation, the increase of LEGR, and the generation of the characteristic frequency 1.4 f n of these physical quantities (pressure fluctuations, Q criterion and LEGR). In runner, it is found that the characteristic frequency of pressure fluctuation is 3.6 f n , which satisfies a linear relationship with the characteristic frequency 1.4 f n in the vaneless region. The vortices, moving in the vaneless and runner region, interact with the blades of runner and mainly contribute to the pressure fluctuation characteristic frequency in the runner. • Unsteady flow and fluctuations of a pump turbine under runaway condition are studied. • Flow separation and vortices cause energy dissipation in vaneless region and runner. • Vortex motion greatly affects pressure fluctuation in vaneless region. • Interaction between blade and vortex causes pressure fluctuation frequency in runner. [ABSTRACT FROM AUTHOR]

By using the Stereo Particle Image Velocimetry (PIV) technique we observe the secondary flow of second kind in a corner of a channel of square cross-section. The boundary layer thickness is much lower than the channel size. Therefore, the flow is still developing, not filling the entire channel cross-section, which is the case more widely reported in literature studied in a very long channel. The non-linear secondary effects of interacting boundary layers are observed as a single stream-wise vortex close to the channel corner in case of laminar boundary layers. In the case of turbulent boundary layers, the secondary flow takes shape of a symmetric pair of counter-rotating vortices. This pattern is observable only in the average velocity field, while the instantaneous ones display large amount of vortices, whose spatial distribution close to the corner in dependence on their orientation leads to statistically emergent net vorticity. At the same time, this pattern is reproduced by using a numerical simulation. • Secondary flow of second kind in a short channel observed by Stereo PIV • For R e L ≤ 1. 0 ⋅ 1 0 5 , the secondary flow is laminar in shape of single vortex in the corner. • The size of laminar secondary flow vortex decreases with R e L as R ∼ R e L − 1 ∕ 2 . • At R e L ≥ 1. 3 ⋅ 1 0 5 , a pair of counter-rotating vortices appears in the averaged sense. • This pattern develops also in a "steady-state" numerical simulation. [ABSTRACT FROM AUTHOR]

Taking advantage of alternative expressions for potential vorticity (PV) in divergence forms, we derive balances between volume integral of PV and boundary conditions, that are then applied to practical computations of PV: • We propose a new method for diagnosing the Ertel potential vorticity from model output, that preserves the balances; • We show how the expression of PV can be derived in general coordinate systems. This is here emphasised with isopycnic coordinates by generalising the PV expression to the general Navier-Stokes equations; • We propose a generalised derivation for the Haynes-McIntyre impermeability theorem, which highlights the role of the bottom boundary condition choice (e.g. no-slip vs free-slip) and mixing near the bottom boundary for the volume integral of PV. The implications of balances between volume integral of PV and boundary conditions are then analysed for specific processes at various scales: • At large scale, we show how an integral involving surface observations (derived from satellite observations) is linked to the integral of PV within a layer (between two isopycnals). This surface integral can be calculated for models and observations and can be used for validation; • At mesoscale or sub-mesoscale, we analyse the relationship between net PV anomalies and net surface density anomalies for idealised vortices and 2D fronts. This can help determining vortex or jet structures for idealised studies or empirical methodologies; • We also confirm and integrate previous results on the modification of PV within a bottom boundary layer into a single diagnostic taking into account the effect of density and velocity modifications by diabatic processes along the topography and diapycnal mixing within the boundary layer. • Ertel potential vorticity (PV) expressions in divergence form improve PV diagnostics in models. • Balances are derived between volume integral of PV and boundary conditions. • Practical numerical computations of PV, preserving balances, are proposed. • The implications of balances are examined for selected processes. [ABSTRACT FROM AUTHOR]

Highlights • A new method is proposed to analyze the vortex. • A new iso-vortex surface is done according to Q criterion. • Tendency of vortex structure is intuitive according to the iso-vortex surface. • Vorticity magnitude has a sudden change near the wall. Abstract This study presents a new method to analyze the vortex of cyclone separator by using the vortex identification of Q criterion. The Phase Doppler Particle Analyzer (PDPA) is used to measure the flow, and the numerical simulation is done by using the Reynolds stress model (RSM). In addition, a new conception of iso-vortex surface according to Q criterion is proposed for identifying the vortex structure. Since the model is validated by agreement between the numerical results and experimental data, and then is used to analyze the flow field. The results show that the tendency of the vortex structure is intuitive according to the new iso-vortex surface. Equivalent diameter of the iso-vortex surface is large in the upper region of the cylinder and decreases along the axis downward. The carrier energy of vortex decay gradually. In the process, vortex develops rapidly, even ruptures. Meanwhile, the vorticity magnitude has a sudden change near r /R = 0.93, and then vorticity magnitude rises up sharply. The vortex line of relative stable region is almost circular distribution, while the vortex line is complex at the upper region of cylinder, which has a hierarchical trend. The trend of the vortex center deviating from the geometric center firstly increases, reaches a peak value, and then reduces gradually. The axial position of peak value increases with the increase of velocity. Furthermore, the vortex structure can be appropriately adjusted to improve the flow instability, which is highly significant. [ABSTRACT FROM AUTHOR]

Abstract The rotary-type energy recovery device (RERD) is a promising energy recovery device which can significantly reduce the energy consumption of the Reverse Osmosis system in seawater desalination. A two-dimensional particle image velocimetry (PIV) measurement has been carried out in our previous study to visualize and understand the flow dynamics in the duct of the RERD. Based on the PIV experimental results, a computational fluid dynamics (CFD) method was applied to study flow characteristics and mixing process in the RERD. Three different turbulence modeling approaches (standard k - ε , RNG k - ε , and Realizable k - ε) were compared and validated by PIV experimental data. As a result, the simulation method was able to capture major features of the flow characteristic in the duct, such as the jet flow and the vortex structure. The Realizable k - ε turbulence model, presenting a good agreement with the experimental data, was recommended for analyzing the detailed information of flow characteristics. The simulation results revealed a swirling flow formed in the duct. This three-dimensional flow structure caused high flow turbulence and significant mass transfer in the mixing zone. These results would be beneficial to researchers in better understanding the mixing process in the duct of the RERD. Highlights • Three different k - ε turbulence model results were compared with experimental data. • Numerical results enriched the finding of the "entrance effect" observed by PIV method. • The Realizable k - ε model showed better predictions of vortex structures in the duct center. • A high concentration gradient and a swirling flow were in the duct transverse plane. [ABSTRACT FROM AUTHOR]

The importance of type-II superconductors with strong pinning comes from their ability to carry large transport currents in the presence of a magnetic field. We report on the results of the bulk magnetization measurements in the superconducting state in clean single crystals of NdFeAsO 0.9 F 0.1 at various temperatures. The zero-temperature superconducting critical current density, J c , where the superconducting transition temperature T c reaches a maximum of 45(1) kelvin, displays a remarkable high value. The field dependence of the critical current density was analyzed within the collective pinning model. A remarkably good agreement between the experimental results and theoretical δ1 pinning curve is obtained. This indicates that pinning in NdFeAsO 0.9 F 0.1 crystal originates from spatial variation of the mean free path. Additionally, the normalized pinning force density, F p , curves verses H/H irr (H irr is the irreversibility field) were scaled using the Dew-Hughes model. Our analysis suggests that point pinning alone cannot explain the observed field variation of F p . [ABSTRACT FROM AUTHOR]

Mixing is an essential operation in many microfluidic devices. Droplet-based micromixers utilize droplets for mixing enhancement. In the present study, a novel three-dimensional simulation is conducted which has the ability to capture not only the mixing process, but also the chemical reactions inside liquid droplets. This two-phase model is used for simulating the reacting flow inside a serpentine microchannel and explores the effects of droplet size and reaction rate on the production and consumption of species in droplets. It is observed that the chemical reaction in each droplet, begins from its front area. Furthermore, it is shown that the production of species does not depend on water fraction (ratio of water flow rate to total flow rate) and for all droplet sizes, only depends on the reaction rate. Moreover, different transient generated vortices and secondary flows are studied in the presence and absence of droplets and explained in details based on the position of droplets. It is found that during the passage of droplets through a microchannel bend, the number of cross-sectional vortices changes and the velocity magnitude in these secondary flows increases dramatically. [ABSTRACT FROM AUTHOR]

The design of turbine blades is a critical issue in the performance of vertical-axis wind-turbines (VAWTs). In a previous study, it is discovered that a loss of thrust in VAWT blades with a wave-like leading edge can be attributed primarily to vortex distribution. This finding prompted us to apply the wave-like blade design to the trailing edge rather than the leading edge. In this study, computational fluid dynamics was used to observe the flow field on straight and tubercle blades in order to predict the resulting thrust and power performance. Increasing the amplitude and wavelength of the tubercle was shown to increase the maximum thrust by as much as 2.31% and the power coefficient by 16.4%, compared to a straight blade. Furthermore, the overall and maximum thrust performance of blades with a modified trailing edge was shown to exceed those of blades with a wave-like leading edge, due to a shift in the location of the vortices by the induced flow. [ABSTRACT FROM AUTHOR]

Through-silicon via (TSV) technology has been quickly improved, however, there are challenges in reducing electrodeposition time. In this research, new conical TSVs, 3 μm diameter and 27 μm depth (3 × 27 μm), were made. The flow pattern of the electrolyte inside a conical TSV and a cylindrical TSV is simulated with a TSV chip attached on a rotating disk electrode (RDE) rotating at 1000 rpm in the electrolyte. The flow pattern inside conical TSVs confirmed advantages of the conical shape over the cylindrical shape. The conical shape forms deeper vortexes and shows higher velocities at the same depth than the cylindrical shape. A character of electrolyte bases on anodic area charge was used to optimize electrolyte bath composition for the extreme fast filling. Without additives, the anodic area charge at 1000 rpm was larger than that at 10 rpm, however, the reverse obtained in the presence of additives. D-optimal designs for quantitative multilevel of additives were applied to create an experiment plan. The result confirmed a strong effect of Cl − concentration and interactions of Cl − with polyethylene glycol (PEG) and bis-(3-sodiumsulfopropyl)-disulfide (SPS) with sulfonated diallyl dimethyl ammonium chloride copolymer (SDDACC) which bring to anodic area charge at 1000 rpm was smaller than that at 10 rpm. In an optimal concentration of SDDACC, the 3 × 27 μm conical TSV was filled within 3 minutes. A new additive acceleration mechanism was proposed from the accumulation of Cu(I) thiolate inside vortexes. [ABSTRACT FROM AUTHOR]

The accuracy, robustness, dissipation characteristics and efficiency of several structured and unstructured grid methods are investigated with reference to the low Mach double vortex pairing flow problem. The aim of the study is to shed light into the numerical advantages and disadvantages of different numerical discretizations, principally designed for shock-capturing, in low Mach vortical flows. The methods include structured and unstructured finite volume and Lagrange-Remap methods, with accuracy ranging from 2nd to 9th-order, with and without applying low-Mach corrections. Comparison of the schemes is presented for the vortex evolution, momentum thickness, as well as for their numerical dissipation versus the viscous and total dissipation. The study shows that the momentum thickness and large scale features of a basic vortical structure are well resolved even at the lowest grid resolution of 32×32 provided that the numerical schemes are of a high-order of accuracy or the numerical framework is sufficiently non-dissipative. The implementation of the finite volume methods in unstructured triangular meshes provides the best results even without low Mach number corrections provided that a higher-order advective discretization for the advective fluxes is employed. The compressible Lagrange-Remap framework is computationally the fastest one, although the numerical error for the momentum thickness does not reduce as fast as for other numerical schemes and computational frameworks, e.g., when higher-order schemes are utilized. It is also shown that the low-Mach number correction has a lesser effect on the results as the order of the spatial accuracy increases. [ABSTRACT FROM AUTHOR]

This paper focuses on the multi-scale wake structures of a new asymmetric side mirror of a race car, which can generate the aerodynamic downforce. The turbulent wake generated by symmetric bluff bodies of triangle and semi-cylinder and an asymmetric bluff body (called CT-cylinder) were experimentally investigated and were compared. Firstly, the instantaneous velocity and vorticity were quantitatively measured by high-speed PIV at a Reynolds number of 14,440 in the circulating water channel. Then a one- and two-dimensional wavelet multi-resolution techniques were used to analyze the instantaneous velocity and vorticity in order to decompose turbulent structures into a number of subsets based on their central frequencies. It is found that the CT-cylinder wake generates the asymmetric flow structures and separation region that is smaller than that of the triangle wake. The contribution to vorticity concentration comes from the large-scale structure of CT-cylinder wake in the ( x , y )-plane, and the intermediate-scale structure of the CT-cylinder wake dominates the flow structure in the ( x , z )-plane. There is no evident difference among the three wakes in the relatively small-scale structures. The large-scale structures of three wakes exhibit the main contributions to the Reynolds shear stress, accounting for almost 82–92%. However, the small-scale structures make less contribution to Reynolds shear stresses. The phase-averaging and spectral analyses indicate that the CT-cylinder wake generates the relatively smaller vortices and smaller separation region and the CT-cylinder wake exhibits lower fluctuation in the shear layer. [ABSTRACT FROM AUTHOR]

Vortex merging is a basic fluid phenomenon which has been much studied for two-dimensional flows. Here we extend such a study to a specific class of three-dimensional flows, namely to vortices possessing a helical symmetry. In addition to the standard Reynolds number, this introduces another dimensionless control number, the pitch, which quantifies the periodicity length along the helix axis. Helical vortices with large pitches merge very much as in a two-dimensional setting. However, their rotation speed is reduced and the merging period is delayed. These effects, caused by the presence of a self-induced velocity in curved three-dimensional vortices, are understood by computing the streamfunction in the frame of reference rotating with the two vortices, and by inspecting the locations of its hyperbolic points. At intermediate pitch values, only viscous diffusion acts, resulting in a slow viscous type of merging. Finally for small pitches, the system is unstable resulting, at the nonlinear stage, in a different type of merging which breaks the initial central symmetry. [ABSTRACT FROM AUTHOR]

Using the nonlinear Ginzburg–Landau equations we study the vortex configurations in a superconducting square with a central square pillar in the presence of a uniform applied magnetic field. The presence of the pillar changes the vortex structure in the superconducting sample considerably. We calculate magnetization, magnetic induction, supercurrent and vorticity, which show different vortex configurations as a function of magnetic field. [ABSTRACT FROM AUTHOR]

In order to study site-selective NMR in chiral p -wave superconductors, we calculate local nuclear relaxation rate T 1 − 1 in the vortex lattice state by Eilenberger theory with and without non-magnetic impurity scattering in the Born limit and unitary limit. The local T 1 − 1 in the NMR resonance line shape is different between two chiral states p ± , depending on whether the chirality is parallel or anti-parallel to the vorticity. In the p − -wave, anomalous suppression of local T 1 − 1 occurs around the vortex core due to the negative coherence term coming from odd-frequency s -wave Cooper pair induced around the vortex. We especially examine the site dependence of the anomalous suppression of local T 1 − 1 , including the applied magnetic field dependence and the impurity effects. [ABSTRACT FROM AUTHOR]

An experimental investigation is conducted to study the effect of middle length and inclination angle of an S-shaped channel on adiabatic two-phase flow patterns. To study this effect, three channel lengths of 0.25, 0.50, and 1.00m, and four bends at the angles of 22.5°, 45°, 67.5° and 90° were used along the test section which is 35.83m long. The channel cross section dimensions were 0.05×0.10m and the test fluids were air and water. The observed flow patterns in 12 cases were studied. Each flow pattern was identified by visual observation and recorded with video camera for further analysis of the collected data points. The momentum transfer phenomenon between the phases for passing the upward middle section along the S-shaped channel was closely investigated. Five flow patterns of vortex plug, vortex slug, vortex wavy-annular, churn and mist packet annular were observed. The vortex type flow patterns were recorded with high speed video camera. The physical mechanisms and momentum transfer related to formation of these flow patterns were explained in detail. Flow pattern maps were obtained and then compared for different middle lengths and inclination angles to perform a parametric study on the S-shaped channel. Moreover, effect of upstream/downstream of the S-shaped channel on the maps and transition lines were discussed. It was revealed that upstream effects reflect the interactions and momentum transfer between the phases. Therefore, flow pattern types mainly depend on upstream behavior. On the other hand, the amount of backward flow resulting in the establishment of countercurrent flow and creation of vortexes along middle section is determined by downstream effects. Increasing the middle length or inclination angle causes the transition to vortex plug flow pattern to occur at higher liquid velocities. It also causes the area with churn flow pattern on the map to expand ending in smaller vortex slug flow pattern area. However, increasing middle length or decreasing inclination angle, ends in smaller area of mist packet annular area. It is also observed that for higher middle lengths, increasing the inclination angle shifts the transition lines more remarkably. A dimensionless analysis of obtained flow pattern maps is also carried out. It is shown that effect of geometry changes along S-shaped pipeline on two-phase flow pattern transitions is determined by either inclination angle or elevation difference between two horizontal lines. The explained flow mechanisms, obtained flow pattern maps and the transition lines can be used to predict the particular flow pattern that will be established for a given mass flux, inclined length and angle along pipelines. [ABSTRACT FROM AUTHOR]

• Four micromixers with double V-shaped baffles are designed. • All mixers achieve high mixing efficiency at the short distance of 3500 μm. • The visualization of vortex formation is conducted to analyze the mixing mechanism. • Different vortex patterns form in the microchannels as inlet velocity varies. Achieving rapid mixing of different liquids in a short distance is important for biochemical applications. Here, a new type of micromixers consisting of double symmetrical V-shaped baffles (MMA and MMB mixers) and double asymmetrical V-shaped baffles (MMC and MMD mixers) is proposed. Numerical simulations are performed to evaluate their mixing performance when the Reynolds number (Re) ranges from 0.1 to 50. Meanwhile, the visualization of vortex formation is conducted to analyze the mixing mechanism. The results show that the mixing efficiency of all mixers approaches 0.8 for all Re at the short distance of 3500 μm. At relatively low Re =0.5, the MMA and MMB perform better than MMC and MMD due to more adequate molecular diffusion; whereas the unbalanced lateral flows and multidirectional vortices in the MMC and MMD induce stronger chaotic convection at high Re =50, leading to a higher performance. Moreover, the disturbance created by the vortex on the mass transfer surface is not only relying on its vorticity but also correlates with the position of vortex/vortex leg. Lastly, all micromixers are fabricated, tested and compared with the simulation results and both of them show a good agreement. [Display omitted] [ABSTRACT FROM AUTHOR]

There is a close relationship between jet shear layer and acoustic refraction, but research on the unsteady characteristics between them is relatively inadequate. Firstly, the relationship between the sound ray direction and the vorticity, which is the classical ray tracing method, was simply derived, and the applicable conditions were focused on to extend it to the unsteady conditions. Then, the concept of effective area of sound refraction was proposed for beamforming sound source identification array. Sound propagation experiment was carried out in a 3/4 open-jet acoustic wind tunnel by using microphone array and controllable sound source with low flow resistance and self-noise, and the unsteady acoustic drift of initial source with different frequencies was measured. After that, the flow structure of same wind tunnel, especially the jet shear layer, was simulated and verified, which proved that the shear layer formed by the straight section of one nozzle side is quasi two-dimensional. In the simulation, 30 monitoring areas were divided along the shear layer, and the variation of vorticity integral value in each area was analyzed. Finally, similar to the Amiet method, the volume of the effective region was ignored, and several conditions that could be satisfied in this paper were introduced to establish a linear relationship between regional vorticity and acoustic drift. Through the comparison between the acoustic drift of direct measurement and vorticity calculation, the validity of the linear relationship at the time average and instantaneous levels is proved. The research of this paper show that the relationship between vorticity and acoustic ray is still valid at unsteady level as long as the conditions of low-speed flow and high frequency sound are satisfied. At the same time, it provided a new idea for non-contact flow measurement in shear layer of jet structure. [ABSTRACT FROM AUTHOR]

For the new sandglass-type floating body with special shape, this paper mainly analyzes the characteristic of the heave damping and its corresponding components by computational fluid dynamics (CFD) method. Firstly, the convergence and accuracy of the CFD method to simulate the heave motion of sandglass-type floating body were validated along with the experimental data. Secondly, the numerical model of the floating body with forced heave motion was created, and then hydrodynamic forces versus different motion amplitudes were calculated and compared to discuss its nonlinear characteristics. Afterwards, based on time-history curves of the hydrodynamic force, heaving added mass and damping of the floating body versus various frequencies were achieved and found to be different from the potential results. In order to analyze the reasons that cause difference, numerical simulations using inviscid and k-ε turbulent models were performed respectively, and the results indicate that the effect of fluid viscosity on the hydrodynamic coefficients is small and not decisive. Then equivalent radiation damping based on CFD wave elevation was deduced to explore the relation between the inviscid CFD and potential models. Finally, local wave field and vorticity field near the body were mainly discussed, and quantitative analysis based on mechanical energy was performed to verify that the vortex due to the body shape is the essential cause of the heave damping for the sandglass-type floating body. • Numerical method to simulate the heave motion of the new sandglass-type FPSO were validated. • Equivalent radiation damping based on CFD solutions was deduced and discussed with the potential results. • Vortex generated by the non-viscous fluid separation was analyzed based on mechanical energy. [ABSTRACT FROM AUTHOR]

We investigate the stability of a composite cyclone representative of Arabian Sea eddies using a high resolution primitive equation model. We observe that the eddy is unstable with respect to a mixed barotropic/baroclinic instability, leading to the growth of an azimuthal mode 2 perturbation. The latter deforms the eddy, which eventually evolves into a tripole after about 4 months of simulation. The presence of a critical level for the most unstable mode generates sharp fronts in the surface mixed layer where the Rossby number is large. These fronts then become unstable, and this generates submesoscale cyclones and filaments. Near these fronts, diapycnal mixing occurs, causing the potential vorticity to change sign locally, and symmetric instability to develop in the core of the cyclonic eddy. Despite the instabilities, the eddy is not destroyed and remains a large-scale coherent structure for the last 6 months of the simulation. Looking at Sea Surface Height only, the composite eddy evolves little, and fairly represents the eddy observed in the altimetry which can live for several months. The study of this simulation thus illustrates the numerous kinds of instabilities which may occur in large cyclonic eddies but cannot be captured directly by altimetric data. • A composite cyclone representative of the Arabian Sea eddies is unstable. • Submesoscale features are generated at the surface via hydrodynamical instabilities. • Currently available data cannot capture the instability occurring in the eddy. [ABSTRACT FROM AUTHOR]