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The evolution of unforced and weakly damped two-dimensional turbulence over random rough topography presents two extreme states. If the initial kinetic energy E is sufficiently high, then the topography is a weak perturbation, and evolution is determined by the spontaneous formation and mutual interaction of coherent axisymmetric vortices. High-energy vortices roam throughout the domain and mix the background potential vorticity (PV) to homogeneity, i.e., in the region between vortices, which is most of the domain, the relative vorticity largely cancels the topographic PV. If E is low, then vortices still form but they soon become locked to topographic features: Anticyclones sit above topographic depressions and cyclones above elevated regions. In the low-energy case, with topographically locked vortices, the background PV retains some spatial variation. We develop a unified framework of topographic turbulence spanning these two extreme states of low and high energy. A main organizing concept is that PV homogenization demands a particular kinetic energy level E#. E# is the separator between high-energy evolution and low-energy evolution. [ABSTRACT FROM AUTHOR]

The Liutex vector is new quantity introduced to represent the rigid-body rotation part of fluid motion and thus to define and identify vortices in various flows. In this work, the intermittency and power-law similarity of the Liutex vector in homogeneous, isotropic turbulence and a turbulent channel are explored. First, we found that the Liutex vector is more intermittent than the vorticity vector in the considered turbulent flows, which indicates that an iso-surface of a Liutex magnitude with an appropriate threshold could capture the major rotating motions or vortical motions of the flow. Second, the three-dimensional energy spectrums of velocity, vorticity (enstrophy spectrum) and the Liutex vector in homogeneous isotropic turbulence are shown to exhibit power laws of −5/3, 1/3 and 1/3 in the inertial subrange, respectively, whilst the Liutex energy spectrum particularly satisfies an additional −10/3 power law in the viscous subrange. This viscous similarity of the Liutex vector is the only power law that survived from the wall presence and is argued to originate from the fact that the Liutex vector represents the rigid part of fluid motion and is free from any shear contamination. The existence of such a viscous similarity law indicates a certain coherence of the small scales of turbulence and could possibly help understand and model turbulence. [ABSTRACT FROM AUTHOR]

Unsteady cavitating turbulent flow around a NACA66 hydrofoil was simulated using a mass transfer cavitation model and a modified filter-based turbulence model in this paper. The modified filter-based turbulence model can accurately predict the pressure coefficient in midplane and shedding frequency of the unsteady cloud cavitation than standard κ – 𝜀 model and filter-based turbulence model. The time evolution of transient cavitation cloud structure predicted by the three-turbulence model was compared. The result which was predicted by the modified filter-based turbulence model is in good agreement with the experimental results. The time evolution of re-entrant jet had been analyzed. The instantaneous wall-pressure evolution on the suction surface (SS) predicted by the modified filter-based turbulence model had been analyzed. The cavitation-vortex interaction had been analyzed in this study. The different effects on the cavitation-vortex interaction of the vortex stretching term, vortex dilatation term and baroclinic torque term in the transport equation of vorticity had been discussed. [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]

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]

Rigid structures, such as groins or spur dikes, are constructed along riverbanks for various purposes, which pose computational challenges for unsteady flow in engineering mechanics. This paper presents a study of turbulent flow past a series of groins in a shallow, open channel by large-eddy simulation (LES). A direct-forcing immersed boundary method (IBM) was implemented to approximate complex boundaries around groins with round heads. The time-averaged velocities and turbulence intensities at the water surface obtained by an experiment using particle image velocimetry (PIV) were employed to validate the LES model, finding a satisfactory agreement between laboratory data and model results. Subsequently, the numerical model was employed to investigate the impact of groin parameters (i.e., head shape, aspect ratio , and length ) on the flow properties. Model results showed that a rectangular-headed groin generates higher turbulence intensities and larger vortices than a round-headed groin. On the other hand, the groin aspect ratio () affects the strength of turbulence intensities, vorticity in the mixing layer, and flow patterns in the groin field. Consistent with previous studies, the groin length () significantly affected the turbulent intensities and the vorticity but had little influence on the streamline in the recirculation zone. Eddies were produced at the groin tips and transported downstream. The shape of the vortex group varied as the vortices were transported downstream by the flow. Coherent structures were visualized by -criterion around the groin tips. [ABSTRACT FROM AUTHOR]

Abstract: In a gas–solid fluidized bed, the particle vortexes play an important role in determining the heat transfer capability, while the coherent structures dominate the hydrodynamic characteristics of the turbulent flow field. A two-dimensional Eulerian–Eulerian model integrating the kinetic theory of granular flow (KTGF) is used to simulate the particle vertical fluctuating velocity in a gas–solid fluidized bed, based on which the continuous wavelet transform technique is for the first time introduced to analyze the particle vortex behavior. The results show that the time scale of a particle vortex has a linear relationship with the wavelet scale. Some particle vortexes perform periodic motion and merge with or separate from others until the end of the vortex period. As the superficial gas velocity and bed height increase, particle vortexes are stretched and their boundaries become unclear. Furthermore, to investigate the intermittency of the gas–solid flow field, the probability density function (PDF) of wavelet coefficients is computed. It is found that the wavelet coefficients and the probabilities of intermittent events both improve as the wavelet scale, superficial gas velocity and bed height increase. Since intermittency is caused by coherent structures in the flow field, the effects of coherent structures on the intermittency are studied through a coherent structure extracting method and the extended self-similarity (ESS) scaling law. Before extraction, wavelet coefficients at the same scale increase as the superficial gas velocity and bed height increase, while the scaling law curve at U g =0.75m/s deviates from those at other superficial gas velocities, but shows no obvious change with the bed height. After extraction, the discriminations of scaling law curves under different operating conditions are eliminated and all curves merge in one. The effects of bubble formation, their size and propagation on particle vortex behavior are also analyzed. Bubble motion becomes more violent as gas velocity and bed height increase, which enhance the particle vortex size and activity and coherent structure generation, leading to stronger intermittency of granular flow field. As gas velocity increases, the amplitude of pressure fluctuation first increases and then decrease, while one sharp peak changes to be uniform lower peaks. As bed height increases, the major frequency decreases slightly and the amplitude increases slightly. This work provides deep insights into the turbulent flow field in a gas–solid fluidized bed by CFD simulation and the continuous wavelet transform technique. [Copyright &y& Elsevier]

Abstract: This work presents a novel swirler with variable blade configuration for gas turbine combustors and industrial burners. The flow dynamics downstream the swirler was explored using Large Eddy Simulation (LES). The resolved turbulence kinetic energy in the region where the flow exhibits the main flow phenomena was well above 80% of the total turbulent kinetic energy of the flow. It was evidently shown that the new swirler produces a central recirculation zone and a Rankine vortex structure which are necessary for swirl flame stabilization. Two Reynolds-averaged NavierStokes (RANS) simulation cases utilizing the standard and realizable k-ε turbulence models were also conducted for two objectives. The first is to demonstrate the validity of RANS/eddy-viscosity models in predicting the main characteristics of swirling flows with comparison to the LES results. The second objective is to comparatively investigate the flow features downstream the new swirler in both co-rotating and counter-rotating blade configurations. The results show that the counter-rotating configuration produces higher turbulence kinetic energy and more compact recirculation zone compared to the co-rotating configuration. [Copyright &y& Elsevier]

Abstract: The multi-scale structures of turbulent wakes generated by three kinds of bluff body, i.e. circular cylinder, square cylinder and compound of cylinder and square (CS) cylinders, have been experimentally investigated in this paper. Firstly, the instantaneous velocity fields and vorticity were measured by the high-speed PIV technique in a circulating water channel. The instantaneous streamlines and corresponding normalized vorticity contours are obtained at a Reynolds number of 5600. Then one- and two-dimensional wavelet multi-resolution technique was used to analyze the instantaneous velocities and vorticity measured by the high-speed PIV. The turbulence structures were separated into a number of subsets based on their central frequencies, which are linked with the turbulence scales. The instantaneous vorticity and Reynolds shear stresses of various scales were examined and compared between the three generators. It is found that the large-scale turbulent structure makes the largest contribution to the vorticity and Reynolds shear stresses for the three wake generators and exhibits a strong dependence upon the initial conditions or the wake generators. The large-scale vorticity and the sizes of vortex in the circular and square cylinders are larger than those in the CS cylinder wake. The contributions to the Reynolds shear stresses from the large-scale turbulent structures account for 90–96% to the measured maximum Reynolds shear stresses for the three wakes. However, the small-scale structures make less contribution to the vorticity and Reynolds shear stresses. [Copyright &y& Elsevier]

Vortices generated at the open end of a planar shock tube are numerically simulated using the AUSM+ scheme. This scheme is known to have low numerical dissipation and therefore is suitable for capturing unsteady vortex motion. However, this low numerical dissipation can also cause oscillations in the vorticity field. Numerical experiments presented here highlight the effect of numerical dissipation on the simulated vortex, as well as the role played by turbulence models. Two turbulence models - the shear-stress-transport (SST) and its modified version for unsteady flows (SST-SAS) - are employed to observe the effect of including turbulence models in such complex flows where the vortex has an embedded shock. [ABSTRACT FROM AUTHOR]

This paper presents construction and performance of a novel hybrid microelectromechanical system (MEMS) vortex flowmeter. A miniature cantilever MEMS displacement sensor was used to detect frequency of vortices development. 3-mm-long silicon cantilever, protruding directly out of a trailing edge of a trapezoidal glass-epoxy composite bluff body was put into oscillatory motion by vortices shed alternately from side surfaces of the obstacle. Verified linear measurement range of the device extended from 5 to 22 m/s; however, it could be broadened in absence of external 50-Hz mains electrical interfering signal which required bandpass frequency-domain digital sensor signal processing. The MEMS vortex sensor proved its effectiveness in detection of semilaminar airflow velocity distribution in a 40-mm-diameter tubular pipe.\hfill[2010-0164] [ABSTRACT FROM AUTHOR]

Abstract: Previous studies have suggested that the asymmetric vortex wake which develops behind an axisymmetric body at angle of attack is the result of the amplification of a disturbance introduced at the tip of the body. The potential-flow vortex wake of a circular cylinder in cross-flow is examined and similarities to the wake of the axisymmetric body at incidence are described. The implications of these similarities for the suggested model are discussed, and an argument is made that the vortex trajectories are fixed at a given angle of attack and shifted upstream or downstream by the initial disturbance. The need for further detailed experimental studies of the asymmetric vortex wake development is noted. [Copyright &y& Elsevier]

Abstract: Hydrogen is a promising energy in the future, and it is desirable to characterize the combustion behavior of its blends with air. The premixed hydrogen/air flame microstructure and propagation in a horizontal rectangular closed duct were recorded using high-speed video and Schlieren device. Numerical simulation was also performed on Fluent CFD code to compare with the experimental result. A tulip flame is formed during the flame propagating, and then the tulip flame formation mechanism was proposed based on the analysis. The induced reverse flow and vortex motion were observed both in experiment and simulation. The interactions among the flame, reverse flow and vortices in the burned gas change the flame shape and ultimately it develops into a tulip flame. During the formation of the tulip flame, the tulip cusp slows down and stops moving after its slightly forward moving, and then, it starts to move backward and keeps on a longer time, after that, it moves forward again. The structure of the tulip flame is becoming less stable with its length decreasing in flame propagation direction. The flame thickness increases gradually which is due to turbulence combustion. [Copyright &y& Elsevier]

Abstract: Measurements and numerical simulations of turbulent flows with free-surface vortex in an unbaffled reactor agitated by a cylindrical magnetic stirrer are presented. Measurements of the three mean and fluctuating components of the velocity vector are made using a laser Doppler velocimetry in order to characterise the flow field at different speeds of the stirrer. A homogeneous Eulerian–Eulerian multiphase flow model coupled with a volume-of-fluid method for interface capturing is applied to determine the vortex shape and to compute the turbulent flow field in the reactor. Turbulence is modelled using a second-moment differential Reynolds-stress transport (RST) model, but for some cases the based shear-stress transport (SST) model is also used. The predictions obtained using the ANSYS CFX-5.7 computational fluid dynamics code are compared with the images of the vortex and the measured distributions of mean axial, radial and tangential velocities and turbulent kinetic energy. The predicted general shape of the liquid free-surface is in good agreement with measurements, but the vortex depth is underpredicted. The overall agreement between the measured and the predicted axial and tangential velocities obtained with the RST model is good. However, the radial velocity is significantly underpredicted. Predictions of the turbulent kinetic energy yield reasonably good agreement with measurements in the bulk flow region, but discrepancy exists near the reactor wall where this quantity is underpredicted. The SST model predictions are generally of the same quality as those of the RST model, with the latter model providing better predictions of the tangential velocity distribution. [Copyright &y& Elsevier]

Climate change redistributes turbulence in both space and time, adding urgency to understanding of turbulence effects. Many analytic and analog models used to simulate and assess effects of turbulence on plankton rely on simple Couette flow. There shear rates are constant and spatially uniform, and hence so is vorticity. Over the last decade, however, turbulence research within fluid dynamics has focused on the structure of dissipative vortices in space and time. Vorticity gradients, finite net diffusion of vorticity and small radii of curvature of streamlines are ubiquitous features of turbulent vortices at dissipation scales but are explicitly excluded from simple, steady Couette flows. All of these flow components contribute instabilities that cause rotation of particles and so are important to simulate in future laboratory devices designed to assess effects of turbulence on nutrient uptake, particle coagulation, motility and predator-prey encounter in the plankton. The Burgers vortex retains these signature features of turbulence and provides a simplified “cartoon” of vortex structure and dynamics that nevertheless obeys the Navier-Stokes equations. Moreover, this idealization closely resembles many dissipative vortices observed in both the laboratory and the field as well as in direct numerical simulations of turbulence. It is simple enough to allow both simulation in numerical models and fabrication of analog devices that selectively reproduce its features. Exercise of such numerical and analog models promises additional insights into mechanisms of turbulence effects on passive trajectories and local accumulations of both living and nonliving particles, into solute exchange with living and nonliving particles and into more subtle influences on sensory processes and swimming trajectories of plankton, including demersal organisms and settling larvae in turbulent bottom boundary layers. The literature on biological consequences of vortical turbulence has focused primarily on the smallest, Kolmogorov-scale vortices of length scale η. Theoretical dissipation spectra and direct numerical simulation, however, indicate that typical dissipative vortices with radii of 7η to 8η, peak azimuthal speeds of order 1 cm s−1 and lifetimes of order 10 s or longer (and much longer for moderate pelagic turbulence intensities) deserve new attention in studies of biological effects of turbulence. [ABSTRACT FROM AUTHOR]

Abstract: The vortex shape in a non-standard partially baffled agitated vessel in the form of a glass-lined, under-baffled stirred vessel has been investigated using both experimental and numerical approaches for an air/water system for different rotation speeds of the agitator. A simple and flexible experimental strategy was developed for determination of the time-averaged location of the unstable free surface using a process involving superimposition of images. CFD simulations were made to predict the vortex shape by using an Eulerian–Eulerian multiphase model coupled with a homogenous turbulence model. The simplifying assumptions of a constant bubble size, a constant drag coefficient and use of the turbulence model were made. An assessment of the capability of the numerical method to predict the vortex shape was carried out through comparison between experimental data and numerical results. Considering for comparison purposes a water isosurface volume fraction equal to 0.9, to account for the existence of air/water mixture present at the interface in the experiments, instead of the classical value of 0.5, gave very good agreement with the experimental data. [Copyright &y& Elsevier]

The article discusses a computational fluid dynamics study of mould powder entrapment caused by vortexing in a continuous steel casting process. Two-phase-flow phenomena was modeled using the Volume Of Fluid (VOF) method. The effect of different turbulence models, mesh densities and initial as well as boundary conditions on the flow field was also examined.

Abstract: We study numerically particle transport in a two-dimensional coherent vortex field. Reasonable agreement exists between previously derived radial transport coefficients for energetic particles (Verkhoglyadova, O.P., le Roux, J.A. Particle diffusion on vortices in nearly incompressible magnetohydrodynamics. Astrophys. J. 602, 1002–1005, 2004a; Verkhoglyadova, O.P., le Roux, J.A. Cosmic ray transport in a vortex flow. IGPP-UCR Conf. Physics of the Outer Heliosphere (Riverside), AIP Conf. Proc., pp. 243–248, 2004b) and results of numerical simulations. Different physical factors controlling particle momentum change and drifts are analysed. It is shown that the vortex electric field is the main cause of trapped particle motion. Drift due to magnetic field inhomogeneity predominantly disturbs free particle gyroorbit along background magnetic field in the vicinity of a vortex. Our simulations show the development of a subdiffusion regime. [Copyright &y& Elsevier]

Images of the electron density of a magnetized electron column illustrate the dynamics of intense vortices moving in a weaker background vorticity. A moving retrograde clump of vorticity generates a spiral wake that then evolves into many long-lived holes, contributing to the late time fluctuations. [ABSTRACT FROM AUTHOR]