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A viscous flux formulation at interior faces and at Dirichlet boundary conditions in the framework of unstructured vertex-centered k-exact finite-volume schemes is presented. It is based on a geometric decomposition of the viscous flux into orthogonal and nonorthogonal parts. Special emphasis is put on the truncation errors and the spectral properties of the scheme, and a connection is established to the well-known α-damping scheme. The latter is also used to introduce discretization coefficients into the method that allow a fourth and a sixth order of accuracy in space on Cartesian grids. The effect of the scheme is presented in terms of canonical diffusion problems, as well as test cases for laminar wall-bounded flows. It is shown that the method preserves a second-order accuracy for the viscous operator, even on highly distorted unstructured grids and in the presence of boundaries and that it strongly enhances the solution accuracy with respect to a conventional scheme for viscous fluxes. [ABSTRACT FROM AUTHOR]

A three-dimensional direct numerical simulation was conducted to investigate the vortex-wing interactions through two NACA 0012 stationary wings placed in tandem at a low Reynolds number Re=10,000. The aerodynamic characteristics and three-dimensional flow structures were analyzed for the tandem wings. The back wing disturbed by the upstream vortices gained an evident increase in aerodynamic performance, where the advantage is related to the suppression of the large-scale vortex formation near the trailing edge. The Liutex method was applied to visualize the vortical structures for investigating the three-dimensional evolution and instability when interacting with the back wing. The upstream wake triggered dual-secondary vortices and intensified the secondary instability on the back wing. The induced vortices contributed to the lift enhancement because they provided an extra low-pressure region when propagating downstream on the suction side of the back wing. Because of the three-dimensional destabilization, the vortex interaction in the evolution process accelerated the transition and injection of the high-momentum flow into the boundary layer attached to the back wing, energizing the turbulent boundary layer and eliminating the large-scale separation near the trailing edge. This study provided a new perspective on the enhanced aerodynamic performance of tandem layout. [ABSTRACT FROM AUTHOR]

Energy harvesting technology plays an important role in converting ambient energy into useful electrical energy to power wireless sensing and system monitoring, especially for systems operating in isolated, abandoned or embedded locations where battery replacement or recharging is not a feasible solution. This paper provides an integrative study of the methodologies and technologies of energy harvesting from fluid flow-induced vibration (FIV). The recent research endeavors contributing to flow-based energy harvesting have been reviewed to present the state-of-the-art issues and challenges. Several mechanisms on FIVs including vortex-induced vibrations (VIVs), flutter, galloping and wake galloping are thoroughly discussed in terms of device architecture, operating principles, energy transduction, voltage production and power generation. Additionally, advantages and disadvantages of each FIV energy harvesting mechanism are also talked about. Power enhancement methods, such as induced nonlinearities, optimized harvester's configuration, hybridization and coupling of aerodynamic instabilities, for boosting the harvester's output are also elucidated and categorized. Moreover, rotary wind energy harvesters are reviewed and discussed. Finally, the challenges and potential directions related to the flow-based energy harvesters (FBEHs) are also mentioned to provide an insight to researchers on the development of sustainable energy solutions for remote wireless sensing and monitoring systems. [ABSTRACT FROM AUTHOR]

Kármán Vortex Street, a fascinating phenomenon of fluid dynamics, has intrigued the scientific community for a long time. Many researchers have dedicated their efforts to unraveling the essence of this intriguing flow pattern. Here, we apply the lattice Boltzmann method with curved boundary conditions to simulate flows around a circular cylinder and study the emergence of Kármán Vortex Street using the eigen microstate approach, which can identify phase transition and its order-parameter. At low Reynolds number, there is only one dominant eigen microstate W1 of laminar flow. At Rec1 = 53.6, there is a phase transition with the emergence of an eigen microstate pair W2,3 of pressure and velocity fields. Further at Rec2. = 56, there is another phase transition with the emergence of two eigen microstate pairs W4,5 and W6,7. Using the renormalization group theory of eigen microstate, both phase transitions are determined to be first-order. The two-dimensional energy spectrum of eigen microstate for W1, W2,3 after Rec1, W4–7 after Rec2 exhibit −5/3 power-law behavior of Kolnogorov’s K41 theory. These results reveal the complexity and provide an analysis of the Kármán Vortex Street from the perspective of phase transitions. [ABSTRACT FROM AUTHOR]

The dynamics of spin–orbit-coupled Bose–Einstein condensate with parity-time symmetry through a moving obstacle potential is simulated numerically. In the miscible two-component condensate, the formation of the Kármán vortex street is observed in one component, while 'the half-quantum vortex street' is observed in the other component. Other patterns of vortex shedding, such as oblique vortex dipoles, V-shaped vortex pairs, irregular turbulence, and combined modes of various wakes, can also be found. The ratio of inter-vortex spacing in one row to the distance between vortex rows is approximately 0.18, which is less than the stability condition 0.28 of classical fluid. The drag force acting on the obstacle potential is simulated. The parametric regions of Kármán vortex street and other vortex patterns are calculated. The range of Kármán vortex street is surrounded by the region of combined modes. In addition, spin–orbit coupling disrupts the symmetry of the system and the gain-loss affects the local particle distribution of the system, which leads to the local symmetry breaking of the system, and finally influences the stability of the Kármán vortex street. Finally, we propose an experimental protocol to realize the Kármán vortex street in a system. [ABSTRACT FROM AUTHOR]

This article presents the comparison between the unsteady flow characteristics of thick (t/c ≥ 6%) and thin (t/c ≤ 6%) airfoils performing pitching motion at Reynolds number Re = 3.6 × 10 5 . For the analysis, the considered airfoils are NACA 0012 as thick and symmetric airfoil and GOE 417A as thin and highly cambered airfoil. The analysis in this study is restricted to only two airfoils because from the preliminary investigations, similar aerodynamic characteristics with slight variation in their flow field development under same conditions were observed. The influence of reduced frequency, pitching amplitude and mean angle of attack on the airfoil's aerodynamic performance in terms of instantaneous force coefficients is thoroughly investigated. Further, the flow surrounding the airfoils is also investigated. It is noticed that the aforementioned parameters substantially affect the instantaneous force coefficients (both qualitatively and quantitatively). Under certain conditions, the formation of reverse Karman vortex street was observed indicating airfoil's thrust force generation. It must be noted that the conditions where the thrust force generation occurs for thick and symmetric airfoils like NACA 0012 are quite different from that of thin and cambered airfoils like GOE 417A. Additionally, a drastic change in the flow field around the airfoils is observed under similar conditions. For NACA 0012 airfoil, the formation and convection of the Leading Edge Vortex (LEV) under certain conditions has detrimental effects on its aerodynamic characteristics as it encourages early flow separation. However, the same cannot be true for thin and highly cambered airfoils like GOE 417A. Under the similar conditions with negligible variation, the LEV contributes toward providing extra suction over the GOE 417A airfoil's suction side. This significantly improves airfoil's aerodynamic characteristics. [ABSTRACT FROM AUTHOR]

The subject of this work is the development of compact analogs of vortex flows models, which are used in the modeling of vortex structures observed during the flight of an aircraft and the motion of a body in a fluid. In particular, two significant misunderstandings prevailing in this area of science are highlighted. The first misunderstanding is that the stationary motion of fluid parcels in a circle is treated as an inviscid vortex. Therefore, any vortex flow model that does not explicitly contain viscosity is considered to describe inviscid vortex motion. It has been proven that this is not so: the stationary viscous motion of fluid parcels in circular orbits corresponds to the self-balance of one force - the force of viscosity. This conclusion, in an explicit form, was made for the first time. This is very important because it changes our ideas about force balance, where two or more forces of different natures must necessarily be present. Overcoming this misunderstanding opens the way for creating compact analogs of existing models of vortex motions. Along the way, one more - the second general misunderstanding in the field of vortex dynamics was eliminated. Wherever we read it, we can see that the compactness of the vortex flow is associated with the compactness of the vorticity field. This is facilitated by the fact that the equations for vorticity and not for velocity are considered. As a result, except for one or two models of vortices, which correspond to the rotation of the entire space, up to infinity, this violates the fundamental law of physics - the law of conservation and transformation of energy. It is about the fact that, as a second misunderstanding, an error is assumed when calculating the kinetic energy of the vortex flows: the Jacobian in cylindrical (polar) coordinates is not considered. As a result, all the mentioned models of vortex flows, which correspond to the hyperbolic law as their asymptotics in the periphery, have infinite kinetic energy. Certainly, this does not correspond to the formation and evolution of compact vortex structures. Therefore, in this work, based on overcoming the aforementioned misunderstandings, many previously obtained models of compact vortex flows, as well as those obtained for the first time, are presented. In particular, this applies to the turbulent vortex flow during the formation of a vortex sheet, which is a compact analog of the Burgers-Rott vortex - both the classical one corresponding to laminar motion and the one consisting of a laminar flow in the core and a turbulent flow on the periphery of the vortex. Research methods are entirely theoretical. Well-known theorems of theoretical mechanics, mathematical theory of field, and calculus of variations, etc. are used. The obtained solutions are compared with the existing corresponding analogs of non-compact flows. Conclusions. Using the methods of calculus of variation, it was possible to show the possibility of the formation of quasi-solid-like rotational motion in a boundary layer of an incompressible fluid. The very presence of viscosity, or rather its taking into account (boundary layer), indicates a possible transition of the flow from plane-parallel motion to the just-mentioned rotational one due to the Kelvin-Helmholtz instability. In addition, two new models of the Burgers-Rott vortex flow were obtained in this study. The first model uses the general solution obtained by Burgers, but this model corresponds to a combined vortex: although the velocity field is continuous, the vorticity field has a discontinuity - at the point of maximum velocity. It is proved that such an approach is quite possible: the equation of motion is satisfied everywhere, i.e., at every point in space, and the tangential stresses are continuous functions. Since the periphery of the Burgers-Rott vortex is an unstable flow, another model is proposed - with a laminar core and a turbulent periphery. Certainly, the motion of fluid parcels in the peripheral region is described by a velocity distribution other than that of Burgers. Finally, the possible use of known models of compact vortex flows to simulate the von Karman vortex street is considered, with an indication of the advantages of these models.

The effect of arrangement of two pipes on the parameters of their force interaction in a fluid flow has been considered. The result of the study in this paper will be selection of the parameters for the mutual arrangement of pipes, which is justified by the calculation of hydrodynamic forces and confirmed by the physical or numerical experiments. The software used is designed for pipe bundles and reproduces hydroelastic excitations with periodic vortex separation. This paper provides a description of the known physical experiments and their comparison with the authors' numerical experiments. [ABSTRACT FROM AUTHOR]

To study the relationship of external dump of opencast coal mine impacting on surrounding micrometeorological factors, and then provide the foundation acknowledge for the ecological restoration in mining area, the airflow movement and water distribution around the mine area were regarded as the influencing factors to furtherly explore the ecological effects of the scale and form of the external dump. Taking Hongshaquan open-pit coal mine as an example, using Fluent fluid simulation software, adopting large eddy simulation and component transfer model, the external dump models with different heights and angles were established for simulation. The monthly temperature and dew point data of the area in 2020 were obtained through the regional meteorological station where Hongshaquan open-pit coal mine located, thereby reaching the monthly condensation height of the area. By analyzing the cloud diagrams of air velocity and moisture mass fraction under different conditions, the influence law of different sizes of external dump on the surrounding air flow movement and water distribution was obtained, and compared with the condensation height in the area. The numerical simulation results showed that: with the increasing dump height, the air climbing speed increased slowly, while the maximum climbing height increased, as well as the moisture mass fraction on the windward slope and the dump top ascended. More and more obvious Karman vortex street phenomenon was formed along the leeward slope. The number of vortices increased and the influence range became larger. Both of the airflow velocity and moisture mass fraction in the vortex area increased and reached their maximum values. The increase in the angle of the external dump presented a significant impact on the maximum climb height of the airflow, but showed little impact on the surrounding micrometeorological factors. Under the condition of 360 m limit height, when the external dump Angle reached the critical value of 26°, the maximum airflow climbing height would reach the summer condensation height in this area. Alternatively, under the condition of 22° limit Angle of the outer dump, when the height of the outer dump reached the critical value of 380 m, the maximum climbing height of airflow would reach the summer condensation height of the region, thereby promoting the precipitation around the mining area

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Atmospheric vortex streets (AVSs) are often observed in the wake of the leeward side of mountainous islands and are considered atmospheric analogs of the classic Kármán vortex street when a fluid flows past a cylindrical obstacle. The prevailing westerlies were observed year-round around the Tibetan Plateau. However, it remains to be understood whether the wake on the leeward side of the Tibetan Plateau exhibits a stable AVS and how the AVS impacts precipitation over the downstream region. In this study, the environmental meteorological factors, spatiotemporal characteristics, and various properties of the AVS on the leeward side of the Tibetan Plateau were examined for the period of 1979–2018 using global reanalysis datasets. The results show that the spatial structure of these AVSs closely resembles that of the classic Kármán vortex street observed in the laboratory. The meteorological factors satisfy the conditions in which a stable AVS can exist year-round. Moreover, various properties of these AVSs, including the aspect ratio and Strouhal number, are similar to those in previous studies of smaller obstacle caused AVS. Thus, these AVSs on the leeward side of the Tibetan Plateau can be interpreted as the atmospheric analog of classic Kármán vortex streets. The results further show that the spatiotemporal structure of precipitation over the wake of the Tibetan Plateau was largely shared by the cyclonic activities in the AVS. Approximately 80–90% of the total precipitation and heavy rain days in the main rainband over the wake of the Tibetan Plateau are closely tied to the seasonal evolution of the AVS. [ABSTRACT FROM AUTHOR]

Transonic planar flows around a circular cylinder are investigated numerically for laminar and turbulent flow conditions with Reynolds numbers of 50 ≤ R e D ≤ 300 and 8890 ≤ R e D ≤ 80,000 and free stream Mach numbers in the range of 0.2 ≤ M a ∞ ≤ 2 . A commercially available CFD tool is used and validated for this purpose. The results show that the flow phenomena occurring can be grouped into eight regimes. Compared to the incompressible flow regimes, several new phenomena can be found. In contrast, at higher M a ∞ of 0.6 ≤ M a ∞ ≤ 0.8 vortices in the wake of the cylinder are suppressed for R e D = 50 . In some cases, M a ∞ = 0.8 and R e D ≥ 300 , λ -shocks are formed in the near cylinder wake. For supersonic M a ∞ , two different phenomena are observed. Beside the well-known oblique and detached shocks, for 50 ≤ R e D ≤ 300 a wake with instabilities is formed downstream of the cylinder. Furthermore, the temporal mean drag coefficient C ¯ D , the Strouhal number S t r , as well as the critical Mach number M a crit are calculated from the simulation results and are interpreted. [ABSTRACT FROM AUTHOR]

In order to solve the shortcomings of traditional TLDs (tuned liquid dampers), such as their narrow frequency bands and their occupation of large amounts of space, based on deep-water theory, a new type of TLD incorporating Karmen vortex street theory is proposed and named as KV-TLD. A shaking table experimental test is conducted to fully analyze its vibration damping characteristics and its effectiveness in terms of vibration control is confirmed. Through comparison with a traditional TLD, the superiority of KV-TLD is verified. By analyzing parameters such as the mass ratio and the frequency ratio, it is found that a larger mass ratio means a better damping effect. When the frequency ratio is close to 1.0, the damping effect is better, while the vibration damping advantage of KV-TLD is much lower. Good performance in terms of installation, a wide frequency band, and damping effect means that KV-TLD is superior in terms of actual engineering application possibilities, and can also act as a good reference for subsequent engineering projects. [ABSTRACT FROM AUTHOR]

A numerical study is conducted to understand the impact of an unsteady freestream on the aerodynamic performance of an oscillating airfoil. The unsteady flow environment is generated by placing a stationary inline circular cylinder array upstream of the oscillating airfoil. The dependence of thrust with variation of Reynolds numbers and Strouhal numbers is investigated, and it is revealed that the unsteady flow environment enhances thrust production of a pitching airfoil. This increased thrust production was related to an effective increase in the Reynolds number experienced by the airfoil. With airfoil-vortex interaction analysis, the increase in average thrust coefficient was shown to be caused by constructive interaction of freestream vortex structures and the oscillating airfoil. Drag-inducing interactions were also observed but were less common than thrust-increasing events, resulting in a higher average thrust. A simple scaling law is expanded to include the effects of unsteadiness, where thrust is found to be linearly dependent on turbulence intensity. It is demonstrated that the thrust generated by the pitching airfoil when operating in highly unsteady flow environments is more accurately represented as a function of Reynolds number, Strouhal number, and turbulence intensity. [ABSTRACT FROM AUTHOR]

The world's first scientific satellite for sustainable development goals (SDGSAT-1) provides valuable data about offshore small-scale ocean phenomena, including the Kármán vortex street phenomenon. Although the simulation of the oceanic vortex street phenomenon is crucial for understanding not only the mechanisms of vortex formation in fluid dynamics but also their impact on the surrounding environment, the traditional simulation relies on the strong theoretical hypothesis of Navier–Stokes equations. Here, we propose a self-supervised neural network with high generalization ability to implement Navier–Stokes equations, simulating realistic oceanic vortex streets. Specifically, the physical informed convolutional neural network is first employed to determine the corresponding pressure and velocity fields, achieving accurate simulation of oceanic vortex streets with lower computational cost; Then, the observational islands in SDGSAT-1 imagery are embedded as obstacles, meanwhile, the marine background field including wind and terrain is synchronously incorporated to achieve more realistic simulation results compared with traditional methods; Finally, the morphological parameters of oceanic vortex streets are calculated and associated analysis are carried out to deepen our understanding of small scale vortex street phenomena. In addition, the experimental results demonstrated our proposed method can obtain promising time efficiency. With this partial differential equation deep learning solver framework combining observation and theory, there will be potential to expedite the cognitive process of oceanic phenomena. [Display omitted] • The inconsistency between observation and simulation is a scientific issue in oceanography. • A self-supervised neural network is proposed to solve the Navier–Stokes equations. • Formation process of oceanic vortex street observed in SDGSAT-1 is simulated. • Complex dynamical characteristic of the vortex is parameterized. • Proposed method yields impressive simulation performance while reducing discrepancies. [ABSTRACT FROM AUTHOR]

To investigate the dynamics of droplet-vortex interactions in particle-laden Karman vortex street flows, the simulations were carried out by using Euler-Lagrange approach, which was validated by the available experiments and numerical results. Then, the particle dispersion and the dimensionless frequency (Strouhal number) of the wake flow were analyzed to evaluate the particle-vortex interactions. The particle dispersion was statistically analyzed from both time and space dimensions and the different instantaneous dispersion patterns were explained by the relative slip velocity. Two independent scaling parameters, Stokes number StL and particle-fluid mass loading ratio Φ were revealed, and the particle mean square displacement and the Strouhal number were modelled by using these two scaling parameters, respectively. Finally, the characteristic lengths of the particle-laden wake flow were researched, and the Strouhal number physical model was developed based on the oscillating fishtail model. The results indicated that, firstly, StL and Φ, which constitute a dominant scaling group, can characterize the dynamics of droplet-vortex interactions in wake flow. Particles gradually separate from the vortex with the increase of StL due to the centrifugal effect, and the vortex intensity and regularity get worse with the increase of Φ, which further disperses the droplets for their momentum exchange with irregular vortex structures. Secondly, the length of the formation region and the width of the free shear layer diffuse are the two simultaneous characteristic lengths of the Strouhal number in oscillating wake. The proposed Strouhal number model gives a physical basis for the frequency determination, and the predicted errors are within ±1.5% error bands with mean absolute percentage error of 0.67%.

Transonic planar flows around a circular cylinder are investigated numerically for laminar and turbulent flow conditions with Reynolds numbers of 50≤ReD≤300 and 8890≤ReD≤ 80,000 and free stream Mach numbers in the range of 0.2≤Ma∞≤2. A commercially available CFD tool is used and validated for this purpose. The results show that the flow phenomena occurring can be grouped into eight regimes. Compared to the incompressible flow regimes, several new phenomena can be found. In contrast, at higher Ma∞ of 0.6≤Ma∞≤0.8 vortices in the wake of the cylinder are suppressed for ReD=50. In some cases, Ma∞=0.8 and ReD≥300, λ-shocks are formed in the near cylinder wake. For supersonic Ma∞, two different phenomena are observed. Beside the well-known oblique and detached shocks, for 50≤ReD≤300 a wake with instabilities is formed downstream of the cylinder. Furthermore, the temporal mean drag coefficient C¯D, the Strouhal number Str, as well as the critical Mach number Macrit are calculated from the simulation results and are interpreted.

The paper is concerned with a doubly infinite vortex array introduced by Weihs (Nature 241:290–291, 1973; in: Wu, Brokaw, Brennen (eds) Swimming and flying in nature, Vol. 2, Plenum Press, New York, 1975) as a model problem employed in order to understand the hydrodynamic and energetic benefits of fish schooling. Weihs considered two different 'modes' of swimming: one where the fish swim in anti-phase and one where they swim in phase. The stability properties for the vortex array corresponding to the anti-phase mode of swimming are well understood; but this is not the case for the in-phase mode. A normal mode analysis of perturbations applied to the corresponding vortex array is carried out. The array is found to be always unstable when subjected to general perturbations, but stable solutions exist if all consecutive vortex streets are subjected to the same perturbation. [ABSTRACT FROM AUTHOR]

Vortex-body interaction noise from a rod-airfoil model has been investigated experimentally in an aeroacoustic wind tunnel. The model configuration varies by changing either the rod cross section (circular, square, rhombic, and elliptical) or the install angle of the airfoil at three directions (attack angle a, sweep angle ß, and lean angle?) independently. This paper aims to assess the effects of these parameters on the characteristics of vortex-body interaction noise. A near-field microphone array was applied to locate the noise sources on the rod-airfoil model, and a far-field microphone array determined the noise level and radiation directivity. The results suggest that, in the test range, both the elliptical rod cross section and the change of the airfoil install angles can visibly reduce vortex-body interaction noise compared to the baseline rod-airfoil containing the circular rod and the airfoil at zero angles. The interaction noise level decreases with the increase of the eccentricity of the elliptical rod. Of the angles, the most effective way is the change of the sweep angle ß, which can achieve a noise reduction of 8 dB when ß reaches 20 deg, but the attack angle a has more influence in reducing the vortex shedding frequency. In addition, the noise source distribution of different frequencies also changes with the variation of rod-airfoil configurations. Flow visualization using particle image velocimetry was conducted to analyze the noise change mechanism. The findings from this study are beneficial for engineers in designing low-noise components in several aeronautical and industrial applications. [ABSTRACT FROM AUTHOR]

A myriad of different blending or switching functions have been developed to blend Reynolds-averaged Navier-Stokes (RANS) and large-eddy-simulation (LES) models properly in order to prevent issues such as modeled stress depletion. This challenge is a large focus of modern hybrid methods along with generating sufficient turbulent statistics in the Reynolds-averaged to large-eddy-simulation interface. This paper investigates these issues by implementing a variety of blending functions in the NASA OVERFLOW computational fluid dynamics solver and comparing them with the existing length-scale blending models. Scenarios considered include a zero pressure gradient flat plate, subcritical flow around a cylinder, and a supersonic reattaching shear layer. Overall, the eddy-viscosity blending methods produce results similar, if not improved, to their length-scale blending counterparts. In particular, it was found that the entropy function gave more control and stability during the transition from RANS to LES. [ABSTRACT FROM AUTHOR]

To investigate the dynamics of droplet-vortex interactions in particle-laden Karman vortex street flows, the simulations were carried out by using Euler-Lagrange approach, which was validated by the available experiments and numerical results. Then, the particle dispersion and the dimensionless frequency (Strouhal number) of the wake flow were analyzed to evaluate the particle-vortex interactions. The particle dispersion was statistically analyzed from both time and space dimensions and the different instantaneous dispersion patterns were explained by the relative slip velocity. Two independent scaling parameters, Stokes number StL and particle-fluid mass loading ratio φ were revealed, and the particle mean square displacement and the Strouhal number were modelled by using these two scaling parameters, respectively. Finally, the characteristic lengths of the particle-laden wake flow were researched, and the Strouhal number physical model was developed based on the oscillating fishtail model. The results indicated that, firstly, StL and φ, which constitute a dominant scaling group, can characterize the dynamics of droplet-vortex interactions in wake flow. Particles gradually separate from the vortex with the increase of StL due to the centrifugal effect, and the vortex intensity and regularity get worse with the increase of which further disperses the droplets for their momentum exchange with irregular vortex structures. Secondly, the length of the formation region and the width of the free shear layer diffuse are the two simultaneous characteristic lengths of the Strouhal number in oscillating wake. The proposed Strouhal number model gives a physical basis for the frequency determination, and the predicted errors are within ±1.5% error bands with mean absolute percentage error of 0.67%. [ABSTRACT FROM AUTHOR]

In order to solve the shortcomings of traditional TLDs (tuned liquid dampers), such as their narrow frequency bands and their occupation of large amounts of space, based on deep-water theory, a new type of TLD incorporating Karmen vortex street theory is proposed and named as KV-TLD. A shaking table experimental test is conducted to fully analyze its vibration damping characteristics and its effectiveness in terms of vibration control is confirmed. Through comparison with a traditional TLD, the superiority of KV-TLD is verified. By analyzing parameters such as the mass ratio and the frequency ratio, it is found that a larger mass ratio means a better damping effect. When the frequency ratio is close to 1.0, the damping effect is better, while the vibration damping advantage of KV-TLD is much lower. Good performance in terms of installation, a wide frequency band, and damping effect means that KV-TLD is superior in terms of actual engineering application possibilities, and can also act as a good reference for subsequent engineering projects.

In electromembrane systems, the transfer of ions near ion-exchange membranes causes concentration polarization, which significantly complicates mass transfer. Spacers are used to reduce the effect of concentration polarization and increase mass transfer. In this article, for the first time, a theoretical study is carried out, using a two-dimensional mathematical model, of the effect of spacers on the mass transfer process in the desalination channel formed by anion-exchange and cation-exchange membranes under conditions when they cause a developed Karman vortex street. The main idea is that, when the separation of vortices occurs on both sides in turn from the spacer located in the core of the flow where the concentration is maximum, the developed non-stationary Karman vortex street ensures the flow of the solution from the core of the flow alternately into the depleted diffusion layers near the ion-exchange membranes. This reduces the concentration polarization and, accordingly, increases the transport of salt ions. The mathematical model is a boundary value problem for the coupled system of Nernst–Planck–Poisson and Navier–Stokes equations for the potentiodynamic regime. The comparison of the current–voltage characteristics calculated for the desalination channel with and without a spacer showed a significant increase in the intensity of mass transfer due to the development of the Karman vortex street behind the spacer.

A staggered distribution of vegetation is very common in nature, which might significantly affect the flow structure. To investigate its impact, three rows of staggered vegetation groups are designed under seven different working conditions. The horizontal and vertical distances between the vegetation groups are constant, while the vegetation density is variable. The flow field is measured by the acoustic Doppler velocimeter (ADV). It is indicated that both the patch exit velocity and the steady wake velocity decrease with the increase of the density, which is also related to the vegetation rows. Except for the first patch, the upstream adjustment regions of all patches are diminished to some extent, due to the effect of the contralateral patch. The steady wake region also shows a decreasing trend, with a reduction of 0.5D each time when the water flows through the patch. This means that the flow structure is also affected by the number of vegetation rows. The intensity of the turbulence increases along the channel and has a non-axisymmetric distribution similar to that of the velocity. The Karman vortex street generated by the patches on the same side would merge into a larger vortex street. Depending on its intensity, the vegetation at the back has different effects on the vortex street. [ABSTRACT FROM AUTHOR]

The whole process of turbulence formation was quantitatively studied by the catastrophe method. The change law of the production of turbulent kinetic energy and its spectral distribution with the transformation of wave number and time factor are mainly studied. In addition, the distribution and production of turbulent kinetic energy in Karman vortex street model are studied by numerical simulation. The results show that the production of turbulent kinetic energy first increases and then decreases with the time factor, and the spectrum at different wave numbers follows this trend. When the time factor g (t) = − 1. 8 , the maximum value appears. In the Karman vortex street model, the simulation results are consistent with our derived values. [ABSTRACT FROM AUTHOR]

In this paper, we study the non-linear dynamic response generated as a result of a fluid–structure interaction between a flexible structure and a flowing fluid, when the structure is subjected to non-linear excitations. In first place, the use of semi-discrete approximations allowed us to show that the motion of a flexible structure coupled with a surrounding fluid flowing could be modelled and analysed via a coupled Complex Cubic Ginzburg–Landau equations (CCGLEs). Through the obtained CCGLEs, we were able to show that modulational instability (MI) is the main mechanism responsible for the generation of vortex shedding. Moreover, we showed that the stability of continuous wave depends on the coupling parameters between the fluid and the structure. Secondly, using a mathematical method, namely the G'/G expansion method, we found that vortex wave trains could be generated as cylindrical waves. These results are highly significant from a theoretical point of view and could be a plus to explain the process of generation of Kármán Vortex as a consequence of unstable coupling between two continuous wave in the fluid–structure system. Moreover, considering the industrial interest, such as floating wind turbines, this work aims to provide an additional understanding of the interactions between a flexible body and a surrounding flow. • Modulational instability in fluid–structure interaction, • Describe flow wake as vortex patterns, • Vortex-induced vibration, • Nonlinear excitations of a damped flexible body, • The dynamic response of immersed structure against fluid effects. [ABSTRACT FROM AUTHOR]

• VIV of a novel functionally graded graphene origami reinforced splitter plate attached to a cylinder is numerically studied. • Dynamic behaviour of the system under VIV is remarkably improved compared with its counterpart without graphene origami. • VIV induced maximum von Mises stress is greatly affected by graphene origami distribution pattern but is insensitive to hydrogen coverage. • Among all graphene origami and hydrogen coverage patterns considered, H GR X-W GR X pattern is most effective in reducing vibration amplitudes. Understanding the interaction between fluid and structures under various fluid conditions is critical for engineering applications, with vortex-induced vibration (VIV) being a significant area of research. This study presents a comprehensive numerical investigation on the VIV behaviors of functionally graded (FG) graphene origami (GOri)-enabled auxetic metamaterial (GOEAM) splitter plates attached to a circular cylinder by employing the finite element analysis. The splitter plate is composed of multiple GOEAM layers with the material properties possessing a gradient variation along the thickness direction. The local mesh quality is monitored and improved by Yeoh smoothing. The present work is focused on the effect of the auxetic properties of GOEAM on plate vibrations, the influence of plate length on wake patterns, and the effect of fluid loads on the cylinder–plate body and vortex pattern. The study reveals that: (1) the plate structure with H GR X-W GR X metamaterial distribution pattern with high elastic moduli in surface layers exhibits low deflection amplitudes, (2) shorter plates intensify vortices while longer plates reduce vortex prevalence, (3) the natural frequencies of both FG-GOEAM and pure metallic plates are found to be well above the vortex shedding frequency and as such, the fluid loads have a negligible effect on the cylinder–plate body and vortex patterns. These insights offer an in-depth understanding of fluid-structure interactions and valuable knowledge for the design and optimisation of graphene based metamaterial structures in engineering applications. [ABSTRACT FROM AUTHOR]

This work presents the results of numerical modeling of Karman vortex street generation performed with ANSYS/FLUENT package application. The influence of the mechanical elements located downstream of the bluff body on the vortex frequency has been found during earlier laboratory investigations. Five various geometrical configurations have been tested. Considerable differences in pictures of distributions of pressure, horizontal and vertical velocities have appeared for various configurations. Qualitative as well as quantitative results are presented in the paper. They confirm the significant dependence of the Karman vortex street parameters on the meter configuration.

An accurate assessment of physical transport requires high-resolution and high-quality velocity information. In satellite-based wind retrievals, the accuracy is impaired due to noise while the maximal observable resolution is bounded by the sensors. The reconstruction of a continuous velocity field is important to assess transport characteristics and it is very challenging. A major difficulty is ambiguity, since the lack of visible clouds results in missing information and multiple velocity fields will explain the same sparse observations. It is, therefore, necessary to regularize the reconstruction, which would typically be done by hand-crafting priors on the smoothness of the signal or on the divergence of the resulting flow. However, the regularizers can smooth the solution excessively and will not guarantee that possible solutions are truly physically realizable. In this paper, we demonstrate that data recovery can be learned by a neural network from numerical simulations of physically realizable fluid flows, which can be seen as a data-driven regularization. We show that the learning-based reconstruction is especially powerful in handling large areas of missing or occluded data, outperforming traditional models for data recovery. We quantitatively evaluate our method on numerically-simulated flows, and additionally apply it to a Guadalupe Island case study—a real-world flow data set retrieved from satellite imagery of stratocumulus clouds.

To clarify the mechanism for aerodynamic sound to be generated from a lifting surface placed in a flow with turbulence, wind-tunnel experiments and numerical simulations have been carried out for a flow around an airfoil subjected to the wake of a circular cylinder. The test airfoil has the NACA0012 profile with a chord length of 150 mm and a spanwise length of 500 mm, and it is set in a flow with the cylinder wake at angles of attack of 0 degree and 9 degrees. The wind velocity is set to 30 m/s, which results in an airfoil Reynolds number of 3.0×105. The circular cylinder is set 100 mm upstream of the leading edge of the airfoil, and its diameter as well as installation position in the transverse direction are varied. The numerical simulations are composed of large eddy simulation (LES) and aeroacoustical simulations that solves the Lighthill equation in the frequency domain with the Lighthill tensor computed by the above-mentioned LES as the acoustical source terms. The sound pressure level is also computed by the Curle’s equation with the fluctuation in the airfoil lift force computed by the LES as the acoustical source and compared with the experiments. The computed static-pressure distributions on the airfoil surfaces and aerodynamic sound spectra are in good agreement with the measured equivalents. The pressure fluctuation near the leading edge of the airfoil is remarkably high when the circular cylinder is placed upstream. When they flow near the leading edge of the airfoil, the vortices in the cylinder wake are stretched due to the acceleration of the main flow, form strong source of sound, and radiate intense sound in the upstream direction.

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Images in the visible range of the MODIS Terra/Aqua and ETM+ Landsat 7/OLI Landsat 8 satellites are used to study vortex streets (VSes) in the region of the Shantar Islands (Sea of Okhotsk​​). Two stable VSes are observed in the Northeast Strait separating Large and Small Shantar Islands. The northern VS forms near Diomede Stones Island, while the southern VS arises near a nameless group of rocks. The northern VS is 9–11 km long and consists of 4–5 pairs of cyclone-anticyclone vortices. The average diameter of a northern vortex is 0.73 ± 0.09 km. Vortices in the southern street are traced at distances of 6–7 km, with 3–5 vortex pairs recorded. The average diameter of a southern vortex is 0.47 ± 0.07 km. Vortices in the chain propagate mainly in the easterly direction at low tide and in the westerly direction at high tide. The spatial and temporal scales of the phenomenon show that both the vortices and VSes in the Northeast Strait are of a sub-mesoscale nature. The remote sensing data allow geometric parameters characterizing the stability of the oceanic VSes to be determined. The results from measurements and calculations can be used as tests demonstrating the possibility of applying the existing theory of similarity to VSes under real oceanic conditions. The parameters of oceanic VSes are compared to the results obtained in laboratory experiments and the characteristics of atmospheric VSes. [ABSTRACT FROM AUTHOR]

As one of the most important electrified railway components, the pantograph-catenary (PC) system plays a key role in the energy transfer for the train through the sliding electric contact. However, the performance of the PC system is severely limited by pantograph arcing, and the study on the dynamic characteristics of pantograph arcing is still far away from satisfactory. In this article, a magnetohydrodynamic (MHD) model of pantograph arcing is first established, which focused on the impact of crosswind as well as the input current on the arcing dynamics. Once the model was validated by laboratory experiment, the influences of the crosswind and input current on the arc temperature and voltage are studied in detail. Two parameters, namely the diameter of the catenary and the gap distance between the pantograph strip and the catenary, were varied to see what effects that might have. An interesting periodic phenomenon for both the temperature and voltage fall has been found, which was explained based on the Karman vortex street of fluid theory. [ABSTRACT FROM AUTHOR]

The flow past a cylinder is a classical problem in flow physics. In a certain range of Reynolds number, there will be Karman vortex street phenomenon in the wake of a cylinder, which will greatly increase the pressure drag of the cylinder. By controlling the vortex shedding phenomenon, drag reduction of the cylinder could be effectively realized. In this paper, a NACA0012 airfoil with pitching oscillation is placed downstream of the cylinder. Based on the tight coupling method, kinematics equations and Navier–Stokes equations in the arbitrary Lagrangian–Eulerian form are solved. Firstly, the effect of airfoil oscillation period and the distance between airfoil leading edge and cylinder center (x/D) are studied respectively, especially considering the aspects of vortex shedding and drag reduction effect. Besides, the vortex interaction in the flow field around the airfoil and cylinder is analyzed in detail. It is found that the NACA0012 airfoil with pitching oscillation can change the period of vortex shedding. Moreover, it can also increase the drag reduction rate to as high as 50.5%, which presents a certain application prospect in the engineering drag reduction field, e.g., for launch vehicles, ship masts, submarine pipelines, etc.

Atmospheric vortex streets quite often form in the wake of tall islands in the marine atmospheric boundary layer. Most satellite pictures of this phenomenon show that two rows of staggered, counter-rotating vortices are aligned more or less in a straight line downstream of the islands, like Kármán vortex streets behind cylindrical obstacles. In this paper however, an unusual downstream behaviour of an atmospheric vortex street in the wake of Heard Island is discussed, where there is a sudden change in orientation of the vortices downwind of the island. The reason for this development can be traced to a special synoptic weather situation. [ABSTRACT FROM AUTHOR]

The effect of electric wind produced by a positive corona discharge on the air flow around a circular cylinder at Reynolds numbers of 2400 and 3200 is investigated. The geometry of cylinder–wire electrodes is considered for two positions of the corona electrode relative to the cylinder: one wire behind the cylinder and two symmetrical wires above and below the cylinder. A direct numerical simulation of the electrohydrodynamic problem is performed using an original unipolar model of the corona discharge. The effect of a thin jet of electric wind directed from the corona electrode to the cylinder on the structure of the vortex wake behind the cylinder and the drag force is considered. It is shown that, when two corona electrodes are located above and below the cylinder, the electric wind prevents the formation of a Karman vortex street and significantly reduces the air drag of the cylinder. If the discharge electrode is located behind the cylinder, the corona discharge and the electric wind lead to the formation and development of large vortices in the wake behind the cylinder, which leads to significant fluctuations in its air drag. It is shown that a corona discharge significantly changes the characteristics of the Karman vortex street: as the voltage increases to 30 kV, the vortex shedding frequency decreases by a factor of 2.5 and the sizes of the vortices and their rotation velocity noticeably increase. The drag force is quasi-periodic and its mean value linearly depends on the corona voltage. [ABSTRACT FROM AUTHOR]

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Dead-water zones (DWZs) in natural open channels, formed by embayment or consecutive groins, can provide favorable growth conditions for aquatic organisms. Although flow hydrodynamics in side-cavity zones have been well studied, the impact of vegetation on recirculating flow is rarely considered. This study adopts large eddy simulation (LES) to examine the flow field in a rectangular embayment zone with different population densities of vegetation. The numerical model is validated by mean streamwise velocity data collected near mid-depth in the physical experiment. Vegetation rearranges the circulation structure in the DWZ and weakens the velocity and turbulent kinetic energy. This negative effect increases with increasing population density. With the development of the shedding vortex induced in the front edge of the channel–embayment interface, the large-scale coherent structure forms in the mixing layer and is hardly affected by the variation of population density. As the population density increases, the mean retention time first decreases and then increases as a result of the combined action of three factors, namely, the large-scale coherent structure, the plant-induced Karman vortex street, and the blocking effect of dense vegetation. [ABSTRACT FROM AUTHOR]

Highlights • The theory of Karman vortex street was adopted to explain the reason of weak region formation. • Two tube configurations of which the section was semicircular and semielliptical were discussed in this paper. • The velocity vector, the flow streamline and the temperature contour of the modified configurations were displayed. Abstract The main objectives were to develop a method for designing an effective finned heat exchanger through changing the configuration to ensure the tubes be streamlined shape. Kinds of inserts including, vortex generator, louver fin, and serrated fin in the heat transfer was used to increase the effect of thermal performance, but also was harmful to the flow characteristics. Consequently, the comprehensive performance was obviously lower than the heat exchangers with no inserts. In order to solve this problem, streamlined tube configuration was adopted in this paper to search a new method to enhance the comprehensive performance by reducing the Karman vortex street effect. Two modified configurations of tube were investigated in this paper. The modified configurations were both made of semicircular tube and semielliptical tube. The simulation was validated by Wang's experiment and Arafat's simulation. The numerical data were presented at Reynolds number in the range between 1300 and 2000. Based on the numerical results, the effect of elliptical axis ratio on the thermal hydraulic performance was discussed. The results showed that the comprehensive performance of the modified configurations was greater than that of the circular tube. In addition, the velocity vector, flow streamline and temperature contour of the configurations were displayed. The Karman vortex street effect was adopted to explain the reason of wake region formation. It can be concluded that long and thin section of heat transfer tube, which was coincident with streamlined design, was beneficial to improve the comprehension performance of the heat exchanger. [ABSTRACT FROM AUTHOR]

An innovative wind-induced energy harvester utilizing a piezoelectric bimorph with a pulling strip was designed based on Kármán vortex street theory to collect wind energy from a rotating machinery for powering some devices. A mathematical model was initially established for the model force analysis, and the voltage generated by the piezoelectric bimorph were calculated. The stress distribution on the piezoelectric bimorph was also examined to prevent the piezoelectric material from cracking. A series of experiments were conducted according to the results of the numerical analysis. Both the numerical analysis and the experiments demonstrate that: first, rotation speed is the most important factor that affects the power generation capacity of the piezoelectric bimorph, and with a suitable installing angle, an output power of 1 mW can be easily acquired when the rotation speed exceeds 140 r/min; and, second, choosing an appropriate installing angle can increase the power generation capacity more effectively than rectifying the diameter of the cylindrical obstacle. [ABSTRACT FROM AUTHOR]

In view of problem that performance of mine-used wind speed sensor is easy to fall because of poor working conditions, a kind of mine-used wind speed sensor based on DDS technology was designed. The sensor adopts DDS technology to design ultrasonic oscillation circuit with high stability resonance frequency, and uses Karman vortex street principle to achieve real-time, accurate measuring of wind speed value. The experiment result shows reliability of the sensor is high, measurement error is smaller, and anti-interference is strong.

The separating and reattaching flows and the wake of a finite rectangular plate are studied by means of direct numerical simulation data. The large amount of information provided by the numerical approach is exploited here to address the multi-scale features of the flow and to assess the self-sustaining mechanisms that form the basis of the main unsteadinesses of the flows. We first analyse the statistically dominant flow structures by means of three-dimensional spatial correlation functions. The developed flow is found to be statistically dominated by quasi-streamwise vortices and streamwise velocity streaks as a result of flow motions induced by hairpin-like structures. On the other hand, the reverse flow within the separated region is found to be characterized by spanwise vortices. We then study the spectral properties of the flow. Given the strongly inhomogeneous nature of the flow, the spectral analysis has been conducted along two selected streamtraces of the mean velocity field. This approach allows us to study the spectral evolution of the flow along its paths. Two well-separated characteristic scales are identified in the near-wall reverse flow and in the leading-edge shear layer. The first is recognized to represent trains of small-scale structures triggering the leading-edge shear layer, whereas the second is found to be related to a very large-scale phenomenon that embraces the entire flow field. A picture of the self-sustaining mechanisms of the flow is then derived. It is shown that very-large-scale fluctuations of the pressure field alternate between promoting and suppressing the reverse flow within the separation region. Driven by these large-scale dynamics, packages of small-scale motions trigger the leading-edge shear layers, which in turn created them, alternating in the top and bottom sides of the rectangular plate with a relatively long period of inversion, thus closing the self-sustaining cycle. [ABSTRACT FROM AUTHOR]

Abstract In order to shed some light upon the mechanism for the free vibrations of the wavy cylinder reported previously (Zhang et al., 2017), we numerically investigate the effect of external forcing on the flow around a wavy cylinder at R e = 5000. The forcing takes the form of sinusoidal transverse motion with the amplitude fixed at A e / D m = 0.2 and the frequency spanning from f e D m / U ∞ = 0.1 to 0.3. Even though the wavy cylinder is optimally designed to annihilate the Kármán vortex shedding in the fixed configuration, the Strouhal frequency is found to resurrect in the forced vibration cases, leading to very similar responses between the normal and wavy cylinders, especially at high forcing frequencies. By sectional analysis it is revealed that the sectional distribution of the force coefficients undergoes significant changes with the increase of the forcing frequency. Statistical tools such as the correlation and coherence reveals further spanwise features of the force coefficients. In addition, the instantaneous three-dimensional vortical structures are visualized by the spanwise voriticity isosurfaces to collaborate the previous discussions. The cessation of the flow control efficacy in the forced vibration is explained by the recession of the mean streamwise vorticity contours. The results presented in the work imply that the vortex-induced vibration of the wavy cylinder might be ascribed to the typical wake-structure interaction as the normal cylinder, although the mechanism for the initial growth of the oscillations still warrants further investigations. Highlights • The forced vibration of a wavy cylinder is systematically studied. • The concealed Karman vortex shedding resurrect in the forced vibration, and may serve as the mechanism for VIV. • The response curves of the drag and lift coefficients, the vortex shedding patterns of the wavy cylinder resemble that of the normal cylinder. [ABSTRACT FROM AUTHOR]

The characteristics of the flow and heat transfer in transitional flow are not clear and especially the lack of some reliable experimental data hinders the numerical study of the heat exchanger. Based on this, a two-dimensional time-resolved digital particle image velocimetry from the American-TSI-corporation was adopted. Experiments were carried out in an open-loop, recirculating water tunnel that operated at steady-state conditions. The experimental object is a three-cylinder with an equilateral triangle configuration having center-to-center distance ratio of 3. Reynolds number based on the tube diameter and the velocity of inlet is set at 70, 90, 120, 150, 200, 250 and 300. The flow parameters were quantified to research the effect of Reynolds number on flow field for supporting some facts to analyze the heat transfer. Time-mean flow field, turbulence statistics and velocity processed by proper orthogonal decomposition were investigated at different Reynolds numbers. The wake closure length, oscillation and Karman vortex in the wake of the three-cylinder model were discussed in detail. [ABSTRACT FROM AUTHOR]

Abstract In the present paper, three-dimensional unsteady flows past a rotating wavy cylinder have been studied through numerical simulations. The behavior of the lift forces acting on a rotating wavy cylinder and a rotating straight cylinder is compared, respectively. The results suggest that the rotation rate of the wavy cylinder has to be about two times that of a straight rotating cylinder in order to obtain the same value of the mean lift. The pressure distribution and the vortex structures in the wake of the rotating wavy cylinder at various rotation rates are also demonstrated and analyzed in this work. As rotation rate of the cylinder increases, the vortex formation length increases and the periodic shedding of the Karman vortex is suppressed. Three-dimensional vortex structures in the wavy cylinder wake are visualized at various rotation rates. Due to the surface rotation effect, a distinct difference exists in the vortex structures between a rotating wavy cylinder and a stationary wavy cylinder. [ABSTRACT FROM AUTHOR]