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A major challenge in the operation and development of the high-head water resources, especially in the mountains with high sediment yield, is the erosion of Pelton turbine components. In this work, a numerical study was carried out based on sediment properties measured in field conditions, such as particle distributions and concentrations, to analyze the erosion mechanism of a prototype Pelton turbine injector. The Volume of Fluid (VOF) method was combined with a Lagrangian particle tracking approach to simulate the air-water-sediment flow, followed by the application of Mansouri’s model to estimate the erosion. The predicted erosion patterns were in good agreement with field observations, especially in physically reproducing the asymmetrical erosion distribution on the needle surface. To elucidate this asymmetry, fundamental analysis of the flow patterns including the vortex structures and the secondary flow on the particle behaviors was carried out. Interestingly results were found about the secondary flow induced by the von Karman vortex shedding, which increased the particle separations, and consequently, enhanced the erosion in shedding areas. The current work may provide important engineering insights to reduce erosion of components with inner obstructions.

A stationary wing placed in the wake of a bluff body experiences lift enhancement. The quasi-periodic flow in the wake causes excitation of the separated flow in the post-stall conditions. The increase in the time-averaged lift force is associated with the flow separation, leading-edge vortex formation and subsequent reattachment in a process similar to the dynamic stall of oscillating wings. The lift enhancement is maximum for an optimal offset distance from the wake centerline. At the optimal location, potential flow oscillations, rather than the direct impingement of large vortices in the wake, provide the excitation. The smaller amplitude flow oscillations lead to a large separation bubble in the time-averaged sense in the post-stall regime. The delayed flow separation in the wake has a similar mechanism and frequency to those of the active flow control methods for separation. The degree of the lift enhancement is remarkable, given that the wake at a Reynolds number of 50,000 is expected to be highly three-dimensional.

A model experiment has been conducted for a hang-off evacuation riser considering the lower marine riser package (LMRP) based on the strain gauge test technology. The finite element eigenvalue method is used to analyze the natural frequency and mode shapes of the hang-off riser considering the axial tension force and the weight of LMRP. The modal analysis method is used to restructure the motion and analyze the vortex-induced vibration characteristics of the hang-off riser. Then, the influence of evacuation velocity is discussed. Results indicate that the mode shapes amplitude of the experimental riser with LMRP is significantly larger than that of the ordinary cantilever. The dominant frequency of each measuring point is the same, and the end-cell-induced vibration small frequency is observed near the bottom end. The vortex shedding in the cross-flow (CF) direction produced a large lift force, which resulted in the vibration frequency of in-line direction dominated by the CF direction. When the reduced velocity (Vr) is less than 40, the frequency response of the hang-off riser model increased with the evacuation velocity, which was subjected to the Karman vortex shedding frequency. When the frequency response of the riser model reached, the second-order natural frequency with Vr exceeds 40. Consequently, the “lock-in” phenomenon was observed and the riser exhibited vibrations with the second-order model.

Within the framework of the generalized time-dependent Ginzburg-Landau equations, we studied the influence of the magnetic self-field induced by the currents inside a superconducting sample driven by an applied transport current. The numerical simulations of the resistive state of the system show that neither material inhomogeneity nor a normal contact smaller than the sample width are required to produce an inhomogeneous current distribution inside the sample, which leads to the emergence of a kinematic vortex-antivortex pair (vortex street) solution. Further, we discuss the behaviors of the kinematic vortex velocity, the annihilation rates of the supercurrent, and the superconducting order parameters alongside the vortex street solution. We prove that these two latter points explain the characteristics of the resistive state of the system. They are the fundamental basis to describe the peak of the current-resistance characteristic curve and the location where the vortex-antivortex pair is formed., Comment: 9 pages, 6 figures. Accepted for publication in Scientific Reports

We study a two-dimensional flow over a cylinder and three ellipses with the aspect ratio $$a / b = 2.04$$ , $$3.65$$ , and $$5$$ at a Reynolds number $$Re = 100$$ based on the inflow velocity and hydraulic diameter using a direct numerical simulation (DNS) and linear stability analysis. The DNS shows that increase in $$a / b$$ leads to stabilization of the flow due to decrease in the recirculation zone. The linear stability analysis based on the time-averaged velocity field shows that the modes of interest describe a Karman vortex street. The oscillation frequency reconstructed from the linear stability analysis is in excellent agreement with the DNS. Solving the linearized adjoint equations made it possible to identify the flow area where direct and adjoint modes overlap. These areas of the field $$S_w$$ , usually called “wavemaker,” change their shape with increase in $$a / b$$ . Non-zero values of $$S_w$$ tend to approach the bottom part of the “tail” of the ellipse due to the asymmetry of the recirculation region, attached to the top side of the ellipse.

The wake of a circular cylinder was forced with a temporally modulated and spatially segmented dielectric-barrier-discharge plasma actuator. The temporal modulation was achieved by modulating the a...

This paper investigates the formation mechanism of the propulsive force of a flapping airfoil, which performs harmonic angular oscillations in a continuous medium flow, in the light of the recently proved general theorem on the added mass of bodies in a viscous incompressible fluid. The flow is described by the Navier–Stokes equations. The calculations were performed by the mesh-free numerical method of viscous vortex domains. This work explains the formation mechanism of a reverse vortex street in the wake behind the flapping airfoil in the forward thrust mode. The dominant contribution of the added mass force to the value of the propulsive force is detected. The obtained results reveal to some extent the hydrodynamic action mechanism of the tail fin of underwater animals.

Effects of the history (Basset) force on the dispersion of solid particles over a cylinder inside a two-dimensional channel flow associated with von Karman vortex shedding are investigated. Particles with Stokes numbers ( St k ) of 0.1, 0.5, 1.0 and 5.0 and particle-to-fluid density ratios of 1.1, 10 and 1000 are considered. Clustering (i.e. preferential concentration) of particles is studied qualitatively by visualising particle distributions, and quantitatively by analysing the preference of particles in sampling the vorticity field and by using Voronoϊ tessellation analyses. It is found that the effects of history force on the distribution of particles are noticeable only at moderate particle-to-fluid density ratios and Stokes numbers, though not very significant. Nonetheless, our results suggest that for flows with strong vortices the impact of the history force can be significant. It has also been observed for the first time that inclusion of the history force at high Stokes numbers, i.e. St k = 5.0 , can increase particle clustering at both moderate and high particle-to-fluid density ratios. At lower Stokes numbers, however, particle clustering decreases when the history force is taken into account. In general, the history force suppresses the influence of particle inertia in all the investigated cases. Finally, the deposition rate of particles, mainly on the front wall of the cylinder, decreases when the history force is included for all cases except when St k = 0.1 .

When the vortices shedding from an upstream rod impinge upon the surface of a downstream airfoil, the resultant vortex–body interaction noise can be significant. The use of “natural rod-base blowing” to reduce this interaction noise is investigated numerically using a 3D hybrid computational aeroacoustics approach. The natural blowing is generated through an internal slot that interconnects the stagnation and base region of the rod. Numerical simulation is performed for a straight blowing at blowing rates (BRs) between 6.4% and 16.3% and an oblique blowing with slot–incidence angles ( θ ) between 0 ° and 15 ° , respectively. Far-field noise evaluation demonstrates that the natural rod-base blowing under tested BRs can reduce significantly the noise emission, whereas loses its efficiencies gradually with the increase of angle θ . The most effective case is the straight blowing ( θ = 0 ° ) at BR ≥ 13.6% where the far-field tonal noise associated with the steady-periodic von Karman vortex shedding is annihilated. The changes in transient flow structures indicate that the natural blowing gradually attenuates the von Karman vortex shedding when BR increases, resulting in a mitigation of vortex–body​ interaction on the airfoil leading surface and hence a reduction of the unsteady lift. At BR=13.6%, as the angle θ reaches 15 ° from below, a new von Karman vortex street reforms in the flow field, leading to another tonal noise. Linear stability analysis based on a local concept of absolute/convective instability suggests that the absolutely unstable region in the near-wake of the rod elongates first and then shrinks with the increase of BR, while the local absolute growth rate decreases monotonously.

To perform flow-related behaviors in darkness, blind cavefish have evolved Lateral Line Systems (LLSs) with constriction canals to enhance hydrodynamic sensing capabilities. Mimicking the design principles, we developed a Canal-type Artificial Lateral Line (CALL) device featuring a biomimetic constriction canal. The hydrodynamic characterization results revealed that the sensitivity of the canal LLS increases with the decrease in the width (from 1 mm to 0.6 mm) and length (from 3 mm to 1 mm) of the constriction canal, which is in accordance with the modeling results of canal mechanics. The CALL device was characterized in Karman vortex streets generated by a cylinder in a laminar flow. The CALL device was able to identify the diameter of the cylinder, with a mean identification error of approximately 2.5%. It also demonstrated the identification ability of wake width using the CALL device, indicating the potential for application in hydrodynamic perception.

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.

The phase-averaged skin-friction analysis based on global luminescent oil film (GLOF) was conducted for periodically fluctuating unsteady phenomena at the frequency of approximately 150 Hz which is estimated based on Karman vortex shedding. An unsteady pressure transducer and a camera were synchronized, and the time-averaged and phase-averaged skin-friction fields were investigated. The time-series image pair data obtained by the camera were decomposed into eight intervals of a phase angle of $$\pi /4$$ with synchronizing the signal of the unsteady pressure. The phase-averaged result shows the periodical pattern corresponding to the vortices structure generated from the edge of the test model which was not resolved by the time-averaged result. The phase-averaged processing was successfully applied to the GLOF measurement, and the results showed the detail information of skin friction at each phase.

The flow past a wall mounted square cylinder, a typical and basic shape of building, bridge or offshore structure, was simulated using URANS computation through adoption of three turbulence models, namely, the k-e model, k-ω model, and the v2-f model. It is well known that this flow is naturally unstable due to the Karman vortex shedding and exhibits a complex flow structure in the wake region. The mean flow field including velocity profiles and the dominant frequency of flow oscillation that was from the simulations discussed earlier were compared with the experimental data observed by Wang et al. (2004; 2006). Based on these comparisons it was found that the v2-f model is most accurate for the URANS simulation; moreover, the k-ω model is also acceptable. However, the k-e model was found to be unsuitable in this case. Therefore, v2-f model is proved to be an excellent choice for the analysis of flow with massive separation. Therefore, it is expected to be used in future by studies aiming to control the flow separation.

A global luminescent oil-film (GLOF) image analysis method to estimate unsteady skin-friction fields in an unsteady flow field is proposed and demonstrated. A governing equation describing the dynamics of the oil film (the thin-oil-film equation) is employed for the unsteady oil-film images. The frequency response of the oil-film movement is analyzed, and a cutoff frequency is defined as a function of the oil-film thickness and the kinematic oil viscosity. The estimating skin-friction vector is defined along with a spatiotemporal weighted window and obtained by solving the overdetermined system of the thin-oil-film equation. The system can be solved by using the weighted linear least-squares method, and the time-resolved skin-friction field can be estimated. The time-resolved GLOF image analysis method is demonstrated on an experiment of a junction flow on a flat surface with a square cylinder. The GLOF images in the Karman vortex shedding bounding the flat surface were acquired, and the time-resolved skin-friction fields were obtained. The results showed that fluctuation in the skin-friction vectors corresponds to the shedding frequency, and the vortices bounding the surface were extracted. The averaged skin-friction field is compared with the result of the previous study based on the time-independent model. The normalized skin friction from both methods showed good agreement, which indicates that the quantitative value will be obtained when a calibration process is involved in a future study.

The aerodynamic noise generated by a circular cylinder in a crossflow is a classical problem in aeroacoustics, which can be found in various practical applications, such as parts of the landing gea...

This work presents a theoretical and experimental study of a flexible plate immersed in a von Karman vortex street. The wake is generated in a water flow using a cylindrical obstacle with a Reynolds number lower than 200. The vortices provoke oscillations of a flexible plate whose leading edge is clamped a few cylinder diameters downstream of the obstacle. The oscillation amplitude of the free edge is examined experimentally as the plate length is varied with respect to the wavelength. The value of the peak of the amplitude and the phase shift between the forcing vortices and the plate deflection are consistent with theoretical predictions. These predictions use an Euler-Bernoulli model for the motion of the plate produced by the pressure difference over the plate due to the combined effect of the vortex street and the deflection of the plate. The ratio between the plate length and the wake wavelength for which resonance occurs is fixed by the condition that the natural frequency of the plate is equal to the vortex frequency.

A square cylinder exposed to a fluid flow is regarded as the simplest model of the structures in sea water or air flow, and known to show the Karman vortex induced vibration (KVIV) and the galloping in separate or overlapping ranges of flow velocity. In this study, wind tunnel experiments were performed on the cross-flow vibration of a square cylinder, and effects of a downstream square plate on the vibration were investigated to develop a convenient passive technique for vibration control. The square plate has a side length w equal to the side length of the square cylinder D (= 26 mm) and set downstream with the gap ratio S/D in the range of 0.12 to 2.90 where S is the separation gap. The results show that the square plate has only little influence on KVIV and enhances the galloping, resulting in the lower critical reduced velocity of galloping Ug i.e. the onset velocity of galloping reduced to about half of the isolated counterpart at S/D = 0.98. Most remarkable effect of the downstream square plate is the enhancement of the vibration near the peak velocity of KVIV at S/D = 1.17, which is around five times larger as compared with the isolated counterpart. This may indicate occurrence of the wake-body-interference fluid elastic vibration.

Experimental studies are conducted on a rotationally oscillating cylinder with an attached flexible filament at a Reynolds number of 150. Parametric studies are carried out to investigate the effect of cylinder forcing parameters and filament stiffness on the resultant wake structure. The diagnostics are flow visualization using the laser-induced fluorescence technique, frequency measurement using a hot film, and characterization of the velocity and vorticity field using planar particle image velocimetry. The streamwise force and power are estimated through control volume analysis, using a modified formulation, which considers the streamwise and transverse velocity fluctuations in the wake. These terms become important in a flow field where asymmetric wakes are observed. An attached filament significantly modifies the flow past a rotationally oscillating cylinder from a Bénard–Kármán vortex street to a reverse Bénard–Kármán vortex street, albeit over a certain range of Strouhal number, $St_{A} \sim 0.25\text {--}0.5$ , encountered in nature in flapping flight/fish locomotion and in the flow past pitching airfoils. The transition from a Kármán vortex street to a reverse Kármán vortex street precedes the drag-to-thrust transition. The mechanism of unsteady thrust generation is discussed. Maximum thrust is generated at the instants when vortices are shed in the wake from the filament tip. At $St_{A} > 0.4$ , a deflected wake associated with the shedding of an asymmetric vortex street is observed. Filament flexibility delays the formation of an asymmetric wake. Wake symmetry is governed by the time instant at which a vortex pair is shed in the wake from the filament tip.

By using model vegetation (e.g., synthetic bars), vortex structures in a channel with vegetation patches have been studied. It has been reported that vortex structures, including both the vertical and horizontal vortexes, may be produced in the wake in the channel bed with a finite-width vegetation patch. In the present experimental study, both velocity and TKE have been measured (via Acoustic Doppler Velocimeter—ADV) to study the formation of vortexes behind four vegetation patches in the channel bed. These vegetation patches have different dimensions, from the channel-bed fully covered patch to small-sized patches. Model vegetation used in this research is closely similar to vegetation in natural rivers with a gravel bed. The results show that, for a channel with a small patch (Lv/Dc = 0.44 and Dv/Dc = 0.33; where Lv and Dv are the length and width of patch and Dc is the channel width, respectively), both the flow passing through the patch and side flow around the patch have a considerable effect on the formation of flow structures beyond the patch. The results of further analysis via 3D classes of the bursting events show that the von Karman vortex street splits into two parts beyond the vegetation patch as the strong part near the surface and the weak part near the bed; while the middle part of the flow is completely occupied by the vertical vortex formed at a distance of 0.8–1 Hv beyond the vegetation patch, and thus, the horizontal vortexes cannot be detected in this region. The octant analysis is conducted for the coherent shear stress analysis that confirms the results of this experimental study.

Recent advances in geostationary imaging have enabled the derivation of high spatiotemporal‐resolution cloud‐motion winds for the study of mesoscale unsteady flows. Due to the general absence of ground truth, the quality assessment of satellite winds is challenging. In the current limited practice, straightforward plausibility checks on the smoothness of the retrieved wind field or tests on aggregated trends such as the mean velocity components are applied for quality control. In this study, we demonstrate additional diagnostic tools based on feature extraction from the retrieved velocity field. Lagrangian Coherent Structures (LCS), such as vortices and transport barriers, guide and constrain the emergence of cloud patterns. Evaluating the alignment of the extracted LCS with the observed cloud patterns can potentially serve as a test of the retrieved wind field to adequately explain the time‐dependent dynamics. We discuss the suitability and expressiveness of direct, geometry‐based, texture‐based, and feature‐based flow visualization methods for the quality assessment of high spatiotemporal‐resolution winds through the real‐world example of an atmospheric Kármán vortex street and its laboratory archetype, the 2D cylinder flow., Key Points: Recently developed high‐cadence geostationary satellite winds enable the Lagrangian analysis of unsteady island wake flows. Good correspondence between Lagrangian Coherent Structures and observed cloud patterns indirectly confirms the fidelity of fluid dynamics. Discussion of benefits and pitfalls of common flow visualization techniques for the analysis of fluid dynamics., Swiss National Science Foundation, Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347, Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659, https://doi.org/10.5281/zenodo.3534276, https://www.avl.class.noaa.gov/, https://github.com/tobguent/vislcs-guadalupe

Numerical simulations of fluid flows can produce a huge amount of data and inadvertently important flow structures can be ignored, if a thorough analysis is not performed. The identification of these flow structures, mainly in transient situations, is a complex task, since such structures change in time and can move along the domain. With the decomposition of the entire data set into smaller sets, important structures present in the main flow and structures with periodic behaviour, like vortices, can be identified. Therefore, through the analysis of the frequency of each of these components and using a smaller number of components, we show that the Proper Orthogonal Decomposition can be used not only to reduce the amount of significant data, but also to obtain a better and global understanding of the flow (through the analysis of specific modes). In this work, the von Kármán vortex street is decomposed into a generator base and analysed through the Proper Orthogonal Decomposition for the 2D flow around a cylinder and the 2D flow around two cylinders with different radii. We consider a Newtonian fluid and two non-Newtonian power-law fluids, with n=0.7 and n=1.3. Grouping specific modes, a reconstruction is made, allowing the identification of complex structures that otherwise would be impossible to identify using simple post-processing of the fluid flow., This research was funded by FCT( Fundacao para a Ciencia e a Tecnologia I.P.), FSE (Fundo Social Europeu), NORTE2020, CEFT (Centro de Estudos de Fenomenos de Transporte) and by FEDER funds through COMPETE2020. Grants: PTDC/EMS-ENE/3362/2014, POCI-01-0145-FEDER-016665, UIDB/00013/2020, UIDP/00013/2020, UIDP/00532/2020, UIDB/00532/2020, SFRH/BD/143950/2019.

In this paper, vortex shedding and suppression are numerically investigated as autonomous and non-autonomous dynamical systems respectively. Lagrangian coherent structures (LCSs) are used as a numerical tool to analyze these systems. These structures are ridges of Finite time Lyapunov exponent (FTLE) which act as material surfaces that are transport barriers within the flow. Initially, the utility of LCSs is explored for revealing the coherent structures of these systems. Finally, an active flow control method, steady rotation is applied to the non-autonomous dynamical system with different speed ratios to mitigate vortex shedding magnitude. This will eventually turn the system into an autonomous system. Fixed saddle points, separation profiles essentially as unstable time variant manifolds attached to cylinder wall and evolution of other unstable manifolds with variant speed ratios are analyzed with reference to LCSs. It is revealed that speed ratio of 2.1 fully suppresses the von Karman vortex street at Reynolds number of 100 and system turns into an autonomous dynamical system with fixed saddle points and time-invariant manifolds.

International audience; A methodology is presented that exploits both data-assimilation techniques and resolvent analysis for reconstructing turbulent flows, containing organized structures,with an efficient set of measurements. The mean (time-averaged) flow is obtained using variational data-assimilation that minimizes the discrepancy between a limited set of flowmeasurements, generally from an experiment, and a numerical simulation of the Navier–Stokes equations. The fluctuations are educed from resolvent analysis and time-resolved data at a single point in the flow. Resolvent analysis also guides where measurements of the mean and fluctuating quantities are needed for efficient reconstruction of a simple example case study: flow around a circular cylinder at a Reynolds number ofRe = 100.For this flow, resolvent analysis reveals that the leading singular value,most amplifiedmodes, and themean profile for 47 < Re < 320 scalewith the shedding frequency and length of the recirculation bubble. A relationship between these two parameters reinforces the notion that a wave maker, forwhich the length scaleswith the recirculation bubble, determines the frequency and regionwhere an instability mechanism is active. The procedure offers away to choose sensor locations that capture the main coherent structures of a flow and a method for computing mean pressure by using correctly weighted resolvent modes.

A second order finite-difference numerical method is used to solve the Navier–Stokes equations of incompressible flow, in which the solid body with complex geometry is immersed into the fluid domain with orthogonal Cartesian meshes. To account for influences of the solid body, interactive forces are applied as boundary conditions at Cartesian grid nodes located in the exterior but in the immediate vicinity of the solid body. Fluid flow velocities in these nodes are reconstructed to track and control the deformation of the solid body, in which the local direction normal to the body surface is employed using the level-set function. The capabilities of this method are demonstrated by the application to fish swimming, and the computed behaviors of swimming fish agree well with experimental ones. The results elucidate that the ability of swimming fish to produce more thrust and high efficiency is closely related to the Reynolds number. The single reverse Kármán street tends to appear when both the Strouhal number and tail-beating frequency are small, otherwise the double-row reverse Kármán street appears. The algorithm can capture the geometry of a deformable solid body accurately, and performs well in simulating interactions between fluid flow and the deforming and moving body.

Simple design Savonius vertical-axis wind turbine can generate energy at low wind speed from any direction. However, its large static torque has a low power factor. Therefore, an innovation was made by providing 16 guide vanes around the shaft outside the blade with the angle is about 45° to a radial line. The specialty of guide vanes is that, they are able to concentrate the wind flow toward the turbine blade from any direction. The fluid motion around the turbine blade that produces torque on the turbine shaft was analyzed utilizing the Computational Fluid Dynamics (CFD) simulation and then verified by tracking actual fluid motion strings of threads attached on each side of the turbine blade. The result shows that without guide vanes the wind flow around the turbine blade generates vortex on the blade and Karman vortex at the downstream. These vortexes descend effectively kinetic energy in the wind flow so that the mechanical energy on the turbine shaft becomes small. At a certain blade position, the vortex becomes stronger and the fluid separation from the blade surface becomes thicker. The stronger vortex tends to descend stronger fluid kinetic energy while the thicker separation tends to reduce the lift on the blade. Consequently, these two flow conditions tend to produce negative torque. Installing guide vanes around the blade, the wind flows are concentrated by the guide vanes to the turbine blade, which effectively reduces vortex around the blade and blocks large vortex outside the guide vanes downstream. Flow separation is suppressed by the concentrated flow producing larger lift. As a result, the power factor increases by 61.6 %. This huge increase in power factor is achieved when the wind speed is 5 m/s though a stable turbine rotation is achieved at a lower speed, Простой вертикальный ветрогенератор Савониуса может генерировать энергию при низкой скорости ветра в любом направлении. Однако большой статический крутящий момент имеет низкий коэффициент мощности. Поэтому было сделано новшество, предусматривающее 16 направляющих лопаток вокруг вала снаружи лопасти с углом около 45° к радиальной линии. Особенность направляющих лопаток заключается в том, что они способны концентрировать на лопасти турбины ветровой поток в любом направлении. Движение жидкости вокруг лопасти турбины, которое создает крутящий момент на валу турбины, было проанализировано с использованием моделирования вычислительной гидродинамики (ВГД), а затем проверялось путем слежения за нитями фактического движения жидкости, прикрепленных с каждой стороны лопасти турбины. Результат показывает, что без направляющих лопаток ветровой поток вокруг лопасти турбины создает вихрь на лопасти и вихрь Кармана в нисходящем потоке. Эти вихри эффективно снижают кинетическую энергию в ветровом потоке, так что механическая энергия на валу турбины становится небольшой. При определенном положении лопасти вихрь становится сильнее, а отделение жидкости от поверхности лопасти становится толще. Более сильный вихрь имеет тенденцию снижать более сильную кинетическую энергию жидкости, в то время как более толстое разделение имеет тенденцию уменьшать подъемную силу на лопасти. Следовательно, эти два режима потока имеют тенденцию создавать отрицательный крутящий момент. При установке направляющих лопаток вокруг лопасти, ветровые потоки концентрируются направляющими лопатками на лопасти турбины, что эффективно уменьшает вихрь вокруг лопасти и блокирует большой вихрь снаружи направляющих лопаток в нисходящем потоке. Разделение потока подавляется концентрированным потоком, создающим большую подъемную силу. В результате коэффициент мощности увеличивается на 61,6 %. Это огромное увеличение коэффициента мощности достигается при скорости ветра 5 м/с, хотя стабильное вращение турбины достигается при меньшей скорости, Простий вертикальний вітрогенератор Савоніуса може генерувати енергію при низькій швидкості вітру в будь-якому напрямку. Однак великий статичний крутний момент має низький коефіцієнт потужності. Тому було зроблено нововведення, що передбачає 16 напрямних лопаток навколо вала зовні лопаті з кутом близько 45° до радіальної лінії. Особливість напрямних лопаток полягає в тому, що вони здатні концентрувати на лопаті турбіни вітровий потік в будь-якому напрямку. Рух рідини навколо лопаті турбіни, який створює крутний момент на валу турбіни, було проаналізовано з використанням моделювання обчислювальної гідродинаміки (ОГД), а потім перевірялося шляхом спостереження за нитками фактичного руху рідини, прикріплених з кожного боку лопаті турбіни. Результат показує, що без напрямних лопаток вітровий потік навколо лопаті турбіни створює вихор на лопаті і вихор Кармана в низхідному потоці. Ці вихори ефективно знижують кінетичну енергію в вітровому потоці, так що механічна енергія на валу турбіни стає невеликою. При певному положенні лопаті вихор стає сильніше, а відділення рідини від поверхні лопаті стає товще. Більш сильний вихор має тенденцію знижувати сильнішу кінетичну енергію рідини, тоді як більш товстий поділ має тенденцію зменшувати підйомну силу на лопаті. Отже, ці два режими потоку мають тенденцію створювати негативний крутний момент. При установці напрямних лопаток навколо лопаті, вітрові потоки концентруються напрямними лопатками на лопаті турбіни, що ефективно зменшує вихор навколо лопаті і блокує великий вихор зовні напрямних лопаток в низхідному потоці. Поділ потоку пригнічується концентрованим потоком, що створює велику підйомну силу. В результаті коефіцієнт потужності збільшується на 61,6 %. Це величезне збільшення коефіцієнта потужності досягається при швидкості вітру 5 м/с, хоча стабільне обертання турбіни досягається при меншій швидкості

Accurate and efficient prediction of separated transitional flows is always a big challenge for computational fluid dynamics (CFD), especially in engineering applications. The recently proposed mul...

Spatially periodic vortex systems that form due to unstable shear flows are called vortex streets. A number of exact and asymptotic solutions of two-dimensional hydrodynamic equations describing nonstationary vortex streets have been constructed. It is shown that the superposition of the flow with a constant horizontal shear and its perturbations in the form of a nonmodal wave provides an exact solution that describes a nonstationary vortex street with rotating elliptic current lines. The width of the zone occupied by such a vortex street has been determined. The equation of separatrix separating vortex cells with closed current lines from an external meandering flow has been obtained. The influence of the quasi-two-dimensional compressibility and beta effect on the dynamics of vortex streets has been studied based on the potential vorticity transport equation. The solutions describing the formation of vortex streets during the development of an unstable zonal periodic flow and a free shear layer have been constructed using a longwave approximation.