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Three-way coupled numerical simulations of particle-laden fluid flow around a square obstacle are performed by a coupled finite-volume and discrete-element method, taking account of the collisions between particles. The fluid flow is described by the Eulerian formalism, while the Lagrangian formalism is used for the solid particles. The dynamics of each phase and the modulation mechanism of the flow field by the inertial particles are explored. The Reynolds number based on the side length of the obstacle is 100, which leads to a typical periodic flow pattern of vortex shedding. Under the action of particles, the flow tends to be chaotic even in the upstream region, and the vortex shedding occurs earlier. The intensity of the vortex is attenuated due to particle dissipation. At sufficiently small Stokes numbers, the particles exhibit flow tracer behavior with relatively high fidelity. However, as the Stokes number and mass loading increase, the interaction between the particles and fluid becomes important, especially in the core region of the channel. With the increase of the particle response time, the spanwise velocity fluctuation is attenuated, while the influence on the streamwise velocity fluctuation seems complicated. No particles are observed to hit the rear surface of the obstacle or deposit in the domain during the flow. When the Stokes number and mass loading are elevated from 0.0036 and 0.065 to 0.5 and 9.024, respectively, the distribution of the particles near the channel centerline varies from being filled in the shedding vortices to surrounding them. Meanwhile, the particle-free zones centered on these vortices become larger. For the Stokes number and mass loading up to 1.5 and 27.073, respectively, the particles are scattered in the downstream area, and the flow pattern of the Karman vortex street is modified. Within the range of the Stokes number considered, the drag and lift coefficients, and the Strouhal number also show a certain trend of variation.

The fan installed in the outdoor unit of air-conditioning (AC) systems is often partially shrouded to increase the inlet area and allow the installation of a larger upstream heat exchanger. However, the effect of radial inflow from the unshrouded part on the aerodynamic performance of the fan is seldom studied. The current paper studies the flow structure in a partially shrouded fan to understand the effects of radial inlet flow on the main flow and the flow in the tip region. The results showed that the flow from the unshrouded region has the same scale as the axial inflow in a partially shrouded fan. The flow field and tip leakage vortex development in the partially shrouded fan are substantially different and the loss corresponding to the TLV is intensified compared to the fully shrouded one. In a partially shrouded fan, the TLV rolls up adjacent to the tip section due to the interaction with radial inflow and follows the tip chord in the unshrouded part. The TLV then moves away from the suction surface in the shrouded part inducing a large blockage and loss at the fan outlet. A vortex street is created outside the fan by the exiting TLV which restricts the main flow from radial expansion and increases the fan backpressure. The reverse flow near the shroud is mainly driven by the TLV and can be controlled by changing the TLV trajectory.

Numerical simulations are carried out to study the flow around two tandem square cylinders (SC) under the effect of spacing ratio(g/D) and splitter plate length (l/D) for a fixed Reynolds number (Re) = 100. The g/D is varied from 0 to 10 and l/D is varied from 0.5 to 10. The splitter plate length is found to have strong effect on vortex shedding and fluid forces. The maximum reduction in mean drag coefficient is observed at l/D = 8, that is 15% and 78% for upstream and downstream cylinders, respectively. The maximum reduction in root-mean-square value of lift coefficient is found at l/D = 10, that is 99%. The flow pattern at both of these points is steady flow. There is 100% vortex shedding suppression for l/D > 5. The observed flow patterns for flow past tandem cylinders without splitter plate are; single bluff body (SBB), steady flow (SF), quasi-steady flow (QSF), fully developed flow (FDF) and fully developed two-row vortex street flow (FDTRVS) regimes. SBB, QSF and SF regimes were observed in presence of splitter plate.

Numerical simulations of flow-induced vibration on isolated and tandem elliptic cylinders with varying reduced velocities are carried out. Immersed boundary multi-relaxation-time lattice Boltzmann flux solver is used as numerical solution method so that the solution procedure can be conducted in a Cartesian grid. The accuracy and rationality of this method are verified by comparison with previous numerical results. For vortex-induced vibration on isolated elliptical cylinder, numerical simulations are conducted for different aspect ratio (0.7 ≤ A R ≤ 1.5) and reduced velocities (4.0 ≤ U r ≤ 10.0). Vibration response mainly contains desynchronization regime, initial branch and lower branch. When reduced velocity drives to lower branch, transverse amplitude reach to the peak value and double vortex street is formed in the wake. For flow-induced vibration on two elliptical cylinders in a tandem arrangement, numerical simulations are conducted for different aspect ratio (0.7 ≤ A R ≤ 1.5) and reduced velocities (3.0 ≤ U r ≤ 10.0) at gap spacing L/D = 3 and 6. Vibration and flow characteristics are more complex compared with a single elliptical cylinder. The beginnings of entering into lock-in region are delayed for both bluff bodies and vibration response of downstream elliptical cylinder is enhanced by coupling interaction at higher reduced velocity. The flow and vibration characteristics of upstream elliptical cylinder are close to that of single elliptical cylinder at larger gap spacing. The tandem system is beneficial to gain more energy for higher reduced velocity. When downstream elliptical cylinder drives into lower branch, the transverse amplitude is the largest and double vortex street is formed in the wake region.

It was experimentally observed that the front of a propagating flame of a well-mixed diluted methane-oxygen mixture at 298 K and 100–300 Torr does not form von Karman vortex shedding behind the obstacle of cylindrical shape of 30–50 mm in diameter, including a perforated cylinder; however, the instability under the same conditions occurs in the flow of hot products. In the perforated cylinder, the occurrence of local primary ignition centers on its inner surface is observed. In the mathematical modeling, the main observed features of the flame front propagation were taken into account: the chain branched mechanism of gaseous combustion and the absence of vortex shedding behind the obstacle at flame propagation. It was shown that a qualitative model of compressible dimensionless non-reactive/reactive Navier–Stokes equations in low Mach number approximation yields both the mode of the emergence of von Karman instability in chemically inert gas and the absence of the instability in the mode of flame propagation in a reacting flow. The model computations confirmed the occurrence of local primary ignition centers on the inner surface of the obstacle.

Flow over a traveling wavy foil attached with a flexible plate has been numerically investigated using the lattice Boltzmann method combined with the immersed boundary method. The influence of the flexibility and length of the caudal fin on the locomotion of swimming fish through this simplified model, whereas the fish body is modeled by the undulating foil and the caudal fin by the plate passively flapping as a consequence of fluid-structure interaction. It is found that the plate flexibility denoted by the bending stiffness, as well as the length ratio of tail and body, plays an important role in improving thrust generation and propulsive efficiency. It is also revealed that there exists a parameter region of the plate length and stiffness, in which positive propeller efficiency can be achieved. The effect of the passively flapping flexible plate on the pressure field and the vortex production on the wake is further discussed. It is found that when the length ratio of caudal fin and body is greater than 0.2, a reverse von Karman vortex street occurs when the bending stiffness is about greater than 1.0, and a great thrust is generated as a result of a large pressure difference occurring across the flexible plate. This work provides physical insight into the role of the caudal fin in fish swimming and may inspire the design of robotic fish.

The fluid-structure interaction (FSI) in case of fish swimming in the vortex street is investigated by numerical simulations. The vortex street is generated by a D-section cylinder. A 2-D fish model is placed on the downstream centerline of the bluff cylinder at a distance of 4 diameters away from the center of the cylinder. To simulate the fish body undulation and movement, the moving mesh is generated by a coupling approach based on the radial basis function and the overset grid technology. The Navier-Stokes equation in the arbitrary Lagrangian-Eulerian form is solved by coupling with the kinematics equation. Three cases are investigated: in a stationary position without deformation, a passive locomotion without deformation, and an active deformation based on the Karman gait model. The results indicate that the fish body is acted by an alternating force and moment when it is located in the centerline of the vortex street. Furthermore, the fish could extract sufficient kinetic energy to overcome the drag under suitable conditions even when it keeps rigid and out of the suction zone. When the fish body undulates based on the Karman gait model, the interaction is evidently shown between the fish body and the vortices. The theoretical analysis demonstrates that the lateral force and the moment acting on the fish body vary in a cosine formula, with the lateral translation and the body rotation as a result. This study focuses on the behavior of the fish body in the bluff cylinder wake and reproduces some phenomena observed in the experiments. Besides, the Karman gait model is also theoretically analyzed, for the further exploration of the FSI mechanism in case of fish swimming.

Physical experiment and direct numerical simulation are employed to study a transitional flow of viscous fluid past a spanwise circular cylinder mounted near the wall of a rectangular channel. The Reynolds number varies in the range of 60 to 480. Spiraling tornado-like motion of fluid directed from the sidewalls toward the center of the channel behind the cylinder is discovered. The evolution of this motion with the Reynolds number is outlined. Formation of large-scale vortical structures (analogue of Karman vortex street) and their turbulent breakdown in the cylinder wake are described. The behavior of the drag coefficient of the cylinder and the nearest wall with the Reynolds number is revealed.

The fluid motion characteristics have an important influence on the evolution of the backflow vortex near the pipe wall. The instantaneous velocity and instantaneous temperature of the outlet flow ...

The Kármán Vortex Shedding is one of the special types of vortex that is generated from asymmetric flow separation. For many years engineers tried to suppress the vortex shedding as it brings unnecessary motion to the static members inside the flow field. A converter model is designed and studied to harness the energy associated with this vortex shedding and convert it into usable form rather than suppressing it. It is a bluff body placed on the free stream incurring vortex-induced vibration and giving out a swinging pendulum motion. This motion is utilized to produce electricity. The model is analyzed on the free stream of water and conversion efficiency of 8.9% is achieved. A theoretical formula is derived regarding the force acting on the bluff body during the motion. Various parameters such as aspect ratio, flow velocity, lock-in delay, frequency of oscillation, etc. as well as their relations are studied by simulating the model in ANSYS FLUENT 18.1 for different configurations. From the simulated results it is obvious that as the lift force on the bluff body increases, more power generation is possible. Also, the experimental results paved the way for further study for practical large-scale implementation of the converter.

A novel model is proposed to estimate the optimal propulsive efficiency and the optimal dimensionless heaving velocity of a flapping foil based on a lift–drag ratio under the assumption of negligib...

This work demonstrates the viability of a novel transverse-gust generator that is capable of producing a controllable time-varying gust without increasing the turbulence level within a large region...

The variable flexibility of a fish body is believed to play a significant role in improving swimming performance. To explore the effect of non-uniform flexibility on the motion performance of fish under biologically relevant conditions, we set up three different flexible distribution modes for a semi-free filament and compared the motion performance of different flexible distribution modes through numerical simulations. The filament is located in the wake of the front flapping foil; it can swing adaptively in the lateral direction according to the flow situation of the surrounding fluid and finally reach a stable position. The results show that the motion state of the filament will alter with a change in the flexibility of the filament, from moving in the vortex street to moving on the side of the vortex street. In the Benard-von Karman (BvK) vortex streets, the drag coefficient of the filament increases as the flexibility of the filament increases, and the value of the drag coefficient is at a minimum when the flexibility of the filament increases linearly along the length of the filament. Further investigation indicates that at 85%–90% of the filament length (starting from the leading edge), the flexibility of the filament begins to increase significantly, and the filament can obtain its best propulsion performance. The results of this work provide new insights into the role of non-uniform flexibility during the process of fish movement and provide a valuable reference for the design of bionic underwater vehicles.

The aerodynamic characteristics are vital for short cylindrical Terminal Sensitive Bullets (TSB) with low aspect ratio, especially in terminal trajectory. Currently, there is little research in terms of the TSB and short cylinder with two free ends, and particularly in this trajectory, where the scanning angle β and roll angle α vary over a broad range between 0° and 180°. In this work, wind tunnel experiments are first conducted to learn the effects of Reynolds number and scanning angle on aerodynamic parameters for short cylinder with aspect ratio L / D = 1 . Similar to infinite cylinder, for the short cylinder with two free ends, the drag crisis phenomenon still exists in the critical regime 1.7 × 10 5 ≤ R e ≤ 6.8 × 10 5 . Then 3D simulations are performed to demonstrate the aerodynamic characteristics of short cylinder and TSB over a broad range of R e , L / D , α and β . The sensitivity analysis of time step and grid are presented as well. When β = 0 ° , for short cylinder, the drag crisis phenomenon was also observed in the simulation, but not as obvious as in the wind tunnel test. In some attitudes, there is an obvious Karman vortex in the wake of short cylinder and TSB. The correlation between time-averaged aerodynamic coefficients and L / D , R e , α & β is discussed. The vortex shedding frequency and shear layer behavior are obtained for quasi-steady and unsteady flow. Finally, the effect of end’s shape on drag reduction and vortex shedding frequency is analyzed.

Purpose) In this study, for the first time, the efficacy of control rods for full suppression of vortex-induced vibrations (VIV) and galloping of an elastically supported rigid square cylinder that vibrates freely in the cross-flow direction is investigated. Design/methodology/approach To this aim, two small control rods are placed at constant angles of ± 45° relative to the horizontal axis and then the influence of diameter and spacing ratios on the oscillation and hydrodynamic response along with the vortex structure behind the cylinder is evaluated in the form of nine different cases in both VIV and galloping regions. Findings The performed simulations show that using the configuration presented in this study results in full VIV suppression for the spacing ratios G/D = 0.5, 1 and 1.5 at the diameter ratios d/D = 0.1, 0.2 and 0.3 (D: diameter of square cylinder, G: distance between rods and cylinder, d: diameter of rods). On the contrary, a perfect attenuation of galloping is only achieved at the largest diameter (d/D = 0.3) and the smallest spacing ratio (G/D = 0.5). In general, for both VIV and galloping regions, with increasing diameter ratio and decreasing spacing ratio, the effect of the control rods wake in the vortex street of square cylinder gradually increases. This trend carries on to the point where the vortex shedding is completely suppressed and only the symmetric wake of control rods is observed. Originality/value So far, the effect of rod control on VIV of a square cylinder and its amplitude of oscillations has not been investigated.

The dripping of molten thermoplastics is a widely observed phenomenon in cable and facade fire, where the large drips can often carry a blue chain flame during the free fall to ignite other flammable materials and escalate the fire hazard. This work simulated the flame evolution behind a falling thermoplastic drip with the DNS model and finite-rate flame chemistry. The accelerated free-fall of drip was modeled by fixing the position of drip, increasing the upward airflow, and setting a fuel jet on the top of the drip. Modeling reproduces the dripping flame and reveals the flame shedding to be a combination of a lifted flame and a vortex street, where the lifted flame caused by the gravity acceleration of drip is identified as the critical factor that governs the shedding formation. As the diameter of drip decreases, the falling drip becomes difficult in forming a stable shedding structure in the wake region, so that the dripping extinction occurs due to the dilution and cooling of airflow, agreeing well with the experimental observation. This work reveals the underlying mechanism of stabilizing the dripping flame and helps evaluate the fire risk and hazard of dripping phenomena.

The present study is a revisit to Ludwig Prandtl’s elimination of von Karman vortex street behind a circular cylinder by using steady suction in the boundary layer. We show in particular the full-time vortex evolutions and vortex dynamics in the wake. The wind tunnel investigations are conducted at the Reynolds number (Re) of 2.0 × 104. Slot suction is implemented symmetrically on both lower and upper boundary layers of the cylindrical test model to modify the flow separation process and to eliminate the downstream vortex street. The boundary layer suction is described by the dimensionless momentum parameter of suction relative to the free-stream airflow. A high-speed particle image velocimetry system is employed in the present study to visualize the flow patterns and capture the wake dynamics of the natural and modified cylinders controlled with boundary-layer suction.

The wake dynamics behind a sinusoidally pitching custom designed symmetrical airfoil is analyzed computationally. The effect of pitching frequency and pitching amplitude is investigated independently at a constant Reynolds number. The wake patterns are found to exhibit a transition from von Karman to reverse von Karman vortex street and from symmetric reverse von Karman vortex street to asymmetric deflected wake as established. For laminar flow under investigation, the contours of time-varying velocity fluctuations are presented. The mean profiles of velocity and the fluctuations at different locations in the wake show that the thrust wakes are generated at the expense of energy of the surrounding fluid. The temporal behavior of the flow field is analyzed using the phase plot. The time series plots of force coefficients show a periodic loading on the airfoil. Proper orthogonal decomposition (POD) analysis of drag, neutral and thrust wakes displayed the evolution of coherent structures in the flow field. The evolution of these structures portray the transitions in the wake of the airfoil. The merging of coherent structures in an asymmetric deflected wake turns out to be an efficient way of identifying the location of deflection.

A numerical study about a droplet impacting onto a deep pool is presented here by employing the Volume of Fluid method to track the interface. In this investigation, two open source solvers, named Gerris and Basilisk, are used to solve the incompressible Navier–Stokes equations with free surface. The results are compared to provide some reference for the researchers involved in this research community. Firstly, the capabilities of the two solvers in simulating the droplet impacting problems are validated against available experimental results. And the adaptive mesh refinement techniques used in the two solvers are also found to be applicable. When simulating the drop impacting problems in an axisymmetric coordinate system, it is found that less meshes are produced in Gerris than that in Basilisk when similar adaptive criterion is adopted in both solvers. However, Basilisk still shows much higher computational efficiency due to its great superiority at parallelization, and thus less CPU time is required for such unsteady problems. When simulating moderate Reynolds number impacting problems, the two numerical solvers show perfect agreement ( R e ≤ 5000 ), however, since higher Reynolds numbers ( R e ≥ 5000 ) require smaller size of mesh in vicinity of the thin impacting region and the concentrated vortex region, it is hard for Gerris to obtain satisfactory results because of its poor computational efficiency. Thus, with such a high parallel capability and computational efficiency, Basilisk is introduced to solve a large number of cases. It is summarized that the flow pattern of the drop impacting problem can be classified to three categories. They are smooth ejecta sheet, main vortex shedding and Von Karman vortex street. Each flow pattern relates to the vortex evolution inside the liquid closely. Basililsk also shows its potential in solving much more complicated three-dimensional problems. They cannot be completed by Gerris owing to its poor computational efficiency.

A study of vortex dynamics is essential in understanding the behaviour of the forces acting in a flow. This is because super positioning of vortex flow on other elementary flows results in generation of finite lift forces. The flow past a triangular cylinder is turbulent. Vortex structures dominate this type of flow. A study of vortex dynamics of this type of flow will reveal the physics involved in the flow. The vortex dynamics of near wake region is responsible for the generation of von Karman Vortex Street. As a result, the vortex flow structures in the near wake region of 60° and 75° triangular cylinders were studied numerically using LES. The study was performed at Re = 520 in the intermediate range. A system of interdependent vortices was discovered in the near wake region. This vortex system exists behind both cylinders. The primary and secondary vortex strengths have sinusoidal variation. This variation is similar in both cases. However, the time-period and amplitude are directly proportional to apex angle. A mathematical model of the variation was presented. This model is applicable to both cylinders. The study also indicated that the time-period and amplitude of the vortex strengths are mathematically related.

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.

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.

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.

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.

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.

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.

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.

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.

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.

A majority of fish and other aquatic animals generate thrust by oscillating their fins and flukes, which creates a highly three-dimensional and complex wake that is characterized by spanwise compre...

The vortex dynamics of thrust and lift generation for periodically plunging airfoils and finite wings at low Reynolds numbers is reviewed. Effects of vortex lock-in phenomenon, leading-edge vortex,...

This work explores the aerodynamics of an airfoil oscillating in a uniform-shear flow at chord Reynolds number of approximately 1.2×104 using complementary two-dimensional Navier–Stokes computation...