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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.

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.

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 ...

Karman vortices forming behind blades or guide vanes may induce severe vibration and noise in hydraulic machinery. To solve these problems, the trailing edge of vanes or blades is usually trimmed as a remedy. However, this type of modification is not always effective. Based on an engineering project, two sides of the tail of an asymmetric guide vane were modified respectively. The influence of the modification on the wake flow under different inflow conditions was studied using numerical simulation. The results show that, for the studied case, a 45°-bevel cut on the side of a vane with larger velocity gradient near the trailing edge can effectively suppress the Karman vortex. However, the effect of trimming on the side with smaller velocity gradient is poor. These two ways of trimming can reduce the amplitude of the vortex-excitation force by 73.6% and 35.4%, respectively. When guide vanes opening or head increases, the trimming effect is weakened. In addition, the inflow angle and velocity magnitude have a significant impact on the wake flow behind a hydrofoil. When the attacking angle decreases from 0° to –5°, the amplitude of the vortex-excitation force doubles. When the attacking angle increases to 5°, the Karman vortex street almost disappears. The strength of the Karman vortex is related to the difference between the velocity gradient on the two sides of a vane. The greater the difference, the more stable the wake flow.

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.

The flow around an staggered arrangement of seven cylinders is numerically studied using the finite element method to solve the two-dimensional and incompressible Navier—Stokes equations. The Reynolds number is 200 and the spacing ratios $$L{\text{/}}D$$ = 1.1–5. In accordance with different hydrodynamic characteristics and vortex street arrangements, three distinct forms of wake flow can be identified, namely, the single bluff-body pattern ( $$1.1 < L{\text{/}}D < 1.5$$ ), the deflected pattern ( $$1.5 < L{\text{/}}D$$ < 3), and the vortex impact pattern ( $$3 < L{\text{/}}D < 5$$ ). For the further in-depth study of the characteristics of different wake patterns, the variations of the hydrodynamic forces on seven cylinders and the underlying mechanisms are analyzed and the wake structures are presented. The results of the analysis show that the gap flow under different wake patterns has different effects on the change of the hydrodynamic characteristics. It is found that the change of the wake structure is closely related to the change of the wake pattern.

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.

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.

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.

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.

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

Purpose Two-dimensional incompressible fluid flows around a rectangular shape placed over a larger rectangular shape at low Reynolds numbers (Re) have been numerically analyzed in the present work. The vortex shedding is investigated at different arrangements of the two shapes allowing the investigation of three possible configurations. The calculations are carried out for several values of Re ranging from 1 to 200. The effect of the obstacle geometry on the vortex shedding is analyzed for crawling, steady and unsteady regimes. The analysis of the flow evolution shows that with increasing Re beyond a certain critical value, the flow becomes unstable and undergoes a bifurcation. This paper aims to observe that the transition of the unsteady regime is performed by a Hopf bifurcation. The critical Re beyond which the flow becomes unsteady is determined for each configuration. A special attention is paid to compute the drag and lift forces acting on the rectangular shapes, which allowed determining; the best configuration in terms of both drag and lift. The unsteady periodic wake is characterized by the Strouhal number, which varies with the Re and the obstacle geometry. Hence, the values of vortex shedding frequencies are calculated in this work. Design/methodology/approach The dimensionless Navier–Stokes equations were numerically solved using the following numerical technique based on the finite volume method. The temporal discretization of the time derivative is performed by an Euler backward second-order implicit scheme. Non-linear terms are evaluated explicitly; while, viscous terms are treated implicitly. The strong velocity–pressure coupling present in the continuity and the momentum equations are handled by implementing the projection method. Findings The present paper aims to numerically study the effect of the obstacle geometry on the vortex shedding and on the drag and lift forces to analyze the flow structure around three configurations at crawling, steady and unsteady regimes. Originality/value A special attention is paid to compute the drag and lift forces acting on the rectangular shapes, which allowed determining; the best shapes configuration in terms of both drag and lift.

The study contributes to the explanation of flow behaviour and the basis of phenomena existing during two-dimensional airflow around an ice-accreted cylinder, representing the section model of a bridge cable. The geometrical features and flow characteristics of the near-wake flow patterns of the iced cable were investigated within the Reynolds number range of 2.2·104–6.4·104 in low and moderately turbulent flow. The experimental procedure was conducted to create ice accretion on the cylindrical model. The shape of the ice was registered using a photogrammetry method. For the aerodynamic investigations the ice-accreted model was reproduced at a smaller scale by means of 3D printing. Representative snapshots of the flow field behind the stationary horizontal simulated iced cable were digitally analyzed by the Particle Image Velocimetry technique. The flow visualization provided quantitative data about the velocity and the direction of flow streams within both the near-wake and the sidewise regions of distributed flow around the model. Evidence of the existence of the vortex excitation process was obtained at three principal angles of attack. The characteristics of the vortex street and the location of the flow boundary layer separation points were recognized. The obtained results were compared with results for a smooth cylinder.

This work focuses on the temperature (passive scalar) and velocity characteristics within a turbulent Kármán vortex using a phase-averaging technique. The vortices are generated by a circular cylinder, and the three components of the fluctuating velocity and vorticity vectors, $u_{i}$ and $\unicode[STIX]{x1D714}_{i}$ ($i=1,2,3$), are simultaneously measured, along with the fluctuating temperature $\unicode[STIX]{x1D703}$ and the temperature gradient vector, at nominally the same spatial point in the plane of mean shear at $x/d=10$, where $x$ is the streamwise distance from the cylinder axis and $d$ is the cylinder diameter. We believe this is the first time the properties of fluctuating velocity, temperature, vorticity and temperature gradient vectors have been explored simultaneously within the Kármán vortex in detail. The Reynolds number based on $d$ and the free-stream velocity is $2.5\times 10^{3}$. The phase-averaged distributions of $\unicode[STIX]{x1D703}$ and $u_{i}$ follow closely the Gaussian distribution for $r/d\leqslant 0.2$ ($r$ is the distance from the vortex centre), but not for $r/d>0.2$. The collapse of the distributions of the mean-square streamwise derivative of the velocity fluctuations within the Kármán vortex implies that the velocity field within the vortex tends to be more locally isotropic than the flow field outside the vortex. A possible physical explanation is that the large and small scales of velocity and temperature fields are statistically independent of each other near the Kármán vortex centre, but interact vigorously outside the vortex, especially in the saddle region, due to the action of coherent strain rate.

The flow structure and the unsteady mechanism of the unsteady cavitating flow are reviewed in this paper. The flow patterns and structures in different cavitation regime, for the attached cavitation and the vortical cavitation, are shown with both the visualization and the quantitative information. The attached cavitating flow around the Clark-Y hydrofoil and the vortical cavitating flow around the Tulin hydrofoil are considered. In particular, the phenomena such as the large-scale vortex structure and the re-entrant flow associated with the cloud cavitation, and the cavitating vortex street’s forming and crumbling are described. The evolution of the cavitation structure in the transient sheet/cloud cavity forming, along with the cavity collapse induced by the re-entrant flow and the shock wave propagation are discussed. The perspective future research of higher fidelity simulations, and the accurate identifications of the cavitating vortex structure is commented.

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

Structures in crossflow are susceptible to vortex-induced vibrations (VIV) when the vortex-shedding becomes synchronised with the structural vibration. Strategies to control VIV often include modifying the separation point on the cylinder surface, such as adding helical strakes, although there remains disagreement regarding the mechanism by which these work. We explore the role of the separation point on VIV acting in the streamwise (drag) direction, by performing high-speed Particle-Image Velocimetry (PIV) measurements of the wake and the structural displacement of a range of cylinders with different cross-sectional shapes, including circular (no fixed separation points), equilateral triangles (fixed separation points) and elliptical cylinders (which act as an intermediate case). None of the non-circular cylinders are found to exhibit VIV, despite having approximately the same experimental conditions (mass ratio, structural damping, Reynolds number range, etc.) as the circular cylinders, which undergo VIV. The phase-averaged PIV measurements of the near wake of the circular cylinder are used to calculate the separation angle throughout the shedding cycle for different wake modes, and it is shown that all the wake modes that are associated with VIV require a periodic movement of the separation point. In contrast, the variation in the separation angle was negligible for the von Karman vortex street observed behind near-stationary circular cylinders and for all non-circular cylinders. The experiments illustrate the great sensitivity of the wake mode and streamwise VIV to modifications of the separation point and demonstrate that even a moderately elliptical cylinder (major to minor axis ratio of 1.54) is sufficient to completely suppress VIV.

Fascinating wake vortex patterns emerge when a circular cylinder is forced to vibrate laterally to a uniform fluid flow, deviating from the well-known Karman vortex street and first reported by Williamson & Roshko (J. Fluids Struct., vol. 2, 1988, pp. 355–381). The two rows of single vortices (2S mode) can suddenly transition to a row of paired vortices and a row of single vortices (P+S mode) as the forcing amplitude is increased. Further increase in amplitude finds another sudden jump back to the 2S mode. Through a series of elegant and carefully crafted numerical simulations, Matharu et al. (J. Fluid Mech., vol. 918, 2021, A42) determine that the transitions occur via bifurcations, but that underlying these observed ‘jumps’, a continuous evolution of the vortex street between the modes is seen along unstable branches connecting the two modes. As the Reynolds number decreases from 100, bistability and the P+S mode are eventually suppressed.

Turbulent-ribbed channels are extensively used in turbomachinery to enhance convective heat transfer in internally cooled components such as turbine blades. One of the key aspects of such a problem is the distribution of the heat transfer coefficient (HTC) in fully developed flows, and many studies have addressed this problem by the use of computational fluid dynamics (CFD). In the present document, large eddy simulation (LES) is performed for a configuration from a test-rig at the Von Karman Institute representing a square channel with periodic square ribs. The whole channel is computed in an attempt to better understand HTC maps in this specific configuration. Resulting mean and unsteady flow features are captured, and predictions are used to further explain the obtained HTC distribution. More specifically turbulent structures are seen to bring cold gas from the main flow to the wall. A statistical analysis of these events using the joint velocity-temperature probability density function (PDF), and quadrant method allows to define four types of events happening at every location of the channel and which can then be linked to the HTC distribution. First, the HTC is very high where the flow impacts the wall with cold temperature whereas it is lower where the hot gas is ejected to the main flow. In an attempt to link the HTC trace on the channel wall with structures in the flow field far-off the wall, the main modes are identified performing power spectral density (PSD) analysis of the velocity along the channel. Dynamic mode decomposition (DMD) of the flow field data is then used to present the spatio-temporal characteristics of two of the identified most dominant modes: a vortex-street mode linked to the first rib and a rib-to-rib mode appearing because of the quasi-periodicity of the configuration. However, DMD analysis of the HTC trace on the wall does not emphasize any dominant mode. This indicates a weak link between the main flow large scale features and the instantaneous and more local HTC distribution.

Three-dimensional (3-D) wake transition for flow past a diamond cylinder is investigated numerically. Detailed 3-D direct numerical simulations (DNS) show that the wake is represented by mode A with global vortex dislocations for Reynolds numbers Re = 121–150, followed by a mode swapping between modes A and B for Re = 160–210 and increasingly disordered mode B for Re ≥ 220. In the mode swapping regime, different characteristics of the dislocation and non-dislocation periods are revealed by decomposing flow properties (e.g. the root-mean-square lift coefficient) into the values corresponding to the dislocation and non-dislocation periods. Such decomposition helps to explain some major differences observed for the cases of a diamond and a circular cylinder. In addition to DNS, Floquet stability analyses are conducted to identify the 3-D wake instability modes of a diamond cylinder up to Re = 300. Phase-averaged base flow is used to eliminate the quantitative uncertainties induced by the aperiodic secondary vortex street of the base flow. Interestingly, a subharmonic instability mode is identified at Re ≥ 285, whereas mode B is absent. The origin of the subharmonic mode is explained. The disagreement between the DNS and the Floquet analysis regarding the existence of mode B and the subharmonic mode is also explained. It is found that the natural 3-D flow involves complex interactions between the streamwise and spanwise vortices, as well as between the 3-D wake transition and the two-dimensional base-flow transition, which excite mode B and suppress the subharmonic mode.

This study identifies and explains the formation mechanisms (FMs) of the secondary vortex street in the far wake of a circular cylinder. Direct numerical simulation and transient growth analysis are performed for a Reynolds number (Re) in the range of 100–200. Unlike most of the earlier studies, which have attributed the formation of the secondary vortices to either the hydrodynamic instability of the mean flow (FM1) or a merging of the two-layered vortices (FM2), the present study demonstrates that both FM1 and FM2 are at play. Specifically, FM1 is observed for Re = 150–160, whereas FM2 is observed for Re = 200. Interestingly, both FM1 and FM2 exist (in an alternate manner) for Re = 170 and 180. The underlying physical mechanism controlling the manifestation of either FM1 or FM2 is explored. In short, secondary vortices that emerge after and before the annihilation of the two-layered vortices are formed from FM1 and FM2, respectively. The variation in the streamwise locations for the emergence of the secondary vortices with Re is quantified and physically explained. Fundamentally, the emergence of the secondary vortices through either FM1 or FM2 is induced by the convective instability of the shear layers in the intermediate wake, which is further induced by an obvious increase in the shear rate of the shear layers as the wake gradually transitions from a primary vortex street to a two-layered vortex street. The general conclusions drawn from a circular cylinder are expected to be applicable to other bluff bodies.

The onset and physical patterns of gap instability (GI), characterised previously as an inviscid, Kelvin–Helmholtz instability, were investigated experimentally and numerically in an eccentric annular channel with an inner-to-outer diameter ratio . The gap vortex street generation mechanism and development was analysed and an improved physical model was proposed.

The influence of weak low-frequency actuation on the three-dimensional turbulent wake of a cantilevered triangular prism of aspect ratio 4 protruding a thin turbulent boundary layer is investigated experimentally at a Reynolds number of 12 000. Results are reported for surface pressure measurements on the leeward face and wake of the obstacle and particle image velocimetry for selected planes in the wake. Zero-net-mass-flux actuation (sinusoidally modulated synthetic jets) is used to excite the flow through two slits spanning the obstacle height along the edges of the leeward face. Vortex shedding lock-on is shown to occur over intervals corresponding to subharmonics of the actuation frequency. The synchronization mechanism is identified, where weak perturbations due to actuation at critical stages of the Karman vortex formation trigger shedding. A phenomenological model is presented, linking the concepts of vortex formation time and circulation transport, to describe lock-on phenomena for one-sided and symmetric two-sided actuation. The model further describes interactions with the synthetic jet leading to the splitting of shed vortices observed in earlier studies. Similarities to results observed for other geometric and actuation configurations suggest a broader relevance of the proposed model and highlight differences between weak and strong forcing.

A set of large eddy simulations for the free surface backward facing step (FSBFS) are carried out to study wave formation behind the step. The volume-of-fluid ghost fluid method is employed to capture the free surface. Previous studies have indicated that the wave physics depend on the step draught-based Froude number (Fr). For small Fr, the rear face of the step (transom) becomes wet, while for large Fr, the wave separates smoothly from the transom. Close to a critical Fr separating wet and dry transoms, both conditions may occur. Here, we study wet, critical, and dry conditions based on the Fr classification with three different inflow boundary layer profiles (ReL=1,2,3×106). For Fr = 1.75 (wet conditions), we observe a weak dependence on the ReL. A proper orthogonal decomposition of the velocity field at Fr = 1.75 shows a coherent vortex street forming beneath the free surface. At Fr = 2.66 (critical conditions), we observe that an increase in the ReL results in a decrease in the wavelength and pronounced gas entrainment due to wave breaking. For Fr = 3.17 (dry conditions), we also observe shorter wavelength at increased ReL. Further, in the dry conditions, a breaking wave is noticed to occur at higher ReL, while breaking waves are not observed for the smallest studied ReL. Based on the results, we conclude that the wave shape for FSBFS cannot be characterized by the Froude number alone.

Two degree-of-freedom (2-DoF) flow-induced vibrations (FIV) of two rigidly coupled cylinders are numerically investigated at the Reynolds number of 3 . 6 × 1 0 6 . Two-dimensional (2D) Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations are performed combined with the k − ω SST turbulence model. A low mass-damping system is considered with a mass ratio of 2 and a damping ratio of zero. The diameter ratio is set to 0.25. The influence of the reduced velocity ( U r ), the position angle ( α ) of the small cylinder relative to the large cylinder and the gap ratio ( G / D ) between the cylinders on the FIV response of the system are analyzed. The simulations are performed for 2 ≤ U r ≤ 12 . To analyze the effect of α , three values are considered, [ α = 0 ° , 90 ° , 180 ° ] with a constant value of G / D = 0 . 1 . The effect of G / D is studied for the α = 90 ° configuration in which the G / D = 0 . 25 and G / D = 0 . 5 configurations are analyzed in addition to the G / D = 0 . 1 configuration. It is found that the lock-in regime extends beyond U r = 12 for the α = 0 ° , 180 ° and α = 90 ° at G / D = 0 . 1 and 0.25 configurations compared with the single cylinder configuration. Extended lock-in range is observed when the small cylinder is placed at α = 90 ° , with G / D = 0 . 1 , and at α = 180 ° . In addition, for the α = 90 ° configuration, the results obtained for C ¯ D , C ¯ L and A y , m a x / D converge to those of the single cylinder with the increase of G / D . A 2T shedding mode is observed for the single cylinder at 6 . 5 ≤ U r ≤ 10 and for the α = 0 ° configuration when U r ≥ 8 . For the α = 90 ° and 180 ° configurations, the wake patterns are irregular. With the increase of G / D , a longer vortex street behind the small cylinder is observed, which creates many vortical structures in the wake of the cylinders.

This paper explores an idealized model of the ocean surface in which widely separated surface-wave packets and point vortices interact in two horizontal dimensions. We start with a Lagrangian which, in its general form, depends on the fields of wave action, wave phase, stream function and two additional fields that label and track the vertical component of vorticity. By assuming that the wave action and vorticity are confined to infinitesimally small, widely separated regions of the flow, we obtain model equations that are analogous to, but significantly more general than, the familiar system consisting solely of point vortices. We analyse stable and unstable harmonic solutions, solutions in which wave packets eventually coincide with point vortices (violating our assumptions), and solutions in which the wave vector eventually blows up. Additionally, we show that a wave packet induces a net drift on a passive vortex in the direction of wave propagation which is equivalent to Darwin drift. Generalizing our analysis to many wave packets and vortices, we examine the influence of wave packets on an otherwise unstable vortex street and show analytically, according to linear stability analysis, that the wave-packet-induced drift can stabilize the vortex street. The system is then numerically integrated for long times and an example is shown in which the configuration remains stable, which may be particularly relevant for the upper ocean.

In this work, the flow over an elliptic cylinder near a moving wall is investigated for Reynolds numbers less than 150. Here, the ratio between the gap (i.e., the distance between the cylinder and the wall) and the length of the semi-major axis of the elliptic cylinder varies from 0.1 to 5. This ratio is hereafter denoted as the gap ratio. The resulting Kármán vortex street, the two-layered wake, and the secondary vortex street have been investigated and visualized. Numerical simulations show that for the steady flow, the wake is composed of two asymmetric recirculation vortices, while a decrease in the gap ratio suppresses the vortex shed from the lower part of the cylinder. For the unsteady flow, the wake can be classified into four different patterns based on the wake structures (the Kármán vortex street, the two-layered wake, and the secondary vortex street). The regions of these wake patterns are given in the gap ratio and Reynolds number space, showing that the critical Reynolds number for the transition between different patterns increases as the gap ratio decreases. An overall increase in the mean drag coefficient with increasing gap ratios is observed, except for a sudden drop that occurs within a small gap ratio range. Moreover, as the gap ratio increases, the onset location of the two-layered wake first decreases due to a decrease in flow velocity in the gap and then increases due to the weakening of the wall suppression effect. This is the authors’ accepted and refereed manuscript to the article. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in (citation of published article) and may be found at (URL/link for published article abstract).

Inertial point particles suspended in a two-dimensional unsteady circular cylinder flow at R e = 100 are studied by one-way coupled three-dimensional numerical simulations. The striking clustering pattern in the near-wake is strongly correlated with the periodically shed Karman vortex cells. The particles are expelled from the vortex cores due to the centrifugal mechanism and coherent voids encompassing the local Karman cells are therefore observed. The particles clustering at the upstream side of each void hole form a smooth edge, where the particle velocity magnitude is consistently lower than at the downstream edge of the voids. The trajectories of these particles originate from the side of the cylinder where the sign of vorticity is opposite to that of the vortex encompassed by the corresponding void hole. The particles are seen to decelerate along a substantial part of their trajectories. Particle inertia is parameterized by means of a Stokes number S k and smooth edges around the void holes still exist when S k is increased, although their formation is delayed due to larger inertia. Increasing inertia contributes to a decoupling of the particle acceleration from the slip velocity, which almost coincided at S k = 1 .

We examine two dimensional properties of vortex shedding past elliptical cylinders through numerical simulations. Specifically, we investigate the vortex formation length in the Reynolds number regime 10 to 100 for elliptical bodies of aspect ratio in the range 0.4 to 1.4. Our computations reveal that in the steady flow regime, the change in the vortex length follows a linear profile with respect to the Reynolds number, while in the unsteady regime, the time averaged vortex length decreases in an exponential manner with increasing Reynolds number. The transition in profile is used to identify the critical Reynolds number which marks the bifurcation of the Karman vortex from steady symmetric to the unsteady, asymmetric configuration. Additionally, relationships between the vortex length and aspect ratio are also explored. The work presented here is an example of a module that can be used in a project based learning course on computational fluid dynamics.

Vortex flow around multiple columns of finite length is ubiquitous in engineering. The present work focuses on the basic fluid physics in terms of the vortex shedding flow patterns and their dependence on structural configurations and flow parameters. Though widely documented in the literature, there is no consensus on certain aspects of the wake characteristics immediately behind the obstacles for a multi-column structure at a relative high Reynolds number range. A comprehensive set of numerical simulations has been conducted to investigate the flow interactions with four square section shaped columns in a diamond configuration, which is complimented by experiments using particle image velocimetry and force measurements in a physical model with Reynolds numbers varying from 3.7 × 10 4 to 6.0 × 10 4 . Horizontal structural members called pontoons were added near the end of the columns to alter the interactions with the surrounding fluid. This work reveals further insights of the fluid physics including the interactions of the vortex shedding processes due to the multi-columns and pontoons. The pontoons are seen blocking the vortices shed from the free end of the column by pushing the recirculation region further away from the free end of each column. In addition to the vortex shedding period being increased, further examination of the wake region indicates that the vortex street tends to be tidier and more structured by adding the pontoons to a basic multi-column structure. The findings will lead to better understanding in vortex shedding fluid physics and improved design in new offshore structure development such as deep-draft semi-submersibles and tension leg platforms.

We present the first application of an artificial neural network trained through a deep reinforcement learning agent to perform active flow control. It is shown that, in a two-dimensional simulation of the Kármán vortex street at moderate Reynolds number ($Re=100$), our artificial neural network is able to learn an active control strategy from experimenting with the mass flow rates of two jets on the sides of a cylinder. By interacting with the unsteady wake, the artificial neural network successfully stabilizes the vortex alley and reduces drag by approximately 8 %. This is performed while using small mass flow rates for the actuation, of the order of 0.5 % of the mass flow rate intersecting the cylinder cross-section once a new pseudo-periodic shedding regime is found. This opens the way to a new class of methods for performing active flow control.

In this study, the numerical analysis of unsteady cavitating turbulent flow behind a two-dimensional wedge-shaped body is performed using the commercial program STAR-CCM+ as a part of a fundamental study on the control fin of supercavitating underwater vehicles. We explore the vortex structures in the near and far wake fields and investigate the effect of cavity growth on the periodic characteristics of wake flow (σ = 1.0 ~ 2.0). Pressure fluctuations above the wedge are converted to sound pressure levels in the frequency domain via the fast Fourier Transform. As a result, we confirm that the shedding frequency of the vortices behind the body is strongly affected by the development of cavitation. As the cavitation number decreases, the frequency of the vortex in the near wake region decreases, and the force accelerating the Karman vortex in the far wake region decreases. In addition, we clearly validate the wake flow characteristics of a two-dimensional wedge-shaped body by comparing our numerical results with the experimental results carried out at the Chungnam National University Cavitation Tunnel (CNU-CT) at three different cavitation numbers (σ = 1.3, 1.5, and 2.0). Observations using a high-speed camera and measurements of pressure fluctuation above the test model are carried out to demonstrate the wake flow characteristics.

Direct numerical simulations were conducted to investigate the effect of two parameters, density ratio and laminar flame speed, on the conditions of the onset of local extinction and blow-off of lean premixed flames, stabilized on a meso-scale bluff-body in hydrogen-air and syngas–air mixtures. A total of six simulation cases were considered as isolated comparison of the two parametric effects of the fluid dynamic instability and flame time scale. For all cases under study, the general flame development towards the blow-off limit showed a sequence of five distinct modes, with possible cyclic patterns among the different modes for a range of velocity conditions. The onset of local extinction was observed during the asymmetric vortex shedding and vortex street mode. As the density ratio is decreased, the flow inunder reviewstability is promoted through the increased sinuous mode, and such behavior was properly scaled by the Strouhal number. Although the blow-off velocity is altered by the fluid dynamic effects, the condition for the onset of local extinction and blow-off was mainly dictated by the competition between flow residence time associated with the lateral flame motion and ignition delay of the local mixtures. Time scale analysis supported the validity of the findings across all the cases investigated.

In the present study, flow behind a square cylinder is investigated using control rods. The control rods are placed in the shear layer and studied using particle image velocimetry, hotwire anemometry and flow visualization techniques. Experiments are conducted in a low-speed wind tunnel for three different Reynolds numbers 485, 805 and 1290. However, detailed analysis of flow is only presented at a Reynolds number of 485. The aim is to modify the wake behind a square cylinder for possible suppression of vortex Street and drag reduction. The control rod diameter is kept constant as 0.2 times of the cylinder diameter. Depending upon the gap between control rods and the cylinder, the flow structure, vortex dynamics, the distance of the shear layer roll up behind the cylinder is modified. An attempt is made to correlate the different flow structures with corresponding drag forces. Several parameters such as time-averaged drag coefficient, Strouhal number, recirculation length, fluctuating velocity, turbulence intensity, and Reynolds stress are measured. To understand the flow phenomena and its intricate structure, flow visualization images are also captured in the same setup with smoke visualization. Maximum up to 22% drag reduction is observed at a particular control rod position.

Dry galloping has been observed on in-service bridges and has been reproduced in several wind tunnel experiments of inclined stay cables. In certain cases, the large-amplitude vibrations caused by dry galloping could not be mitigated with damping levels specified by the Post Tensioning Institute (PTI). Wind tunnel investigations were conducted for IHI Corporation on a 1:1 scale sectional model of an inclined bridge stay cable in the 3 m × 6 m Wind Tunnel at the National Research Council Canada in 2015. The purpose of the investigation was to reproduce large amplitude cable galloping, which had been observed at an existing cable-stayed bridge in Japan. The experiment was designed to investigate the influence of different damping levels and the ability of a 5 mm-diameter helical fillet to mitigate dry cable vibrations. The current work identified several important factors that contributed to dry galloping and large-amplitude cable motions. It was shown that the physical mechanisms leading to the onset of large-amplitude motion in the critical and supercritical Reynolds number regimes are distinct. In the critical Reynolds number regime, the drop in drag, increase in lift, and fluctuations in the laminar separation bubbles along the cable destabilize the low pressure lobes, allowing the changes in pressure along the cable to become synchronized with the cable motion in a highly-correlated manner. This type of galloping could not be mitigated with damping greater than that required by the PTI. At high Reynolds numbers, the boundary layer state changed and was associated with the upstream movement of the separation point and an asymmetric pressure distribution, Karman vortex shedding re-emerged and appeared to combine with low-frequency variations to induce large-amplitude motion. The large-amplitude motion at high Reynolds numbers could be mitigated with additional damping.

Purpose When a reflected shock interacts with the boundary layer in a shock tube, the shock bifurcation occurs near the walls. Although the study of the shock bifurcation has been carried out by many researchers for several decades, little attention has been devoted to investigate the instability pattern of the bifurcation. This research work aims to successfully capture the asymmetry of the whole flow field, and attempt to achieve the instability mechanism of the shock bifurcation by a direct numerical simulation of the reflected shock wave/boundary layer interaction at Ma = 1.9. In addition, the reason for the formation of the bifurcated structure is also explored. Design/methodology/approach The spatial and temporal evolution of the shock bifurcation is obtained by solving the two-dimensional compressible Navier–Stokes equations using a seventh-order accurate weighted essentially non-oscillatory (WENO) scheme and a three-step Runge–Kutta time advancing approach. Findings The results show that the formation of shock bifurcation is mainly because of the shock/gradient field interaction, and the height of the bifurcated foot increases with the growth of the shock intensity and the gradient field. The unsteady asymmetry of the upper and bottom shock bifurcated structures is because of the vortex shedding with high frequency in the rear recirculation zone, which leads to the fluctuation of the recirculation area. The vortex shedding process behind the bifurcated structure closely resembles the Karman vortex street formed by the flow around the cylinder. The dimensionless vortex shedding frequency varies between 0.01 and 0.02. In comparison to the scenario at Ma = 1.9, the occurring time of instability is delayed and the upper and bottom bifurcated feet intersect in a relatively short time at Ma = 3.5. The region behind the bifurcated shock is a transitional flow field containing obvious cell structures and “isolated islands.” Originality/value This paper discovers an unsteady flow pattern of the shock bifurcation, and the mechanism of this instability in the reflected shock/boundary layer interaction is revealed in detail.

A wall-resolved Large Eddy Simulation (LES) is performed for a loosely packed bare rod bundle with Reynolds number of 54,650. The simulation includes heat transfer in a liquid metal with the Prandtl number of 0.016. The rod bundle configuration corresponds to the ALFRED lead-cooled reactor design. The ALFRED design employs nuclear fuel assemblies with triangular arrangement of the fuel rods. In the present study, two important thermal-hydraulics aspects of the ALFRED rod bundle configuration have been analysed. Firstly, the proposed design should not lead to any undesirable vibration issues and, secondly, the heat transport should assure sufficient cooling with no hot spots. The obtained flow and thermal fields are analysed and compared with available data. A detailed investigation of the flow field has revealed the existence of a relatively weak gap vortex street in the loosely packed rod bundle. Hence, this considered configuration is unlikely to cause any vibration issues. In addition, the observed thermal field has shown a specific logarithmic trend, representing the heat transfer in wall bounded flow configurations for a wide range of Peclet numbers. In general, no hot-spots are observed for the considered design parameters.

The turbulent flow in the subchannels around an outer rod of a scaled up (12.9:1) model of a 60° section of the CANDU reactor 37-rod bundle with a pitch-to-diameter ratio of 1.149 was documented for three bulk Reynolds numbers (50,000, 100,000 and 130,000). Reported results include measurements of the mean axial velocity, the Reynolds stresses, the turbulent kinetic energy, the anisotropy tensor components, the integral length scale, the Taylor microscale, the Kolmogorov microscale, the wall shear stress, the friction factor and the characteristics of the gap vortex street forming in the vicinity of a rod-wall gap. In general, normalised turbulence properties showed some weak dependence on Reynolds number, whereas the normalised Strouhal number, convection speed and wavelength of the vortex street were insensitive to the Reynolds number. The gap vortex street was found to form fairly close to the rod-bundle inlet, which was fitted with an endplate. The objective of this work was to elucidate the effect of Reynolds number on the turbulence properties and the vortex streets that form in tightly-packed rod bundle flows. The reported measurements have been documented in sufficient detail to serve as a reliable database for the validation of computational fluid dynamics analyses of such flows.

Herein, experiments were carried out with two connected circular cylinders that are free to rotate around a pivot in different arrangements including both cylinders on the downstream, both on the upstream and a cylinder on each side of the pivot while the other cylinder is at the pivot point. The gap between the two cylinders is varied from 0 to 4.9D (D being the diameter of the cylinder) and the center of gravity (cg) is varied from −2.5D to +2.5D from the pivot point. The hydrodynamic forces on the cylinders are measured with a load cell and the surrounding flow is measured simultaneously using particle image velocimetry (PIV). The Reynolds number is varied in the range of 500 R e 2 . 4 × 1 0 4 during the test to find the maximum possible displacement amplitude for each configuration. Then, an spectral analysis is performed on the measured velocity field to have information about the vortex shedding frequency of the unsteady flow in the gap region and cylinder wake. When the gap between the cylinders (g) is zero, the cylinders behave like a single bluff body (SBB) in cross-flow. However, depending on the arrangement, the wake patterns are different. If both cylinders are at the downstream side of the pivot, the cylinders act as a single body with an extended surface and alternating vortexes shed behind the cylinders in each cycle forming a single vortex street on the wake side (SBBV). Sliding both cylinders to the upstream side shifts the vibration mechanism to the single body galloping region (SBBG). The shear layer behind the two cylinders is stretched along the fluid flow and vortexes are formed at a greater distance downstream comparing to the SBBV. Having one cylinder on the pivot changes the vibration to a typical wake induced vibration if the other cylinder is at the downstream. If the other cylinder is at the upstream, the vibration is similar to a single cylinder system but the cylinder on the pivot disturbs the wake and changes the stagnation point on the upstream cylinder. When both cylinders are at an equal distance from the pivot point, three major flow fields are observed. For larger gaps (g > 3.9D), there is no reattachment of the shear layers downstream and two synchronized vortex streets form behind the two cylinders. By decreasing the gap, the downstream cylinder starts to interact with the vortexes shed from the upstream cylinder. If 2.5D g 3.9D, the gap flow pattern is split-gap where the vortexes split inside the gap before reaching the downstream cylinder. As the gap decreases, vortex excitation (VE) is less effective and gap-switching induced vibration (GSIV) dominates the vibration mechanism. Reducing the gap to G 0 . 9 turns the split-gap flow to an entire-gap flow pattern that forms a sharp velocity gradient inside the gap region and increases the total lift force dramatically on the system which maintains a larger vibration amplitude.

This paper proposes a new generalized semi-empirical model to simulate fluid–structure interaction forces experienced by a transversely oscillating square-section cylinder by a more comprehensive approach. The total fluctuating transverse aerodynamic forces determined by this new modeling approach are composed of three main unsteady excitation modules. The first component relates to the conventional fluid inertia effect; the second component represents the varying incidence angle effect, including the wavelength-changing effect of downstream wake undulation by introducing a circulation lag function; and the last one denotes the excitation effect due to the strong resonant interaction between wake undulation and Karman vortex shedding. In addition, the new model establishment process follows the “simple-to-use” principle, so it can be applicable even when few experimental data are available. Good agreements are observed between the predictions by this model and unsteady force coefficient results taken from harmonically forced vibration experiments. Moreover, the new model developed in this study can not only simulate the interaction effects between the vortex-induced vibration (VIV) and galloping, but also retain the predictive capability for the cases of pure vortex resonance and pure galloping phenomena. Finally, some significant findings are also presented in the context of the aeroelastic instability phenomena.

The wavy cylinders have been proven effective in suppressing the Karman vortex shedding and mitigating the aerodynamic forces. As yet their performance in the inclined incoming flow has not been well understood. In the current work, large eddy simulations are carried out to unveil the aerodynamics of the flow around inclined wavy cylinders at the Reynolds number of 5000. Three inclination angles, 0 ° , 30 ° and 45 ° , together with 2 × 2 combinations of shape parameters, namely λ ∕ D m = 2 and 6, a ∕ D m = 0 . 1 and 0.15, have been taken into consideration. Firstly, the aerodynamic force coefficients in terms of both the span-wise averaged and sectional values are discussed at length. It is revealed that the growing inclination angle invites not only a surge in their span-wise averaged values, but also an enlargement in the sectional difference of the force coefficients. The span-wise correlation of the lift force is found to be enhanced for the wavy cylinders with the increase of inclination angle, while the opposite is true for the normal cylinders. By examining the mean wake properties, it is disclosed that the increase in the span-wise averaged drag force coefficients is closely related to the shrinkage in the vortex formation length and the regression of the base pressure, whereas the sectional distribution of the drag coefficients is largely affected by the stagnation pressure. The mean wall shear stress fields are exploited to shed light on the flow topology of the cylinder surfaces. It is discovered that the wavy cylinders, especially the short-wavelength one, exhibit significant span-wise variation in the separation structure in the inclined flow. Besides, the chaotic surface flow in the rear side of the non-inclined cylinders could be regulated to maintain symmetry by the secondary axial flow in the near wake of the inclined cylinders. The instantaneous three-dimensional vortical structures are visualized by the Q isosurfaces at last to collaborate the previous discussions. A preliminary mechanism for the cessation of flow control efficacy for the wavy cylinders in inclined flow is also presented.

This paper focuses on the characteristics of the regime C flow (Tatsuno & Bearman, J. Fluid Mech., vol. 211, 1990, pp. 157–182) around an oscillating circular cylinder in still water. The regime C flow is characterised by the formation of large-scale vortex cores arranged as opposed von Kármán vortex streets, resulting from a regular switching of vortex shedding directions with respect to the axis of oscillation. Both Floquet analysis and direct numerical simulations (DNS) are performed to investigate the two- (2-D) and three-dimensional (3-D) instabilities. The present study reveals that the low-wavenumber 3-D instability can emerge slightly before the 2-D instability in regime C. In total, five spanwise vortex modes were identified: (i) standing-wave pattern, S-mode; (ii) travelling-wave pattern, T-mode; (iii) mixed ST-mode; (iv) X-type vortex pattern, X-mode; and (v) U-type vortex pattern, U-mode. The modal analysis conducted in this study demonstrates that the vortex patterns and the corresponding spatial and temporal modulations of the dynamic loads of the S-, T- and mixed ST-modes are mainly induced by the 3-D instability of a single wavenumber. The characteristics of the X-mode are due to the superposition of the 3-D instabilities of multiple wavenumbers. The U-mode is dominated by a 2-D instability and its interaction with 3-D instabilities. The domain size dependence study demonstrates that the regime C flow is very sensitive to the spanwise length of the computational domain. The subcritical nature of the regime C flow is responsible for the discrepancy in the marginal stability curves obtained by independent Floquet stability analysis, DNS and physical experiments.

A wind tunnel study has been carried out to investigate flow control around a hollow circular cylinder using passive jets driven by naturally occurring pressure differences. Flow enters the cylinder through spanwise holes along the stagnation line and exits through a spanwise distribution of holes at$\pm 65^{\circ }$. The diameter of the entry and exit holes were 1 % and 0.5 % of the cylinder diameter, respectively. Reynolds numbers were at the upper end of the subcritical regime and ranged from$3\times 10^{4}$to$2.8\times 10^{5}$. Jet spacings of 10 % and 20 % of the cylinder diameter were investigated, and the ratio of the average jet exit velocity to the cross-flow velocity at the boundary layer edge was found to rise to approximately 0.35 and 0.4, respectively, above a Reynolds number of$1.5\times 10^{5}$. Findings based on using the surface oil flow technique revealed a repeating, organised cellular pattern downstream of adjacent jet exit holes consisting of a primary counter-rotating vortex pair structure, followed by a secondary weaker pair. Downstream of adjacent exit holes, and centred midway between them, there exists a separation bubble which delays final flow separation compared with the flow directly downstream of a jet. The variation in the angular position of boundary layer separation across the span had the effect of suppressing von Kármán vortex shedding. This resulted in a drag coefficient, at the upper end of the Reynolds-number range studied, 14.5 % lower than that found using trip wires to initiate boundary layer transition.

The aerodynamic damping of forward and backward inclined prisms has been identified through a series of forced vibration wind tunnel tests. The test results show that the amplitude of vibration and reduced wind velocity both have a significant effect on the aerodynamic damping of the inclined prisms, especially at reduced wind speeds where von Karman vortex lock-in occurs. The effect of forward and backward inclination on the characteristics of aerodynamic damping in inclined prisms is dissimilar to a vertical prism, and the characteristics have been discussed in terms of the unsteady aerodynamic force and Strouhal number in this study. This study not only advances our understanding on aerodynamic damping of inclined prisms, but also provides values that can be used to estimate aerodynamics and aeroelasticities of inclined prisms.