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Flow around a cylinder is an important research problem in fluid mechanics. However, studies on traveling wave wall (TWW) flow control for the wake generated by the flow around a 3D cylinder remain limited. This study performs large eddy simulation (LES) to investigate the wake characteristics of flow around a three-dimensional (3D) circular cylinder with the TWW. The influence of the main control parameters of TWW on the aerodynamic forces and wake of the 3D cylinder is analyzed. The wake characteristics of the 3D cylinder at Re = 4 × 104 are obtained, and the traveling wave propagating downstream is achieved on the rear surface of the 3D cylinder using dynamic mesh. The control effects of wave amplitude, number of waves, and wave velocity of the traveling wave on the aerodynamic forces of the cylinder are analyzed, and the optimal control parameter combination is determined. The results demonstrate that the TWW control method completely eliminates the spanwise 3D flow characteristics of the cylinder when the amplitude ratio, number of waves, and velocity ratio are 0.02, 4, and 1.5, respectively. Moreover, it effectively suppresses the flow separation on the cylinder surface, eliminates the wake vortex, and suppresses vortex-induced vibration (VIV). [ABSTRACT FROM AUTHOR]

We experimentally investigate the effect of material, length, and spacing of trailing-edge extensions on controlling the flow over an airfoil based on our recent experimental work. Force measurements and flow field quantifications were carried out to investigate the aerodynamic performance and flow structures in the wake of an airfoil and, thus, to reveal differences in control effectiveness and mechanisms. Moreover, multi-scale proper orthogonal decomposition and spectral proper orthogonal decomposition are employed to extract coherent flow structures in the flow field. The results indicate that the owl feather can improve the aerodynamic performance, while artificial materials lead to decreased lift-to-drag ratio. However, nylon has optimal adaptability and robustness in controlling turbulent fluctuations, including Reynolds stress and turbulent kinetic energy at different angles of attack (AOAs). The length sensitivity is highly associated with the AOA, i.e., the optimal length increases with the increase in AOA. In addition, the spacing sensitivity correlates with the Reynolds number (Re), i.e., the optimal spacing decreases with higher Re at high AOA. These differences root in the competition effect between the increasing adverse pressure gradient and the interference on regular vortex shedding. It is concluded that nylon with mediate length (L = 0.2D) and relatively large spacing (S = 0.5B) is recommended for wake control and noise attenuation of the S833 airfoil. [ABSTRACT FROM AUTHOR]

Porous coating and blowing jets are both effective flow control methods for a bluff body. In the present study, we conducted wind tunnel experiments to investigate the combined control effects on a circular cylinder. The flow control was achieved by active steady blowing flows through the structured porous surface on the leeward side of the cylinder. The Reynolds number Re in the experiments, based on the cylinder outer diameter, was 1.0 × 10 4 . The control effects were evaluated by a non-dimensional blowing momentum coefficient C μ , which was determined by various blowing mass flow rates, incoming wind speed, and the geometry of the porous surface. Reduced-order models, including proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD), were employed to analyze the wake stabilization effects of the secondary jet flows. We found that, under the control of secondary flows ejected from the porous region of the cylinder, POD modal characteristics in the global flow wake were changed; temporal and spatial properties of DMD transformed; frequency and mode of the vortex shedding process shifted; statistical turbulent flow characteristics ameliorated; and the estimated drag coefficients restrained. Experimental results in the present study demonstrated that the secondary flow ejected from the structured porous surface and the resultant small-scale vortices could stabilize the cylinder wake with proper C μ values. [ABSTRACT FROM AUTHOR]

In the current research, a small circular rod is placed upstream of the main cylinder. This is used to change the incoming flow and, thus, impact the vortex shedding process of the wake as well as the resultant aerodynamic forces. The experiments are carried out in a wind tunnel with a subcritical Reynolds number of Re = 32 000. The sectional pressure distribution around the main cylindrical model is obtained to calculate unsteady aerodynamic forces. The experimental results show that the gap ratio markedly affects the aerodynamic forces exerting on the main cylinder. The optimal gap ratios for drag reduction are found to be G/D = 1.5 and 2.0, where the average drag and unsteady lift forces exerting on the main cylinder are suppressed. In addition to pressure measurements, we also apply the high-speed particle image velocimetry system to obtain flow characteristics of the cylinder wake as well as the gap flow. Two different modes of gap flow are found, i.e., the cavity mode and the wake impingement mode, and the transition value between them is around G/D = 1.5. Moreover, at the appropriate gap ratio, the wake vortex shedding pattern of the main cylindrical model changes from antisymmetric to symmetric. [ABSTRACT FROM AUTHOR]

We experimentally investigated the attenuation of the von Kármán vortex street behind a circular cylinder by the use of active suction through a structured porous surface in wind tunnel tests. The Reynolds number Re, based on the outer diameter of the cylinder D, is set to be 1.0 × 10 4 . The structured porous surface of the test model is made of resin by the three-dimensional printing technique to obtain well-organized porous structures. The active suction control effectiveness is quantified by a non-dimensional suction coefficient CQ, which is determined by the suction flow rate Q, the porous structure of the cylinder's surface, and the free stream velocity U ∞ . A high-speed particle image velocimetry measurement system is utilized to acquire global and detailed wake flows behind the baseline and controlled cylinders. In addition to directly perceived time-averaged characteristics, flow analysis methods including proper orthogonal decomposition, spectral analysis, and linear stability analysis are employed to study the underlying nature of cylinder wakes with and without distributed suction control. Experimental results show that multi-scale coherent structures in the cylinder wake are homogenized and the near wake is regularized under the control of suction. With a proper CQ, the alternating vortex street behind the circular cylinder is found to be greatly attenuated and the vortex shedding mode completely switched. [ABSTRACT FROM AUTHOR]

• An active jet produced by an air velocity regulator installed on a box girder is used to alleviate the unsteady aerodynamic force and alternative vortex shedding. • Significant mitigation of vortex-induced vibration (VIV) on a box girder is achieved. • A coupled model is successfully developed to predict the VIV of a box girder with an active jet. Owing to the availability of the jet flow control strategy in mitigating strongly alternating vortex motion and shedding in the wake flow structure, an active jet produced by an air velocity regulator installed on a box girder is proposed to alleviate the fluctuating aerodynamic force imposing on a fixed box main girder model. The pressure distributions on the top and bottom surfaces of the deck's two sections were recorded using a digital miniature pressure scanner system. The investigation manifests that the fluctuation of the outer surface pressure (OSP) distribution of the deck is alleviated, and the mean value is stable in all test cases. Wavelet coherence analysis of the OSP between two sections on the deck was performed to determine the relationship between the surface pressures varying with frequency over time. Based on the OSP distribution, the aerodynamic force was obtained to macroscopically display the availability of the active jet. Moreover, an active jet was applied to a free-vibrating box girder model to study its ability in suppressing vortex-induced vibration (VIV). The results of the oscillation response obtained by a laser displacement apparatus show that the box girder with the active jet has a lower vibration amplitude, and the VIV can be entirely suppressed when the non-dimensional jet momentum coefficient reaches a certain value. In addition, a coupled model of VIV was developed to predict the vibration response of the deck. The calculated results of the vibration response of the deck obtained by the coupled model are close to those of the experiments. [ABSTRACT FROM AUTHOR]

This study experimentally investigated an active flow control method with combined jets at the front and the rear stagnation points of a square cylinder to suppress the unsteady wake flow. The Reynolds number (Re) was 1.67 × 10 4 , based on the incoming speed of airflow and the diameter of test model. The square cylinder model was manufactured with two narrow slots symmetrically positioned at the centerline of the front and rear surfaces. The strength of the jets is characterized with a dimensionless momentum coefficient C μ . We obtained the dynamic wake flow regimes by employing the particle image velocimetry technique. Then, with the method of proper orthogonal decomposition and linear stability analysis, the time-averaged flow characteristics, e.g., turbulence kinetic energy (TKE) and the Reynolds shear stress (RSS) distributions, and the dynamic wake flow behind the square cylinder were analyzed in detail. Results of flow visualization suggested that at low momentum coefficient C μ the wake flow regime showed no notable modifications to the wake. As C μ increased to 0.0948, the periodic shear layers from the square cylinder were found to be pushed to the farther wake. Meanwhile, the time-averaged wake flow region was found to be greatly modified in the streamwise direction with a notable decrease in TKE and RSS distributions. The experimental results indicated that unsteadiness of vortex shedding in the wake flow experienced notable suppression. For higher C μ up to 0.2133 and 0.3793, unsteady vortex shedding from the square cylinder and the dynamic wake flow were further suppressed in the near wake. A linear stability analysis was also employed to reveal the underlying nature of wake modification by the combined jets. [ABSTRACT FROM AUTHOR]

A parametric study of the passive-suction-jet control scheme was experimentally performed for the cylinder model. The dependence of the control effectiveness on the gap thickness was determined for a cylinder model controlled by passive-suction-jet pipes at Reynolds number (Re) values ranging from 20,000 to 60,000. The fluid forces and flow structure were measured on a cylinder model without any control and used to evaluate the control effectiveness. Proper orthogonal decomposition (POD) analysis, phase average, and triple decomposition were applied to the flow fields to explore the control mechanism. The results show that the pipe control was most effective when the gap thickness was 0.075 D. Too large or too small a gap thickness was not beneficial for controlling the resistance level, and the fluctuating lift was reduced to a tiny level as the thickness increased to 0.075 D. The steady jets ejected from the leeward slots rolled up into several small-scale standing vortices. These vortices modified the streamline topology of the model, developing a new saddle point in the wake. Moreover, the roll-up of the separated shear layers was restrained by the pipe, which inhibited the vortex formation and shedding processes and the unsteady forces exerted on the model. • A passive suction jet scheme for controlling the flow past a circular cylinder was investigated. • The influence of the thickness of passive suction jet pipes on the control effectiveness was analyzed. • Aerodynamic forces and wake velocity distributions were determined in a wind tunnel. • The velocity field was analyzed using POD analysis, phase-average method, and triple decomposition scheme. [ABSTRACT FROM AUTHOR]

We carry out wind tunnel investigations to study the flow of a circular cylinder modified with two rigid splitter plates hinged along its stagnation points. The equal-sized and symmetrically placed splitter plates are both parallel to the incoming airflow, and their single-sided length in the streamwise direction varies from 0 to 2.0D (where D is the cylinder diameter). The wind tunnel experiments are conducted at the Re of 3.33 × 104. In addition to bilaterally arranged plates, two other configurations of splitter plates, i.e., front-plate-only and rear-plate-only, are also investigated. By employing the sectional measurement of surface pressure in the midspan slice, we evaluate typical aerodynamic parameters, including pressure distribution, instantaneous drag and lift forces, frequency spectra of the unsteady lift forces, mean drag, and root-mean-square lift coefficients acting on the cylindrical test models. A particle image velocimetry (PIV) system is used to visualize and quantify the vortex shedding process and the dynamic interactions of the natural and modified cylinders. Experimental results of the surface pressure measurement and PIV measurement results are then combined to reveal the effects of rigid plates with different configurations (bilateral, front-only, and rear-only) on the circular cylinder flow. [ABSTRACT FROM AUTHOR]

Highlights • The first, second and third mode VIVs of a flexible cable model are reproduced and the frequency lock-in phenomenon is observed for different modes of VIVs. • It is feasible to reproduce multi-mode RWIVs of a flexible cable in a wind tunnel. • Water rivulets on the cable surface excite RWIVs, and higher-mode RWIVs develop at higher wind speeds. • Local distributions of upper rivulet water flow excite multi-mode RWIVs. • RWIVs and VIVs may have causal links. Abstract We reproduce multi-modal vortex-induced vibrations (VIVs) and multi-modal rain–wind-induced vibrations (RWIVs) of an inclined and yawed cable in wind tunnel tests. The flexible cable model has low mass and low damping. First, the cable model is kept dry and exposed to uniform airflow; it experiences first-, second- and third-mode VIVs with the increase of wind speed. The structural responses of VIVs are analysed and the frequency lock-in phenomenon is observed for different modes of VIVs of the flexible cable. In addition to VIVs of the dry cable, RWIVs are excited by guiding water rivulets on the cable surface from a water tube. The first-, second- and third-mode RWIVs of the flexible cable are observed and identified at a much higher range of incoming wind speeds than that of the VIVs. To further explore the origin of the multi-modal behaviours of RWIVs, the upper rivulet is guided and restricted to form locally along the cable. Experimental results reveal that the higher- and multiple-mode RWIVs can be excited by a local rivulet, even under a lower wind speed. Finally, RWIVs are compared to VIVs to uncover their underlying similarities and relationships. [ABSTRACT FROM AUTHOR]

This paper numerically investigates the distributed blowing control of the flow around a square cylinder at a low Reynolds number of R e = 200. The air holes are placed at the windward or leeward stagnation points. The control effect of the unsteady aerodynamic forces is notably different with different spanwise distributions of air holes, which is represented by the dimensionless air hole area ratio G. The simulation results indicate that the windward blowing control (WBC) shows better control effect than the leeward blowing control (LBC) in reducing the time-averaged drag coefficient C d ¯ of the square cylinder while the fluctuating lift coefficient C l ′ can be significantly suppressed by the LBC. Besides, the surface pressure distribution, the turbulence kinetic energy (TKE) and Reynolds shear stress (RSS) distributions and the shedding vortices in the wake are analysed and compared in detail under different control conditions. Finally, through the instantaneous 3D vortex structures and the spanwise fluctuating velocity w ′ distribution in the flow field, the suppression of the unsteady aerodynamic forces is mainly related to the 3D vortex pairs induced by the blowing jet. • Distributed blowing was employed to control the flow around a square cylinder. • The spanwise distribution of air holes notably affected the steady blowing flow control. • A 'Six-Vortices Type' vorticity distribution appeared in the flow field under windward blowing control. [ABSTRACT FROM AUTHOR]

We experimentally investigated the control effectiveness of circular cylinder flow and the mechanism of self-adaptive microfibres inspired by bird feathers. The nylon wires were placed at the front stagnation point of a circular cylinder at a Reynolds number of Re = 2.67 × 10 4 . Surface pressure measurement was employed to analyse the unsteady aerodynamic forces around the circular cylinder, and a high-speed particle image velocimetry measurement system was used to visualise and analyse the wake flow structures. The results indicate that the nylon wires at a high length ratio L/D evidently suppress the Reynolds stress and turbulent kinetic energy, thus suppressing the fluctuating lift and mean drag forces of a circular cylinder. In addition, the nylon-induced vortex at high L/D values can inhibit the interaction of the original shear layers to moderate the von Kármán vortex street behind the cylinder. However, the control effect is limited at low L/D values because the nylon-induced vortex structures cannot reach the wake field and interact with the shear layer. [ABSTRACT FROM AUTHOR]

For a suspension bridge, suspenders in the wake of bridge tower interfered by the shedding vortices, would experience a large-amplitude vibration, named as tower-wake-induced vibration (TWIV). At specific wind speeds, the vibrating frequency of suspenders during TWIV would be captured by the shedding frequency of the tower wake, which is the lock-in of TWIV. The present research establishes the governing equations of flexible suspenders, considering the tension variations caused by the deformation, also adopts the aerodynamic model of TWIV, to study the lock-in phenomenon and the TWIV characteristics. To explore the variation trend of nonlinear frequency with amplitude of suspenders, the difference of dynamic parameters of suspenders in two vibrating motions are recognized through free vibration motions at first. Then, a formula for the equivalent amplitude-dependent (EAD) frequency of the suspender is raised. The lock-in phenomenon of TWIV is reproduced in these two motions as well. Eventually, TWIVs of four suspenders at various positions in the rear of bridge tower are simulated based on practical engineering, considering the variation of aerodynamic forces along the altitude due to the wind profile. The conclusions indicate that TWIV of suspenders is a low-frequency and low-order-mode vibration at high wind speeds. • The TWIV calculation of suspender is realized combined with nonlinear suspender equation and TWIV aerodynamic model. • The lock-in mechanism of the suspender TWIV is revealed. • TWIV of suspenders is validated as a low-frequency and low-order-mode vibration at high wind speeds. [ABSTRACT FROM AUTHOR]

This paper numerically investigates the effectiveness of the control of steady suction on a stationary circular cylinder with several isolated suction holes on the surface at a subcritical Reynolds number. The control effectiveness as a function of the azimuthal position, spanwise spacing and suction flow rate of the suction holes on the control of the aerodynamic forces on the cylinder and the suppression of alternate vortex shedding are taken into account. The study of the azimuthal location of the suction holes indicates that azimuthal angles of θ =90° and 270°, which are close to the separation point, provide the most substantial decreases in the aerodynamic forces. When restricted to the most effective azimuthal angle, a remarkable control effectiveness can be achieved when the axial spacing between two neighboring suction holes is less than a minimum value even under a small suction momentum coefficient. However, if the axial spacing exceeds the minimum spacing, the control effectiveness will not be saturated even under a very large suction momentum coefficient. Thus, the cause of the effective aerodynamic force control is suggested to be a result of obvious three-dimensional phenomenon in the near wake, which is characterized by the generation of a convergent flow between two neighboring suction hole sections and a stronger, larger three-dimensional vortex pair adjacent to the convergent flow. It has been suggested that this strongly three-dimensional flow pattern is induced by the strong interaction between two neighboring but counter-rotating three-dimensional vortices separately produced by two neighboring suction holes. Moreover, the effects of such three-dimensional flow patterns are investigated in detail based on variations in the flow field and sectional aerodynamic forces in different cross sections. Finally, the upper limit of the axial spacing between two neighboring suction holes to form such a three-dimensional flow pattern is suggested to be between 0.75 D and 1.5 D when the suction flow rate exceeds a certain value. [ABSTRACT FROM AUTHOR]

In this study, circular cylinders with bionic surfaces were developed to control the flow fields around cylindrical structures such as stay cables of bridges. The bionic cylinder geometries were characterized by slanted elliptical governing sections and wave surfaces. Wind tunnel experiments were conducted to identify the surface pressure distributions, aerodynamic forces, and flow field characteristics of bionic circular cylinders at a Reynolds number (Re) of 3.8 × 104. The results show that the aerodynamic forces of the bionic cylinders are sensitive to the wind attack angle (γ) and wave surface wavelength. The control effects were excellent at γ = 0°, 90°, and 180°, whereas they vanished at γ = 45° and 135° compared with a baseline circular cylinder. When the wavelength range of the bionic cylinder is 4 D –5 D , the maximum reductions in the mean drag and fluctuating lift were 27% and 93%, respectively. The vortex formation region was elongated, and the alternative vortex shedding process was suppressed. Proper orthogonal decomposition was performed to determine the coherent structure of the wake field and the boundary vorticity flux analysis demonstrates the difference in the strength of the wake vortex from the viewpoint of boundary vorticity diffusion between the fluid and wall. Spatial–temporal correlation analysis of the velocity profiles behind different cross-sections shows that the wave surface troughs contribute significantly to disrupting the flow consistency in the spanwise direction. • A bionic cylinder characterized by wave surface and slanted elliptical governing sections was investigated. • The bionic surface could reduce the mean drag and fluctuating lift of the cylinder. • The flow structures, POD modes, and boundary vorticity flux were analyzed to explore the control mechanism. • The control method could decrease the spanwise correlation of the wake velocity and enhance the characteristics of the three-dimensional flow. [ABSTRACT FROM AUTHOR]

Fluid flow around a circular cylinder is ubiquitous in nature and in various industrial applications. The periodic von Kármán vortex shedding from the cylinder is one of the primary factors contributing to flow-induced vibrations of structural components in the ocean, coastal, and bridge engineering. Because flow-induced vibrations cause structural damage, it is essential to develop effective ways to suppress alternating vortex shedding and manipulate circular cylinder flow. Therefore, active control schemes for mitigating flow-induced vibrations have garnered attention from various researchers in recent decades. In contrast to passive control schemes, active control schemes have the advantages of controllability and maneuverability. This selective literature review provides a retrospect and discusses the recent progress in several active control schemes for circular cylinder flow, including suction/blowing, synthetic jets, electromagnetic forcing, rotation, forced oscillation, thermal effects, acoustic excitation, moving surfaces, and feedback control strategies. In addition, some perspectives on AI-based active control are proposed and discussed briefly. • This selective literature review gives a retrospect and recent progress in classical active control schemes for circular cylinder flow. • Some perspectives of artificial intelligence-based active control are also proposed and discussed briefly. [ABSTRACT FROM AUTHOR]

A passive-suction-jet control scheme was adopted to manipulate the periodic vortex shedding of two side-by-side circular cylinders. The baseline case (both cylinders without control), Case #1 (only the lower cylinder was controlled), and Case #2 (both cylinders were controlled) were numerically simulated at five spacing ratios (S R) of 1.5, 2.0, 2.5, 3.0, and 4.0. The results show that the cylinders in all three cases experience a biased flow pattern at S R = 1.5 and 2.0 and tend to behave as isolated cylinders at S R > 2.0. The flow structures in Case #2 reveal that at S R = 1.5 and 2.0, the jets blown from the pipes roll up to form a large-scale vortex behind one cylinder and a series of small-scale vortices behind the other, eliminating the periodic vortex shedding of the two cylinders completely and yielding low fluctuating lifts. At S R > 2.0, the vortices formed by the jets weaken the influence of the wake vortices on the cylinders, contributing to a 94% reduction in lift fluctuation on average. Moreover, the holes centered at θ = 75°–135° were revealed to play an important role in continuously pouring positive vorticities into the fluid, which manipulate the periodic vortex shedding. • The control effectiveness of passive-suction-jet pipes on the flow over two side-by-side cylinders was examined. • The jets eliminated the periodic vortex shedding when both cylinders were controlled at spacing ratios of 1.5 and 2.0. • The lift fluctuation was reduced by 94% on average when both cylinders were controlled at spacing ratios lager than 2.0. • The boundary vorticity flux distribution reveals the control mechanism of the pipes. [ABSTRACT FROM AUTHOR]

We experimentally investigated the dynamic wake evolution of flow around a square cylinder controlled with steady jet ejecting from the rear stagnation point in a wind tunnel at Reynolds number of 1.33 × 104. For diversified jet momentum coefficients C m , modal analyses, dynamic vortex shedding process, time-averaged turbulence kinetic energy (TKE) and Reynolds shear stress (RSS) were obtained from high-speed Particle Image Velocimetry (PIV) measurements, proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) analyses. As C m grew, it was found that TKE distribution was restrained and RSS was shifted towards the outlet of the steady jet. Variations of dominant frequencies in POD and DMD modes were observed. The pattern of vortex shedding from the shear layer were transferred from antisymmetric to a symmetric one. The vortex sheets were also observed to be elongated downstream in the wake. Experimental results indicated that the dynamic wake of the square cylinder could be controlled significantly by the steady jet from the rear stagnation point. • The flow around a square cylinder is controlled with steady jet positioned at its rear stagnation point. • The nondimensional jet momentum coefficient is found to play an important role in flow control effectiveness. • Steady jet stabilizes the dynamic wake and alleviate the unsteady vortex shedding from the square cylinder. [ABSTRACT FROM AUTHOR]

Active control of leading-edge suction and trailing-edge jet (LSTJ) on the main girder interrupted the strong correlation of spanwise vortex (i.e., von Kármán vortex), which changed the quasi-periodic vortex shedding process in the main girder wake. This indicates that the flow field presented a strong 3D-flow characteristic and the wake vortex could be lengthened. Thus, the crucial fluctuation of the pressure distribution and the unsteady aerodynamic force can be alleviated by using LSTJ. • An active control of LSTJ was investigated to suppress the fluctuating surface pressures and unsteady aerodynamic force of box girder. • A reduction in the TKE was achieved and the spanwise vortices were perturbed. The energy amplitude of main mode is decreased. • The LSTJ has control effect on alleviating the unsteady aerodynamic force at various angles of attack. An experimental study was conducted to investigate the control effectiveness and underlying mechanism of the leading-edge suction and trailing-edge jet (LSTJ) for a box girder at a Reynolds number of 2.08 × 104. Four circular holes were machined in the leading edge and the trailing edge of the box girder to induce steady LSTJ flow. The strength of the LSTJ is described by a dimensionless suction/jet momentum coefficient. Pressure taps around the surface associated were used with a digital sensor array system to acquire the pressure distribution of the test model. The aerodynamic forces, which macroscopically represent the control abilities of the LSTJ, were calculated based on the surface pressure distribution. The pressure measurements indicate that the fluctuating pressure on the surface near the trailing edge was attenuated and the root-mean-square value of the lift, drag, and moment forces reduced with LSTJ control. In addition to the surface pressure measurements, particle image velocimetry (PIV) was used to obtain the flow fields of the box girder model with and without LSTJ control. Both the pressure measurement and PIV results show that a certain value of the suction/jet momentum coefficient can ensure optimum control effectiveness of the LSTJ. Moreover, the LSTJ was found to push the unsteady flow structure further downstream in the model wake. A data-processing method of dynamic mode decomposition was used to illustrate the LSTJ mechanism. The energy and Strouhal number of the main mode of the LSTJ controlled case were found to differ from the uncontrolled case. The topological structure of the main mode extracted from the flow field also showed a significant variation. Linear stability analysis of the wake flow field indicates that the control method of LSTJ changes the region of unstable flow notably. Moreover, the control effectiveness of the LSTJ at various wind angles of attack was confirmed in the present study. [ABSTRACT FROM AUTHOR]

Wind tunnel tests were conducted to investigate the effect of the self-issuing jet flow control on the vortex-induced vibrations (VIVs) of a streamline single-box girder at various angles of attack (AOAs). Spanwise periodic passive slits were symmetrically arranged on the windward and leeward lower inclined panels of the girder. The Reynolds number varied from 4.19 × 104 to 1.14 × 105, and the AOA varied from −5.0°to 5.0°with increments of 2.5°. The vibration response characteristics of the girder with different slit arrangements were discussed, and the interactions between the shedding Kármán vortices (KVs) and passive jets were investigated. The results show that the self-issuing jet control method could effectively suppress the VIV; the slits on the upper part of the lower inclined panel showed the best effect (Case I) at various AOAs. In addition, the large shedding KV were modified into two kinds of vortices: a reduced shedding KV and standing vortex at the trailing bottom corner of the girder. When the shedding vortex generation was displaced downstream, a second instability occurred in the near-wake. The self-issuing jets worked better at positive AOAs because more than one slit rows controlled the shedding vortices. Finally, the flutter performance of the single-box girder with self-issuing jets was investigated; Case I had a positive effect on the flutter behaviour at AOAs of −2.5°to 5°; the same flutter performance was observed at −5°. [ABSTRACT FROM AUTHOR]

In this study, vortex-induced vibrations (VIVs) of a spring system-supported circular cylinder model are suppressed via passive-jet flow control in wind tunnel experiments. A laser displacement measurement system is employed to obtain the vibration response of the test model with and without a passive-jet control scheme. The results indicate that the VIV oscillation amplitudes and frequency lock-in ranges of the controlled models are smaller than those of the baseline case. In addition to the vibration measurements, the pressure distribution in the test model is measured with a digital pressure measurement system. According to the surface pressure results, the mean pressures of the leeward surface are lifted, and the root-mean-square (RMS) pressures on the cylinder surface are suppressed compared with those of the baseline case. Moreover, in the controlled cases, the mean drag and lift fluctuations are remarkably alleviated, and the Reynolds shear stress in the cylinder wake decreases considerably. All results indicate that the effectiveness of the passive-jet flow control in this investigation is determined by the spacing of two neighboring jet pipes. Moreover, a linear correlation analysis of the surface pressures reveals that the correlation of the external surface pressure is tremendously disturbed by the passive-jet flow control. The passive-jet flow control by jet pipe equipping on the periphery of a elastically-supported circular cylinder forms the windward suction and leeward jets to interrupt the strongly correlation of external surface pressure on the test model and suppress the quasi-periodic vortex shedding process in the cylinder wake. As a result, the vortex-induced vibrations of the circular cylinder are mitigated. Image 1 • A passive-jet flow control method was investigated to suppress the vortex-induced vibrations of a circular cylinder. • A notable reduction in the fluctuations of both surface pressures and aerodynamic coefficients were achieved. • The periodic vortex shedding was alleviated, and the separation point was shifted with the passive-jet flow control. [ABSTRACT FROM AUTHOR]

• A passive jet method was experimentally investigated to manipulate the unsteady vortex shedding around a rectangular column. • A notable reduction in the fluctuations of both the surface pressures and aerodynamic forces were achieved. • The main separation vortices were stretched by the small-scale reverse vortices induced by the jet flow. • The descent of the low-order mode energy corresponded to the suppression of the vortex shedding. A passive jet method was experimentally investigated to manipulate the unsteady vortex shedding around a rectangular column. This method employed a circuit channel connecting the windward and leeward sides of the column to implement the windward passive suction and leeward passive blowing synchronously to modify the flow regime around the column. A bare column model with a side ratio of SR = 1.3 was utilized as a baseline case and the same column models covered with passive suction/jet rings served as controlled cases. Based on different angles of attack (AoAs) of the column models, the Reynolds number varied from 29,300 to 48,200 at an oncoming wind velocity of 10 m/s. At different intervals of suction/jet rings, the surface pressure distributions, aerodynamic forces, and flow structures of the column models were measured to evaluate the control effectiveness of the passive jet method. The results indicated that a notable reduction in the fluctuations of both the surface pressures and aerodynamic forces was achieved. Meanwhile, the mean values of drag forces were also evidently decreased. The flow regime was modified in the vicinity of passive jet holes. The main separation vortices were stretched by the small-scale reverse vortices induced by the jet flow. In addition, the TKE level in the wake flow was considerably decreased in the controlled cases. By extracting eigenvalues and mode coefficients, the POD analysis revealed that the descent of the low-order mode energy corresponded to the suppression of the vortex shedding. The series of phase reconstruction and POD-filtered flow fields demonstrated that the passive jet flow could interact with the shedding vortices and weaken the strength of the vortex shedding process. [ABSTRACT FROM AUTHOR]

• Twisted elliptic (TE) cylinders with aspect ratios (ARs) of 1.1 and 1.2 were studied. • The Gaussian characteristics of the fluctuating pressure were analyzed. • The phase difference and coherence between the pressure signals were analyzed. • The Reynolds stresses behind a TE cylinder with an AR of 1.2 were eliminated. In this study, we performed an experimental study to explore the surface pressures and flow structures around twisted elliptic (TE) cylinders at a Reynolds number of 3.8 × 104. Two TE cylinders, denoted as TE11 and TE12, with aspect ratios of 1.1 and 1.2 for elliptical cross-sections, respectively, were selected to compare the performance of vortex shedding suppression. The distribution characteristics of the fluctuating pressure at different spanwise positions and the phase difference and coherence function between the pressure signals were analyzed. The results showed that TE12 cylinder was not dominated by antiphase shedding vortices, as in the baseline and TE11 cylinders, but rather by low-frequency components in the flow field, yielding a Gaussian distribution of the fluctuating pressure. The pressure pulsations were significantly asymmetric for the TE12 cylinder due to the earlier flow separation on the upper surface, but this asymmetry was not apparent for the TE11 cylinder. The Reynolds shear stress distribution revealed that the transition to turbulence in the separated layer was significantly delayed by the TE surface. Moreover, for TE12 cylinder, zero turbulence fluctuations in the near wake explained the low-frequency domination of the pressure fluctuations, resulting in 93.8% attenuation of the lift fluctuations. [ABSTRACT FROM AUTHOR]

An active control method was used to attenuate the unsteady vortex shedding in the wake of a square cylinder. Distributed jets were applied to the rear porous surface at a Reynolds number of 1.07 × 104. The strength of the jets was measured using the dimensionless jet momentum coefficient C Q. The dynamic mode decomposition, instantaneous vorticity evolution, and time-averaged characteristics, e.g., turbulent kinetic energy, streamlines, velocity profiles and Reynolds stress distributions of the wake flow field were used to analyze the control effects. The results suggest that with increasing C Q , two jet vortices gradually form near the rear cylinder wall and compete with the vortex shedding. Consequently, the vortex shedding pattern became nearly symmetrical, whereas the jet vortices remained asymmetrical. The vortex shedding process was pushed downstream. The fluctuations in the wake, particularly near cylindrical walls, were significantly suppressed. Thus, this scheme was successful, indicating potential for drag reduction and energy efficiency. Better control effects were associated with higher C Q in the present study. • A square cylinder wake is controlled via distributed jets over the coated rear porous surface. • The formed jets vortices manipulate the wake vortex shedding process. • The turbulent kinetic energy and the Reynolds stress distributions are restrained and the wake flow stabilized. [ABSTRACT FROM AUTHOR]

In the current study, a new passive flow control strategy characterized by 3D printing structured porous surface was introduced and experimentally investigated to manipulate the dynamic wake evolution and flow characteristics around a circular cylinder at a Reynolds number of 2.02 × 10 4. With the 3D printing structured porous surface, parts of the incoming flow could go through the cylinder and communicate with the wake flow spontaneously. As a result, the vortex shedding was suppressed and the drag was reduced. From POD analysis results, we observed shifted modes and decreased vorticity distribution, indicating that the periodic vortex shedding process was weakened. This was also verified by the analysis of dynamic wake evolution and by the power spectrum analysis. In addition, experimental results showed that the time-averaged turbulent kinetic energy (TKE) and the Reynolds shear stress (RSS) distributions were restrained in the wake of the porous cylinder. Moreover, the mean estimated drag coefficients witnessed a significant decrease up to 49.47%. • The flow around a cylinder is modified via structured porous surface. • The porous cylinder stabilizes the dynamic wake and alleviate the unsteady vortex shedding from the cylinder. • The mean estimated drag coefficient is estimated to be decreased up to 49.47%. [ABSTRACT FROM AUTHOR]

The aerodynamic parameters and flow field characteristics of cylinders with and without a porous media coating (PMC) were experimentally investigated in a wind tunnel. This research explored the potential of using a PMC to control vortex-induced vibration (VIV). The PMCs were manufactured with four pores-per-inch values (PPI = 15, 20, 30, and 40) and three depths, (d / D = 0. 125 , 0.25, and 0.5, where d is the depth of the coating and D is the diameter of the bare cylinder). The instantaneous drag and lift forces were measured in static tests. The results showed that all the PMCs could decrease the fluctuation of the forces. The root-mean-square lift force was reduced by up to 98% with a coating of PPI = 15 and d / D = 0. 5 at Re = 5.2 × 104. The flow fields were visualized using a particle image velocimetry (PIV) system and analyzed with time-averaged velocity fields, wake velocity profiles, and turbulence kinetic energy distributions. For PMCs with smaller PPI values and greater depths, the wake vortex formed further away from the cylinder. Then the dynamical responses were tested for elastically supported models. Not all the PMCs could suppress the VIV. When PPI < 30, the PMC decreased the VIV amplitude. For each PPI, a greater depth resulted in a lower amplitude. The VIV amplitude for a coated cylinder was reduced to 4% of that for the bare cylinder. When PPI > 30, the PMC enhanced the VIV, with a greater depth resulting in a higher amplitude. When PPI = 30, the PMC could either increase or decrease the VIV amplitude, depending on the depth. The flow fields around the oscillating cylinder were also analyzed to reveal the effect of the PMC on the wake vortex shedding behavior. • Porous media coatings are very efficient in reducing the fluctuations of the lift and drag forces. • Wake vortices form further away from the cylinders with coatings. • Vortex-induced vibration can be significantly suppressed or enhanced by the coating. [ABSTRACT FROM AUTHOR]

• An upstream splitter plate is used to control the circular cylinder flow. • Vortical structures rolled up on the upstream splitter plate and flow separation change the incoming flow of the cylindrical model. • Drag reduction is generally realized but fluctuating-lift suppression needs to be careful. In the present study, we place a short splitter plate onto the upstream side of a circular cylinder to modify its incoming flow conditions and the wake-vortex patterns and to attenuate the aerodynamic forces. Wind-tunnel investigations are performed at a Reynolds number of Re = 2.66 × 10 4 . Special interests are paid to the length sensitivity of the rigid splitter plate. The 2D particle image velocimetry (PIV) technique is employed to measure the flow patterns around the natural and controlled cylinders to reveal the great modifications of the upstream plate to the cylinder flow. Except for PIV tests, sectional pressure measurements are also conducted to obtain the pressure distribution around the cylinder surface so as to estimate the aerodynamic forces, i.e. drag and lift forces of the natural and controlled cylinders. Results of the surface-pressure measurement demonstrate clearly the control effectiveness and the surrounding flow topologies obtained by PIV system help to uncover the mechanism of the passive upstream-plate control. [ABSTRACT FROM AUTHOR]