Effect Of Vortex Shedding Research Articles

Viscous effects on the hydrodynamic performance of a two-body wave energy converter with a damping plate

In this study, the hydrodynamic forces and power absorption performance of an autonomous underwater vehicle (AUV)-based two-body wave energy converter (2BWEC) are investigated. A theoretical model is developed within the framework of linear potential flow to solve for added mass, radiation damping, and wave excitation force using the matched eigenfunction expansion method (MEEM). A computational fluid dynamics (CFD) model is employed to account for vortex-shedding effects of the floater and inner cylinder with a damping plate under various excitation conditions. Empirical formulas for supplementary added mass and drag coefficients caused by flow separation are proposed based on curve-fitting the differences between CFD results and MEEM calculations. These formulas are integrated into motion equations to enhance accuracy in evaluating the power absorption of the 2BWEC. It has been found that in the context of viscous flow, both the added mass and damping coefficients are increased, particularly for the inner cylinder with a damping plate. In addition, the viscous hydrodynamic coefficients exhibit strong dependence on the Keulegan–Carpenter number, while showing insensitivity to changes in the frequency parameter β. The supplementary (viscous) added mass provides additional inertia for the AUV with a limited mass itself, which is advantageous for the power absorption of the AUV-based 2BWEC. Conversely, the presence of viscous damping from the damping plate impedes wave energy capture.

Analysis of tower structure for fatigue taking into account wind vortex shedding

Vortex shedding is manifested in all tower structures with a solid or lattice cross-section with relatively equal width and depth of the cross-section. The phenomenon is associated with the alternate breakdown of vortices from the edges of the structure, which cause it to oscillate under the influence of inertial masses in a plane perpendicular to the wind. The main condition for the manifestation of vortex shedding is the action of wind with a speed exceeding a certain critical one. As shown by calculations of a real tower structure, oscillations according to the first natural frequency and shape occur already under the influence of a moderate wind. This requires attention, assessment of the number of oscillation cycles and the implementation of criteria for taking into account the combined effect of frontal wind and vortex shedding on the structure and calculation of the structure for endurance. The paradox is that this phenomenon is not widely known in engineering practice, and instructions on the mandatory calculation of structures for vortex excitation appeared only with the implementation of amendment 2 to DBN B.1.2-2:2006 "Loads and influences. Design norms" in 2020. In this analyzed state of research on this issue, an available method for calculating vortex excitation, introduced in EN 1991-1-4:2005, is presented. The calculation and analysis of the McDonald's advertising totem with a height of 25.575 m, implemented in Kyiv in 2016, was performed. Calculations showed that this structure is characterized by oscillations due to vortex shedding. From the available sources of meteorological data, an approximate number of oscillation cycles was determined, criteria for calculating the structure's durability were determined, and generalizations were made that can be applied to all structures. In particular, it is pointed out the need to limit stresses beyond the endurance limit in two ranges - parametric resonance from the frontal wind, and sign-changing cyclic oscillations from vortex excitation. An analysis of the effective cross-sections and effective structural forms of building details when working under the action of a frontal wind with vortex shedding was carried out.

Revealing the influence of wind vortex shedding on the stressed-strained state of steel tower structures with solid cross-section

This work investigated the effect of vortex shedding during the action of wind on tower structures with a solid cross-section. Failure to take into account this influence, together with the manifestation of the phenomena of physical wear of structures during long-term operation, can lead to building accidents. The object of the research are tower structures with a solid cross-section, which are widely used in modern infrastructure ‒ advertising pylons with a transparent advertising structure with a height of about 12, 22, 25 m, and a flagpole with a height of 48 m. The most frequent manifestations of oscillations according to the first natural frequency and shape have been considered, which occur even with moderate winds and when a large number of oscillation cycles occur. In this work, the number of oscillating cycles per year was determined and estimated based on the archive of meteorological observations. It was found that the number of oscillating cycles due to the action of wind excitation for the example of the structures under study is from 2.6 to 14.4 million per year, which requires the mandatory limitation of stresses in the parts of structures during the design to enable their durability. The magnitude of forces from vortex shedding for the studied structures ranges from 2.9 to 43.5 % of the forces caused by the influence of the frontal wind, depending on the height of the structure. Thus, it was found that the influence of vortex shedding is very insignificant for structures up to 12–15 m high and increases for structures 20 m and higher. Rational forms of the cross-sections of structures were established to reduce the influence of vortex shedding ‒ these are cross-sections of a circular and a cross-section close to it. In a general formulation, these are sections for which peak stresses relative to forces in one plane fall into the neutral zone for stresses from forces in another plane. Recommendations have been also given on the simultaneous consideration of forces from the action of frontal wind on the structure and vortex shedding since both manifestations of wind action on tower structures cannot be separated

Comparison of 2D and 3D Simulations on Predicting the Performance of a Savonius Wind Turbine

The performance of Savonius rotors plays a vital role in harnessing wind energy for various applications. This study aims to investigate and compare the performance characteristics of Savonius rotors using two-dimensional (2D) and three-dimensional (3D) analysis approaches. The primary objective is to determine the optimal analysis method for evaluating the rotor's performance. The simulations were conducted using ANSYS software, considering six different tip-speed ratios (TSRs) ranging from 0.2 to 1.2, and three wind speed categories (7 m/s, 5 m/s, and 3 m/s). The hybrid shear-stress transport (SST) k-omega was used as the turbulence model. The results indicate that the 2D analysis approach, which simplifies the rotor's geometry by assuming rotational symmetry, provides reasonable estimations of the rotor's performance. However, the 3D analysis captures the intricacies of the rotor's actual geometry, accounting for the effects of non-uniform flow and vortex shedding, which can significantly influence the rotor's performance. The comparative analysis reveals that the 3D analysis predicts higher torque and power coefficients than the 2D approach, especially at higher wind speeds. At TSR 1.0, the torque and power coefficient obtained from the 3D approach are 6.56 Nm and 0.102, respectively meanwhile 2D approach gains 6.28 Nm torque and 0.098 for power coefficient. This research contributes to a better understanding of the performance characteristics of Savonius rotors and highlights the importance of considering three-dimensional effects in their analysis. The findings can guide the design and optimization of Savonius rotor systems, leading to improved wind energy conversion efficiency and enhanced utilization of renewable energy resources.

A characterization of unsteady effects for transonic turbine airfoil limit loading

To better understand the effect of vortex shedding on the nature of the trailing edge shock system during airfoil limit loading a computational fluid dynamic investigation was performed for a transonic turbine airfoil at sublimit, limit and supercritical conditions. Four modeling strategies were employed: steady state RANS, unsteady RANS, DDES, and turbulence model free. The influence of transient modeling approaches on the predicted mass-flow averaged total pressure loss coefficient, mass-flow averaged flow angles, and on the limit loading pressure ratio were found to be insignificant with the exception of the URANS model during critical loading. It was found that the URANS modeling approach failed to predict the transition from near trailing edge dominated vortex formation to base pressure vortex formation resulting in a drastic rise in predicted total pressure loss. Surface isentropic Mach distributions were predicted similarly for all modeling strategies, with the exception of the trailing edge base pressure region and points of shock impingement along the suction surface. A detailed review of the boundary layer states at the trailing edge was performed. It was found that all of the modeling approaches predicted laminar boundary layer profiles along the pressure surface trailing edge and turbulent profiles along the suction surface. The predicted base pressure distributions were also reviewed, showing the base pressure to decrease with increasing pressure ratio. The unsteady simulation approaches consistently predicted lower average surface pressures than the steady state RANS simulations. Qualitative images of the numerical Schlieren contours were presented and reviewed showing large differences in the prediction of vortex shape, size, and subsequent shock influence.

Research on the directional characteristics of wind noise emitted by bionic rods

In this paper, the directional characteristics of wind noise emitted by different bionic rods were studied based on a hybrid computational aeroacoustics method. The noise reduction mechanism of surface grooves, pits, and bumps was analyzed, respectively. The basic principle of noise reduction is to reduce the influence of the vortex shedding on the rod by changing the shape of the rod or passive control technology to reduce the dipole sound source. The unsmooth transverse surface will increase the loss of flow on the leading edge of the rod and reduce the vertical effect of vortex shedding on the rod. The convex leading edge of the rod can help to transfer the vertical noise from low frequency to high frequency and reduce the vertical effect of wake vortex shedding to reduce the peak sound pressure level. The cost of those was the increase in the aerodynamic drag and the increase in noise in the flow direction (the increase in the amplitude of drag fluctuation). In particular, the longitudinal v-groove structure on the surface of the elliptical rod can reduce the circumferential aerodynamic noise while keeping the aerodynamic drag coefficient unchanged.

A Novel Integrated Optimization-Driven Design Framework for Minimum-Weight Lateral-Load Resisting Systems in Wind-Sensitive Buildings Equipped with Dynamic Vibration Absorbers

The increasing rate of urbanization in recent decades has resulted in a global surge in the construction of slender high-rise buildings. These structures are prone to excessive wind-induced lateral vibrations in the crosswind direction owing to vortex shedding effects, causing occupant discomfort and, ultimately, dynamic serviceability failure. To reconcile the worldwide accelerated trend in constructing tall buildings with the sustainable building sector agenda, this paper proposes a novel bi-objective integrated design framework that leverages dynamic vibration absorbers (DVAs) to minimize the required material usage in the wind load-resisting structural systems (WLSSs) of occupant comfort-governed tall buildings. The framework couples structural sizing optimization for minimum-weight WLSS design (objective 1), with optimal DVA tuning for floor acceleration minimization to satisfy codified wind comfort design requirements by using the smallest DVA inertia (objective 2). Furthermore, a versatile numerical strategy is devised for the efficient solution of the proposed bi-objective optimization problem. For illustration, the framework is applied to a 15-storey steel building equipped with one of two different DVAs: a widely considered top-floor tuned mass damper (TMD) and an innovative ground-floor tuned inerter damper (TID). The derived Pareto optimal integrated (WLSS-plus-DVA) designs demonstrate that significant reduction in both structural steel usage and embodied carbon emissions can be achieved using either one of the two DVAs with moderate inertia. It is concluded that the proposed optimization-driven design framework and numerical solution strategy offer an alternative innovative approach to achieve material-efficient high-rise buildings under wind hazards.

Design and Analysis of Bladeless Wind Turbine

The idea of bladeless windmills is based on the vortex shedding effect hypothesis. A wind-powered generator with the fewest moving elements is the vortex bladeless windmill. The oscillation or vibration caused by the wind is used to generate the electric current. As a result, piezoelectric material or a linear alternator are used to generate electricity. In this project effort, we attempted to increase the vortex-induced vibrations of the turbine built of Epoxy Carbon UD (230GPa) by altering the design of the mast and base. The maximum deflection is 0.22775m (condition 7) at 10 m/s2 acceleration.

Investigation of hybridized bluff bodies for flow-induced vibration energy harvesting

Small-scale wind energy harvesting based on flow-induced vibration (FIV) mechanisms has attracted lots of research interest in recent years. Vortex-induced vibration (VIV) and galloping energy harvesters usually outperform each other in different wind-speed ranges. To combine the advantages of VIV and galloping harvesters, this paper explores the idea of using a hybridized bluff body constituting of two cylindrical and one cuboid segment for wind energy harvesting. The total length of the hybridized bluff body was fixed. The cuboid segment length was varied to investigate the effect on the FIV behavior of the bluff body. The results show that, when the cuboid segment is short in length, the bluff body exhibits VIV-like behavior in the low wind-speed range and galloping-like behavior in the high-speed range. In the medium wind-speed range, galloping-VIV coupling appears. However, if the cuboid segment is longer, the galloping-VIV coupling phenomenon disappears; the hybridized bluff body behaves just like a cuboid one and only exhibits a galloping motion. In addition to experiments, computational fluid dynamics (CFD) simulations are also conducted to provide more insights into the aerodynamics of the hybridized bluff body. The simulation results reveal that introducing hybridization into the bluff body changes the vorticity flow behind it and alters the vortex-shedding behavior. The vortex-shedding effect, in turn, affects the vibration of the bluff body, as well as the performance of the harvester.

Effects of nonlinearity and viscous damping on the resonant responses in two-dimensional moonpools with a recess

In this study, the nonlinear and viscous damping effects on the resonant responses inside a two-dimensional moonpool with a recess are investigated. Based on a fully nonlinear potential flow (FNPF) model, the nonlinear moonpool responses excited by the forced heave motion of a structure are simulated in the time domain. To consider the vortex-shedding damping effects in the moonpool, induced by both piston-type motion (the first harmonic component) and sloshing-type motions (the higher harmonic components), a pressure drop model based on Faltinsen and Timokha (2015) is developed and applied to the FNPF model (FNPF-V for short). The response amplitude operators (RAOs) of the higher harmonics, by which the nonlinear effects are evaluated, are computed by the FNPF-V model. It is found that the higher harmonics are noticeable for the excitation frequencies ω n / ( n + 1 ) ( n is a positive integer, ω n denotes the natural frequency of the n th sloshing mode), where secondary resonances can be triggered. In addition, it is found that the strong nonlinear interactions between the first and higher harmonics can lead to a significant amplification of the moonpool responses. The nonlinear behaviour (the secondary resonance, as well as the sum- and difference-frequency effects) is substantially enhanced as the excitation amplitude increases. Moreover, it is found that the length of the recess influences the nonlinear response of the moonpool in two respects with opposite effects. On the one hand, for the same first harmonic response, moonpools with a longer recess are more apt to exhibit nonlinearity. On the other hand, for the same external excitation, as the length of the recess increases, the first harmonic responses at the excitation frequencies ω n / ( n + 1 ) decrease, suppressing the nonlinear behaviour. • This paper studies the secondary resonances triggered by the piston-mode response. • A pressure drop model is introduced to consider the vortex-shedding effects. • The nonlinear interactions between different harmonic components are investigated. • The effects of recess length on the nonlinear response are analysed and interpreted.

Tuned tandem mass dampers-inerters for suppressing vortex-induced vibration of super-tall buildings

Modern super-tall buildings with rectangular floor shapes are vulnerable to significant vibrations due to the vortex shedding effect formed around structural edges. Tuned tandem mass dampers-inerters (TTMDI) is extended in this study to suppress vortex-induced responses of super tall buildings for occupant comforts. The optimal TTMDI tuning problem for a 400 m super tall building exposed to crosswind loads is solved through frequency-domain analysis. Meanwhile, the control performance of TTMDI is explored and contrasted with that of the tuned mass damper (TMD), tuned tandem mass dampers (TTMD) and tuned mass damper‐inerter (TMDI) from various perspectives. A crosswind-resistant design framework of the TTMDI equipped super-tall building system is then devised to meet comfort requirements. Finally, results are further assessed in the time -domain using a virtual real time hybrid simulation (VRTHS) system based on the proposed hybrid model predictive controller. It is found that there are several optimal inertance coefficient redistribution points (i.e. R points) in the design process and the R points have a great impact on the optimal internal structure of TTMDI. Results clearly demonstrate that TTMDI has better effectiveness and robustness than TMD, TTMD, and TMDI. Moreover, the total demand for total damping coefficients of TTMDI is significantly smaller than TMDI after the R points, indicating that it is easier to implement in engineering practices.

Partially-covered fractal induced turbulence on fins thermal dissipation

The impacts of partially-covered fractal grids induced turbulence on the forced convective heat transfer across plate-fin heat sink at Reynolds number ReDh = 22.0 × 103 were numerically and experimentally investigated. Results showed that partially covered grids rendered a higher thermal dissipation performance, with partially-covered square fractal grid (PCSFG) registering an outstanding increase of 43% in Nusselt number relative to the no grid configuration. The analyzation via an in-house developed single particle tracking velocimetry (SPTV) system displayed the findings of unique “Turbulence Annulus” formation, which provided a small degree of predictivity in the periodic annulus oscillations. Further assessments on PCSFG revealed the preferred inter-fin flow dynamics of (i) high flow velocity, (ii) strong turbulence intensity, (iii) vigorous flow fluctuations, (iv) small turbulence length scale, and (v) heightened decelerated flow events. These features stemmed from the coupling effects of multilength-scale fractal bar thicknesses in generating a veracity of eddy sizes, and a vertical segmentation producing heightened mass flow rate while inducing favourable wake-flow structures to penetrate inter-fin regions. Teeming effects of such energetic eddies within plate-fin array unveiled a powerful vortex shedding effect, with PCSFG achieving fluctuation frequency f = 18.5 Hz close to an optimal magnitude. The coaction of such traits limits the growth of fin boundary layers, providing superior thermal transfer capabilities which benefits the community in developing for higher efficiency heat transfer systems.

Experimental Study of the Aerodynamic Characteristics of a Suspended Monorail Vehicle-Bridge System Under Crosswinds

Using wind tunnel tests, the aerodynamic characteristics of a suspended monorail transport (SMT) are systematically investigated under crosswinds for the first time. Two practical vehicle–bridge combination forms (single-line and double-line) are considered. As a contrast, single vehicle and bridge models are also studied to further reveal their aerodynamic characteristics. The mean and root mean square (RMS) wind pressure coefficients, aerodynamic coefficients and power spectrum density (PSD) distributions are used to numerically investigate the effects. The paper confirms that: (1) the mean wind pressure coefficients of the single vehicle and bridge models are close to unity at the center point of the windward side and gradually decrease from the center point to both sides. The maximum value of the RMS wind pressure coefficients mainly occurs at the top or bottom surface of the testing models due to the effect of vortex shedding; (2) the aerodynamic coefficients and the mean and RMS wind pressure coefficients are significantly increased for both the vehicle and bridge models when considering the aerodynamic coupling effects; (3) the aerodynamic characteristics of the bridge can be significantly affected by the location of the running vehicle, but the fluid flow can maintain relative stability during the meeting of the two vehicles.

Liquid vortexes and flows induced by femtosecond laser ablation in liquid governing formation of circular and crisscross LIPSS

Orientations of laser induced periodic surface structures (LIPSS) are usually considered to be governed by the laser polarization state. In this work, we unveil that fluid dynamics induced by femtosecond (fs) laser ablation in liquid (fs-LAL) can easily break this polarization restriction to produce irregular circular-LIPSS (CLIPPS) and crisscross-LIPSS (CCLIPSS). Fs laser ablation of silicon in water shows formation of diverse LIPSS depending on ablation conditions. At a high power of 700 mW (repetition rate of 100 kHz, pulse duration of 457 fs and wavelength of 1045 nm), single/twin CLIPSS are produced at the bottom of macropores of several microns in diameter due to the formation of strong liquid vortexes and occurrence of the vortex shedding effect. Theoretical simulations validate our speculation about the formation of liquid vortex with an ultrahigh static pressure, which can induce the microstructure trenches and cracks at the sidewalls for fs-LAL of Si and tungsten (W) in water, respectively. At a low power of 50 mW, weak liquid vortexes are produced, which only give birth to curved LIPSS in the valleys of grooves. Consequently, it is deduced that liquid vortex plays a crucial role in the formation of macropores. Mountain-like microstructures induce complex fluid dynamics which can cause the formation of CCLIPSS on them. It is believed that liquid vortexes and fluid dynamics presented in this work open up new possibilities to diversify the morphologies of LIPSS formed by fs-LAL.

Numerical study on drag reduction and wake modification for the flows over a hydrofoil in presence of surface heterogeneity

In this article, direct numerical simulations of flow around NACA0012 hydrofoil with superhydrophobic surface (SHS) is presented. Surface heterogeneity takes into account by periodic no-slip and shear-free grates on the surface. The study is conducted for a range of [Formula: see text] degree angles of attack and at fixed Reynolds number ( Re) 1000. SHS leads to effective Navier slip on the hydrofoil surface and corresponding changes in velocity profile. Bubble separation delays for higher gas-fraction ( G.F), and minimizes the vortex shedding effects. As a result, flow remains two-dimensional (2D) for higher angles of attack as compared to flow over three-dimensional (3D) hydrofoil with no-slip boundary. Both the drag reduction and enhancement of lift force are observed. Maximum lift are observed at [Formula: see text], while the drag force continues to decrease with the gradual increase in angle of attack and patterned micro-grates fraction. Mode C with smaller wavelength is observed at [Formula: see text]. Furthermore, increase in gas-fraction leads to increase in slip length which thickens the boundary layer and mimics the vorticity implies drag reduction. Thus, replacement of the no-slip surfaces by superhydrophobic surfaces can be treated as a promising drag reduction technology.

A preliminary research on the periodic characteristics of a ventilation system with fluid oscillator

The fluid oscillator is a device that produces periodic alternating jets by supplying a certain pressure of the fluid. In this paper, the fluid oscillator is applied to the field of building ventilation. The supply air direction will change periodically and create a vortex shedding effect in the ventilated environment. By establishing a three-dimensional model of the double feedback fluid oscillator, CFD numerical simulation technology was used to study the indoor flow field characteristics when the fluid oscillator was used for ventilation and the slit air supply. The results show that the influence area of slotted air is small, and the energy is concentrated and distributed. Only a few eddies are generated on both sides of the main jet, and many stagnation zones are generated indoors. Fluid oscillator ventilation into the indoor airflow has a broader range of influence. The flow field distribution is uniform, and the jet energy dissipates quickly. The fluid oscillator will generate many eddy currents inside the room, effectively reducing the airflow stagnation zone in the room. Because the fluid oscillator can accelerate the disturbance of the flow field inside the room, it has a particular reference value to optimize the indoor air distribution.

Liquid vortexes and flows induced by femtosecond laser ablation in liquid governing formation of circular and crisscross LIPSS

Orientations of laser induced periodic surface structures (LIPSS) are usually considered to be governed by the laser polarization state. In this work, we unveil that fluid dynamics induced by femtosecond (fs) laser ablation in liquid (fs-LAL) can easily break this polarization restriction to produce irregular circular-LIPSS (CLIPPS) and crisscross-LIPSS (CCLIPSS). Fs laser ablation of silicon in water shows formation of diverse LIPSS depending on ablation conditions. At a high power of 700 mW (repetition rate of 100 kHz, pulse duration of 457 fs and wavelength of 1045 nm), single/twin CLIPSS are produced at the bottom of macropores of several microns in diameter due to the formation of strong liquid vortexes and occurrence of the vortex shedding effect. Theoretical simulations validate our speculation about the formation of liquid vortex with an ultrahigh static pressure, which can induce the microstructure trenches and cracks at the sidewalls for fs-LAL of Si and tungsten (W) in water, respectively. At a low power of 50 mW, weak liquid vortexes are produced, which only give birth to curved LIPSS in the valleys of grooves. Consequently, it is deduced that liquid vortex plays a crucial role in the formation of macropores. Mountain-like microstructures induce complex fluid dynamics which can cause the formation of CCLIPSS on them. It is believed that liquid vortexes and fluid dynamics presented in this work open up new possibilities to diversify the morphologies of LIPSS formed by fs-LAL.

The effect of time-varying axial tension on VIV suppression for a flexible cylinder attached with helical strakes

Flexible cylindrical structures, such as top-tensioned risers (TTRs) and tethers of tension leg platforms (TLPs), usually vibrate under the combined excitation of vortex shedding and time-varying axial tension. Helical strakes are widely used to suppress vortex-induced vibration (VIV). The VIV suppression effectiveness of helical strakes for a flexible cylinder under axial tension excitation is still unknown. Model tests were conducted to investigate the effect of time-varying axial tension on VIV suppression for a flexible cylinder fitted with helical strakes. Three tension amplitude ratios (Tv/Tc = 0.1, 0.2 and 0.3, where Tv is the amplitude of the varying tension and Tc is the constant axial tension) and four tension frequency ratios (fv/f1 = 0.5, 1.0, 2.0 and 4.0, where fv is the frequency of the varying tension and f1 is the fundamental natural structural frequency) were considered. The vibration displacements were reconstructed using the measured strains by the modal analysis method. The dominant mode, response frequency and displacement amplitude were determined to show the influence of axial excitation on the dynamic characteristics of the flexible cylinder attached with helical strakes. When the reduced velocity is low, the second-order mode vibrations of the straked cylinder with axial excitation are excited. Due to the axial excitation, the straked cylinder has large cross-flow (CF) and in-line (IL) displacements at low reduced velocities. The displacement contours in the case of fv/f1 = 1.0 are remarkably different from those in other cases. When the reduced velocity increases, the effect of vortex shedding is enhanced. Both fv induced by axial excitation and the fn induced by vortex shedding are prominent, meaning that both vortex shedding and axial excitation affect the vibration of the straked cylinder. Under axial excitation, the IL displacements of the straked cylinder are enlarged, especially in the case of fv/f1 = 1.0. Axial excitation can reduce the VIV suppression efficiency of helical strakes.

Effect of NACA0012 Airfoil Pitching Oscillation on Flow Past a Cylinder

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

Effect of dual vortex shedding on airfoil tonal noise generation

To improve understanding of dual vortex shedding over a trailing edge with regard to airfoil tonal noise generation, synchronized velocity and noise measurements are conducted on a NACA (National Advisory Committee for Aeronautics) 0012 airfoil at angle of attack near zero and a Reynolds number of 220 000. Instantaneous flow fields obtained by particle image velocimetry show the development of separated shear layer, vortex roll-up, and vortex breakup near the airfoil trailing edge. The time-averaged flow fields feature separation bubbles on both sides, and the root mean square values of streamwise velocity fluctuations show triple peak structures. The velocity spectra agree well with the noise spectra in terms of broadband humps and discrete tones. Through proper orthogonal decomposition (POD) analysis, the most energetic modes are identified, which represent global structures in the flow field. The trailing edge noise being an integral effect of the velocity fluctuation near the trailing edge, the POD analysis provides an alternative view for understanding the noise generation mechanism. The first and second modes are dominated by out-of-phase vortex shedding along the airfoil surface and in the near wake, which dominates the high amplitude noise emission process. The third and fourth modes represent in-phase vortex shedding, which dominates the low amplitude noise emission process. The noise source region is determined by the correlation between the velocity and sound pressure, which shows approximately the same periodic pattern as the first and second POD modes.