Vortex Street Research Articles

Laminar post-stall wakes of tapered swept wings

While tapered swept wings are widely used, the influence of taper on their post-stall wake characteristics remains largely unexplored. To address this issue, we conduct an extensive study using direct numerical simulations to characterize the wing taper and sweep effects on laminar separated wakes. We analyse flows behind NACA 0015 cross-sectional profile wings at post-stall angles of attack $\alpha =14^\circ$ – $22^\circ$ with taper ratios $\lambda =0.27$ – $1$ , leading-edge sweep angles $0^\circ$ – $50^\circ$ and semi aspect ratios $sAR =1$ and $2$ at a mean-chord-based Reynolds number of $600$ . Tapered wings have smaller tip chord length, which generates a weaker tip vortex, and attenuates inboard downwash. This results in the development of unsteadiness over a large portion of the wingspan at high angles of attack. For tapered wings with backward-swept leading edges, unsteadiness emerges near the wing tip. On the other hand, wings with forward-swept trailing edges are shown to concentrate wake-shedding structures near the wing root. For highly swept untapered wings, the wake is steady, while unsteady shedding vortices appear near the tip for tapered wings with high leading-edge sweep angles. For such wings, larger wake oscillations emerge near the root as the taper ratio decreases. While the combination of taper and sweep increases flow unsteadiness, we find that tapered swept wings have more enhanced aerodynamic performance than untapered and unswept wings, exhibiting higher time-averaged lift and lift-to-drag ratio. The current findings shed light on the fundamental aspects of flow separation over tapered wings in the absence of turbulent flow effects.

Far field behaviour of wingtip vortices under synthetic jet-based control

Wingtip vortices have been proven to be detrimental to both aircraft efficiency and safety due to their adverse effects such as wake hazard, blade vortex interaction noise and induced drag. Despite the extensive literature on the subject, the number of experimental works featuring far field velocity measurements under active control is very limited. The present work deals with the experimental investigation of the effectiveness of synthetic jet actuation on the control of wingtip vortices and their wake hazard. In order to preserve the mutual induction of the counter-rotating vortices during their evolution, an unswept, low aspect ratio, squared-tipped, finite-span wing is employed. The synthetic jet actuation is based on triggering the inherent instabilities of Crow and Widnall at different values of the momentum coefficient Cμ with the goal of reducing the vortex strength and obtaining an anticipated vortex break up. Different exit geometries of the synthetic jets have been tested to analyze the effects of the jet velocity and position on the wingtip vortices. Phase-locked measurements of the velocity field in the far wake at a distance from the wing trailing edge from 26 to 80 chord lengths have been performed via stereoscopic particle image velocimetry. The effects of blowing at high momentum coefficient Cμ=1% are demonstrated to be remarkable on the mitigation of the wingtip vortices. On the other hand, both the time and phase-averaged results suggest that, at relatively low values of Cμ (0.2%), using a larger synthetic jet exit section area allows to greatly affect the wingtip vortices' features causing a striking alleviation of the vorticity distribution up to 90% with respect to the baseline reference case. In fact, due to the actuation at low frequency, the vortex instability is prematurely risen up and amplified, leading to early vortices linking and their consequent dissipation.

Model Verification and Experimental Study of The Turbomachinery Wake Flow Field Reconstruction

Abstract A clear understanding of the turbomachinery wake flow field is one of the key problems in developing turbomachinery aerodynamics. The theoretical model of helical vortices is established in cylindrical coordinates (r,θ,z), and the model corresponds to the experimental scheme, which verifies the feasibility of reconstructing flow field by interpolation in r-z plane under different θ. This model shows that theta should not exceed 20 degrees in order to reconstruct the flow field more accurately. Taking rotors as the representative of the turbomachinery, Particle image velocimetry (PIV) is used to measure the velocity field in the rotor wake. Interpolating reconstruction of spacing between different measurement planes, the reconstruction results are relative to the theory, which plane spacing should be less than 20 deg. Using the vorticity weighted average method to calculate the central position of the vortex core, the wandering motion of the tip vortex is obtained. On this basis, a correct conditional average algorithm is proposed to eliminate this effect. Through the analysis method of the circulation, it is proved that the point where the second derivative of the circulation with respect to the radius is 0 corresponds to the radius of the vortex core, and the evolution law of the growth of the vortex core radius is proportional to the square root of the vortex ages. The results provide a reliable experimental method for the quantitative study of the spatio-temporal evolution of complex vortex structures in the three-dimensional flow of turbomachinery wakes.

Trailing-edge fringes enable robust aerodynamic force production and noise suppression in an owl wing model

As one of the unique owl-wing morphologies, trailing-edge (TE) fringes are believed to play a critical role in the silent flight of owls and have been widely investigated using idealized single/tandem airfoils. However, the effect of TE fringes and associated mechanisms on the aeroacoustics of owl wings, which feature curved leading edges, wavy TEs, and several feather slots at the wingtips, have not yet been addressed. In this study, we constructed two 3D owl wing models, one with and one without TE fringes, based on the geometric characteristics of a real owl wing. Large-eddy simulations and the Ffowcs Williams‒Hawkings analogy were combined to resolve the aeroacoustic characteristics of the wing models. Comparisons of the computed aerodynamic forces and far-field acoustic pressure levels demonstrate that the fringes on owl wings can robustly suppress aerodynamic noise while sustaining aerodynamic performance comparable to that of a clean wing. By visualizing the near-field flow dynamics in terms of flow and vortex structures as well as flow fluctuations, the mechanisms of TE fringes in owl wing models are revealed. First, the TE fringes on owl wings are reconfirmed to robustly suppress flow fluctuations near the TE by breaking up large TE vortices. Second, the fringes are observed to effectively suppress the shedding of wingtip vortices by mitigating the flow interaction between feathers (feather-slot interaction). These complementary mechanisms synergize to enhance the robustness and effectiveness of the TE fringe effects in owl wing models, in terms of aerodynamic force production and noise suppression. This study thus deepens our understanding of the role of TE fringes in real owl flight gliding and points to the validity and feasibility of employing owl-inspired TE fringes in practical applications of low-noise fluid machinery.

Ground test and numerical investigation on aerodynamic performance of a quad tilt-rotor aircraft in ground and water effects

Ground and water interferences significantly affect the safety of takeoff and landing for Quad Tilt-Rotor (QTR) aircraft. However, the mechanism is extremely complex. An unsteady numerical simulation method suitable for amphibious QTR aircraft is established and verified through ground tests. This study explores the distinct aerodynamic characteristics of the QTR aircraft in water and ground effects (IWE and IGE) and compares them to those out of ground effect (OGE). Results indicate that the wing, ground or water surface strongly affects the QTR's aerodynamic performance by influencing the downwash flow form the rotors. The lift and torque coefficients exhibit irregular fluctuations during IGE compared with OGE. However the distribution of lift and torque coefficients become more periodic and regular during IWE due to the flexible water surface, which serves as a flow rectifier accelerating the dissipation of ground vortices and rotor tip vortices. The soft water surface is impacted to form a “drainage area”, which increases the distance between the rotor and the water. Therefore, the water effect is not as strong as the ground effect. The higher the height of rotors above the surface, the weaker the interference effects. The water effect vanishes when the vehicle is at least 0.9 times the rotor diameter above the water surface while the ground effect vanishes when the vehicle is at least one rotor diameter above the ground.

Significant influence of fluid viscoelasticity on flow dynamics past an oscillating cylinder

This study focuses on numerically investigating the impact of fluid viscoelasticity on the flow dynamics around a transversely forced oscillating cylinder operating in the laminar vortex shedding regime at a fixed Reynolds number of $Re = 100$ . Specifically, we explore how fluid viscoelasticity affects the boundary between the lock-in and no lock-in regions and the corresponding wake characteristics compared with a simple Newtonian fluid. Our findings reveal that fluid viscoelasticity enables the synchronization of the vortex street with the cylinder motion at lower oscillation frequencies than those required for a Newtonian fluid. Consequently, the lock-in region boundary for a viscoelastic fluid differs from that of a Newtonian fluid and expands in the non-dimensional cylinder oscillation amplitude and frequency parameter space. In the primary synchronization region, the wake of a Newtonian fluid exhibits ‘2S’ (two single vortices) and ‘P+S’ (a pair of vortices and a single vortex) shedding modes. In contrast, a ‘2P’ (two pairs of vortices) vortex mode is observed for a viscoelastic fluid within the same region. To gain a deeper understanding of the differences in the coherent flow structures and their associated frequencies between the two fluids, we employ the data-driven reduced-order modelling technique, known as the dynamic mode decomposition (DMD) technique. Utilizing this technique, we successfully extract and visualize the two competing fundamental frequencies (cylinder oscillation and natural vortex shedding frequencies) and their associated flow structures in the case of the no lock-in state, whereas only the dominant cylinder oscillation frequency and associated flow structure in the case of the lock-in state. Furthermore, we propose that the presence of excess strain resulting from the stretching of polymer molecules in viscoelastic fluids leads to a distinct difference in the wake structure compared with Newtonian fluids. This observation aligns with the findings obtained from the $Q$ -criterion and vorticity transport analysis of the wake.

Effects of blade tip thicknesses on hydrodynamic and noise performance of ducted propellers

Propeller noise plays an important role in underwater radiated noise. The radiated noise of a ducted propeller is closely related to the vorticity intensity in the gap between the blade tip and the inner wall surface of the duct. To investigate the influence of blade tip thickening on the hydrodynamic and noise performance of ducted propellers, six different models of ducted propellers with varying blade tip thicknesses are presented. The Reynolds-averaged Navier–Stokes (RANS) method is used to simulate the flow around the ducted propellers, and a combination of Detached Eddy Simulation (DES) and acoustic boundary element method (BEM) is employed to predict the noise. The results show that thickening the blade tips leads to a 4.15% efficiency loss at J = 0.6, but improves efficiency by 17.61% at J = 0.8. As the blade tip thickness increases, the length of the blade tip vortex gradually decreases, and the turbulent kinetic energy (TKE) in the chordwise middle region of the blade tip decreases. Additionally, the peak values of thrust and torque fluctuations of the blades primarily occur at the blade passing frequency (BPF) and its harmonics, with the minimum amplitudes of the BPF at the maximum blade tip thickness. Thickening the blade tips results in a maximum overall sound pressure level reduction of 0.50 dB for the ducted propeller, with a maximum sound pressure level (SPL) reduction of 4.24 dB at certain dominant frequencies.

Investigation of Hydrokinetic Tidal Energy Harvesting Using a Mangrove-Inspired Device

There is a trend towards harvesting tidal energy in shallow water. This study examined how tidal energy can be harvested using a device of oscillating cylinders inspired by the roots of mangroves. A specific focus was placed on optimising the configuration of these devices, informed by the computational fluid dynamics (CFD) analysis of wake interference in the von Kármán vortex street of the cylinders. A maximum efficiency of 13.54% was achieved at a peak voltage of 16 mV, corresponding to an electrical power output of 0.0199 mW (13.5% of the hydrokinetic energy of the water) and a power density of 7.2 mW/m2 for a flow velocity of 0.04 m/s (Re=239). The configuration of upstream cylinders proved to have a significant impact on the power generation capacity, corroborated further in CFD simulations. The effect of wake interference was non-trivial on the magnitude and quality of power, with tandem arrangements showing the largest impact followed by staggered arrangements. Though with comparatively low energy densities, the device’s efficiencies found in this study indicate a great potential to harvest tidal energy in shallow water, which provides a consistent baseload power to supplement intermittent renewables (e.g., solar and wind).

Adaptive Mesh Refinement for Trailing Vortices Generated by Propellers in Interaction with Slipstream Obstacles

The investigation of cavitating trailing vortices emerging from marine propellers is of great interest in the industry. With the help of computational fluid dynamics (CFD), studying the cavitating trailing vortices may be facilitated. However, limitations in computational power raise the necessity to execute numerical simulations as efficiently as possible. The time-efficient simulation of cavitating trailing vortices interacting with rigid bodies is especially challenging due to the continuous change of cavity locations. This study investigates the usability, capability, and practicability of automatic adaptive refinement at every calculation time step for transient Reynolds-averaged Navier–Stokes (RANS) and large eddy CFD simulations of the cavitating tip and hub vortices, utilizing the Schnerr–Sauer cavitation model, in the presence of a rudder located in the propeller slipstream and for an isolated propeller, with additional focus on the computational effort necessary for using high frequency updating adaptive mesh refinement (AMR). It is found that AMR is suitable for resolving cavities with relative motion to the propeller and in interaction with slipstream obstacles. However, the computation time is significantly increased, which renders this method useful only if a classic AMR is not possible due to geometrical limitations. Even in the cases that benefit from the automated AMR, numerical instabilities may lead to unphysical pressure fluctuations, which reduce the suitability of the method for the evaluation of underwater radiated noise.

A new propulsion enhancement mechanism in metachronal rowing at intermediate Reynolds numbers

Metachronal rowing is a biological propulsion mechanism employed by many swimming invertebrates (e.g. copepods, ctenophores, krill and shrimp). Animals that swim using this mechanism feature rows of appendages that oscillate in a coordinated wave. In this study, we used observations of a swimming ctenophore (comb jelly) to examine the hydrodynamic performance and vortex dynamics associated with metachronal rowing. We first reconstructed the beating kinematics of ctenophore appendages based on a high-speed video of a metachronally coordinated row. Following the reconstruction, two numerical models were developed and simulated using an in-house immersed-boundary-method-based computational fluid dynamics solver. The two models included the original geometry (16 appendages in a row) and a sparse geometry (8 appendages, formed by removing every other appendage along the row). We found that appendage tip vortex interactions contribute to hydrodynamic performance via a vortex-weakening mechanism. Through this mechanism, appendage tip vortices are significantly weakened during the drag-producing recovery stroke. As a result, the swimming ctenophore produces less overall drag, and its thrust-to-power ratio is significantly improved (up to 55.0 % compared with the sparse model). Our parametric study indicated that such a propulsion enhancement mechanism is less effective at higher Reynolds numbers. Simulations were also used to investigate the effects of substrate curvature on the unsteady hydrodynamics. Our results illustrated that, compared with a flat substrate, arranging appendages on a curved substrate can boost the overall thrust generation by up to 29.5 %. These findings provide new insights into the fluid dynamic principles of propulsion enhancement underlying metachronal rowing.

On the characteristics of the wake of a wind turbine undergoing large motions caused by a floating structure: an insight based on experiments and multi-fidelity simulations from the OC6 project Phase III

Abstract. This study reports the results of the second round of analyses of the Offshore Code Comparison, Collaboration, Continued, with Correlation and unCertainty (OC6) project Phase III. While the first round investigated rotor aerodynamic loading, here, focus is given to the wake behavior of a floating wind turbine under large motion. Wind tunnel experimental data from the UNsteady Aerodynamics for FLOating Wind (UNAFLOW) project are compared with the results of simulations provided by participants with methods and codes of different levels of fidelity. The effect of platform motion on both the near and the far wake is investigated. More specifically, the behavior of tip vortices in the near wake is evaluated through multiple metrics, such as streamwise position, core radius, convection velocity, and circulation. Additionally, the onset of velocity oscillations in the far wake is analyzed because this can have a negative effect on stability and loading of downstream rotors. Results in the near wake for unsteady cases confirm that simulations and experiments tend to diverge from the expected linearized quasi-steady behavior when the rotor reduced frequency increases over 0.5. Additionally, differences across the simulations become significant, suggesting that further efforts are required to tune the currently available methodologies in order to correctly evaluate the aerodynamic response of a floating wind turbine in unsteady conditions. Regarding the far wake, it is seen that, in some conditions, numerical methods overpredict the impact of platform motion on the velocity fluctuations. Moreover, results suggest that the effect of platform motion on the far wake, differently from original expectations about a faster wake recovery in a floating wind turbine, seems to be limited or even oriented to the generation of a wake less prone to dissipation.

Experimental investigation on the turbulent wake flow in fully established transonic buffet conditions

The transonic flight regime is often dominated by transonic buffet, a highly unsteady and complex shock-wave/boundary-layer interaction involving major parts of the flow field. The phenomenon is associated with a large-amplitude periodic motion of the compression shock coupled with large-scale flow separation and intermittent re-attachment. Due to the resulting large-scale variation of the global flow topology, also the turbulent wake of the airfoil or wing is severely affected, and so are any aerodynamic devices downstream on which the wake impinges. To analyze and understand the turbulent structures and dynamics of the wake, we performed a comprehensive experimental study of the near wake of the supercritical OAT15A airfoil in transonic buffet conditions at a chord Reynolds number of 2×106\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2\ imes 10^{6}$$\\end{document}. Velocity field measurements reveal severe global influences of the buffet mode on both the surface-bound flow field on the suction side of the airfoil and the wake. The flow is intermittently strongly separated, with a significant momentum deficit that extends far into the wake. The buffet motion induces severe disturbances and variations of the turbulent flow, as shown on the basis of phase-averaged turbulent quantities in terms of Reynolds shear stress and RMS-values. The spectral nature of downstream-convecting fluctuations and turbulent structures are analyzed using high-speed focusing schlieren sequences. Analyses of the power spectral density pertaining to the vortex shedding in the direct vicinity of the trailing edge indicate dominant frequencies one order of magnitude higher than those associated with shock buffet (Stc=O(1)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$St_c=\\mathcal {O}({1})$$\\end{document}) vs. Stc=O(0.1)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$St_c=\\mathcal {O}({0.1})$$\\end{document}). It is shown that the flapping motion of the shear layer is accompanied by the formation of a von Kármán-type vortex street of fluctuating strength. These wake structures and dynamics will impact any downstream aerodynamic devices affected by the wake. Our study, therefore, allows conclusions regarding the incoming flow of devices such as the tail plane.

Modeling of Main Parameters for Near Wake from Pair of Side-by-Side not too Close Cylinders

A one-dimensional model of the near wake produced by a pair of not-too-close cylinders in a viscous incompressible fluid is developed on the basis of the Stuart–Landau model and perturbation theory. The whole wake is considered as two interacting partial von Kàrmàn vortex streets or two coupled Stuart–Landau oscillators, when the nonlinear nature of the interaction of these streets is taken into account. The observable spectrum of the global modes of the wake is obtained, the calculated and experimental eigenfrequencies of the oscillation modes being in agreement with respect to the modes.

Prediction method of tip vortex circulation based on hydrofoil load

In this paper, a theoretical derivation from the perspective of the force on the slender bodies arranged along the hydrofoil span is performed to explain why the vortex circulation Γvortex should theoretically be equal to the maximum bound circulation Γ0 around the hydrofoil. Then, a more accurate and operable prediction formulation based on hydrofoil load, Γvortex=kΓ0‾, is proposed, where k = 0.9–1. Γ0‾ is the averaged hydrofoil circulation along spanwise direction and its value can be obtained easily from hydrofoil load. To validate our new formulation, two typical vortex flows, i.e., tip leakage vortex (TLV) flow and tip vortex (TV) flow, are simulated with large eddy method. Our numerical results confirm that the variations of hydrofoil load along span would lead to streamwise vorticity, and then the rolling-up of latter leads to the vortex formation (Green, 1995). However, since the influential region of vortex is limited that not all streamwise vorticity could be entrained into the vortex and there are viscous and dissipation effects, the actual Γvortex cannot reach Γ0 but it is quite close to Γ0‾. The prediction formulation agrees well with the numerical results and available measured data.

Nested numerical simulation of multi-scale flow characteristics around monopile foundation of offshore wind farm at Weihai coastal water, China

When constructing an offshore wind farm under tidal current conditions, flow field characteristics without structures are often used for design. However, the presence of structure will greatly affect the flow field around structures. Therefore, it is important to study the flow characteristics around the wind farm foundations under the tidal flow conditions to ensure safe construction and operation. Due to the spatial-temporal scale differences of different physical processes, it is difficult to apply the existing numerical models to accurately simulate the flow characteristics around the offshore monopile. Therefore, this study will link the large-scale TELEMAC with the small-scale OpenFOAM code to establish a multi-scale spatial nesting model to consider the temporal and spatial flow characteristics of real ocean environmental conditions. The nested model is validated using the measurements. The validated model is then applied to conduct some parametric study to evaluate the effect of the wind speed and direction and tidal flow on the flow characteristics around the monopile. Results reveal that the tidal flow velocity could be significantly suppressed by strong opposite wind and vice versa. Tidal flow separation occurring at the lee side of the pile generates the so-called von Karman vortex street downstream of the pile.

Investigation on the behavior of flow and aerodynamic noise generated around the tandem seal-vibrissa-shaped cylinder

Through comparing with the tandem circular and elliptic cylinders with the same characteristic dimensions, the behavior of flow and flow-induced noise generated around the tandem seal-vibrissa-shaped cylinder is studied based on delayed detached-eddy simulation model and acoustic analogy approach. The co-shedding pattern of flow developed around the tandem cylindrical-like bars is investigated. The spatial modes, mode energy, and mode coefficients of turbulent flow around the geometries are analyzed through spectral proper orthogonal decomposition. Results show that the lift fluctuations of downstream bar are stronger than those of upstream bar, and more aerodynamic noise is radiated from the downstream bar than from the upstream bar. The alternative arrangement of nodal and saddle planes of seal-vibrissa-shaped cylinder introduces three-dimensional flow separations and suppresses the shear layer interactions, inhibiting the regular vortex shedding of Kármán vortex street occurring in the tandem cylinder wake. The reversed vortex shedding generated by two adjacent saddle surfaces in the wake of seal-vibrissa-shaped cylinder balances the lateral force and mitigates the lift fluctuations greatly, thereafter reduces the aerodynamic noise generated by wall pressure fluctuations introduced by unsteady fluctuating forces exerting on the surfaces of geometries. Compared to the tandem circular and elliptic cylinders, the good noise reduction effect with sound pressure level reduced at main frequency range has been achieved from the tandem seal-vibrissa-shaped cylinder. The calculated spectra and amplitude levels of aerodynamic noise agree well with the experimental measurements from the anechoic wind tunnel, verifying the accuracy of the numerical simulations.

Wing flexibility effect on aerodynamic performance of different flapping wing planforms

Uniform downwash of a revolving or flapping wing is a key indicator of aerodynamic efficiency. Although the slack-angle of flexible wings can significantly alter the downwash due to the inherent aerodynamic twist, its effect on the downwash distribution for a change in planform remains unknown. In this study, the effect of wing flexibility (slack-angle β) on the unsteady aerodynamic characteristics is investigated using three wing planforms. The study reveals that ββ > 5° can suppress the wing planform effect, leading to the generation of the same amount of lift at mid-stroke. To emphasize on the role of wing flexibility, further comparisons are made with rigid cases for each change in planform. A change in planform for the rigid cases significantly affects the position of the tip vortices (TiVs). But, the TiV positions of the flexible wings remained unchanged, helping to generate a wider downwash area to enhance the lift in contrast to that of the rigid-wing counterparts. The study found that the flexible and rigid wings can generate two distinct downwash patterns, which are respectively near uniform and tip-oriented, during the flapping motion. The ability of the flexible wings to sustain the wider near uniform downwash irrespective of the selected planform is paramount for enhanced aerodynamic performance.

The Effect of Blade Coning on the Flow Field of Wind Turbines under Yaw Conditions

Most wind turbines operate under yaw conditions, which can reduce the output power of wind turbines. In order to improve the power of wind turbines under yaw conditions, this paper adopts a method of blade taper to study its impact on the wind turbine flow field. A 2000W wind turbine is chosen as the research object and SOILDWORKS software is employed to establish a model of blade taper. Three cone angles of 4 °, 8 °, and 12 ° were used, and numerical simulations were conducted using STARCCM+. It is found that under 30 ° yaw conditions, the blade with a cone angle of 12 ° had the highest output power, followed by 8 ° and 4 °. Compared to 4 °, the output power of 12 ° and 8 ° increased by 2.32% and 5.99%, respectively. At the same time, appropriately increasing the blade cone angle can also reduce the dissipation of tip vortices and center vortices in the flow field. By analyzing the limit streamline diagram, it can be concluded that the vorticity at an 8 ° cone angle is less than the other two cone angles under 30 ° operating conditions. Therefore, in this simulation, the comprehensive performance of the 8 ° cone angle is better, which not only improves the output power under 30 ° yaw, but also makes the operation of the wind turbine more stable.

Aerodynamic analysis for different operating states of floating offshore wind turbine induced by pitching movement

The floating offshore wind turbine (FOWT) may experience dynamic stall and complex rotor-wake interaction, such as the vortex ring state (VRS) or propeller working state (PWS), under large platform movements, which can lead to intricate alterations in both aerodynamic parameters and flow states. In this paper, we propose a reliable criterion based on the velocity field of FOWT to distinguish its operating states (VRS or PWS). The aerodynamic characteristics and flow mechanism of the FOWT are considered in details as different operating states induced by a large pitch motion are experienced. Excessive angle of attack (AOA) will cause a dynamic stall, generating a large number of stall vortices and inducing violent fluctuations in the normal and tangential forces of the blade. When the wind turbine experiences VRS or PWS, the continuity of the tip vortex is broken by the blade–vortex or vortex–vortex interaction, complicating the flow state and even resulting in a negative thrust and AOA. In comparison with the remainder of the blade, the blade tip is more likely to experience VRS or PWS owing to its higher movement velocity.

Flow-induced noise from a seal-vibrissa-shaped cylinder

Inspired by the special structure of seal vibrissae and their ability to effectively suppress vortex-induced vibrations, this paper studies the characteristics of flow and aerodynamic noise generated around a seal-vibrissa-shaped cylinder at a Reynolds number (based on the hydrodynamic diameter and freestream velocity) of 6 × 104 using the delayed detached-eddy simulation model combined with the acoustic analogy approach. The far-field aerodynamic noise has also been measured at Reynolds numbers of 6 × 104∼1.2 × 105 in an anechoic wind tunnel to compare the seal-vibrissa-shaped cylinder with cylindrical and elliptical bars of the same characteristic dimensions. The calculated spectra of aerodynamic noise agree well with the wind tunnel measurements, verifying the accuracy of the numerical simulations. It is found that the three-dimensional flow separations introduced by the alternative saddle and nodal planes of the seal-vibrissa-shaped cylinder inhibit shear layer interactions and reduce the spanwise coherence, resulting in a wake with no dominant coherent structures and thus suppressing pressure fluctuations on the solid surfaces. The configuration of seal-vibrissa-shaped cylinder eliminates the regular Kármán vortex street which generally occurs in a cylinder wake, and consequently inhibits the tonal peak in the aerodynamic noise. The sound pressure level is also reduced in most frequencies. The consistency of the normalized spectra of the far-field aerodynamic noise and the vortex shedding frequency at different inflow speeds demonstrates the high accuracy of the anechoic wind tunnel measurements. Compared with cylinders with circular or elliptical cross sections, the seal-vibrissa-shaped cylinder effectively reduces the flow-induced noise by, respectively, 13 dB or 11 dB for a far-field receiver of 90° at Reynolds number of 6 × 104, showing great potential in reducing aerodynamic noise.