Vortex Street Research Articles

Analysis of NREL-5MW wind turbine wake under varied incoming turbulence conditions

The offshore environment is an ideal site for wind farm development as it offers abundant wind energy resources. Investigating offshore wind turbine wake under different turbulence conditions is essential for enhancing energy generation efficiency and promoting sustainable development. This paper utilizes a synthetic turbulence method called Discretizing and Synthesizing Random Flow Generation (DSRFG) to generate four categories of turbulence environments based on International Electrotechnical Commission design requirements for offshore wind turbines. A standalone NREL offshore 5-MW baseline wind turbine is then simulated using the actuator line method coupled with large eddy simulation. Proper Orthogonal Decomposition is employed to analyze the wake characteristics under different incoming turbulence conditions. For the overall wake behaviors, the flow mode, in which some tip vortices flow back towards the hub, become more significant with higher incoming turbulence intensity. Those vortices disturb the turbine and result in unfavorable effects. Meanwhile, when the detailed characteristics of the wake are studied, each primary mode experiences an increase in the number of small-scale vortices with higher incoming turbulence intensity. It results from the interaction between the wake and outer flow, which accelerates wake recovery and benefits the downstream turbine.

Reconstruction and prediction of the rotor wake flow field in hovering state

A clear understanding of the flow structure and evolution of the rotor wake flow field is one of the key problems to be solved in the development of rotor aerodynamics. The tip vortices are the primary structure of the rotor wake flow field. A theoretical model of the vortex ring was established to verify the feasibility of flow field reconstruction. Particle image velocimetry (PIV) is used to measure the two-dimensional velocity field in the rotor wake. Because of the wandering motion of tip vortices, a new conditional average algorithm is proposed to deal with the flow field. On this basis, it is proposed that the plane interval of the rotor wake flow field reconstructed by interpolation should be less than 20 deg. The vortex core radius is obtained by the analysis method based on the circulation. A Back Propagation (BP) neural network was established to predict the axial and radial positions of the tip vortices and the radius of the vortex core. The inputs are the vortex ages measured by the experiment, and the outputs are the position and radius of the vortex core under any vortex ages. The predicted values were in good agreement with the experimental results. The study provides further analytical methods and new insights for the characteristics and evolution of tip vortices.

Maximizing wind farm power output with the helix approach: Experimental validation and wake analysis using tomographic particle image velocimetry

AbstractWind farm control can play a key role in reducing the negative impact of wakes on wind turbine power production. The helix approach is a recent innovation in the field of wind farm control, which employs individual blade pitch control to induce a helical velocity profile in a wind turbine wake. This forced meandering of the wake has turned out to be very effective for the recovery of the wake, increasing the power output of downstream turbines by a significant amount. This paper presents a wind tunnel study with two scaled wind turbine models of which the upstream turbine is operated with the helix approach. We used tomographic particle image velocimetry to study the dynamic behavior of the wake under the influence of the helix excitation. The measured flow fields confirm the wake recovery capabilities of the helix approach compared with normal operation. Additional emphasis is put on the effect of the helix approach on the breakdown of blade tip vortices, a process that plays an important role in re‐energizing the wake. Measurements indicate that the breakdown of tip vortices and the resulting destabilization of the wake are enhanced significantly with the helix approach. Finally, turbine measurements show that the helix approach was able to increase the combined power for this particular two‐turbine setup by as much as 15%.

Photoelectron jets and interference structures in ionization beyond dipole approximation: carrier- envelope phase effects and vortex streets.

We have analyzed photoelectron jet formation in strong-field ionization using the hydrodynamic picture of quantum mechanics. We showed that von-Kármán-like vortex streets emerge in between jets in photoelectron momentum distributions. The spatial orientation of the jets can be controlled by tailoring the carrier-envelope phase of the driving laser pulse. This indicates that it is possible to experimentally measure emitted photoelectrons off the optical axis, which opens up new possibilities for high-frequency laser pulse diagnostics.

Analysis of the wake mechanism in external flow around tandem bluff bodies with different aspect ratios

The interaction mechanism of external flow with two inline rectangular cylinders having different aspect ratios under the impact of gap spacing (G) is the subject of this research. The gap spacing between the cylinders was varied from 0.25 to 20 times their size. Both cylinders were vertically mounted, with the first having a higher aspect ratio than the second. The results revealed five distinct flow patterns under the influence of G: single slender body, shear layer reattachment, intermittent shedding, binary vortex street, and single-row vortex street. The mean pressure on both cylinders was found to vary due to changes in flow patterns. Both cylinders bore the same shedding frequency but had different pressure variations. The second cylinder placed in the wake of first experienced negative average drag force for some spacing values, while the first cylinder had positive average drag values for all chosen G. Due to the change in flow pattern from shear layer reattachment to intermittent shedding flow, the negative drag force on the second cylinder jumped to a positive drag. It was also observed that the rms values of drag and lift force coefficients, as well as their amplitudes for the second cylinder, were mostly higher than corresponding values for the first cylinder at all selected G. This study revealed that G = 4 and 8 are the critical gap spacing values due to sudden changes in fluid force parameters.

Analysis of Aerodynamic Loads on Heliostats at Operation Position Using Large Eddy Simulation and the Consistent Discrete Random Flow Generation Method

Abstract This study utilizes the large eddy simulation model (LES) and a synthetic method based on the Fourier technique called consistent discrete random flow generation (CDRFG) to analyze the peak aerodynamic loads on heliostats due to the atmospheric boundary layer. With the CDRFG technique, key flow parameters, including mean velocity profile, turbulent intensities, integral length scales, and turbulent spectra generated in wind tunnels, can be replicated while also satisfying the divergence-free condition. A three-facet heliostat with an elevation angle of α = 45 deg and the rear aligned to the inflow was analyzed. The heliostat behaves like a lifting surface in this orientation, accentuating the aerodynamic effect. The methodology proposed in this study can accurately reproduce flow statistics and predict the peak loads. Compared to experimental data, differences of 2.62% for drag, 7.43% for lift, and 11.0% for overturning were observed. Furthermore, the simulation reveals the generation of wingtip vortices on the sides of the heliostat, which contribute to the aerodynamic load. Overall, this technique has been demonstrated to be effective in replicating the atmospheric boundary layer and predicting the aerodynamic coefficients of heliostats.

Influence of an upstream attached flat plate on the newtonian flow past a square cylinder

In this study, thelattice Boltzmann method is used to analyze the flow around a single square cylinder with a flat plate attached atfront side of the cylinder. In order to perceive the impact of the attached flat plate on flow characteristics of the square cylinder, the dimensionless plate lengths are variedfrom 0.1 to 6, while the Reynolds number is fixed at 150. Corresponding to all plate lengths the well-known von Karman vortex street is observed in the cylinder’s wake region. However, the increase in plate’s length results in reducing recirculation regions, enlarging the wake length and thus stabilizing the flow. The fluid forces and velocity field are significantly affected with increment in plate length. A maximum of 65%, 76%, 62% and 11% reduction is observed in the mean drag coefficient (CDm), root-mean-square values of drag coefficient (CDrms) and lift coefficient (CLrms) and amplitude of lift coefficient (CLAmp), respectively, as compared to single cylinder without a flat plate. Pressure is also found to be significantly reduced due to presence of plate at front surface of cylinder.

Chaotic dynamics of two-dimensional flows around a cylinder

We study flow around a cylinder from a dynamics perspective, using drag and lift as indicators. We observe that the mean drag coefficient bifurcates from the steady case when the Karman vortex street emerges. We use standard dynamics metrics to analyze the drag/lift phase diagram. We also find a jump in the dimension of the drag/lift attractor just above Reynolds number of 200, indicating a breakdown of periodicity. We compare the simulated drag values with experimental data obtained over the last hundred years. Although there is substantial agreement, we note a Reynolds number range where there is a substantial, unexplained discrepancy with data from over a hundred years ago. Our simulations suggest that a vibrational resonance in the cylinder would be unlikely for Reynolds numbers greater than 1000, where the drag/lift behavior is fully chaotic.

Numerical study on aerodynamic performance during the process of entering the formation flight

. Two or more planes can fly in close formation similar to migratory birds, making use of the wing-tip vortex of the leader plane to increase lift and reduce drag, thereby effectively improving the flying range. By conducting wind tunnel tests and numerical simulations, the aerodynamic performance of formation flight at different relative positions can be obtained, the optimal formation position can thereby be solved. However, significant nonlinear and unsteady aerodynamic characteristics, induced by the interference between the follower plane and the wing-tip vortices of the leader plane, will affect the flight safety of the whole formation. At present, there is no effective prediction methods. Numerical simulations adopting adaptive grid refinement and dynamic overset grid were conducted for the dynamic entering process of the formation flight containing two Ty-154 planes. The aerodynamic characteristics and vortex interferences were analysed considering the effects of approaching direction and speed. The results indicate that there is deterioration of stability at the position where maximum lift gain is reached; Compared to the entering speed, the impact of directions on dynamic aerodynamic characteristics is more significant.

Contribution of interphase force curl to rigid vorticity transport in water-sand two-phase flow with fine particles

Rigid vorticity transport equation is an effective tool for describing the intuitive vortex evolution characteristics. Compared to single-phase flows, the curl of the interphase force appears as a new source term of this equation under the condition of multiphase flows, which may cause additional contributions. However, the effects of the interaction force on rigid vorticity transport in water-sand two-phase flows with fine particles are still unclear. In this article, taking the Karman vortices induced by a hydrofoil as a typical case, the distributions of rigid vorticity in single-phase flows and two-phase flows were compared, and the dynamics mechanism of the dominant interaction force was analyzed. The following notable results are obtained. Firstly, the drag force can be regarded as the dominant interaction force. Secondly, the effect of the drag force on a vortex tube is mainly manifested as inducing normal strain and the contribution is relatively low. Thirdly, there are only slight differences in the waveform, amplitude, frequency of rigid vorticity and apparent vortical structures between the single-phase flows and the fine-particle two-phase flows. These new findings are helpful for understanding the vortex evolution in water-sand two-phase flows with fine particles.

Dynamic hysteresis process in tip vortex cavitation

In this Letter, experiments are conducted on tip vortex cavitation (TVC) over a stationary elliptical hydrofoil with the cross section of NACA (National Advisory Committee for Aeronautics) 662−415 in a water tunnel. The state of TVC at various cavitation numbers, inflow velocities, and angles of attack is tested. Several macroscopic phase transition processes of TVC are discovered within the hysteresis loop with the operating conditions unchanged, and these new phenomena are defined as the dynamic hysteresis (DH) processes. In addition, the noise characteristics are analyzed to further study the DH. Similar noise spectra are obtained for the DH processes under different working conditions, with a major peak of the same frequency regardless of whether vortex singing occurs or not.

Large eddy simulation of end effects on a cylinder rotor

Due to the limited length of cylinders, their use in practical engineering inevitably involves end effects, which results in three-dimensional flows at the ends of cylinders. These flow fields are the main factor influencing the aerodynamic and flow field characteristics of cylinders. Regarding a finite-length cylinder rotor, a specific pattern of tip vortices will form under the action of rotation, resulting in notable differences in the aerodynamic characteristics between an ideal two-dimensional cylinder and a finite-length cylindrical rotor. In the present study, the large eddy simulation method is used to systematically investigate cylinder rotors with various aspect ratios. By analyzing the sectional aerodynamic and flow field characteristics, the variation laws of the aerodynamic force, wind pressure, and flow field characteristics of cylinder rotors under the influence of end effects were summarized. The results show that the influence range and intensity of the tip vortices on the rotor flow field and sectional aerodynamic characteristics are dominated by the dimensionless rotating speed, which in turn affects the range of the end effects. The development trend of the tip vortices is analyzed and discussed from multiple aspects, including sectional aerodynamics, the pressure coefficient, and the flow correlation, and an attempt is made to explain changes in the phenomenon.

Numerical analysis of accelerating and decelerating ducts modified by camber on the unsteady flow field of pump-jet

Utilizing the Reynolds-averaged Navier-Stokes equations method in conjunction with Detached Eddy Simulation model, this paper investigates effects of duct camber (f) on transient hydrodynamic characteristics, particularly propulsion performance and flow field behavior. The research begins by validating the numerical methodology through experimental and numerical assessments of propeller VP1304 and a pump-jet operating under mooring conditions. The exploration commences with examining propulsion characteristics, followed by analyzing time-domain and frequency-domain data, wherein thrust fluctuations and pulsating pressures are scrutinized via fast Fourier transform (FFT). The pressure distribution and velocity field are subsequently presented to unveil the mechanisms triggered by variations in f. Comparative findings highlight that in cases of pump-jets with lower camber, the outlet velocity exceeds the inlet velocity, a trend contrary to scenarios involving higher f values. Additionally, by analyzing vorticity magnitude distribution and vortices, this study attains comparative insights into the effects of accelerating and decelerating ducts on rotor and stator trailing vortices, offering a window into the flow instability mechanism amidst diverse duct configurations. The results showcased to demonstrate the substantial impact of f variation on hydrodynamic properties. This comprehensive investigation yields practical guidance for the optimal design of pump-jets, potentially informing future design endeavours.

Influence of posture during gliding flight in the flying lizard Draco volans

The agamid lizards of the genus Draco are undoubtedly the most renown reptilian gliders, using their rib-supported patagial wings as lifting surfaces while airborne. Recent investigations into these reptiles highlighted the role of body posture during gliding, however, the aerodynamics of postural changes in Draco remain unclear. Here, we examine the aerodynamics and gliding performances of Draco volans using a numerical approach focusing on three postural changes: wing expansion, body camber, and limb positioning. To this aim, we conducted 70 three-dimensional steady-state computational fluid dynamics simulations of gliding flight and 240 two-dimensional glide trajectory calculations. Our results demonstrate that while airborne, D. volans generates a separated turbulent boundary layer over its wings characterized by a large recirculation cell that is kept attached to the wing surface by interaction with wing–tip vortices, increasing lift generation. This lift generating mechanism may be controlled by changing wing expansion and shape to modulate the generation of aerodynamic force. Furthermore, our trajectory simulations highlight the influence of body camber and orientation on glide range. This sheds light on how D. volans controls its gliding performance, and conforms to the observation that these animals plan their glide paths prior to take off. Lastly, D. volans is mostly neutral in pitch and highly maneuverable, similar to other vertebrate gliders. The numerical study presented here thus provides a better understanding of the lift generating mechanism and the influence of postural changes in flight in this emblematic animal and will facilitate the study of gliding flight in analogous gliding reptiles for which direct observations are unavailable.

Heat Transfer Enhancement of a Mini Channel Heatsink by Utilizing Flexible Beams Placed Downstream of a Cylindrical Obstacle

Several studies have demonstrated that flexible vortex generators exhibit higher hydrothermal efficiency than their rigid counterparts. The present study focuses on the implementation of a cylindrical rigid obstacle located in the vicinity of the inlet of a mini channel. Downstream, several flexible beams are attached to the channel walls, oscillating due to the forces exerted by the Von-Karmann vortex street generated by the cylinder. The Reynolds number is sufficiently low to maintain a laminar regime throughout the mini channel. A parametric analysis is conducted to explore the performance of various configurations of flexible beams with differing numbers, heights, and inter-distances. The hydrodynamic, thermal, and hydrothermal performance of the mini channel is evaluated using the time average friction factor, Nusselt number, and thermal efficiency factor. Results indicate that the presence of flexible beams significantly enhances heat transfer. The best configuration increases the overall thermal efficiency by 45%, while elevating the heat transfer and mechanical loss by 3.05 and 8.52 times, respectively, in comparison with an empty mini channel.

Hydrodynamic analysis of fin–fin interactions in two-manta-ray schooling in the vertical plane

This study investigates the interaction of a two-manta-ray school using computational fluid dynamics simulations. The baseline case consists of two in-phase undulating three-dimensional manta models arranged in a stacked configuration. Various vertical stacked and streamwise staggered configurations are studied by altering the locations of the top manta in the upstream and downstream directions. Additionally, phase differences between the two mantas are considered. Simulations are conducted using an in-house developed incompressible flow solver with an immersed boundary method. The results reveal that the follower will significantly benefit from the upstroke vortices (UVs) and downstroke vortices depending on its streamwise separation. We find that placing the top manta 0.5 body length (BL) downstream of the bottom manta optimizes its utilization of UVs from the bottom manta, facilitating the formation of leading-edge vortices (LEVs) on the top manta’s pectoral fins during the downstroke. This LEV strengthening mechanism, in turn, generates a forward suction force on the follower that results in a 72% higher cycle-averaged thrust than a solitary swimmer. This benefit harvested from UVs can be further improved by adjusting the phase of the top follower. By applying a phase difference of π/3 to the top manta, the follower not only benefits from the UVs of the bottom manta but also leverages the auxiliary vortices during the upstroke, leading to stronger tip vortices and a more pronounced forward suction force. The newfound interaction observed in schooling studies offers significant insights that can aid in the development of robot formations inspired by manta rays.

Influence of rotor blade flexibility on the near-wake behavior of the NREL 5 MW wind turbine

Abstract. High-fidelity computational fluid dynamics (CFD) simulations of the National Renewable Energy Laboratory (NREL) 5 MW wind turbine rotor are performed, comparing the aerodynamic behavior of flexible and rigid blades with respect to local blade quantities as well as the wake properties. The main focus has been set on rotational periodic quantities of blade loading and fluid velocity magnitudes in relation with the blade tip vortex trajectories describing the development of those quantities in the near wake. The results show that the turbine loading in a quasi-steady flow field is mainly influenced by blade deflections due to gravitation. Deforming blades change the aerodynamic behavior, which in turn influences the surrounding flow field, leading to non-uniform wake characteristics with respect to speed and shape.

On the simulation of a wing with detached end-plates using an actuator line based on an anisotropic Gaussian kernel

The results of an actuator line method (ALM) using an anisotropic Gaussian kernel for both, velocity sampling and force projection, are compared to the wall-resolved results of a NACA 0015 rectangular wing at an angle of attack of 10°. The rectangular wing is simulated both, with and without, detached end-plates to show that the ALM is capable of accounting for the presence of a narrow gap of 0.01 c between the tip of the wing and the end-plate. The anisotropic kernel shape is varied in the chordwise, thickness-wise, and spanwise directions for different grid sizes and number of actuating points. The body forces are truncated and regularized to enforce proper spacing between the actuating line and the end-plate. ALM and wall-resolved results are compared on the basis of the integrated lift and drag coefficients, the sectional lift and drag coefficients, as well as the tip vortex core size, position, and circulation. Recommendations are made regarding the optimal kernel shape for a given number of actuating points and mesh resolution.

Extensive study of flow characters for two vertical rectangular polygons in a two-dimensional cross flow

Fluid dynamics problems have a significant impact on the growth of science and technologies all over the world. This study investigates viscous fluid’s behavior when interacting with two rectangular polygons positioned vertically and aligned in a staggered configuration. Two physical parameters, Reynolds Number and Gap spacings, are discussed using the Lattice Boltzmann Method for two-dimensional flow. Results are discussed in vortex snapshots, time trace histories of drag and lift coefficient, and power spectra analysis of lift coefficient. Nine distinct flow vortex streets are identified based on increasing gap spacings between the pair of two rectangular polygons. The vortex shedding mechanism is disturbed at small gap spacings and becomes optimal at large gap spacings. Different physical parameters of practical importance, like mean drag coefficient, root mean square values of drag coefficient, root mean square values of lift coefficient, and Strouhal number, approach the single rectangular polygon value at large gap spacings.

Effects of kinematic parameters and corrugated structure on the aerodynamic performance of flexible dragonfly wings

The effects of unsteady motions of flapping flat plates and corrugated structures in different parameters are studied using fluid–solid coupling and overlapping grid methods. Based on the dragonfly’s right forewing and right hindwing model, these actions include sweeping and pitching, take-off acceleration, and tandem wings cruising in the reverse phase at 180°. The results show that when the advance ratio J=0.36, the “inflow deflection” improves the aerodynamic force in two degrees of freedom compared to simple flapping. When considering only the impact of flexibility, the aerodynamic forces of flexible flat plates and corrugated structures are better than those of the rigid wing models. Considering the effect of corrugated structures, the lift of flexible corrugated wings diminishes, but more thrust is generated. From the perspective of vortex street, vortex rings materialize only in the downstroke stage, while the attachment effect of leading-edge vortices is noticeable in several models. In the same phase flapping, the two wings combine to form a giant wing, which generates significant forward flight momentum. In out-of-phase flapping mode, the series wings generate two lifts and two or three thrust peaks to attain the required forward flight speed while sustaining a high lift.