Vortex Structures In Wake Research Articles

Transverse vortex-induced vibration of a sphere with the application of Lorentz force at low Reynolds number

In this study, the transverse vortex-induced vibration (VIV) of an elastically mounted sphere with the application of a streamwise Lorentz force is investigated through direct numerical simulation. The research parameter range is 300 ≤ Re ≤ 1100 and −0.8 ≤ N ≤ 1, where Re is the Reynolds number and N is the interaction parameter of the Lorentz force. The dependence of sphere responses, forces, and wake structures on Re and N is illustrated in detail. Within this range, two oscillation patterns are identified: VIV and desynchronization. Three wake patterns are identified: two-sided hairpin vortex emerges in the VIV region, while one-sided hairpin vortex and double-threaded wake structures are observed in the desynchronization region. The evolution of these wake patterns is related to the motion of the rear stagnation point (RSP) and separation line (SL) on the sphere surface. A large positive or negative Lorentz force suppresses the motion of RSP and SL, leading to the one-sided hairpin vortex or double-threaded wake structures replacing the two-sided hairpin vortex. Finally, the oscillation patterns are summarized on a map of amplitude response contours in the Re-N space.

A study on the wake structure of an ascending submersible with silk flexible appendages using continuous wavelet transform and dynamic mode decomposition

This study proposed a new method for installing silk flexible appendages on the surface of the submersible to modify the wake structure of ascending submersibles, and explored a method of drag reduction of ascending submersible. A comparative analysis of the flow structure of submersibles with varying appendage lengths was conducted to understand the disturbance characteristics of the wake flow structure of submersible, and the high-speed particle image velocimetry (PIV) measurement experiment was conducted at a Reynolds number of 6456. Analysis of the time-averaged flow field showed that the flexible appendages could disrupt the two large-scale vortices in the wake of the submersibles during the sailing process, but this ability diminished with increasing appendage length. Furthermore, continuous wavelet transform (CWT) and dynamic mode decomposition (DMD) were used to analyze the mechanism behind this phenomenon. The analysis results showed that flexible appendages based on CWT had an inhibitory effect on large-scale flow, and this effect gradually decreased with increasing appendage length. The results indicate that the flexible appendages can disrupt the wake vortex structure, reduce vortex energy, and facilitate the transition from large-scale vortex to small-scale vortex. Additionally, excessive disturbance is generated in the wake region when the flexible appendage is too long, hindering the shedding of small-scale vortices and resulting in an increase.

Reynolds and Mach Number Effects on Propulsive Performance for a Pitching Airfoil

In this work, numerical simulations are conducted to investigate the effects of Reynolds (Re) and Mach (Ma) numbers on thrust characteristics for a pitching airfoil. The results show that as Re increases, the thrust performance is improved. The inverse square root of Reynolds number is demonstrated to be a universal scaling law for both thrust coefficients and drag-to-thrust crossovers across Re=1×103∼1×106 and Ma=0∼0.3. In contrast, as Ma increases above 0.1, the thrust performance is significantly reduced, especially when Ma approaches 0.3. The thrust coefficients follow the square law of the Mach number, which also exhibits universality. Furthermore, the incompressible vortex projection method and the compressible vortex projection method are employed for thrust decomposition. Before this, the full expression of the compressible vortex projection method is derived. It is revealed that as Re increases, the improvement of thrust performance is primarily attributed to the reduced flow viscous effect and, meanwhile, benefits from the formation of more intense boundary layers and vortex structures in near wake. As Ma increases, the enhancement of local flow compressibility ultimately leads to the divergence of added mass drag and thereby reduces the thrust performance.

Experimental and numerical study on characteristics of wake and vortex structure of cluster personnel

The purpose of this study is to study the characteristics of unsteady flow field caused by human activities in crowded public space. This study analyses the wake and vortex structure characteristics of cluster personnel through experiments and numerical simulations. The wake velocity field and vortex structure of 2 (columns) × 5 (rows) human walking were analysed by experimental measurement and large eddy simulation (LES). Personnel movement is simulated by using the dynamic mesh model of computational fluid dynamics (CFD). The results show that the wake of the leg is fully developed, and the wake end presents a “swallow-tail shaped”. The rear row personnel will make a secondary disturbance to the wake of the front row personnel in the wake of the cluster personnel. Therefore, there will be a process of mutual transformation of large-scale vortices and small-scale vortices in the wake field. In addition, the wake flow significantly affects the diffusion and propagation of respiratory pollutants.

A high-resolution numerical investigation of unsteady wake vortices for coaxial rotors in hover

High-resolution numerical simulations for wake vortical flows have long been a challenge in rotor aerodynamics. A novel spectrum-optimized sixth-order Weighted Essentially Non-Oscillatory (WENO) scheme is proposed to discretize inviscid fluxes on moving overset grids, and the Improved Delayed Detached Eddy Simulation (IDDES) is employed to resolve turbulent vortices. The integration of these methods facilitates a comprehensive numerical investigation into the unsteady vortical flows over coaxial rotors in hover. The results highlight the substantial improvement in numerical resolution, in terms of both spatial structure and temporal evolution of unsteady multiscale wake vortices. Coaxial rotors in hover manifest three primary scales of wake vortex structures: (A) the helical evolution of primary blade tip vortices and the periodic occurrence of strong Blade-Vortex Interactions (BVI); (B) the continuous shedding of small-scale horseshoe-shaped vortices from the trailing edges of rotor blades, forming the vortex sheets; (C) the emergence of small-scale secondary vortex braids induced by interactions between rotor tip vortices and the vortex sheets. These vortex structures and their interactions cause high-frequency oscillations in rotor disk loads and induce unsteady perturbations in the local flow field. Interactions among these primary vortices, coupled with the generation of secondary vortices, result in the dissipation, distortion, and breakup of the rotor tip vortices, ultimately forming a vortex soup. Notably, a substantial quantity of seemingly weak small-scale secondary vortex braids significantly contribute to energy dissipation during the evolution of wake vortices for coaxial rotors in hover.

Wake characteristics and vortex structure evolution of floating offshore wind turbine under surge motion

Offshore wind energy is booming and floating offshore wind has been proved to be a promising clean energy. Wake effect is one of the biggest challenges in large-scale floating wind farm which results in power loss and increased fatigue load. In this paper, actuator line model (ALM) coupled with unsteady Reynolds-averaged Navier-Stokes (URANS) is adopted to investigate the wake characteristics and evolution of floating wind turbine under motion. Besides, modal analysis is adopted to extract typical features of turbine wake. Velocity fluctuation is the prominent characteristic of floating turbine wake. The increase of surge frequency results in the decrease of velocity fluctuating region and large surge amplitude leads to bigger velocity deficit fluctuations. In addition, the surge motion brings a faster recovery rate than fixed wind turbine. The induced velocity field generated by vortex ring structure is the key to understanding complex evolution of turbine wake and the vortex ring structure is analyzed in this paper. With the deep understanding of FOWT wake, an innovative vortex ring structure model is proposed in this paper. This work is the basic work of developing the engineering wake model of floating wind turbine, to cope with the wake effect challenge in the large-scale floating wind farm.

Enhancing numerical accuracy in the prediction of rotor wake vortex structures

In modern high-fidelity computational fluid dynamic simulations, the primary vortex system in hover often breaks down into secondary vortices. The sources of numerical error influencing the prediction of the vortex system were studied by performing high-fidelity simulations of the wake of a two-bladed rotor and comparing the predictions to stereoscopic particle image velocimetry measurements in different measurement planes. Various numerical inputs, including sub-iteration convergence, blade pitch offset, and grid resolution, were varied to resolve discrepancies between the measured and predicted vortex characteristics from a previous study done by the authors. A parametric study on near- and off-body solver sub-iteration convergence demonstrated that although the secondary vortex characteristics converged as the sub-iteration convergence of both solvers increased, a large discrepancy in the number of secondary vortices remained. This discrepancy was investigated by varying the thrust, where it was found that the breakdown of the primary vortex is directly linked to the number of secondary vortices. Dissimilarities in the blade pitch angle, which could not be avoided in the experiment, were modeled by intentionally using an offset in the blade pitch angle of the two blades. It was shown that as blade pitch angle offset increases, vortex pairing becomes more distinct. When vortex pairing occurred in both the experiment and simulation, the decay of secondary vortices in the experiment and simulation agreed best. To better match the experimental resolution, grid resolution was increased and comparing the two simulations, the finer mesh simulation agreed best with the measured primary and secondary vortex characteristics.

Numerical study of the evolution of unsteady cavitation flow around hydrofoils with leading-edge tubercles

Humpback leading-edge (LE) tubercles are applied to the cavitation control of hydrofoils, and the effect of LE tubercles on hydrofoil cavitation characteristics under different cavitation numbers (σ) is discussed. The results show that LE tubercles can promote hydrofoil initial cavitation, with cavitation appearing first in the groove. This is because the separation effect of LE tubercles induces flow from peak to trough, resulting in an accelerated flow rate and a local low-pressure area. The quasiperiodic properties of the cloud-cavitation stage are not improved, but LE tubercles considerably reduced hydrofoil cavitation, resulting in a cavitation volume reduction of roughly 16.5%–20.4% and maximum cavitation volume reduction of roughly 10.5%–21.8%. The flow field at the tubercle was analyzed, and it was found that vortex cavitation was induced by the spiral vortex. The pressure pulsation on the hydrofoil is highly related to the cavitating evolution. The dominant frequency of the pressure pulsation increases with the decrease of σ and is not affected by the LE tubercles. Finally, LE tubercles are observed to facilitate the transformation of laminar flow to turbulent flow, hence increasing wake disturbance and facilitating the disintegration of the wake vortex structure.

Wake flow structure and hydrodynamic characteristics of flow around a C-shaped cylinder with variable attack angle at low Reynolds numbers

A numerical investigation is conducted on the flow over a C-shaped cylinder in the low Reynolds number range of Re = 40–160. The effect of attack angle (α) ranging from 0° to 180° is examined simultaneously. Wake evolution and vortex structure as well as the hydrodynamic characteristics are analyzed. Seven flow patterns are identified based on the location of boundary layer separation points and the evolution of near-wall vortices. The boundary layer separation points lock on the two ends of the C-shaped cylinder, resulting in the typical Karman vortex street (Pattern I). A separation point shifts to the curved surface in Pattern II-1 and Pattern II-2, and a quasi-stagnation vortex (QS) is formed within the groove in Pattern II-2. In Pattern III-1 and Pattern III-2, the QS fills the groove. The subordinate vortex is observed in the groove close to the lower end (Pattern IV). The complicated vortex merging occurs around the lower end in Pattern V. The separation points lock on the two ends, exhibiting a pair of counter-rotating vortex shedding downstream of the two ends (Pattern VI). No vortex shedding is found in Pattern VII. Additionally, the characteristic parameters and the hydrodynamic coefficients are related, and they are associated with the flow pattern partition. Four types of vortex street are identified in the wake of the C-shaped cylinder, including no vortex street, 2S vortex mode and decayed vortex street, 2S vortex mode and secondary vortex street (2S-SVS), and P + S vortex mode and secondary vortex street in vortex evolution (P + S-SVS).

Numerical investigation of wind turbine wake characteristics using a coupled CFD-CSD method considering blade and tower flexibility

With the increase in wind turbine sizes, careful investigation into wind turbine wake effect is crucial for operational safety. This study proposes a two-way fluid-structure interaction (FSI) model combining large eddy simulation (LES) and computational structural dynamics (CSD) to investigate wind turbine wake characteristics. Examining the effect of blade and tower flexibility and inflow conditions, the study reveals negligible influence of blade and tower flexibility on mean velocity deficit under both inflow conditions. However, they significantly impact turbulence intensity under uniform inflow condition. The tower motion produces a maximum increase of approximately 45% while the blade flap-wise motion has the second-largest impact, increasing the maximum turbulence intensity by approximately 33% when compared with the fully rigid case. Additionally, the study quantitatively examines wake vortex inclination based on the inclination angle of the second-circle wake vortex structure. Finally, the study concludes that the streamline near the blade tip follows the blade surface, while the flow separation occurs near the blade root, generating two vortices on the suction side. As the tower motion is considered, the larger vortex exhibits alternate vortex shedding while the smaller vortex remains stable in both uniform and turbulent winds.

Numerical Study of Aircraft Wake Vortex Evolution under the Influence of Vertical Winds

Separating wake vortices is crucial for aircraft landing safety and essential to airport operational efficiency. Vertical wind, as a typical atmospheric condition, plays a significant role, and studying the evolution characteristics of wake vortices under this condition is of paramount importance for developing dynamic wake separation systems. In this study, we employed the SST k-ω turbulence model based on an O-Block structured grid to numerically simulate the simplified wing model. We analyzed the variations in the wake vortex structure and parameters of the Airbus A320 during the near-field phase under different vertical wind directions and speeds. The results indicate that favorable vertical winds cause a “flattening” deformation in the wake vortex. Vertical winds reduce the initial vortex strength, accelerate the rate of vortex decay, and influence the trajectory of the vortex core. Notably, under wind speeds of 1~3 m/s, the decay rate is more significant than under 4 m/s. When vertical wind speeds are substantial, it can lead to irregular motion and interactions within the vortex core, forming secondary vortices.

Influences of the wall distance and initial shape on the dynamic behaviors of near-wall bubbles

The phenomenon of buoyancy-driven bubbles near a fluid-solid interface is common in both natural settings and industrial processes, such as oil and gas exploration and production. The intricate dynamics of bubbles are profoundly shaped by their interactions with adjacent solid walls. In this study, a direct numerical simulation of bubbles rising near a wall is conducted using the volume of fluid method. The motion behavior and wake vortex structure of bubbles with three types of migration trajectories (linear, zigzag, and spiral) are analyzed. The influences of the initial shape and wall distance of the bubbles on their motion trajectory, rising velocity, and wake structure is investigated. The results show that the presence of a nearby wall obstructs the diffusion of eddies across a bubble surface, and the repulsive force induced by the wall increases as the distance between the bubble and the wall decreases. Remarkably, at a dimensionless wall distance of 0.6, a change in the lateral lift direction triggers a collision between a zigzagging bubble and the wall, consequently setting the bubble into a bouncing motion mode. In the case of spiral-moving bubbles, proximity to the wall enhances the likelihood of oscillations within the bubble's migration trajectory along the x-y plane while maintaining stability along the z-y plane. While variations in the initial aspect ratio of bubbles have marginal impacts on the migration paths of linear and zigzagging bubbles, the initial shape affects the migration trajectory of spiral bubbles to some extent. The bubble migration trajectory first experiences oscillations when the initial deformation reaches its maximum.

Characteristics of aerodynamic interference and flow phenomenology around inclined square prisms

This study conducts large eddy simulations (LES) to investigate the aerodynamic interference effects and flow field characteristics of the flow around square cylinders, taking into account the inclination of the disturbed structure. The configurations of the structures involve tandem and side-by-side arrangements with the inclination angles of the disturbed structure including +15°, 0°, and −15°. The identification of flow field characteristics involves the examination of multiple components, particularly time-averaged velocity streamlines, axial flow patterns, instantaneous spanwise vortices, and time-averaged wake vortex structures. The results indicate that the vortex structure features of the flow field are significantly influenced by the arrangement type and the inclination angle of the disturbed structure. In contrast to the tandem arrangement, structures arranged in the side-by-side arrangement undergo a considerably reduced intensity of influence from aerodynamic interference effects. The blocking effect of the tandem arrangement and the channel effect of the side-by-side arrangement are undermined when the inclination angle is positive (α > 0). This study enhances the comprehension of aerodynamic interference in inclined prisms and simultaneously establishes a theoretical foundation for the wind resistance design of building structures.

Quantifying the surge-induced response of a floating tidal stream turbine under wave-current flows

Understanding the impact of dynamic effects induced by wave-current conditions on the hydrodynamic performance of a floating horizontal axis tidal turbine (HATT) is crucial toward developing floating tidal turbines to harness tidal energy in deep water sites. The complicated of the wake of a HATT has not yet been fully understood. In this paper, a Computational Fluid Dynamics (CFD) model used to study the performance of a turbine supported by a moored floating platform, due to surge only and in wave-current flows. The CFD model is compared against piled turbine tests, providing an error of 1.36% in power coefficients at the studied TSR = 3.9. It was shown that surge motion caused by even a small wave height or wavelength has a significant effect on the thrust and torque of the rotor. The wave height and period have a remarkable effect on the fluctuation of the hydrodynamic performance due to the induced rotor velocity. The surge motion affects the wake velocity recovery and vortex structure development. For a fuller understanding, future work could explore the negative effects such as induced blade fatigue and platform instability arising from surge motion, as to design cost-efficient turbines, supports and arrangements.

Flow past two rotationally oscillating cylinders

Wake interaction of two rotationally oscillating cylinders in side-by-side configuration is studied experimentally at a Reynolds number of 150. Five spacing ratios, $T/D$ (ratio of centre-to-centre spacing to cylinder diameter), are considered, namely, 1.4, 1.8, 2.5, 4.0 and 7.5. Both in-phase and antiphase forcing are investigated. Oscillation amplitude is varied from ${\rm \pi} /8$ to ${\rm \pi}$ , and forcing frequency, $FR$ (ratio of the oscillation frequency to the vortex-shedding frequency of a stationary cylinder) is varied from 0 to 5. The experimental investigation is done using laser-induced fluorescence, hot film anemometry and particle-image velocimetry (PIV). The interaction between the two cylinders under forcing results in new wake modes and vortex structures and a comprehensive study from the wake visualisations is conducted. Quantitative results are presented in terms of streamwise and cross-stream mean velocity profiles, centreline velocity recovery, peak velocity deficit, wake width, fluctuation intensity, circulation, vorticity contours and drag coefficient. The magnitude of streamwise velocity deficit and cross-stream velocity variation is strongly affected by the presence of second cylinder. The recirculation region behind the cylinders is found to extend further downstream with increase in the forcing. Scaling analysis is carried out to express the peak velocity deficit variation with forcing. It is observed that the relative strength of the vortices shed from inner and outer shear layers depends on the phase of oscillation. An experimental set-up for direct force measurement is designed and the drag force acting on the oscillating cylinders assembly is directly measured and the effect of forcing on the variation of ${C}_{d}$ is studied. An estimate for drag coefficient is also made from the PIV data following a detailed control volume analysis. It is observed that the set of forcing parameters that correspond to maximum and minimum drag also yield extrema in the values of circulation and fluctuation intensity.

Performance improvement of flapping propulsions from spanwise bending on a low-aspect-ratio foil

Inspired from deformable caudal fins, continuous bending motion is utilized to improve the propulsive performance of flapping foils for underwater vehicles. The propulsive performance of a flapping foil with spanwise bending motion is studied numerically using the immersed boundary-lattice Boltzmann method. Simulations are conducted by varying the bending parameters and phases of motion at Re = 200 in uniform flow. It is found that the spanwise bending significantly affects vortex evolution and wake structures, which results in the variation of the flapping propulsion. For the present model with an aspect ratio of two, the most powerful (highest thrust) bending is achieved when fixing the phase lag between bending and flapping at 150°, while the most efficient (highest propulsive efficiency) bending is reached at a phase lag of 270°. With proper phase lags, the propulsive performance could be improved into 137% in thrust and 111% in efficiency from the rigid one. Under the most efficient phase lag, the efficiency could be furtherly improved into 112% through adjusting the bending amplitude due to the weak vortex tangle. The propulsive performance is also sensitive to the bending shape. This study presents a numerical guide for the high-performance flapping foil design with spanwise bending movement.

An investigation on the wake vortex structure and trajectory characteristics of the vehicle launched underwater

ABSTRACT The Improved Delayed Detached Eddy Simulation (IDDES), VOF (Volume of Fluid) multiphase flow model, and overlapping grid technology were adopted in this paper. The results show that the interaction between the high-speed regions and the low-speed flow increases, as the Kelvin–Helmholtz instability phenomenon occurs in the mixing fluid during traveling underwater. Under the effect of the inertial force, the tail bubble expands successively, resulting in an increase in cavity volume and a decrease in pressure. In addition, the wake vortex is composed of the main vortex and secondary vortex, which is a cone of the major influencing factors causing the disturbance of the flow field. As the vehicle continuously passes through the free surface, the buoyancy center of the vehicle successively moves downward, resulting in a change in the direction of buoyancy moment and aggravating the deflection amplitude of the vehicle.

Experimental investigation on the wake structure of teardrop-shape slender body

Flows around the different head length diameter ratio (LE/D = 0.5, 1, and 2, where LE and D refer to the head length and mid-body diameter, respectively) of the teardrop-shaped slender body of revolution with a constant Reynolds number (ReD = 4.1 × 103) were studied using particle image velocimetry. The aim was to explore how the flow geometry affects the hydrodynamic characteristics. Through the time-averaged statistical characteristics of the wall and wake of slender body determined the dynamics performance. Using the power spectral density (PSD) function, proper orthogonal decomposition (POD), and the phase-averaged method, we explored the wake vortex structure, distribution, and strength. The wake of the teardrop-shaped slender body obeys the classical −2/3 decay law at 6 <x/D < 9 and evolves into a self-similar manner. The PSD and POD analyses imply that the POD Modes 1 and 2 are closely related to the large-scale vortex shedding, and the important frequencies of the POD Modes 1 and 2 coincide with the vortex shedding frequency. At the reconstructed flow field of the POD Modes 1 and 2, the analysis of the phase average indicates that the evolution process of a large-scale vortex should involve a periodic three-dimensional helical vortex structure.

Dynamic Analysis of Moving Train Wake Vortex Under Different Infrastructure Scenarios Based on Modal Decomposition

Vortex shedding at the tail of a high-speed train changes the aerodynamic characteristics of the train, which affects the safety and stability of train operation. This paper takes CR400AF as the research object, and uses dynamic monitoring points to realize the whole process monitoring of the flow field at the tail of the train running in open air and in tunnel for the first time. The wake of the train in different infrastructure scenarios is analyzed by the proper orthogonal decomposition method. The study found that the wake vortex structure is quite different when the train runs in different scenarios, and the turbulent kinetic energy intensity of the wake in tunnel is higher than that of the open air running. Modal decomposition method can identify flow structures that have a large impact on train aerodynamics. Through frequency analysis, it is found that the modal frequency obtained from the decomposition is higher when running in open air than when running in tunnel. With the increase of train speed, the modal strouhal number increases when the train is running in open air, and decreases when the train runs in tunnel. After the train enters the tunnel, the reverse movement of the air around the train body suppresses the development and separation of the boundary layer, which is the main reason for the low frequency of wake vortex shedding in tunnel. The stability of the train running in tunnel is worse than that when running in open air, which is closely related to the more complex flow structure around the car body and the drastic change of aerodynamic force when running in the tunnel.

Comparative Analysis of the Hydrodynamic Performance of Arc and Linear Flapping Hydrofoils

In order to improve the hydrodynamic performance of flapping hydrofoils and solve the problem of insufficient hydrodynamic force in plain river network areas, in this study, we consider the more realistic swing of fish tails and propose an arc flapping method, the coupled motion of which has three degrees of freedom: heave, pitch, and lateral displacement. Two flapping methods, positive arcs and negative arcs, were derived on the basis of the lateral displacement direction. By using the finite volume method (FVM) and overlapping grid technology, a numerical simulation was conducted to compare and analyze the pumping performance of three types of flapping hydrofoil, namely, linear, positive arcs, and negative arcs, in order to further provide guidance for the structural optimization of bionic pumping devices. The results showed that the wake vortex structures of the three flapping modes all had anti-Kármán vortex streets, but the wake vortex of linear flapping deflected upward, and the wake vortex of positive arc flapping tended to be further away in the flow field. In one cycle, thrust was always generated by the positive arc flapping hydrofoil and the linear flapping hydrofoil, but the thrust coefficient curve of the positive arc flapping hydrofoil was more stable than that of the linear flapping hydrofoil, and the peak value was reduced by 46.5%. In addition, under the conditions of a flow rate of 750 L·s−1 and an average head of 0.006 m, the pumping efficiency of the positive arc flapping hydrofoil reached 35%, thus showing better pumping performance than the traditional linear flapping hydrofoil under conditions with ultra-low head.