Trailing Edge Vortex Research Articles

Analysis of fluid-structure coupling during the flapping of dragonfly hind wings under different flexible distributions

Abstract To investigate the influence of flexible distribution on the aerodynamic characteristics of a dragonfly hind wing during flapping flight, this study establishes a simplified plate model of the dragonfly hind wing based on the actual size, while neglecting the wrinkled microstructure. Six different flexible distribution patterns are proposed for the model. The overlapping grid technique is employed to achieve fluid-structure coupling calculations during the flapping motion of the dragonfly hind wing, by customizing the Young’s modulus in the solid region to represent different levels of flexibility. The results show that gradually increasing the flexibility along the chordwise direction of the dragonfly wing can induce a stronger leading-edge vortex during flapping, intensifying the accumulation of vorticity near the wingtip and resulting in a higher peak lift coefficient. The flexible wing has a positive impact on the formation and development of the trailing-edge vortex, leading to an increase in the thrust coefficient peak and time-averaged thrust coefficient of the dragonfly hind wing. Consequently, all aspects of the aerodynamic characteristics are significantly improved.

Influence of flow interactions on the aerodynamic characteristics of tandem self-propelled flapping wings with a nonzero angle of attack

It is common in nature for birds or insects to fly in flocks. This study sought to understand the interaction mechanism between complex flows and the aerodynamic characteristics of flocks of flying organisms by employing the lattice-Boltzmann method to investigate tandem self-propelled flapping wings with an angle of attack of 10°. The effects of the initial heaving phase, the initial spacing between the fore and hind wings, and the phase difference between the heaving motions of the fore and hind wings were investigated. It was found that when the fore and hind wings flap in phase, the initial heaving phase and initial spacing can influence the final locomotive state of the tandem system, resulting in three modes: stable flight, collision, and separation. When the tandem system eventually achieves stable flight, only one equilibrium state is observed. In this equilibrium state, the trailing-edge vortex generated by the fore wing reattaches to the lower surface of the hind wing, resulting in 82.3% lower lift efficiency but 19.9% higher propulsive efficiency when compared to a single wing. When the fore and hind wings flap nearly out of phase, the tandem system has better lift characteristics while maintaining good propulsive performance. These findings improve the understanding of the principles of lift and thrust generation in flock flight and will help guide the design of bionic micro air vehicles.

Numerical investigation on the energy extraction characteristics of parallel multi foils arranged in columns

Abstract The oscillating foil is a focus device in the clean energy utilization field. The presence of an oscillating foil structure within the flow field significantly impacts the energy extraction capabilities of the system. In this paper, three types of parallel multi-foil structures arranged in columns are proposed. The energy extraction characteristics of diverse multi-foil structures under various oscillation conditions are analyzed numerically. The flow field structure and the impact of parallel multi foils on the aerodynamic characteristics are investigated. Results demonstrate that the multi-foil structure with a phase difference of 180° achieves the highest level of energy extraction performance, and the maximal mean power coefficient achieves 0.894, which is 11.03% higher than that of the structure with a smaller phase difference. The size of the trailing edge vortex under the structure with a phase difference of 180° is significantly larger, leading to an obvious fluctuation in the aerodynamic lift.

Flowfield Analysis of Vortex Interactions During Large Sharp-Edged Gusts

Uncrewed air vehicles in urban environments will have flights during terminal operations that are dominated by strong transient aerodynamics. These vehicles are not only lighter and smaller than traditional rotorcraft and helicopters, but in many instances they may be hybrid configurations with lifting surfaces similar to those of fixed-wing aircraft. These differences require further understanding of the physics of these transient aerodynamics, specifically large-amplitude transverse gusts and the resulting vehicle response, where classic indicial theory is no longer valid. This is crucial to the safety and certification of these air vehicles near buildings and populations. Prior efforts identified that gust responses depart from traditional linear theory when the leading-edge vortex (LEV) forms as a distinct feature and departs from the lifting surface, resulting in flow nonlinearities. This paper expands our understanding of the interactional physics of these nonlinear transverse gusts with flow separation. LEV and trailing-edge vortex (TEV) behavior are correlated, and these vortex interactions are studied to understand the impact on flow separation and subsequent aerodynamic behavior. Larger gust ratios are observed to increase the LEV normal translation, while flow separation is driven by the location and magnitude of the TEV.

Quasi-steady aerodynamic modeling and dynamic stability of mosquito-inspired flapping wing pico aerial vehicle.

Recent exploration in insect-inspired robotics has generated considerable interest. Among insects navigating at low Reynolds numbers, mosquitoes exhibit distinct flight characteristics, including higher wingbeat frequencies, reduced stroke amplitudes, and slender wings. This leads to unique aerodynamic traits such as trailing edge vortices via wake capture, diminished reliance on leading vortices, and rotational drag. This paper shows the energetic analysis of a mosquito-inspired flapping-wing Pico aerial vehicle during hovering, contributing insights to its future design and fabrication. The investigation relies on kinematic and quasi-steady aerodynamic modeling of a symmetric flapping-wing model with a wingspan of approximately 26 mm, considering translational, rotational, and wake capture force components. The control strategy adapts existing bird flapping wing approaches to accommodate insect wing kinematics and aerodynamic features. Flight controller design is grounded in understanding the impact of kinematics on wing forces. Additionally, a thorough analysis of the dynamic stability of the mosquito-inspired PAV model is conducted, revealing favorable controller response and maneuverability at a small scale. The modified model, incorporating rigid body dynamics and non-averaged aerodynamics, exhibits weak stability without a controller or sufficient power density. However, the controller effectively stabilizes the PAV model, addressing attitude and maneuverability. These preliminary findings offer valuable insights for the mechanical design, aerodynamics, and fabrication of RoboMos, an insect-inspired flapping wing pico aerial vehicle developed at UPM Malaysia.

Hydrodynamic characteristics of hyperbolic cylinder under periodic pulsating flow at moderately high Reynolds number (Re=3900)

Hydrodynamic characteristics of hyperbolic cylinder under periodic pulsating flow at moderately high Reynolds number (Re=3900)

Fluid–structure–surface interaction of a flexibly mounted pitching and plunging flat plate in proximity to the free surface

This experimental study investigates the fluid–structure–surface interactions of a flexibly mounted rigid plate in axial flow, focusing on flow-induced vibration (FIV) response and vortex dynamics of the system within a reduced velocity range of $U^*=0.29\unicode{x2013}8.73$ , corresponding to a Reynolds number range of $Re=518\unicode{x2013}15\,331$ . The plate, with one and two degrees of freedom (DoFs) for pitching and plunging oscillations, is examined at various submerged heights near the free surface. Results show that the plate exhibits divergence instability at low reduced velocities in both 1DoF and 2DoF systems. As the flow velocity surpasses a critical reduced velocity, periodic limit-cycle oscillations (LCOs) occur, increasing in amplitude until a second critical reduced velocity is reached. Beyond this point, LCOs are suppressed, and the plate experiences an increased static divergence angle with further flow velocity increase. The proximity to the free surface significantly influences the FIV response, with decreasing submerged heights leading to reduced LCO amplitudes and a shift of instabilities to higher reduced velocities. Vortex dynamics are analysed using time-resolved volumetric particle tracking velocimetry and hydrogen bubble flow visualisation. The analysis reveals disruptions in the symmetric flow field near the free surface, causing elongation and fragmentation of vortices in the wake of the plate, as well as vortex coupling. Proper orthogonal decomposition (POD) identifies dominant coherent structures, including leading-edge and trailing-edge vortices, captured in the first and second paired modes. On the other hand, higher POD modes capture the interaction of vortices in the wake and near the free surface.

Revisiting the surface-mounted cube: An updated perspective of the near wake and near-wall flow field

Revisiting the surface-mounted cube: An updated perspective of the near wake and near-wall flow field

Energy dissipation mechanism of tip-leakage cavitation in mixed-flow pump blades

Tip leakage flow is one of the significant factors influencing the internal flow stability of mixed-flow pumps, and in severe cases, it can lead to channel blockage and energy loss. In order to gain a deeper understanding of the energy dissipation mechanism induced by tip leakage vortex cavitation, this study is based on the Wray–Agarwal (WA) turbulence model and the homogeneous flow model, investigating the cavitation flow characteristics of mixed-flow pumps. Additionally, the entropy production theory is employed to evaluate the energy losses within the mixed-flow pump and analyze the components of energy loss in the impeller and guide vanes. The research results reveal that with increasing cavitation intensity, the low-pressure region at the leading edge of the blade extends toward the trailing edge, influencing the static pressure distribution on the blade's pressure side. Leakage flow and the spatial distribution of leakage vortices move closer to the suction side of the blade with increasing cavitation intensity. Cavitation primarily affects the energy losses in the impeller region, with turbulent dissipation being the main source of energy loss. High turbulent dissipation zones are concentrated at the trailing edge of the blade, correlating with recirculation vortices and trailing-edge vortices. This study provides theoretical insights with practical implications for enhancing the cavitation performance of mixed-flow pumps, offering valuable guidance for design and operation.

Serration of the duct trailing edge to affect the hydrodynamics and noise generation for a pump-jet propulsor

Inspired by the silent gliding feather of owl wings, the trailing edge of the duct of a pump-jet propulsor was designed with a similar serrated structure in order to reduce noise generation. Two distinct serrated structures were proposed and evaluated using the detached eddy simulation method with the shear stress transport k−ω turbulence model. The findings indicated that while the hydrodynamic efficiency changed within 1% upon the inclusion of the serrated trailing edge, a significant alteration existed in vortex structures of the wake. More horseshoe and secondary vortices were generated since large-scale vortices induced by the duct were disrupted circumferentially. This phenomenon expedited the distortion and mixing of trailing-edge vortices, causing flow instability. Furthermore, the serrated trailing-edge structure led to noise reduction. Particularly in the 0–1000 Hz range, the sound pressure level behind the duct showed a maximum reduction of 4.43 dB.

A novel dragonfly dual-wing hovering flight model

During the hovering flight of dragonflies, the coupling interaction between the forewings and hindwings leads to a reduction in the lift of each wing. Numerous scholars have reached a unanimous conclusion that under the coupling effect, the lift of the hindwings is significantly decreased. Meanwhile, the coupling of the forewings and hindwings enhances the controllability of dragonfly flight. In this article, a novel hovering flight model termed the partial advanced dual-wing model (PADM) is proposed. This model is capable of increasing the lift of both the forewings and hindwings. The maximum average lift of the forewings is increased by 18.09%, and the maximum average lift of the hindwings is increased by 41.58%. In addition to the shared advantage of enhanced positive pressure on the rear half of the wing surface due to the advanced rotation, the superior performance of the hindwings compared to the forewings is attributed to the hindwings cutting off the trailing-edge vortex ring formed by the coupling of the fore and hind wings during the downstroke phase. The vertical force and energy consumption exhibit a linear relationship with the partially advanced time, independent of the coupled aerodynamic effects. The PADM model not only sustains the weight of the dragonfly but also plays a controlling role in transitioning from a hovering flight model to a vertical leap flight model. Furthermore, it enables dragonflies and micro air vehicles to maintain hovering flight while carrying additional loads.

Inhibited swimming capacity of fish entrained in wake vortices behind a semi-cylinder

Inhibited swimming capacity of fish entrained in wake vortices behind a semi-cylinder

Aerodynamic Performance Assessment of Distributed Electric Propulsion after the Wing Trailing Edge

Distributed electric propulsion (DEP) with four propellers distributed along the rear edge of the wing (pusher DEP configuration) promote aerodynamic interactions to a higher level. To study the aerodynamic performance of DEP with the rear wing through simulations and experiments, the multi-reference frame (MRF) with sliding grid is combined with wind tunnel tests. The obtained results demonstrate that the lift and drag of DEP increase with the angle of attack (AoA) and are related to the relative position of the propellers and wing. The propeller has no significant effect on the lift of the wing, and the lift and the AoA remain linear when the AoA is less than 16°. By contrast, the lift coefficient is much higher than the baseline (isolated wing), and the lift is greatly improved with the increasing drag when the AoA is greater than 16°. This is because the flow around the wing of the pusher configuration remains attached due to the suction of the inflow of the propeller on the trailing edge vortex. In addition, the acceleration effect on the free flow improves the kinetic energy of the airflow, which effectively delays the separation of the airflow in the slipstream region.

Investigation of the aerodynamic effects of bio-inspired modifications on airfoil at low Reynolds number

A numerical study was performed to investigate flow behaviors around bio-inspired modified airfoils compared with NACA 4412 airfoil at Re=5.8x104 by solving the two-dimensional, RANS equations with k-ω STT turbulence model. The obtained results reveal a rather abrupt decrease of lift at stall for the NACA 4412 airfoil in contrast to the mild stall depicted by the top-modified airfoil. As compared to the experimental results of the profiled airfoil in the literature, the characteristic behavior of the variation in the lift coefficient shows resemblance. It is seen that from the velocity distribution results, fluid flowed smoothly along the streamlined nose of NACA 4412 airfoil until α=4⁰ and streamlines adhered well for both airfoils at low angles (0⁰, 2⁰). Smaller circulation bubbles were noticed to settle in the canyons of the corrugated cross-section of the top-modified airfoil. In the wake region of the modified airfoil, there is no obvious large flow separation or circulation region at low angles of attack. However, the blue regions of the dimensionless velocity over the NACA 4412 airfoil and bottom-modified airfoil were narrower than over the top-modified airfoil. The recirculation zone over the airfoil started to enlarge, and the rolling up of the trailing-edge vortex appeared. After α=12⁰, the adverse pressure gradient on the suction side of the airfoils became more intense. In the wake zones, it was seen that the circulation regions grew remarkably and became largest as the angle of attack rose to α=16⁰, which pointed out increased drag forces of airfoils.

Three-dimensional transition and force characteristics of low-Reynolds-number flows past a plunging airfoil

The three-dimensional (3-D) transition of the leading-edge vortex (LEV) and the force characteristics of the plunging airfoil are investigated in the chord-based Strouhal number $St_c$ range of 0.10 to 1.0 by means of experimental measurements, numerical simulations and linear stability analysis in order to understand the spanwise instabilities and the effects on the force. We find that the interaction pattern of the LEV, the LEV from a previous cycle (pLEV) and the trailing-edge vortex (TEV) is the primary mechanism that affects the 3-D transition and associated force characteristics. For $St_c \leq 0.16$ , the 3-D transition is dominated by the LEV–TEV interaction. For $0.16 < St_c \leq 0.44$ , the TEV lies in the middle of the LEV and the pLEV and therefore vortex interaction between them is relatively weak; as a result, the LEV remains two-dimensional up to a relatively high Reynolds number of $Re = 4000$ at $St_c = 0.32$ . For $0.44 < St_{c} \leq 0.54$ , and at relatively low Reynolds numbers, the pLEV and the TEV tend to form a clockwise vortex pair, which is beneficial for the high lift and stability of the LEV. For $0.49 \leq St_c$ , the pLEV and TEV tend to form an anticlockwise vortex pair, which is detrimental to the lift and flow stability. In the last $St_c$ range, vortex interaction involving the LEV, the TEV and the pLEV results in an unstable period-doubling mode which has a wavelength of about two chord-lengths and the 3-D transition enhances the lift.

Effect of spanwise distributed camber morphing on dynamic stall characteristics of a finite-span wing

This paper investigates the influence of the spanwise-distributed trailing edge camber morphing on the dynamic stall characteristics of a finite-span wing at Re = 2 × 105. The mathematical model of the spanwise-distributed trailing-edge camber morphing is established based on Chebyshev polynomials, and the deformed wing surface is modeled by a spline surface according to rib's morphing in the chordwise direction. The computational fluid dynamics method is adopted to obtain flow-field results and aerodynamic forces. The shear-stress transportv-γ model is introduced and the overset mesh technique is adopted. The numerical results show that the spanwise distributed trailing edge morphing obviously changes the aerodynamic and energy transfer characteristics of the dynamic stall. Especially when the phase difference between the trailing edge motion and the wing pitch is −π/2, the interaction between the three-dimensional leading-edge vortex and trailing-edge vortex is strengthened, and the work done by the aerodynamic force turns negative. This indicates that the trailing edge deformation has the potential to suppress the oscillation amplitude of stall flutter. We also found that as the trailing-edge camber morphing varies more complex along the spanwise, and the suppression effect decreases accordingly.

Investigation on transition characteristics of laminar separation bubble on a hydrofoil

The phenomenon of water–jet pump stall can be ascribed to the development of blade boundary layer separation with the transition process playing a significant role in this separation. The hydrofoil is usually used as a simplified model of the water–jet pump impeller blade, and its flow field characteristics have important reference values for analyzing the impeller flow. Based on the transition model and the dynamic mode decomposition method, this article presents the results of a study that was carried out on the stall characteristics of the NACA0009 blunt trailing edge hydrofoil. The transition characteristics of hydrofoil surfaces at different angles of attack (AoA)and Chord-based Reynolds numbers (ReL) are obtained. The hydrofoil boundary layer transition is dominated by natural transition as the AoA is less than 4°, while the transition is dominated by leading-edge separation-induced transition as the AoA is greater than 4°. The investigation yields the dynamic properties of the LSB (Laminar Separation Bubble) as the AoA is varied. The phenomenon known as the deep stall is distinguished by the movement of the stall vortex toward the upstream direction near the trailing-edge region, where it merges with the LSB in the leading-edge region. This phenomenon leads to oscillations in the lift and drag coefficients. The relationship between the LSB and the trailing-edge stall vortex is established using DMD (Dynamic Mode Decomposition) methods. As the phenomenon of the deep stall occurs, it can be observed that the modal energy of the leading-edge LSB is comparatively higher than the modal energy of the trailing-edge stall vortex, inducing the dominant role of the LSB and the movement toward the trailing-edge region and, consequently, the phenomenon of trailing-edge vortex shedding in the hydrofoil. The findings of this study could be guidance for the design of fluid machinery blades.

Flow structure and vortex dynamics of blade surface transitional flow in the near-tip region of a compressor cascade

The mean flow topology and vortex dynamics of the blade surface transitional flow in the near-tip region of a C4 compressor blade were investigated using particle image velocimetry measurements with two configurations. The experiment was conducted in a compressor cascade at a chord Reynolds number of 24 000 and an incidence angle of 0°, and a laminar separation bubble was detected on the aft portion of the blade. In the half-span region of the blade, the separation is essentially two-dimensional without reattachment. The vortex dynamics are dominated by the periodic shedding of separated shear layer vortices and their interaction with the trailing edge vortices. The progressive spanwise evolution in the flow structures and vortex dynamics occur near the blade tip (70%–80% blade height), leading to an advanced, thinner separation. In contrast, the tip leakage vortex dominated region is restricted to approximately 20% of blade height from the blade tip. In this region, secondary flow effects are intensive enough to prevent laminar separation. Between the above two regions, there is a transition region (90% blade height), where the influence of the tip flow on the blade surface flow is relatively slight that merely suppresses the vortex shedding of the separated boundary layer, nor the whole shear layer. In the transition region, the velocity fluctuations are significantly reduced.

Laminar flow over a rectangular cylinder experiencing torsional flutter: Dynamic response, forces and coherence modes

This study investigates the flutter response of a rectangular cylinder model with an aspect ratio of 5 at the Reynolds number Re = 100 via direct numerical simulation. The effects of two key parameters, i.e., the moment of inertia and reduced flow velocity, on the aerodynamic performance and dynamic responses of the cylinder in the state of torsional flutter are investigated. To reveal the flutter mechanism, the high-order dynamic mode decomposition (HODMD) analysis is conducted to decompose the flow field. The results show that both an increase in the moment of inertia and a higher reduced flow velocity lead to a larger torsional amplitude and a corresponding decrease in torque. At the same time, the primary frequency decreases and the size of the shedding vortex gradually enlarges. The vortices shed from the leading edge and the trailing edge of the model form a 2P wake pattern. The leading-edge vortex is significantly larger than the trailing-edge vortex in terms of strength and size. The leading edge plays a dominant role and only contributes to the odd-order HODMD modes while the even-order modes are deemed inconsequential. As the moment of inertia increases, the total energy of the higher-order modes increases, which has the same results as the power spectral density of torque, reflecting increased nonlinearity and complexity of the system. Similarly, increasing the reduced flow velocity at the same moment of inertia has similar excitation effects.

Assessment of icing effects on the wake shed behind a vertical axis wind turbine

To shed light on the effect of the icing phenomenon on the vertical-axis wind turbine (VAWT) wake characteristics, we present a high-fidelity computational fluid dynamics simulation of the flow field of H-Darrieus turbine under the icing conditions. To address continuous geometry alteration due to the icing and predefined motion of the VAWT, a pseudo-steady approach proposed by Baizhuma et al. [“Numerical method to predict ice accretion shapes and performance penalties for rotating vertical axis wind turbines under icing conditions,” J. Wind Eng. Ind. Aerodyn. 216, 104708 (2021)] was implemented, which enables the utilization of appropriate approaches for handling turbine rotation and turbulence prediction for each solver. Proper orthogonal decomposition (POD) was utilized to perform a deep analysis of the wake and aerodynamics of the wind turbine for the clean and iced turbines with large eddy simulation turbulence method. Icing causes the leading edge vortex and trailing edge vortex to separate faster than the clean case resulting in a steeper drop in the power coefficient. As for POD modes, those of the streamwise component of velocity illustrated more difference in the amount of modal energy especially at the first modes proving that the icing phenomenon mainly affects the vortex shedding of the flow structures with larger energy and size. The modes of the transversal component of velocity of the clean and iced cases demonstrated more similarity in essence, which could also be understood from the accumulated energy curve.