Wake Vortex Research Articles

Analysis of Wind Turbine Wake Dynamics by a Gaussian-Core Vortex Lattice Technique

The development and deployment of the next generation of wind energy systems calls for simulation tools that model the entire wind farm while balancing accuracy and computational cost. A full-system wind farm simulation must consider the atmospheric inflow, the wakes and consequent response of the multiple turbines, and the implementation of the appropriate farm-collective control strategies that optimize the entire wind farm’s output. In this article, we present a novel vortex lattice model that enables the effective representation of the complex vortex wake dynamics of the turbines in a farm subject to transient inflow conditions. This work extends the capabilities of our multi-physics suite, CODEF, to include the capability to simulate the wakes and the high-fidelity aeroelastic response of multiple turbines in a wind farm. Herein, we compare the results of our GVLM technique with the LiDAR measurements obtained at Sandia National Laboratories’ SWiFT facility. The comparison shows remarkable similarities between the simulation and field measurements of the wake velocity. These similarities demonstrate our model’s capabilities in capturing the entire wake of a wind turbine at a significantly reduced computational cost as compared to other techniques.

Energetic Stratified Turbulence Generated by Kuroshio–Seamount Interactions in Tokara Strait

Abstract Generating mechanisms and parameterizations for enhanced turbulence in the wake of a seamount in the path of the Kuroshio are investigated. Full-depth profiles of finescale temperature, salinity, horizontal velocity, and microscale thermal-variance dissipation rate up- and downstream of the ∼10-km-wide seamount were measured with EM-APEX profiling floats and ADCP moorings. Energetic turbulent kinetic energy dissipation rates and diapycnal diffusivities above the seamount flanks extend at least 20 km downstream. This extended turbulent wake length is inconsistent with isotropic turbulence, which is expected to decay in less than 100 m based on turbulence decay time of N−1 ∼ 100 s and the 0.5 m s−1 Kuroshio flow speed. Thus, the turbulent wake must be maintained by continuous replenishment which might arise from (i) nonlinear instability of a marginally unstable vortex wake, (ii) anisotropic stratified turbulence with expected downstream decay scales of 10–100 km, and/or (iii) lee-wave critical-layer trapping at the base of the Kuroshio. Three turbulence parameterizations operating on different scales, (i) finescale, (ii) large-eddy, and (iii) reduced-shear, are tested. Average ε vertical profiles are well reproduced by all three parameterizations. Vertical wavenumber spectra for shear and strain are saturated over 10–100 m vertical wavelengths comparable to water depth with spectral levels independent of ε and spectral slopes of −1, indicating that the wake flows are strongly nonlinear. In contrast, vertical divergence spectral levels increase with ε.

Rotor Power Savings and Pitch-Link Load Reductions with Spanwise-Varying Active Camber Morphing

The effect of spanwise-varying active-camber morphing on helicopter rotor power and pitch-link loads was investigated computationally for advance ratios ranging from hovering to high-speed forward flight (μ=0.35) and was compared to results obtained with spanwise uniform camber actuation. Different actuation strategies were applied, such as constant (0P) deflection and superimposed harmonic actuation based on a 0P deflection combined with 1P (one per revolution) and 2P harmonics. A comprehensive rotor aeromechanics model of an isolated Bo-105 rotor, including elastic blade modeling and free vortex wake aerodynamics, was used for this analysis. Two different active-camber section sizes were investigated, one ranging from 0.22R to 0.9R and one from 0.5R to 0.9R. The greatest benefits from active-camber actuation were found in hover and high-speed flight. It was possible to reduce both the rotor power consumption and the peak-to-peak pitch-link loads. A relevant contribution of the spanwise variability to these power gains was obtained in hovering flight. Toward higher flight speeds, the effect of spanwise variable camber morphing became less important in terms of rotor power savings. However, it allowed a notable reduction of the actuation amplitude far outboard on the rotor blade.

Reviews and Responses for Aircraft Wake Vortices Affecting Airport Wind Measurements

See detailed reviews and responses in the PDF file.
 DOI for the original paper: https://doi.org/10.59490/joas.2023.7212

Numerical Optimization on Aircraft Wake Vortex Decay Enhancement

Blowing air at the end of the airport runway can accelerate the decay of the near-ground aircraft wake vortex, thereby reducing the negative impact of the vortex on the following aircraft. However, the benefits of accelerating wake dissipation vary for different blowing parameters, so it is necessary to set appropriate parameters in order to obtain better acceleration results. Because of the high cost of traditional optimization methods, this research uses a Kriging surrogate model to optimize the blowing parameters. The results show that the current optimization algorithm can deal with the global optimization problem well. After obtaining 205 sample points, the response surface model of the blowing parameters and blowing yield was accurately established. A relatively optimal parameter setting range was given, and numerical simulation shows that the current parameter setting can obtain improved benefits from accelerated vortex dissipation. In addition, since the optimization process is partially dimensionless, the above optimization results can be used to achieve multi-objective design, that is, the same set of blowing devices can efficiently accelerate the dissipative process of the tail vortices of different aircraft types, thus improving the engineering feasibility of the current blowing method.

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.

Impacts of the Bottom Vortex on the Surrounding Flow Characteristics of a Semi-Submerged Rectangular Cylinder Under Four Aspect Ratios

Abstract Vortex has attracted attention because it is the main factor affecting the flow characteristics around offshore structures. This paper researches the flow field variations around a semi-submerged rectangular cylinder (SRC) under four aspect ratios, thereby revealing the bottom vortex effects on the flow field at Reynolds number = 36,300. According to the particle image velocimetry (PIV) data, the renormalization group (RNG) k–ε model is selected as an applicable turbulence model for studying SRC flow characteristics. From the results, the bottom vortex generation and streamwise velocity are inseparable. As the draft increases, the increased range of the bottom vortex would directly interfere with the flow characteristic around the SRC. Since the wake vortex forms a reattachment state when the SRC pierces the free surface, its flow field could be disturbed by the bottom vortex. Furthermore, the bottom vortex also affects the extreme point position of the boundary layer characteristic at the SRC bottom.

Analysis of the effect of arrangement positions on the propulsion performance of dual hydrofoils of wave-powered boats

Wave-powered boats can harness wave energy to drive hydrofoils to generate propulsion, and the arrangement position between dual hydrofoils has an obvious effect on the magnitude of propulsion generated. In this paper, the effects of the arrangement position between dual hydrofoils on the propulsion of wave-powered boats are studied. An instantaneous equivalent model of dual hydrofoils based on the local frame at the equivalent position is developed according to the relative motion principle, and the propulsion of the rear hydrofoil is calculated, and the reliability of the equivalent model results was verified through experiments. Through the augment optimization calculation based on the validated model mentioned above, the propulsion of the rear hydrofoil with different arrangement positions is obtained and the corresponding positions of the hydrofoil with larger propulsion are listed. Subsequently, a fluid-structure coupling model is established on the basis of the results obtained from the optimization calculation, and the dynamic response of the wave-powered boat is solved under different arrangement positions. It is evident that different horizontal and vertical arrangement positions of the dual hydrofoils have different effects on the propulsion performance. This is primarily due to the different influences of the wake vortices generated by the front hydrofoil acting on the rear hydrofoil, causing different fluctuations during its rotation process, thereby affecting propulsion. Overall, a reasonable arrangement position can effectively improve the propulsion of the wave-powered boat. The relevant research in this paper can provide a reference for similar hydrofoil arrangement designs of the wave-powered boat.

Effect of stroke plane inclination on the hovering aerodynamic performance of tandem flapping foils.

The four-winged form of dragonfly and damselfly allows them to fly with great agility and endurance, which are accomplished by independently controlling the kinematics of each wing. In this study, we performed numerical simulations of two tandem airfoils oscillating along an inclined stroke plane atRe=157. We investigated the effects of the stroke plane angle (β)of forefoil and hindfoil on the aerodynamic performance of dragonflies (or damselflies) hovering flight. Simulations were conducted for parallel and non-parallel stroke planes of forefoil and hindfoil oscillating with three phase differences:ϕ=0o,90oand180o. For parallel stroke planes, the results show that the total lift increases withβ, whereas the total thrust decreases in same condition. In addition, the total lift and thrust decrease with an increase inϕ. The forefoil performance is affected by the flow induced by hindfoil's leading-edge vortices, and the hindfoil interactions with forefoil's wake vortices significantly affect the hindfoil's performance. The findings demonstrate that non-parallel stroke planes exert detrimental effects on the total lift forϕ=0oand90o. However, forϕ=180o, the lift augmentation of 46% is obtained in the case where the forefoil stroke plane angle is smaller than the hindfoil. The results obtained during this investigation can help in optimizing the wing kinematics during the micro air vehicles development.

Vortex-induced vibration response of the cylinder inspired by Terebridae

Vortex-induced Vibration (VIV) responses of the three-dimensional cylinders inspired by Terebridae are numerically investigated at the Reynolds number ranges of 0.8 × 104 ≤ Re ≤ 8.0 × 104. By Terebridae, we mean that inspired by its slender structural shape, and whether it can be used to improve the VIV suppression performance of the traditional structures. Three different cylinders are considered, including the smooth cylinder (T0), the Terebridae-inspired cylinder with four-start helical ribs (T1), and the Terebridae-inspired cylinder with six-start helical ribs (T2). The VIV responses of T1 and T2 is effectively suppressed, and T1 shows better suppression performance. The results show that the maximum cross-flow amplitude ratio of T1 is reduced by 70.7 % compared with that of T0, and the maximum in-line amplitude ratio of T1 is reduced by 85.7 % compared with that of T0. Compared with the maximum mean-drag coefficient of T0, it is reduced by 44.0 % for T1. In the high Reynolds number ranges, the galloping phenomenon does not occur for T1 and T2, and the relationship between the mean-drag coefficients of T0, T1 and T2 is Cd‾ (T2) > Cd‾ (T1) > Cd‾ (T0). It is found that the changes of Q-criterion vortex structure and wake flow play an important role in the VIV response. The Q-criterion vortex of T0 is a slightly curved linear shaped, and the Q-criterion vortices of T1 and T2 are break up. The three-dimensional geometry of the Terebridae-inspired cylinder undermines the correlation of vortex-shedding along the span. The wake flow of T0 can develop normally. The wake flow of T1 quickly rolls up after passing through the top separation point, and the reattachment of the wake vortex destroys the development of the original wake flow. The continuous development of the boundary layer is destroyed by multiple Terebridae-inspired ribs of T2, resulting in the broken wake. Through the comparison of vortex force, it can be seen that the vortex force of T1 is significantly weakened compared to that of T0, and this also indicates that the three-dimensional geometry of T1 can effectively reduce the intensity of the wake flow.

Study on the efficient heat transfer mechanism of microchannel pin-fin arrays under low pumping power

The efficient heat transfer with low pressure loss has long been a critical challenge in the microchannel heat sinks (MCHS) design. Further study is required for the quantitative analysis of the longitudinal vortex heat transfer enhancement mechanism. In this research, a microchannel heat transfer experiment and corresponding simulations are conducted. Based on the common Reynolds number range in engineering applications of MCHS, the underlying causes of heat transfer enhancement and pressure loss increase in MCHS with respect to Reynolds numbers is clarified. The results show that when the Reynolds number is 400, the cylindrical wake reduces the synergistic angle between the velocity and the temperature gradient at the streamwise gap, which greatly enhances the heat transfer performance of MCHS. In addition, when the Reynolds number is 900, the early separation of the cylindrical wake vortex leads to a significant reflux near the center of the spanwise gap, which significantly increases the pressure loss. A Different Reynolds number Efficiency Evaluation Criterion (DEEC) is proposed for evaluating the performance of MCHS. It is found that the flow state of cylindrical wake with Reynolds number of 400 has the best heat transfer efficiency. The concept of introducing longitudinal vortices into the pin–fin array can serve as a key guideline in the heat transfer enhancement optimization design of MCHS.

Influence of turbulent coherent structures on the performance and wake of a wind turbine

Wind energy in the atmospheric boundary layer serves as the primary source for energy absorption and structural load on wind turbines. However, the impact of turbulent coherent structures on the aerodynamic performance and wake characteristics of wind turbines has not been comprehensively evaluated. In this study, the proper orthogonal decomposition (POD) method is employed to assess the influence of turbulent coherent structures of varying scales on the aerodynamic performance and wake characteristics of wind turbines in the neutral atmospheric boundary layer. The results show that turbulent coherent structures are the main factor that determines the wind velocity fluctuation, aerodynamic performance and wake characteristics of wind turbine in the atmospheric boundary layer. When considering the 13th or lower order POD mode, the wind velocity fluctuation increases with the increase of energy content (more POD modes) of the turbulent coherent structures. When considering the first 19 POD modes, the dynamic loads and power of wind turbine fluctuate with high frequencies, the thrust fluctuates in an amplitude range between 2.4 % and 13.9 % around the mean value, and the power fluctuates from 4.5 % to 28.6 % of the mean value. When considering the first 40 POD modes, the average power generation of the wind turbine increases by 26 % compared to the case with no turbulent structures considered. The study of turbine wake shows that turbulent coherent structures can expand the wind turbine wake approximately to a width of 2.5D and a height of 3D (D is the diameter of the wind turbine), offset the wake approximately to 2D, and move forward the position of the wake vortex beginning to dissipation approximately to 7D behind the wind turbine. In addition, turbulent coherent structures can accelerate the wake velocity recovery by increasing the momentum exchange between the atmospheric boundary layer and wind turbine wake.

Effect of Roll Angle Configurations of a Reverse Delta Type Add-on Device on Wing Tip Vortex Alleviation

An add-on device in the shape of a reverse delta has shown the ability to alleviate wake vortices. The present work studies the interaction of the wing tip vortex and the reverse delta type add-on device vortices. This paper looks at the vortex interactions generated downstream of the wing tip in planes perpendicular to the free stream direction and their dependence on roll angles φ at a mean chord-based Reynolds number of Rec=2.75×105. The study reveals that the add-on device causes a reduction in the tangential velocity V and vorticity of the resultant vortex by up to 44.1% and 59.4%, respectively. Also, it is found that the resultant vortex core radius increased by 305%. The results indicate that the reverse delta type add-on device implants countersign vorticity into the wing tip vortex and modifies its roll-up process.

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.

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).

Unsteady Flow Characteristics in a Centrifugal Compressor

Abstract The geometry of the vortex shell significantly affects the non-axisymmetric flow in centrifugal compressors. This study explores the static pressure characteristics of the vortex shell through experimental data and numerical analysis using finite element software. Findings indicate that the vortex tongue region critically influences the cascade structure. At high flow rates (0.40 kg/s), flow field parameters at the blade and vortex shell inlets alter markedly near the tongue side. Conversely, at lower velocities (0.26 kg/s), flow distribution at the blade inlet becomes more uneven, and changes in the inflow angle are more pronounced. Additionally, blade pulsation intensifies at higher velocities, and the jet wake vortex exhibits structural variations at different velocities. High-pressure fluctuations are notably transmitted upwards through the vortex shell, with periodic fluctuations affecting the impeller load. These changes significantly impact the flow field under various operating conditions.

Fish can use coordinated fin motions to recapture their own vortex wake energy.

During swimming, many fishes use pectoral fins for propulsion and, in the process, move substantial amounts of water rearward. However, the effect that this upstream wake has on the caudal fin remains largely unexplored. By coordinating motions of the caudal fin with the pectoral fins, fishes have the potential to create constructive flow interactions which may act to partially recapture the upstream energy lost in the pectoral fin wake. Using experimentally derived velocity and pressure fields for the silver mojarra (Eucinostomus argenteus), we show that pectoral-caudal fin (PCF) coordination enables the circulation and interception of pectoral fin wake vortices by the caudal fin. This acts to transfer energy to the caudal fin and enhance its hydrodynamic efficiency at swimming speeds where this behaviour occurs. We also find that mojarras commonly use PCF coordination in nature. The results offer new insights into the evolutionary drivers and behavioural plasticity of fish swimming as well as for developing more capable bioinspired underwater vehicles.

Tiltrotor Simulations with Coupled Flight Dynamics, State-Space Aeromechanics, and Aeroacoustics

This article describes the development, implementation, and validation of a generic tilt-rotor simulation model with coupled flight dynamics, state-variable aeromechanics, and aeroacoustics. A major novelty of this work lies in the integration of the flight dynamics with a state-space free-vortex wake code that adopts a near-wake vortex-lattice model. This way, the flight dynamics are augmented by the vortex wake dynamics so that the coupled flight and wake dynamics are self-contained and inherently linearizable. The model is implemented for a Bell XV-15 tiltrotor and validated against U. S. Army/NASA XV-15 flight-test data and other data in the literature. Flight control design is performed to provide desired stability, performance, and handling-quality properties and to allow for a fully autonomous transition between hover in helicopter mode and highspeed flight in aircraft mode. The simulation model has clear applications in the development and testing of advanced flight control laws, aeromechanics analysis, and the prediction of aerodynamically generated noise in generalized maneuvering flight.

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

The loss of fish swimming capacity induced by those energetic vortical structures is detrimental, and limited research on the mechanism of fish entrainment in the vortex has been reported. The present study investigated the entrainment of a fish in vortices generated in wake of a semi-cylinder. The swimming fish was modeled using an undulatory NACA0012 airfoil behind semi-cylinders with a diameter D from 0.1 to 2.4 times the fish body length (L). It was found that the fish couldn't get higher propulsion from the low momentum flow for D > 0.8L. The wake of the fish was distorted by the shedding vortices and trailing-edge vortices (TEVs) arose at fishtail. The length- and time-scale of TEVs were positive correlated with these of oncoming co-directional vortices. The uncontrollable hydrodynamics induced by TEVs could elucidate the disruption of fish stability by surrounding vortices, manifested as a loss of propulsion, the generation of a unidirectional torque, and increased power consumption. The fish head gradually got energy from the turbulent flow of oncoming vortices, but fails to compensate adverse effects of the TEVs. The recovery of swimming capacity occurred only in the less disturbed flow between the wake vortices.

Numerical investigation on the wake and energy dissipation of tidal stream turbine with modified actuator line method

This study evaluates correction methods for the actuator line method in tidal stream turbine calculations. Findings indicate that three-dimensional Gaussian distribution coefficients align closely with experimental results when ε = 2dr. Tip loss correction addresses abnormal axial force coefficient elevation at the blade tip, while stall delay correction minimally impacts standard turbine operation. Root loss correction slightly affects the root vortex but notably delays the disappearance of the nacelle after tail flow's bimodal region to 4D, contrary to observations. Using a cylinder correction at the blade root improves the root vortex representation. Enhanced ALM, combined with LES, simulates wake and energy loss across varying turbulence intensities. At low turbulence, the spiral vortex structure is clearer, with fewer stray vortices and a more regular circular shape. Increasing turbulence blurs the structure, introducing more turbulent vortices and losing regularity. Entropy production concept represents energy losses as total, direct, and turbulence entropy production. Direct entropy production is minimal, localized at blade tip, nacelle, and support structures due to significant velocity gradients. Indirect entropy production causes substantial energy loss, resembling wake flow's vortex pattern due to turbulent pulsation. With higher turbulence, EPR, EPDD, and EPTD decline as wake vortices are disrupted, reducing energy loss.