Vortex Interaction Research Articles

Tailoring OAM spectrum of high-order harmonic generation driven by two mixed Laguerre–Gaussian beams with nonzero radial nodes

The extreme ultraviolet (XUV) light beam carrying orbital angular momentum (OAM) can be produced via high-order harmonic generation (HHG) due to the interaction of an intense vortex infrared laser and a gas medium. Here we show that the OAM spectrum of vortex HHG can be readily tailored by varying the radial node (from 0 to 2) in the driving laser consisting of two mixed Laguerre–Gaussian (LG) beams. We find that due to the change in spatial profile of HHG, the distribution range of the OAM spectrum can be broadened and its shape can be modified by increasing the radial node. We also show that the OAM mode range becomes much wider and its distribution shape becomes more symmetric when the harmonic order is increased from the plateau to the cutoff when the driving laser has the nonzero radial nodes. Through the map of coherence length and the evolution of harmonic field in the medium, we reveal that the favorable off-axis phase-matching conditions are greatly modified due to the change of intensity and phase distributions of driving laser with the radial node. We anticipate this work to stimulate some interests in generating the XUV vortex beam with tunable OAM spectrum through the gaseous HHG process achieved by manipulating the mode properties of the driving laser beam.

Developing characteristics of subsonic-supersonic shear layers induced by different lobed mixers in the RBCC

The rapid mixing of air and rocket gas is pivotal for enhancing combustion performance in the ejector mode of rocket-based combined-cycle engines. This study experimentally investigated developing characteristics of subsonic-supersonic shear layers induced by sinusoidal- and rectangular-lobed mixers using a subsonic-supersonic shear layer experimental system and an ice-cluster-based planar laser scattering technique. Both sinusoidal- and rectangular-lobed mixers evidently bolstered mixing through the promotion of transverse flow and vortex interactions. Under equivalent conditions, the sinusoidal-lobed mixer offered superior mixing effects. The mixing enhancement of a sinusoidal-lobed mixer exhibited a positive correlation with the wavelength, with excessively small wavelengths potentially impeding mixing. Based on the dimensionless wavelength ratio, a criterion was identified for evaluating the mixing enhancement of the sinusoidal-lobed mixer: a positive mixing enhancement occurred when the dimensionless wavelength ratio was at least 1. Notably, in this study, the shear layer thickness growth rate increased by 12 % when the dimensionless wavelength ratio was 1.905. These findings can offer valuable insights for improving combustion performance via enhanced mixing.

Numerical studies of shock–vortex interaction over a wedge during shock-wave diffraction—A new approach

In this study, shock wave diffraction has been investigated through a numerical simulation of a moving normal shock incident on a sharp-edged wedge. Schardin's problem is revisited using some existing and new mathematical tools. Two-dimensional compressible Navier–Stokes equation is solved using a higher-order version of the rhoCentralFoam solver in the OpenFOAM platform. Overall flow structures are captured with high efficacy. The divergence of the Lamb vector is used to probe the interaction between vorticity bearing and fluid straining motion, which increases dramatically inside the primary vortex after collision with the reflected Mach stem and increases the turbulent kinetic energy (TKE). In the separated shear layer that emerges from the wedge tip, there is a reduction of TKE after the collision between the lambda shock and accelerated shock. The vorticity pumping into the mean flow by the baroclinic torque is dominant in the separated shear layer before the above collision, whereas after collision it is only dominant inside the primary vortex. A new vector M is introduced here, which is the cross product of the vorticity vector and the pressure gradient vector. The divergence of M shows the interaction between the pressure gradient and the vorticity gradient. This interaction affects the separation bubble formed over the base wall of the wedge due to the shock-wave/boundary layer interaction induced by the lambda shock. Dynamic mode decomposition shows a dominant mode at a frequency of 125.7 Hz, which is due to low-frequency shock oscillation.

Kelvin–Helmholtz Instability “Tube” and “Knot” Dynamics. Part I: Expanding Observational Evidence of Occurrence and Environmental Influences

Abstract Multiple recent observations in the mesosphere have revealed large-scale Kelvin–Helmholtz instabilities (KHI) exhibiting diverse spatial features and temporal evolutions. The first event reported by Hecht et al. exhibited multiple features resembling those seen to arise in early laboratory shear-flow studies described as “tube” and “knot” (T&K) dynamics by Thorpe. The potential importance of T&K dynamics in the atmosphere, and in the oceans and other stratified and sheared fluids, is due to their accelerated turbulence transitions and elevated energy dissipation rates relative to KHI turbulence transitions occurring in their absence. Motivated by these studies, we survey recent observational evidence of multiscale Kelvin–Helmholtz instabilities throughout the atmosphere, many features of which closely resemble T&K dynamics observed in the laboratory and idealized initial modeling. These efforts will guide further modeling assessing the potential importance of these T&K dynamics in turbulence generation, energy dissipation, and mixing throughout the atmosphere and other fluids. We expect these dynamics to have implications for parameterizing mixing and transport in stratified shear flows in the atmosphere and oceans that have not been considered to date. Companion papers describe results of a multiscale gravity wave direct numerical simulation (DNS) that serendipitously exhibits a number of KHI T&K events and an idealized multiscale DNS of KHI T&K dynamics without gravity wave influences. Significance Statement Kelvin–Helmholtz instabilities (KHI) occur throughout the atmosphere and induce turbulence and mixing that need to be represented in weather prediction and other models of the atmosphere and oceans. This paper documents recent atmospheric evidence for widespread, more intense, features of KHI dynamics that arise where KH billows are initially discontinuous, misaligned, or varying along their axes. These features initiate strong local vortex interactions described as “tubes” and “knots” in early laboratory experiments, suggested by, but not recognized in, earlier atmospheric and oceanic profiling, and only recently confirmed in newer, high-resolution atmospheric imaging and idealized modeling to date.

Wake physics of two-dimensional flapping-hydrofoil turbines

We present a numerical study on the wake of two-dimensional flapping-hydrofoil turbines using Reynolds averaged Navier–Stokes method with shear stress transport k–ω model. The adaptive mesh refinement was applied for vortex simulations. The pitching amplitude ranges from 50° to 90°, and the reduced frequency ranges from 0.10 to 0.20. By varying the reduced frequency and pitching amplitude, we visualized three different types of wakes, and they are the von Karman wake, the mixed wake, and the chaotic wake. We found that there is a critical value of the reduced frequency to determine whether the wake will eventually develop into a standard von Karman wake. When the vortices leave the hydrofoil, they first form a classical staggered arrangement and then develop into a stable double-row configuration. The regular motions of vortices along specific trajectories are explained by analyzing the resultant velocity using velocity polygon, taking into account the effects of vortex interactions and environmental factors. The main component of vortex induced velocities at specific locations is always opposite to the freestream velocity, which is the cause of velocity attenuation in the wake. With the increase in the reduced frequency and pitching amplitude, the velocity attenuation is getting worse. The maximum velocity attenuation usually occurs farther downstream from the hydrofoil for large reduced frequencies and large pitching amplitudes. The wake of a flapping-hydrofoil turbine is divided into four feature zones by studying the time-varying characteristics of velocities, which deepens the understanding of the wake and can provide important references in selecting the optimal location for downstream turbines.

Four-dimensional particle tracking velocimetry measurements of unsteady three-dimensional vortex onset and progression for 5415 straight ahead, static drift, and pure sway

Large-scale towed system four-dimensional particle tracking velocimetry (4DPTV) implementation and measurements are presented for towing tank tests for a naval combatant 5415 ship model for straight ahead, static drift β = 10°, and pure sway βmax = 10° conditions. The results, for the first time, provide instantaneous volumetric flow field data around the ship model including near the hull surface, and a complete description of the sonar dome 3D unsteady vortex separation onset and progression due to the 4DPTV larger measurement volume and higher data rates in comparison with previous results using tomographic particle image velocimetry (TPIV); thereby, providing the essential data required for the assessment of current hybrid Reynolds-averaged Navier–Stokes/large eddy simulation turbulence modeling capabilities and guidance for the necessary modeling improvements. The 4DPTV system is summarized and compared with the previous TPIV, including the camera calibration procedures, trigger systems, and synchronization with the sway motion. Analysis procedures, including data reduction procedures and the 3D vortex core detection and voxel labeling techniques, are described. The identification of the complex vortex separations and breakdown is aided using complementary detached eddy simulations. The vortex–vortex interaction process is visualized from instantaneous volumetric datasets. The pros and cons of the 4DPTV vs TPIV, the comparison between the second- and third-generation vortex visualization technique, and the statistical convergence error analysis of sonar dome port vortex for β = 10° are discussed. Plans for increased spatial resolution of the 4DPTV system and additional data reduction techniques for detailed turbulence analysis are also discussed.

Coupling of OpenFOAM with a Lagrangian vortex particle method for external aerodynamic simulations

In the field of computational aerodynamics, it is vital to develop tools that can accurately, but also efficiently, simulate the flow around bluff objects and calculate the aerodynamic forces acting on them. When strong body–vortex interactions take place, the simulations become more demanding, since complex phenomena appear. To address this issue, hybrid Eulerian–Lagrangian solvers have been developed and are increasingly used in the field. In this paper, a Vortex Particle Method (VPM) is coupled with the OpenFOAM software. The Eulerian solver (OpenFOAM) resolves the regions close to the solid boundaries, while the vortex particles evolve the wake downstream, significantly reducing artificial diffusion. The coupling strategy and the validation results of a hybrid code based on the domain decomposition technique are presented. This work is the first to couple OpenFOAM with a Lagrangian solver in the framework of a hybrid solver. Our objective is twofold: to verify the capability of OpenFOAM to run with a VPM and to validate the hybrid solver using benchmark cases. We demonstrate the validation of the solver on the Lamb–Oseen vortex case, the dipole case in the unbounded domain, and the flow around a cylinder at Re = 550. Our results show that coupling OpenFOAM with a VPM can be achieved without complications and efficiently reproduces the results of pure Eulerian simulations.

The Effect of Hydrogen on Nonlinear Flame Saturation

Abstract We investigate the effect of increasing levels of hydrogen enrichment on the nonlinear response and saturation of premixed bluff-body stabilized methane/hydrogen flames submitted to acoustic forcing. The thermal power is kept approximately constant to preserve the nozzle velocity while increasing the flame speed through hydrogen enrichment. The flame describing function (FDF) is measured for a fixed frequency and three hydrogen–methane blends ranging from 10% to 50% by power, corresponding to 25% to 75% by volume. We show that when the flame is forced at the same frequency at similar power and bulk velocities, increasing levels of hydrogen enrichment increase the saturation amplitude of the flame. To provide insight into the flame dynamics responsible for the change in the global nonlinear response and saturation amplitude, the flames were investigated using high-speed imaging in combination with OH planar laser-induced fluorescence (OH-PLIF) at a range of forcing amplitudes. At lower hydrogen concentrations, the flame is stabilized along the inner shear layer and saturation in the heat release rate (HRR) occurs at lower forcing amplitudes due to large-scale flame–vortex interactions causing flame annihilation as observed in several previous studies. At increased levels of hydrogen enrichment, distinctly different flame dynamics are observed. In these cases, the flame accelerates and propagates across to the outer shear layer, which acts to suppress large-scale flame annihilation during roll-up of both the inner and outer shear layers. This results in a coherent increase in flame surface area with forcing amplitudes significantly increasing the saturation amplitude of the flame. These results show that high levels of hydrogen increase the amplitude response to acoustic forcing leading to higher saturation amplitudes. This suggests that substituting natural gas with hydrogen in gas turbines increases the risk of much higher limit-cycle amplitudes if self-excited instabilities occur.

Evolutions of hydrodynamic and electromagnetic wakes induced by underwater vehicles

The electromagnetic wake induced by the movement of stratified conductive seawater in the presence of the geomagnetic ambiance contains vital information for non-acoustic detection. However, the intricate mechanism among hydrodynamics, ions transports, and vortex evolution has not been systematically explored, which synergistically generate the electromagnetic wake. In this context, an interdisciplinary model involving continuity, momentum, heat and mass transfer, and Maxwell's equations is constructed by the ANSYS Fluent software to address the evolution of wake characteristics behind a full-scale submarine-propeller vehicle. The instantaneous visualizations including velocity distributions, vortex interactions, electromagnetic signatures, and vortex dynamics during downstream evolution are reproduced by the high-fidelity numerical simulations. Tip vortex pairing in the circumferential direction and secondary vortex system originating from the local distortions in the helical morphology due to the wake of the sails are observed and described. Flow fields and electromagnetic features at different axial and radial positions in the wake are also discussed. The results manifest that the pronounced intensity of the induced magnetic field can attain 0.5 nT, which can be detected by precise magnetometers. Moreover, velocity components and induced magnetic fields are thoroughly examined to reveal the mechanism of induced electromagnetic field generation, which is closely linked to the flow velocity and background geomagnetic field. Subsequently, dynamic analyses by power spectral densities are executed to further inspect the frequency characteristics. These scientific findings provide beneficial insights for the research on hydrodynamic and electromagnetic wake, especially considering the potential application of submersible non-acoustic detection.

Performance, Aerodynamics, and Aeroacoustics of Side-by-Side Rotors Using High-Fidelity Computational Fluid Dynamics

This paper investigates the performance, aerodynamics, and aeroacoustics of NASA’s side-by-side urban air mobility (UAM) aircraft in hover conditions, utilizing high-fidelity computational fluid dynamics (CFD) simulations. High Performance Computing Modernization Program CREATE™-AV Helios is employed for the CFD simulations, while acoustic predictions are performed by PSU-WOPWOP. The simulations apply overset structured grids, the Spalart–Allmaras detached eddy simulation turbulence model, and adaptive mesh refinement (AMR) on the off-body mesh. The investigation covers four rotor overlap scenarios and five collective pitch angles, all maintaining a consistent rotational speed. Results indicate that as rotor overlap distance increases, a minor improvement in the figure of merit is noticed. Nevertheless, a substantial reduction in blade loading within the overlap region becomes apparent. The blade–vortex interaction (BVI) intensifies between both rotors as the overlap region broadens. The 25% overlap configuration is shown to yield the highest overall sound pressure level (OASPL) among all configurations due to the more pronounced BVIs at the entrance and exit of the overlap area. It is observed that the OASPL exceeds 62 dBA at an altitude of 500 ft (152.4 m) for all overlap cases, exceeding the suggested UAM aircraft noise guideline. Additionally, noise generated by all overlap scenarios is compared against various background noise levels. Results suggest that the noise produced by the side-by-side rotor configuration is not fully masked by any of the different background noise levels at an altitude of 500 ft (152.4 m).

Experimental study on passive plate pitching and coherent structures in a square channel by TR-PIV

The interaction of plate pitching and vortex may damage the accumulator isolation passive valve in nuclear power plant. The plate pitching and turbulent flow in a square channel were studied by high-speed camera (HSC) and time-resolved particle image velocimetry (TR-PIV) at Reynolds numbers (Re) from 1474 to 58952.The plate angle captured by HSC were analyzed by edge detection algorithm. The plate is inclined in hydraulic equilibrium without oscillation when Re is less than 14738. The plate pitching start to be periodic at Re = 22107, and then the periodicity weakens with Re increasing from 22,107 to 51583. The time-averaged pitching angle increases from 0.08467 rad to 1.454 rad with Re increasing from Re = 1474 to Re = 51583. The standard deviation of pitching angle increases from zero with Re less than 14,738 to 0.1294 rad at Re = 22107, and then it decreases to 0.006536 rad at Re = 51583. The peak frequency of plate pitching is the same as that of turbulent flow extracted from the power spectral density of instantaneous velocity downstream of the pitching plate, indicating the passive plate pitching is determined by turbulence. The plate pitching is driven by lift force torque due to the leading-edge-vortex shedding from the pitching plate.The interaction between passive plate pitching and fluid generates complicated turbulence. The time-averaged flow structures are typical shear-flow due to small plate inclination angle without oscillation when Re is less than 14738. The Reynolds stresses are highly enhanced in the shear flow. At Re = 22107, the time-averaged flow structures behave as vortex bubble over the plate and shear flow surrounding the plate, and the Reynolds stresses downstream of the plate are obviously enhanced by shedding vortices from the plate. With Re increasing from 22,107 to 58952, the time-averaged vortex bubble becomes smaller and shifts downwards to the plate trailing end, while the Reynolds stresses with similar distribution patterns decrease fast. The peak frequency of vortex shedding increases with Re increasing. The large spatial and temporal length scales are induced by shear flow at low Re less than 14738. The large length scales are dominated by von Kármán street shedding from the plate at Re from 22,107 to 58952, which are identified by proper orthogonal decomposition (POD) analysis, because the first two modes are the same mode of von Kármán street with a phase difference.The experimental work improves understandings of plate-fluid-interaction of the isolation passive valve and supports validations of numerical simulations.

Diode effects on street canyon ventilation in valley city: Temperature inversion and calm geostrophic wind

In urban areas, especially in valleys, challenges from heat and pollution are common. The situation worsens due to temperature inversions, where warm air traps pollutants. A new study employed multi-scale computational fluid dynamics (CFD) to assess ventilation in valley cities, specifically during temperature inversions and calm winds. Parameters like air change rate (ACH), local mean age of air (LMAA), and purging flow rate (PFR) were used to gauge street canyon ventilation efficiency. Factors such as atmospheric stability, city-slope positioning, and surface materials are altered to analyze impacts on urban heat island (UHI) intensity and air quality.Results indicated that katabatic winds had varied effects on pollution in different city regions, influenced by atmospheric conditions. Higher temperature lapse rates led to enhanced pollutant removal near slopes due to vortex interactions, but distant regions experienced thicker inversion layers, limiting pollutant dispersion. Conversely, anabatic winds improved ventilation with distance. Under inversion conditions, slope-city distance and surface materials played roles.For katabatic winds, city positioning significantly reduced UHI intensity, lowering LMAA by around 440s. Anabatic winds' UHI impacts ranked as atmospheric conditions, city location, and surface materials. A neutral atmosphere reduced UHI by 4.5K, but increased distance between city and slopes strengthened UHI by 4.04K. Ventilation efficiency was mainly tied to atmospheric conditions, while city location impacted pollution mitigation.This research aids in understanding air quality and airflow patterns during extreme weather events, aiding urban canyon design in valleys, especially with recurring heatwaves.

Analysis of the Force Characteristics of Two Tandem Cylinders by Internal Waves over Slope Topography

Large-amplitude internal waves (IWs) with strong lateral shear forces can cause destructive effects on marine engineering structures. In this study, a large eddy simulation (LES) method was employed to simulate the generation and propagation of IWs in a three-dimensional numerical wave tank, and the pressure distribution, flow field characteristics, and force behaviors of two tandem cylinders under the coupling effect of the IWs and slope terrain were studied. The influence mechanism of the normalized value of center-to-center spacing (L) and the diameter of the cylinder (D), i.e., (L/D), on the strength of the vortex disturbance between cylinders was studied by comparing the simulation results of two tandem cylinders with those of a single cylinder (SC) to further explore the mechanical response characteristics of the upstream cylinder (P1) and downstream cylinder (P2). The simulation results showed that the vortex interaction between cylinders is the critical factor that affects the forces acting on the cylinders. The strength of the vortex disturbance could be distinguished by the dimensionless critical center-to-center spacing between cylinders (Lc/D = 3.0). When L/D ≤ Lc/D, the vortex disturbance was severe, causing P1 and P2 to experience significant horizontal positive forces and negative forces, respectively. In the case of L/D > Lc/D, the forces acting on both cylinders gradually returned to those on a single cylinder.

Turbulence Model Effects on Vortex Interaction Prediction on a Multiswept Delta Wing

In this study, vortical flows around a generic triple-delta wing geometry were investigated at transonic speeds. The focus of this work was the investigation of vortex interaction at medium to high angles of attack. To this end, numerical simulations were performed at two different Mach numbers, M=0.5 and M=0.85. To assess the performance of different turbulence models in predicting the complex flowfield, comparisons with experimental data were performed. In addition to the integral forces and moments, surface pressures from pressure-sensitive paint and velocity field data from particle image velocimetry were used. Across all investigated test cases, the best agreement with experimental results was achieved by the Menter shear stress transport turbulence model.

Cyclic Analysis of In-Cylinder Vortex Interactions Based on Data-Driven Detection and Characterization Framework

Abstract The complex vortex flow interactions are critical to affect the fuel–air mixing and combustion stability in direct-injection engine. However, due to the strong cyclic variations inside engine, the multiscale swirl flow characteristics with cyclic details are difficult to be sufficiently revealed. Therefore, a vortex detection and characterization framework, including physical and data-driven methods, is implemented to elucidate the cyclic vortex interaction process. In this study, a high-speed time-resolved particle image velocimetry is applied to record the spatiotemporal flow behavior under three different swirl ratio conditions. First, the presence of vortex motion is detected at each crank angle for each engine cycle. Results show that the vortex interaction processes under different swirl ratio conditions exhibit distinctive characteristics. The presence of multiple vortices and their interactions are found to trigger dramatic changes and variations in swirl flow behavior. Then, the individual-cycle analysis of the vortex interaction effects on flow characteristics is conducted. The vortex characteristics including vortex location, strength, and size are examined with cyclic detail using data-driven unsupervised clustering. Results indicate that the vortex merging is the main source inducing the vortex characteristics variations. Furthermore, the occurrence and duration of the vortex merging process are found to be closely related to the intake swirl ratio and valve lift profile. Increased swirl ratio and valve lift cause vortex to merge earlier and reduce the merging duration. This finding provides a potential idea to alleviate the cyclic variation issue by controlling the vortex merging process.

Aerodynamic Performance of an Unmanned Aerial Vehicle Wing For Varied Wing Geometric Parameters

The aerodynamic performance characteristics of a small unmanned aerial vehicle wing are evaluated computationally at a moderate Reynolds number of Re = 2.8 x 105. The flow exhibits laminar-turbulent transition and features a laminar separation bubble that exists for a substantial portion of the wing span. The flow near the wing tip is three-dimensional and exhibits complex flow patterns due to the interaction of the wing tip vortex with the laminar separation bubble. A series of systematic parametric studies are conducted by varying the wing geometric parameters like the span of the rectangular inboard wing, sweep of the outboard wing and wing dihedral angle. From the parametric studies, it is found that the wing with an outboard sweep of 17° has a 4.54 % improvement in endurance factor compared to the baseline wing at an angle of attack of 5°. The drag of this wing is 19.2 drag counts lower compared to the baseline wing. Further, flow transition is delayed for this wing. The present results show that wing sweep has a significant influence on the improvement of aerodynamic performance of unmanned aerial vehicle wings at moderate Reynolds numbers.

Experimental and numerical analysis of the aerodynamics and vortex interactions on multi-swept delta wings

The flow field around a generic multi-swept delta wing configuration is investigated under transonic flow conditions, both experimentally and numerically. A special focus is on the analysis of vortex/vortex and vortex/shock interactions at moderate angles of attack. In the present study, the Mach number is varied between textrm{Ma} = {0.50} and textrm{Ma} = {1.41} and the angle of attack is varied between alpha = 8^circ and alpha = 28^circ. Numerical results are validated using experimental surface pressure data from pressure taps, as well as forces and moments based on strain gauge measurements. For selected cases, velocity field data from particle image velocimetry (PIV) measurements are available as well. Over a broad range of angle of attack and Mach number, strong vortex/vortex interactions, including vortex braiding and vortex merging, occur. The location of vortex merging is moving downstream with increasing angle of attack and increasing Mach number. Additionally, at textrm{Ma} = {0.85}, vortex/shock interaction occurs above the wing. For moderate angles of attack, shock-induced vortex breakdown is observed.

Large Eddy Simulation of Cavitation Jets from an Organ-Pipe Nozzle: The Influence of Cavitation on the Vortex Coherent Structure

High-speed water jets are widely used in deep mining and the in-depth study of jet characteristics helps to improve drilling efficiency. Three-dimensional Large Eddy Simulation is used to simulate turbulent flows generated by an organ-pipe nozzle. The simulation is validated with existing experimental data and is focused on the evolution and interaction of cavitation bubbles and vortices. Dynamic mode decomposition is performed to extract structural information about the different motion modes and their stability. Results show that the dominant fluid frequency is positively correlated with inlet pressure while unrelated to the divergence angle. Meanwhile, jets’ oscillation is amplified by a large divergence angle, which facilitates the occurrence of cavitation. Results about the flow field outside of an organ-pipe nozzle advance the understanding of the basic mechanism of cavitation jets.

Vortex dynamics in a spin valve nanopillar having hybrid polarizer and magnetostatic coupling

This study investigates the observation and control of vortex gyration in the free layer by utilizing polarized current. Modifying the current alone proves challenging in adjusting the frequency and amplitude, as the orbit radius of the vortex core remains constant. To address this, a spin valve nanopillar model with hybrid polarizer is introduced and analyzed, enabling support and gyration of the vortex core in the nanodisk under spin polarized current. Manipulating current density and fine-tuning of hybrid polarizer parameters adjusts system frequency and amplitude. However, with hybrid polarizer, while vortex gyration and higher-frequency signal production increase at higher current densities, the amplitude generally decreases. To mitigate this behavior, the study explores the vortex interactions influenced by magnetostatic coupling in the free layers of spin valve nanopillar. The objective of this paper is to advance the understanding of vortex physics for applications that require greater amplitude microwave signals.

On the estimation of the angle of attack for vertical axis wind turbines

PurposeThis paper aims to investigate the problem of estimating the angle of attack (AoA) and relative velocity for vertical axis wind turbine (VAWT) blades from computational fluid dynamics data.Design/methodology/approachTwo methods are implemented as function objects within the OpenFOAM framework for estimating the blade’s AoA and relative velocity. For the numerical analysis of the flow around and through the VAWT, 2 D unsteady Reynolds-averaged Navier–Stokes (URANS) simulations are carried out and validated against experimental data.FindingsTo gain a better understanding of the complex flow features encountered by VAWT blades, the determination of the AoA is crucial. Relying on the geometrically-derived AoA may lead to wrong conclusions about blade aerodynamics.Practical implicationsThis study can lead to the development of more robust optimization techniques for enhancing the variable-pitch control mechanism of VAWT blades and improving low-order models based on the blade element momentum theory.Originality/valueAssessment of the reliability of AoA and relative velocity estimation methods for VAWT’ blades at low-Reynolds numbers using URANS turbulence models in the context of dynamic stall and blade–vortex interactions.