Vortex Decay Research Articles

The formation and post-formation processes of vortex rings discharged from laminar starting jets

Motivated by biological systems, vortex rings can enhance the propulsive efficiency in industrial systems. To study the vortex properties during the formation and post-formation stages, impulsively starting jets are investigated by simulations. The effects of the stroke ratio and nozzle geometry are studied at a fixed jet Reynolds number of 2500. The stroke ratio at the formation number is found to be not enough to produce a vortex ring with maximum circulation. The stroke ratio is suggested to be about twice as large as the formation number. An alternative criterion based on the circulation ratio is proposed to describe the onset of pinch-off. This criterion states that the pinch-off would start when the vortex ring attains about 80% of the total jet circulation. During the formation stage, the scaling laws for vortex trajectories and circulation are proposed for continuous formation. By combining the suggested scaling relations and Saffman's velocity formula, the evolution of non-dimensional energy can be predicted. During the post-formation stage, the scaling laws for vortex properties (e.g., the vortex ring diameter, translational velocity, and circulation) are found to be independent of both the nozzle configuration and the vortex Reynolds number. On the grounds of the invariance of impulse in vortex decay, the scaling laws of vortex motion are derived for the non-dimensional energy, circulation, and diffusivity scale of the vortex core. In consequence, the normalized energy and circulation found in experiments can be successfully derived from the similarity model for both nozzle configurations.

Hydrodynamics and Sediment Transport Under Solitary Waves in the Swash Zone

Swash–swash interaction is a common natural phenomenon in the nearshore region, characterized by complex fluid motion. The characteristics of swash–swash interaction are crucial to sediment transport, subsequently affecting the beach morphology. This study investigates the hydrodynamics and sediment transport in swash–swash interaction under two successive solitary waves using a two-phase Smoothed Particle Hydrodynamics (SPH) model. The effects of the time interval between the two waves are examined. It is shown that the time interval has a minor effect on the breaking and swash–swash interacting patterns as well as the final beach morphology but influences the run-up of the second wave and the instantaneous sediment flux. Under wave breaking in the swash–swash interaction, there is significant sediment suspension due to strong vortices, and the suspended sediment forms a plume upward from the bed. The sediment plumes gradually settle down as the vortices decay. These insights enhance the understanding of sediment transport and beach morphology under complex swash–swash interaction.

Vibration control of the straight bladed vertical axis wind turbine structure using the leading-edge protuberanced blades

This study investigates the impact of structural responses resulting from alterations in aerodynamic characteristics caused by incorporating leading-edge protuberance (LEP) blades in vertical-axis wind turbines (VAWTs) under various wind speeds. The current experimental investigation was conducted on a scaled-down straight-blade VAWT with multiple wind speeds at a fixed pitch angle and uniform blade configuration. The unsteady structural excitation caused by the decay of vortices due to the different aerodynamic loads and its effect on the VAWT structure is examined in great detail. The presence of counter-rotating vortices from the LEP profile, however, enhances the aerodynamics of the blades at higher angles of attack. It is crucial to understand the structural characteristics resulting from changes in aerodynamics. The paper presents the effects of the LEP on the structural excitation of the VAWT by analyzing the acceleration and displacement data obtained from the static structure of VAWT for various operational wind speeds. The results showed decreased acceleration and displacement amplitudes for the VAWT with LEP at medium and higher wind speeds. Significant aerodynamic and structural damping was observed in the VAWT with LEP blades. The results obtained were in close agreement with the time and frequency domains of the measured displacement and acceleration from the structure. The bidirectional vibration control was observed for the VAWT with LEP, which was found to have a reduction ratio of 70 %.

Combined Reynolds-averaged Navier-Stokes/Large-Eddy Simulations for an aircraft wake until dissipation regime

A new methodology has been devised to simulate the aerodynamic wake of an airliner during cruise flight using Reynolds-averaged Navier-Stokes (RANS) and Large Eddy Simulations (LES). The wake evolution is simulated considering the full geometry of the aircraft and spans from the jet regime to the dissipation regime. First, the jet regime is computed using RANS modeling and state-of-the-art anisotropic mesh adaptation techniques. Second, the vortex and dissipation regimes are solved using temporal LES with flow field initialization from the previous RANS calculation. RANS information on turbulent quantities is transferred to the LES domain using synthetic turbulence methods. The methodology is first tested on a NACA-0012 wing. It is then applied to the NASA Common Research Model (CRM) geometry on cruise flight conditions. Jet/vortex interaction is studied during the jet regime up to twenty wingspans behind the aircraft. The influences of atmospheric turbulence, jet turbulence, and stable atmospheric stratification are investigated for the vortex and dissipation regimes. It is observed that atmospheric turbulence intensity and stratification both accelerate the vortex decay. Moreover, the secondary wake structure is found to be much more turbulent for numerical RANS initialization than for classical analytic initialization, showcasing the importance of early aerodynamics on wake evolution.

The lower-hybrid drift instability during the evolution of Kelvin-Helmholtz instability

Abstract The lower-hybrid drift instability (LHDI) is a pivotal phenomenon in astrophysics, playing a critical role in energy transfer, macroscopic structures, and evolutionary processes between the magnetosheath and magnetosphere. Using 2D two-fluids numerical simulation, we investigate the spatiotemporal distribution of LHDI during the evolution of Kelvin-Helmholtz instability (KHI) at Earth’s dusk-flank magnetopause. The numerical simulation results show that, during the linear phase of KHI, the LHDI, whose duration time is approximately ∆t1∼10tA, appears around the high-density arms. During the nonlinear phase of KHI, the LHDI appears around the KH vortexes, with a duration of about ∆t2∼5tA. The LHDI disappears with the decay of KH vortex.

Numerical investigation of the vortical structures in the near wake of a model wind turbine

It is well known that the vortex system in a horizontal axis wind turbine wake is highly relevant in terms of fatigue loads and performance of wind turbines located in the wake of other wind turbines. The breakdown process of tip vortices particularly influences the mixing process of the low-speed wake region with the undisturbed flow outside the wake. As a collaboration with the Technische Universität Berlin (TUB), a major goal in a joint research is to study the effects involved in tip vortex breakdown. TUB designed a model turbine that will be towed through a large water tank to analyse and to control the tip vortex decay. To accommodate the measurement equipment, the ratio of blade length to nacelle length is unconventionally small, leading to uncertainty regarding the effect on the breakdown of the tip vortex. Due to the dimensions of the model wind turbine the root vortex system and the nacelle wake are not comparable to former studies and should therefore be investigated in detail. To do so computational fluid dynamics, in particular delayed detached eddy simulations, are conducted for the model turbine with the compressible flow solver FLOWer and the two-equation Menter-SST turbulence model. Two simulations are conducted with and without the nacelle. The results indicate that the root vortices propagate downstream and interact with each other. Additionally, these vortical structures are also influenced by the geometry of the turbine nacelle which causes faster decay of the root vortices compared to a configuration without the nacelle. However, the root vortex breakdown does not influence the tip vortices as the turbulent intensity matches for the area where tip vortices are present. In short, this investigation shows that the influence of the nacelle geometry and root vortex system on the tip vortices is negligible. Thus, the model wind turbine designed by TUB is suitable for the investigation of tip vortices.

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 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.

The Impact of the Tropical Sea Surface Temperature Variability on the Dynamical Processes and Ozone Layer in the Arctic Atmosphere

Tropical sea surface temperature (SST) variability, mainly driven by the El Niño–Southern Oscillation (ENSO), influences the atmospheric circulation and hence the transport of heat and chemical species in both the troposphere and stratosphere. This paper uses Met Office, ERA5 and MERRA2 reanalysis data to examine the impact of SST variability on the dynamics of the polar stratosphere and ozone layer over the period from 1980 to 2020. Particular attention is paid to studying the differences in the influence of different types of ENSO (East Pacific (EP) and Central Pacific (CP)) for the El Niño and La Niña phases. It is shown that during the CP El Niño, the zonal wind weakens more strongly and changes direction more often than during the EP El Niño, and the CP El Niño leads to a more rapid decay of the polar vortex (PV), an increase in stratospheric air temperature and an increase in the concentration and total column ozone than during EP El Niño. For the CP La Niña, the PV is more stable, which often leads to a significant decrease in Arctic ozone. During EP La Niña, powerful sudden stratospheric warming events are often observed, which lead to the destruction of PV and an increase in column ozone.

Critical velocity and arrest of a superfluid in a pointlike disordered potential

Superfluid flow past a potential barrier is a well studied problem in ultracold Bose gases, however, fewer studies have considered the case of flow through a disordered potential. Here we consider the case of a superfluid flowing through a channel containing multiple point-like barriers, randomly placed to form a disordered potential. We begin by identifying the relationship between the relative position of two point-like barriers and the critical velocity of such an arrangement. We then show that there is a mapping between the critical velocity of a system with two obstacles, and a system with a large number of obstacles. By establishing an initial superflow through a point-like disordered potential, moving faster than the critical velocity, we study how the superflow is arrested through the nucleation of vortices and the breakdown of superfluidity, a problem with interesting connections to quantum turbulence and coarsening. We calculate the vortex decay rate as the width of the barriers is increased, and show that vortex pinning becomes a more important effect for these larger barriers. Published by the American Physical Society 2024

Initial stage of formation of vortex flow in an inductor with counter-rotating magnetic fields

In this article, unsteady flows of electrically conductive fluid in a cylindrical cavity under the action of differently directed rotating magnetic fields are investigated. The relevance of the study is due to the presence of such a flow in the flow meter for liquid-metal heat carrier which is currently under development. The subject of study is the process of formation of the spin-up flow stage in a combined inductor of a rotating magnetic field. The acceleration or the spin-up stage is characterized by a significant change in the angular velocity of the liquid and begins with the formation of low pressure areas due to centrifugal forces. The alternation and movement of low and high pressure areas lead to velocity pulsations. For small values of the control parameter, which is the Taylor number, the flow energy gradually increases. In this case, the steady flow consists of two azimuthal and several poloidal vortices with clearly defined structures. When the Taylor number exceeds a critical value, strong pulsations occur in the flow, resulting in the decay of large-scale vortices. The intensity of the current is characterized by the Reynolds number. A power-law relationship with an exponent of 1.57 has been established between the Reynolds and Taylor numbers. It has been found that the time of formation of a steady flow varies from several seconds to tens of seconds, depending on the value of the Taylor number. The estimates of the control parameters have been obtained for a flowmeter operating in a typical mode. It has been shown that for the correct measurement of flow velocity in the flowmeter, it is necessary that that the Taylor numbers be higher than 108. The study is carried out using mathematical modeling and the obtained results are verified by experiment. The average calculated profiles are located within the confidence intervals of the experimental profiles.

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.

Numerical simulation of biomagnetic pulsatile flow through a channel

The study of biological fluids in the presence of a magnetic field is known as biomagnetic fluid dynamics (BFD). The research work in BFD has been rapidly growing due to its applications in developing magnetic devices used for cell separation, targeted drug delivery and cancer tumor treatment. This study aims to examine the biomagnetic fluid flow with pulsatile conditions through a channel when subjected to a magnetic field that varies in space. The nondimensional continuity and momentum equations are solved with the effect of the magnetic field added as a body force. A two-dimensional computational model is developed using the finite volume method and is implemented on a staggered grid system with the help of the semi-implicit fractional step method. The code is written using MATLAB. Numerical simulations are performed by varying the Magnetic, Reynolds and Womersley numbers. Pulsatile flow results indicate the periodic growth and decay of vortices near the source of the magnetic field. With an increase in the magnetic number from 100 to 150, 250 and 500, the maximum vorticity increases by 48.04%, 149.84% and 402.68%. A similar relation is found when varying the Reynolds number, while almost no change is found when varying the Womersley number.

Large-Eddy Simulation of Wake Vortices at Tokyo/Haneda International Airport

A large-eddy simulation (LES) of wake vortices in ground effect is conducted under the influence of airport hangar buildings to investigate the airport-specific behavior of wake vortices. We consider a landing at Tokyo/Haneda International Airport in Japan, where a wake of hangar buildings covers a landing path and touchdown zone under particular wind conditions. We combine an LES code and the Weather Research and Forecasting numerical weather prediction model to retrieve realistic wind conditions around the airport. The behavior of wake vortices is then investigated in the retrieved wind field using the adaptive mesh capability of the LES code for capturing the tiny vortex core of the wake vortex. At the crosswind on the order of a vortex descent speed, the hangar wake reduces the crosswind near the runway, enhancing the windward vortex’s rebound. The effect is alleviated by doubling the crosswind to twice the vortex descent speed. On the other hand, the wind fluctuations caused by the hangar buildings only slightly affect the wake vortex decay.

Simulation Study of the Effect of Atmospheric Stratification on Aircraft Wake Vortex Encounter

The atmospheric environment is an important factor affecting aircraft wake vortex decay and wake separation. In this paper, numerical calculations and strip modeling are combined to complete an analysis of wake encounter under three atmospheric stratifications expressed in Brunt–Väisälä (BV) frequencies. The SST k-ω turbulence model was chosen for the numerical simulation to complete the evolution of the wake vortex field of the A330 aircraft. The A320 and ERJ190 were selected as the aircraft for wake encounter analysis. The dimensionless roll moment coefficient (RMC) was selected as the indicator to construct a three-dimensional hazard zone for wake encounters and then calculate the wake separation. The results show that the higher BV frequencies correspond to faster wake vortex decay and hazard zone dissipation, and a slower decrease in the height of the hazard zone. The risk level of the ERJ190 is higher than that of the A320 with the same wake intensity, and the wake separation of the A320 following the A330 is 5516 m, which is 25.5% less than the ICAO RECAT separation standard, while the wake separation of the A330 following the ERJ190 is 5803 m, which is 37.6% less than the ICAO RECAT separation standard.

Numerical investigation of the effect of blade distortion laws on the corner flow separation of the axial-flow fan

To study the effect on performance of different blade distortion laws of axial flow fans, the radial distributions of blade stagger angle were fit as Bézier curves, and five distribution laws were established. Three-dimensional numerical simulations were carried out for the fan schemes. The results show that distortion near the blade root can effectively restrain the corner separation vortex from climbing along the blade suction surface, so that the low-energy fluid accumulates in the corner region. Tip distortion rises the pressure difference between the pressure and the suction surface, and the turbulence intensity of the tip leakage is increased. Root distortion inhibits the CV diffusion, resulting the decay of tip leakage vortex, thereby improving the stall margin. Relative to a uniform distortion blade, the blade-root distortion scheme increases the radial load of the blade, while the tip distortion scheme results in a decrease of the blade load coefficient. The maximum load position of the end-zone distorted blade moves to the root area, while the maximum load position of the middle distorted blade shifts to the tip area. The end-zone distortion improves the efficiency and total pressure of the fan without changing the efficient working range, and is considered to be the optimal scheme. The maximum pressure coefficient of the end-zone distortion scheme is increased by 4.97% and the maximum efficiency is increased by 1.28%.

The Dynamics of Megafire Smoke Plumes in Climate Models: Why a Converged Solution Matters for Physical Interpretations

AbstractAs the climate system warms, megafires have become more frequent with devastating effects. A byproduct of these events is the creation of smoke plumes that can rise into the stratosphere and spread across the globe where they reside for many months. To gain a deeper understanding of the plume dynamics, global climate simulations of a megafire were performed at a wide range of grid spacings from 2.0° down to 7 km, including a 7 km nonhydrostatic experiment. The analysis focuses on how the resolved dynamics affects the specification of the plume characteristics such as injection height and black carbon (BC) mass. Prior studies initialize the smoke plume at one or a few grid points and this is shown here to produce severely dissipative dynamics. In order to validate such simulations with observations, enhancements of the plume characteristics to offset the dissipation is necessary. Using a numerically converged simulation, sensitivity tests show that to approximate the observed stratospheric lifetime, a reduction in BC fraction by 50% is necessary for external mixtures. The vorticity dynamics of the plume is also analyzed with a Lagrangian budget to understand the mechanisms responsible for the evolution of a collocated anticyclonic vortex. The results can be distilled down into a simple conceptual model. As the plume rises, the air diverges at the top of the updraft where the largest concentrations of smoke are found. This divergence induces a dilution of the background cyclonic absolute vorticity producing an anticyclonic vortex. Vortex decay occurs from opposite arguments.

The turbulence development at its initial stage: A scenario based on the idea of vortices decay

In this paper, a model of the development of a quantum turbulence in its initial stage is proposed. The origin of the turbulence in the suggested model is the decay of vortex loops with an internal structure. We consider the initial stage of this process, before an equilibrium state is established. As result of our study, the density matrix of developing turbulent flow is calculated. The quantization scheme of the classical vortex rings system is based on the approach proposed by the author earlier.

Enhanced Aircraft Wake Decay Under Crosswind Conditions

The starting vortex generated by air blowing from the ground destroys the aircraft wake vortex within a short period of time. Thus, air blowing might shorten flight separation and promote airport operating efficiency. A crosswind is a common phenomenon affecting airplane landing, and it can greatly affect air blowing. In this paper, a large-eddy simulation was used to study the interference exerted by different blowing zones on a wake vortex. The best-performing blowing strategy was estimated by ascertaining the relationship between the blowing zone aspect ratio and the corresponding starting vortex strength. It was found that, without the crosswind, the blowing effect remained almost constant for an aspect ratio of between one and 16. A passive tracer released from the different blowing zones proved that the mechanisms of enhanced vortex decay, with or without crosswind, were identical. However, the results showed that crosswind caused asymmetrical and reduced artificial wake decay enhancement. When the crosswind was imposed, the development of the upwind vortex was sensitive to the aspect ratio, and the blowing zone with an aspect ratio of 16 showed the best performance.

How Do Dust Devil-Like Vortices Depend on Model Resolution? A Grid Convergence Study Using Large-Eddy Simulation

Dust devils are organized convective vortices with pressure drops of hundreds of pascals that spirally lift surface material into the air. This material modifies the radiation budget by contributing to the atmospheric aerosol concentration. Quantification of this contribution requires good knowledge of the dust devil statistics and dynamics. The latter can also help to understand vortex genesis, evolution and decay, in general. Dust devil-like vortices are numerically investigated mainly by large-eddy simulation (LES). A critical parameter in these simulations is the grid spacing, which has a great influence on the dust devil statistics. So far, it is unknown which grid size is sufficient to capture dust devils accurately. We investigate the convergence of simulated convective vertical vortices that resemble dust devils by using the LES model PALM. We use the nesting capabilities of PALM to explore grid spacings from 10 to 0.625 m. Grid spacings of 1 m or less have never been used for the analysis of dust devil-like vortices that develop in a horizontal domain of more than 10 km^2. Our results demonstrate that a minimum resolution of 1.25 m is necessary to achieve a convergence for sample-averaged quantities like the core pressure drop. This grid spacing or smaller should be used for future quantifications of dust devil sediment fluxes. However, sample maxima of the investigated dust devil population and peak velocity values of the general flow show no convergence. If a qualitative description of the dust devil flow pattern is sufficient, we recommend a grid spacing of 2.5 m or smaller.