Vortex Strength Research Articles

Vortex-induced vibrations of a cantilevered blunt plate: POD of TR-PIV measurements and structural modal analysis

Vortex-induced vibrations sustained by immersed bodies may induce early structural fatigue and acoustic noise radiation. The present study consists of an experimental characterization of the vortex shedding and induced vibrations of a cantilevered blunt rectangular aluminium plate of chord to thickness ratio 16.7, immersed in a uniform water flow in the hydrodynamic tunnel of the French Naval Academy Research Institute. Experiences have been conducted at zero degrees incidence for Reynolds numbers Rec (based on chord length) ranging from 2.5×105 to 9.5×105, leading to turbulent wake conditions. The hydrodynamic properties of the wake have been evaluated by statistical analysis, Proper Orthogonal Decomposition (POD) and vortex core identification of Time Resolved Particle Image Velocimetry fields. The structural response of the plate has been examined by laser vibrometry through modal analysis. The fluid–structure interactions have been analysed for three different vibration regimes: out of resonance, lock-off resonance and lock-in resonance with the 1st twisting mode. The features of the first nine POD modes were analysed according to the different vibration regimes to understand the physics proper to the near wake. In the out of resonance regime, the strength of the primary Karman vortices is preferentially controlled by the Strouhal and Reynolds numbers. The thickness based Strouhal number is 0.195. A secondary hydrodynamic excitation source of characteristic Strouhal number 0.227 coexists with the primary Karman vortex shedding mode. At the lock-off resonance, this secondary excitation source is identified as a second Karman vortex shedding mode which is coupled with the natural frequency. A local maximum of vibration magnitude is attained for this regime. The coexistence of these two Karman modes is responsible for the early occurrence of resonance. The lock-in resonance occurs when the primary vortex shedding mode locks with the natural frequency. It is characterized by an enlargement of the wake and by a reinforcement of the primary Karman vortices strength.

Heat transfer performance of internal flow by inserting punched and non-punched vortex generators

This article proposed a new arrangement of planar vortex generators (VGs) and punched VGs within a heated tube. The airflow and heat removal performance of the VGs with new configurations were studied by numerical simulation and experimental methods in turbulent flow (6060∼21,210), and compared with two commonly used arrangements of VGs in tubes (tube fitted with all planar VGs and all punched VGs). The punched VGs used were with three perforation indexes of PI=7.8%,15.2%,46.6%, and three-pitch ratios of PR=1.63,3.27,4.89. An interesting discovery in the case of tubes fitted with VGs in the new configuration was that the strength of main longitudinal vortices displayed a periodic change from weak to strong along the mainstream direction, which presented relative low-pressure loss and poor thermal performance as well. The results indicated that when VGs had the same area holes, there was little difference between the heat enhancement factor (TEF) value of tubes fitted with three arrangements VGs when Re>10,000. Moreover, the increased area of punched hole resulted in poor heat removal rate and lower pressure loss and the smaller the PI was, the larger the TEF. Additionally, the TEF value increased with the decrement of PR. In most Re number ranges, the TEF values were lower than 1.0. The highest TEF of 1.44 was reached by utilizing the punched VGs with PI=7.8%,PR=3.27 at Re=6060.

Investigation of airflow onto uncooled and cooled perpendicular substrates for bioaerosol sampling

A better understanding of the mechanics of condensation is needed to devise active bioaerosol samplers. Here, dropwise water condensation produced by directing humid air flow perpendicularly on ambient (25 °C) and cooled (4 °C) copper plate substrates was studied. Numerical data obtained from solving the two-dimensional incompressible Navier-Stokes equations with low Reynolds numbers (= 40) showed significant vorticity strength developing at the edges of the copper plate. This resulted in a higher degree of aerosol deposition and an increased likelihood of drop nucleation at the impaction zone. When a drop is already present at the edge, simulations predicted that the high vorticity region shifted to the apexes of drops with concomitant increase in the magnitude of vorticity strength. These results explained the experimentally-observed dropwise condensation of larger drops at the edges of the ambient substrate. Analyses of drop size distributions at the center and edges of ambient and cooled substrates showed that drop growth was enhanced by improved condensation on the cooled substrate surface in addition to the flow vorticity effect. Preliminary findings indicate that the recovery of viable aerosolized Escherichia coli from ambient and cooled substrate was found to be invariant, portending its utility for sampling when the electrical power available for cooling is limited.

Vibration induced by active nematics

Active elements in active nematics can impose forces on immersed bodies and move them accordingly. We numerically investigate the vibrational motion of a cantilever beam placed in active nematics. The continuous energy transfer from vortices to the beam results in beam oscillation, whose direction and amplitude depend on the vortex strength, size and position. Referring to the kinetic-energy spectrum, we indicate that both the large- and small-scale vortices are the primary mechanism for the energy transfer between the fluid and beam, leading to the beam oscillatory motion, with the contribution from the large-scale vortices being higher. We investigate the effect of fluid properties such as activity, viscosity and elastic constant on the oscillation frequency. We show that the intensification of the activity increases peak frequency, and there is a linear correlation between the peak frequency and activity. We further demonstrate the reciprocal relationship between viscosity and peak frequency. Subsequently, we relate the increase and decrease in the peak frequency to the energy injection/dissipation by activity/viscosity. Moreover, we reveal the negligibly small dependency of beam peak frequency on the elastic constant and discuss free energy's role in accounting for this behaviour. The findings clearly demonstrate that active fluids can impose an oscillatory motion on flexible bodies, which might be used as a novel method for measuring the critical properties of active nematics.

Aerodynamic characteristics of a super high-speed elevator with different toe guards

To reduce the drag and noise of a super high-speed elevator, a certain type of elevator in operation with different toe guards was taken as the subject to conduct research on its aerodynamic characteristics. In this study, a numerical simulation model was established and used to carry out experiments, and by comparing the theoretical and experimental results, the validity of the method was verified. First, a 3-D unsteady numerical model was established based on improved unstructured dynamic grid method, and the Reynolds average N–S equation was solved by SIMPLEC algorithm. Then, the whole running process of the elevator was simulated, including the velocity distribution, vorticity strength, and pressure gradient in the flow field were analyzed. Finally, the aerodynamic characteristics of the elevator with different vertical heights and four structure layouts of the toe guard were analyzed. The results show that the drag reduction effect is quite apparent when toe guards are installed at the car's top and bottom. In addition, it was found that decreasing the vertical height of the toe guard is conducive to reducing resistance and saving space in the elevator car. The inclusive analysis not only explores the aerodynamic characteristics of super high-speed elevators but also recommends structural optimization design.

Numerical study on aerodynamic resistance reduction of high-speed train using vortex generator

Vortex generator (VG) is one of the potential technical tools to reduce the aerodynamic resistance of high-speed trains. The passive control resistance reduction research of a high-speed train is carried out by using VG. Three typical installation locations, including flow separation point, boundary layer mutation and streamline transition location, and several nearby locations were selected to study the effect of VG location on the train aerodynamic characteristics. The results show that the tail car drag is significantly reduced when the VG is arranged at the boundary layer mutation, the tail car’s resistance can be reduced by 15.42%. The tail car reduces resistance by 3.87% when VG is set at the flow separation point. By analyzing the flow field structure, we found that the VG arranged in front of the separation point triggers the flow separation, which destroys the balance between the separation and longitudinal vortex. It effectively reduces the strength of the separation vortex, thereby reducing the tail car’s aerodynamic drag. The research provides a new thought for the aerodynamic resistance reduction of high-speed trains, and is of great significance to break the limitations of traditional aerodynamic drag reduction technology of trains.

Control mechanism of secondary flow in a turbine cascade with non-axisymmetric endwall profiling under Co-rotating incoming vortex

Abstract Upstream vortex has a significant effect on the secondary flow structure of the downstream turbine in the stage environment. This study investigates the secondary flow structure with non-axisymmetric endwall profiling (NAEW) under the interaction of co-rotating incoming vortex (Vic). A half-delta wing vortex generator is utilized to model Vic. The turbine cascade case which exhibited maximum reduction of the cascade loss with NAEW under no incoming vortex is studied. The mechanism of loss reduction with NAEW under the interaction of Vic is analysed. Vic could decrease the secondary flow near the endwall region by affecting the horseshoe vortex transport in the cascade. However, its loss reduction was lower than the loss increments of Vic itself. The arrival of Vic at the leading edge of the cascade increased the strength of the horseshoe vortex, resulting in a significant increase in loss. Under the interaction of Vic, NAEW decreased the blade loading near endwall region, which resulted in the reduction of cascade loss.

A Review of the Current Unmanned Aerial Vehicle Sprayer Applications in Precision Agriculture

Highlights A comprehensive review of the current Unmanned Aerial Vehicle Sprayer in precision agriculture applications. Comparison of manned and unmanned aerial sprayers in precision agriculture. Latest developments of commercialized UAV sprayers available on the market. Abstract. Unmanned Aerial Vehicles (UAVs) are becoming more broadly used for improving agricultural spraying applications. However, compared with the nearly 100 years of data accumulated on manned aerial applications, UAV sprayers are relatively new, and associated technologies are in the early stages of development. The objective of this paper is to give a comprehensive review of the current UAV spraying platforms with a comparison to manned aerial sprayers and a discussion of their application, performance, and efficiency. A total of 213 peer-reviewed and non-peer-reviewed articles, extension papers, government websites, and company websites were reviewed and cited in this study. We also discuss factors that could influence the effectiveness of aerial spraying applications, such as release height, wind speed, vortex strength, and droplet size. Finally, we review the latest UAV sprayers available worldwide and present technology gaps in those platforms. We highlight areas that require improvement, particularly in autonomous navigational controllers and spraying systems. Keywords: Droplet distribution, Plant protection, Precision agriculture, Spot spraying, UAV sprayer.

An Improved Wake Vortex-Based Inversion Method for Submarine Maneuvering State.

As the noise reduction performance of submarines continues to improve, it is difficult to detect and track submarines through acoustic detection techniques. Therefore, nonacoustic submarine detection techniques are becoming more and more important. The submarine movement will leave a wake vortex, and the information of the wake vortex can be used to invert the maneuvering state of the submarine. However, the wake vortex is constantly dissipated in the evolution process, and the strength of the wake vortex is constantly reduced, resulting in the gradual weakening of the characteristics of the wake vortex, which makes the inversion of submarine operating state difficult and less accurate. In order to solve the above problems, this paper proposes an improved wake vortex-based inversion method for submarine maneuvering state. Firstly, a random finite set of submarine wake vortex observation features is established to obtain the feature with the highest correlation degree with submarine maneuvering state in the random finite set. Secondly, the multiscale fusion module and attention mechanism are used to re-encode the weak features of the wake vortex image, and the salient features of the wake vortex image are extracted. Finally, the manipulation state of the wake vortex image is retrieved by the extracted salient features. The experimental results show that the average inversion accuracy of the proposed algorithm is improved by 1.27% in terms of manipulating state inversion of weak feature wake vortex images. The algorithm in this paper can realize the inversion of submarine maneuvering state in the case of weak submarine wake vortex image features and incomplete feature information. It provides the basis for the detection technology based on the submarine wake characteristics.

The Role of Sea-Surface Conditions in Southern-Hemisphere Polar Vortex Strength and Associated Wave Forcing Revealed by a Multi-member Ensemble Simulation with the Chemistry–Climate Model

The Role of Sea-Surface Conditions in Southern-Hemisphere Polar Vortex Strength and Associated Wave Forcing Revealed by a Multi-member Ensemble Simulation with the Chemistry–Climate Model

Tip vortex cavitation suppression and parametric study of an elliptical hydrofoil by water injection

Tip vortex cavitation (TVC) affects hydrodynamic performance and can cause drastic vibration and noise; therefore, it is crucial to predict the evolution of TVC, understand its generation mechanism, and determine methods to control it. In this work, a large eddy simulation was performed to resolve unsteady turbulence, and the Schnerr–Sauer cavitation model was used to capture transient cavitating flow. Both wetted and cavitating conditions were used in the first step to validate the numerical methods. The mechanism of TVC development and the interactions between the tip vortex and TVC were also revealed. Next, active control by water injection was performed to suppress TVC, and the side and top injection circumstances were explored and compared. Parametric studies were conducted for the side injection condition by changing the injection velocity and angle. The results showed that both side and top injections had remarkable effects on TVC control. Flow field analysis demonstrated that the top injection flow affected the local velocity magnitude and direction of the incident flow of the tip vortex, thus reducing the vortex strength and TVC. For the side injection condition, the injection flow directly influenced the incepted structures of the tip vortex. As a result, injection flow deeply deformed the tip vortex and decreased the generation and intensity of TVC. Furthermore, increasing the injection velocity or the component of the velocity in the cross-streamwise direction could effectively increase the cavitation inhibition rate.

Thermo-hydraulic performance of a cryogenic printed circuit heat exchanger for liquid air energy storage

• The nature of local flow and heat transfer was analyzed using vortex identification. • The reverse flow suppresses local heat transfer and increases the flow resistance. • The air side has larger thermal resistance (∼ 58%) in the absence of phase change. • The best comprehensive performance can be achieved at an inclined angle of 45°. • The pressure drop and heat transfer correlations for the best angle were developed. The liquid air energy storage assisted by liquefied natural gas is a promising large-scale storage method, but its development is limited by the lack of thermo-hydraulic data on the cryogenic printed circuit heat exchanger. A simplified model for conjugate heat transfer was built and solved by a commercial code FLUENT 14.5. Model validation was conducted by comparing results with experimental data. The nature of flow and heat transfer was revealed by quantifying the vortex strength and visualizing the vortex core. The influence of the inclined angle was investigated, and the Nusselt number and friction factor correlations for the best angle were developed. The minimum internal temperature approach for optimum system operation is approximately 6 K, located in the middle of the heat exchanger. Results indicate that the evolution of Dean vortices plays a crucial part in the augmentation of heat transfer. The secondary flow caused by elbows forces the fluid cores to strike the windward side periodically, which disturbs the boundary layer and enhances the fluid mixing. Entropy production analysis shows that the thermal entropy generation has a dominant share of total irreversibility. The best performance is achieved at an inclined angle of 45° since the irreversibility is reduced by augmenting the heat transfer.

Influence of Tip Clearance on Cavitation Characteristics of an Inducer of Turbopump: CFD Study

The tip clearance, a compact gap between the inducer blade tip and casing wall, is critical to both the liquid leakage and cavitation-induced forces of a turbopump. In this study, we numerically investigate the effect of tip clearance on the cavitation characteristics of an inducer. Six different tip clearances, 0.1, 0.3, 0.5, 1.0, 1.5, and 2 mm, namely Models A–F, were designed to evaluate the cavitation performance, cavity structure, blade loading, radial force, etc. Model D (1.0 mm) had the relatively highest head coefficient and smallest cavity area on each blade as compared to all other models. The pressure coefficient distribution and blade loading further confirmed that Model D can maintain a higher pressure head and better suppress the cavitation onset than the other models. The radial force signals in the time and frequency domains show that Model D has an intermediate force magnitude with slightly higher noises at the rotating frequency and its harmonic frequencies. Model D also has a relatively smaller vortex region and smaller vortex strength (λ2 criterion). In short, all results show that Model D is the best alternative to balance the complex interactions of the bulk flow and tip leakage flow, compromising the hydraulic head and rotating cavitation.

Blade-Tip Vortex Noise Mitigation Traded-Off against Aerodynamic Design for Propellers of Future Electric Aircraft

We study noise generation at the blade tips of propellers designed for future electric aircraft propulsion and, furthermore, analyze the interrelationship between noise mitigation and aerodynamics improvement in terms of propeller geometric designs. Classical propellers with three or six blades and a conceptual propeller with three joined dual-blades are compared to understand the effects of blade tip vortices on the noise generation and aerodynamics. The dual blade of the conceptual propeller is constructed by joining the tips of two sub-blades. These propellers are designed to operate under the same freestream flow conditions and similar electric power consumption. The Improved Delayed Detached Eddy Simulation (IDDES) is adopted for the flow simulation to identify high-resolution time-dependent noise sources around the blade tips. The acoustic computations use a time-domain method based on the convective Ffowcs Williams–Hawkings (FW-H) equation. The thrust of the 3-blade conceptual propeller is 4% larger than the 3-blade classical propeller and 8% more than the 6-blade one, given that they have similar efficiencies. Blade tip vortices are found emitting broadband noise. Since the classical and conceptual 3-blade propellers have different geometries, especially at the blade tips, they introduce deviations in the vortex development. However, the differences are small regarding the broadband noise generation. As compared to the 6-blade classical propeller, both 3-blade propellers produce much larger noise. The reason is that the increased number of blades leads to the reduced strength of tip vortices. The findings indicate that the noise mitigation through the modification of the blade design and number can be traded-off by the changed aerodynamic performance.

Comparative Analysis of the Self-Propelled Locomotion of a Pitching Airfoil near the Flat and Wavy Ground.

In this paper, a pitching airfoil near flat and wavy ground is studied by numerical simulations. The kinematic features of the airfoil and the flow field around it are analyzed to reveal unsteady vorticity dynamics of the self-propelled airfoil in ground effect. The optimal pitching periods at different initial heights above flat ground are obtained, which make the pitching airfoil achieve the maximum lift-to-drag ratio. Compared with flat ground, at the same initial height, the optimal pitching periods vary with the shape of ground. The structure and the strength of the wake vortices shedding from the airfoil are adjusted by the wavelength of ground. This leads to the changes of amplitude and occurrence times of the peak and valley of lift and drag force. The results obtained in this study can provide some inspiration for the design of underwater vehicles in the ground effect.

The interaction of longitudinal vortex pairs with a turbulent boundary layer

In this study, we demonstrate an efficient approach to investigating the interaction of vortex pairs on a turbulent boundary layer. Our aim is to assess how vortex characteristics impact the downstream flow. Wall-modelled large eddy simulations are used together with an inlet defined by a Batchelor vortex model superimposed on a turbulent boundary layer profile, generated using synthetic turbulence and a precursor Reynolds-averaged Navier–Stokes calculation. This set-up allows for the efficient testing of multiple configurations whilst providing adequate resolution of outer boundary layer and the vortex–vortex interactions. After validating the methodology, we report a set of simulations for both co- and counter-rotating vortex pairs at different separations and asymmetric strengths. The separation distance between the vortices was found to have a significant effect upon the merging of the vortices. Asymmetric strength vortex pairs, analogous to vortex generators in yaw, demonstrate performance independent of flow direction for small angles. Detailed analysis of this flow provides insight into turbulence generation mechanisms and Reynolds stress anisotropy – valuable reference data for the development of lower-order models. Skin-friction enhancement is shown to be more effective for counter-rotating vortex pairs than co-rotating pairs of the same strength and spacing. Additionally, a wider spacing between the initial vortex positions results in a faster rise in the skin-friction coefficient.

Thermal mixing characteristics of T-junction incident along central axis based on Large-Eddy simulations

A large eddy simulation model was used to explore the temperature mixing characteristics of a T-junction in which a branch pipe extends into the main and the branch fluid is injected along the central axis of the main pipe. The influence of the incident velocity of the branch tube on the temperature fluctuation of the fluid mixing is considered. In the main mixing region, the dimensionless time-averaged temperature（T∗¯）is positively related to the dimensionless root mean square（Trms∗）, while the connecting pipe region shows the opposite trend. With the increase of the velocity ratio, the influence of the branch pipe structure on the main flow field gradually decreases. The rebound vortex accelerates the rupture of the vortex ring, and the higher injection speed of the branch pipe leads to an increase in the vortex strength and the range of mixing area, which enhances the mixing of the fluid and makes the temperature distribution of the flow field more uniform. Finally, through the power spectral density analysis of temperature fluctuations, it is not only found that the high-energy vortex ring falls off to the near-wall region, dominating the temperature fluctuation in this region, but also this type of T-junction greatly reduces the risk of thermal fatigue failure.

Non‐Linear Response of the Extratropics to Tropical Climate Variability

AbstractThere are established El Niño‐Southern Oscillation (ENSO) and Quasi‐biennial Oscillation (QBO) teleconnections to the extratropics that alter the strength of the polar vortex. Here, we demonstrate non‐linearity in the stratospheric polar vortex response to ENSO and QBO combined, in climate model simulations and observational data. We show that the combined effect of El Niño and QBO easterly phase (QBO‐E) on the polar vortex is greater than the linear combination of the individual responses to El Niño and QBO‐E separately. Observations support the model results, despite a relatively small sample of observed cases. We propose that non‐linearity in the teleconnection to ENSO and QBO is caused by the combined effects of an increase in wave driving from El Niño and more poleward propagation of wave activity during QBO‐E. Our results have implications for extreme winters when both El Niño and QBO‐E are active.

Effects of rotation on collection characteristics of fine particles by droplets

Removing particles dispersed in fluid through drops is widely presented in various fields, and the critical factor is particles captured by droplets. Drop rotation effects play a dominant role in the capture process. However, their influences on collection characteristics remain unclear. Thus, a particle collection model was developed to simultaneously consider rotation and translation effects on fine particles captured by an individual droplet. The finite volume method was used to solve for flow field and collection efficiency, and the proposed model was verified by comparison with experimental and published results. The Liutex method was used to identify the vortex structure, on which dimensionless droplet rotation rates ranged from 0 to 0.1. Velocity, drag coefficient, radial position, and captured particle velocity distribution and collection efficiency were also investigated in relation to the rotation effect. The results show that the established model is reasonable. Vortex strength increases with increased rotation speed where the increment can be up to 480, and fluid rotation strength depends on the competitive relation between the increase in the rotation rate and the vortex movement. Radial velocity increases in regions where the angle between the positive x axis and the normal vector of drop surface ranges from 115° to 180° but decreases in regions where the angle ranges from −180° to −120°, and corresponding regions produce a comparative relation for improving particle capture. Increasing the rotation rate can increase the drag force coefficient by about 0.025, hindering droplet–particle collision. Average radial velocity of particles with higher than 3.7 mm/s is necessary at high rotation rates, while collection efficiency decreases at increased droplet rotation rates.

On the acoustic emission characteristics of airfoils with different trailing edge configurations

The present study experimentally and numerically investigates the flow past flat plates and NACA0012 airfoils with the different trailing edge (i.e., blunt, sharp, and rounded) and both end-rounded configurations on the far-field acoustic emissions and complex flow phenomena to determine the best edge geometry for low noise. The studies are conducted for various jet speeds of 20, 30, and 40 m/s (chordwise Reynolds numbers of 1.812 × 105, 2.72 ×105, and 3.62 ×105). It is observed that all the edge-modified flat plates show the highest directivity at an emission angle of 60° for all jet velocities studied. The comparison of power directivities between realistic and flat plate airfoils reveals that the realistic airfoils radiate lower acoustic emissions as compared to flat plate airfoils, albeit they show a common feature of downstream directivity. In general, the far-field acoustic emissions of both end-rounded plates are observed to be lower as compared to blunt, sharp, and rounded trailing edge geometries. The leading edge modification is also observed to reduce airfoil self-noise. The likely reason for the lower far-field acoustic emissions provided by both end-rounded trailing edged foils is due to the modification in the boundary layer characteristics owing to the presence of smooth flow around the rounded leading edge as well as reduced vortex strength as compared to the other foil geometries. Thus, the present study demonstrates that passive modifications of leading edge profiles are essential along with trailing edge for achieving lower acoustic emissions and higher noise reductions.