Vortex Structures Research Articles

Machine learning-assisted discovery of flow reactor designs

Additive manufacturing has enabled the fabrication of advanced reactor geometries, permitting larger, more complex design spaces. Identifying promising configurations within such spaces presents a significant challenge for current approaches. Furthermore, existing parameterizations of reactor geometries are low dimensional with expensive optimization, limiting more complex solutions. To address this challenge, we have established a machine learning-assisted approach for the design of new chemical reactors, combining the application of high-dimensional parameterizations, computational fluid dynamics and multi-fidelity Bayesian optimization. We associate the development of mixing-enhancing vortical flow structures in coiled reactors with performance and used our approach to identify the key characteristics of optimal designs. By appealing to the principles of fluid dynamics, we rationalized the selection of design features that lead to experimental plug flow performance improvements of ~60% compared with conventional designs. Our results demonstrate that coupling advanced manufacturing techniques with ‘augmented intelligence’ approaches can give rise to reactor designs with enhanced performance.

Effect of jet splitting using passive strut on the performance and thermoacoustic characteristics of a scramjet combustor

A scramjet engine offers a potential route to achieve supersonic speeds using airbreathing engines. Achieving proper mixing and combustion poses a challenge due to the supersonic inflow of air. Researchers have explored multi-strut configurations to tackle this issue. However, multiple struts supplying fuel inefficiently can lead to fuel loss and reduced efficiency. Alternatively, utilizing a multi-strut setup passively could enhance combustion and mixing efficiency. In this study, two types of jet splitting passive strut configurations were investigated computationally with the improved delayed detached-eddy simulation turbulence model. Implementation of passive strut altered vortical structures, influencing mixing and combustion performance. The splitting of the jet introduces large-scale vortices downstream. Strategically placing the passive strut in the wake of the combustion zone was found to improve both mixing and combustion efficiency. Acoustic loading was seen to increase with the introduction of passive strut. It was observed that the diamond-shaped passive strut has the highest combustion efficiency; however, it suffers from higher acoustic loading. The dynamic mode decomposition analysis revealed the coupling frequency of fluctuating pressure and heat release rate, which causes thermoacoustic loading. Overall, passive strut placement significantly influenced combustion, mixing, and thermoacoustic properties, highlighting the importance of considering passive strut configurations in design optimization for scramjet engines.

Direct Numerical Simulation of Tandem-Wing Aerodynamic Interactions at Low Reynolds Number

A three-dimensional direct numerical simulation was conducted to investigate the vortex–wing interactions through two NACA 0012 stationary wings placed in tandem at a low Reynolds number Re=10,000. The aerodynamic characteristics and three-dimensional flow structures were analyzed for the tandem wings. The back wing disturbed by the upstream vortices gained an evident increase in aerodynamic performance, where the advantage is related to the suppression of the large-scale vortex formation near the trailing edge. The Liutex method was applied to visualize the vortical structures for investigating the three-dimensional evolution and instability when interacting with the back wing. The upstream wake triggered dual-secondary vortices and intensified the secondary instability on the back wing. The induced vortices contributed to the lift enhancement because they provided an extra low-pressure region when propagating downstream on the suction side of the back wing. Because of the three-dimensional destabilization, the vortex interaction in the evolution process accelerated the transition and injection of the high-momentum flow into the boundary layer attached to the back wing, energizing the turbulent boundary layer and eliminating the large-scale separation near the trailing edge. This study provided a new perspective on the enhanced aerodynamic performance of tandem layout.

Flow dynamics and heat transfer characteristics of flows past a zigzag cylinder in a bounded domain

A comprehensive numerical and experimental study is conducted to investigate the flow dynamics and heat transfer characteristics of flows past a novel wavy-axis circular cylinder placed symmetrically between two parallel walls. For comparison, the flow past a corresponding straight-axis cylinder within the same bounded domain is also studied. The investigations were primarily conducted at Reynolds numbers of 626 and 680, based on the average velocity and diameter of the cylinder, with a blockage ratio of 0.42, based on the widest area of the channel. At these flow conditions, the straight cylinder expectedly showed a “reverse” von Kármán type wake. However, the special zigzag cylinder showed fundamentally distinct flow patterns at the wake leading to a significant drag reduction and a heat transfer enhancement, compared to the performance of the straight cylinder. The pressure drop of the zigzag cylinder is 14 % and 19.4 % lower than that of the straight cylinder, with a higher convective heat transfer coefficient of 24 % and 9 %, respectively, for Reynolds numbers of 626 and 680. The substantial drag reduction is attributed to the formation of steady flow pattern in the wake indicating the suppression of unsteady vortex shedding. The heat transfer enhancement was mainly caused by the formation of elongated vortical structures principally aligned in the streamwise direction. The creation of these streamwise vortex tubes tends to more efficiently mix the fluid between the near-wall area and the core region of the channel than the near-planar vortex shedding associated to the straight cylinder. It was shown that the 3D shape of the zigzag cylinder generates a spanwise velocity component, which results in the creation of a dominant streamwise vorticity component that eventually develops into sustained vortex tubes.

Comparison of vortex formation in expanded and curved aortic sinuses: The effect of sinus curvature

Aortic valve disease is a significant health issue. Research on the effects of aortic sinus geometry and aortic valve motion during a heartbeat is imperative, as numerous valvular diseases are associated with blood flow near the heart valve. Changes in the internal space of the sinus and the formation of blood clots have been reported after the implantation of a transcatheter aortic valve, an artificial heart valve. Although the sinus shape and leaflet motion significantly affect the hemodynamic characteristics and platelet aggregation, the blood flow behaviors near the heart valve associated with clot formation have not been fully elucidated. This study conducted in vitro experiments to investigate the flow behavior near an aortic sinus model based on the sinus shape. Additionally, hemodynamic changes associated with variations in the aortic sinus geometry and leaflet length within an aortic sinus model under pulsatile-flow conditions were elucidated. In a curved sinus model, a large single vortex formed near the center of the sinus region. Meanwhile, the center of the vortex in the sinus region shifted during a cycle in an expanded sinus model. Furthermore, a small vortex remained where the leaflet was initiated, thus increasing the likelihood of thrombus formation. Thus, the curved sinus model is advantageous for preventing material accumulation by maintaining a large vortex structure. The experimental results confirmed that the aortic sinus shape and leaflet length affect the likelihood of thrombus formation inside the aortic sinus.

Numerical investigation of a three-dimensional flow structure influenced by the lateral expansion of a partially distributed submerged canopy

This study employed numerical simulations to investigate the three-dimensional hydrodynamic structure of a channel obstructed by submerged rigid canopies. After validating the model using existing experimental data, a series of numerical experiments varying the canopy density and the blocking ratio were conducted. The results indicate that submerged canopy, different from emergent canopies, triggers the generation of three-dimensional coherent vortices. When canopies expand laterally, the scale, location, and evolution characteristics of cross-sectional secondary flows vary. The direction of secondary circulation at the canopy top of the fully covered vegetation channel is affected by the river width/depth ratio. Three-dimensional coherent vortices are jointly controlled by vertical and transverse Reynolds stresses. When the channel is seriously blocked, the degree of momentum exchange in the mixing layer does not change significantly. The boundary friction effect dominates the momentum exchange process near the wall. An equation coupling the drag length scale and blocking ratio with the vertical coherent vortex penetration length is proposed, indicating that the penetration length increases exponentially with both variables. Higher canopy density exhibits a stronger resistance to the vertical coherent vortex penetration length. In the process of canopy lateral expansion, the evolution time of the outer vortex to scale stability is longer than that of the inner vortex. The significance of research on the three-dimensional vortex structure of rigid submerged vegetation is established based on previous studies and existing conclusions.

The effect of swirling injector geometry on the flow dynamics of precessing vortex core

Precessing vortex core (PVC), a typical vortex structure, universally exists in swirling flow fields and originates from the Kelvin–Helmholtz instability of the swirling shear layer. In the combustion chamber, pressure pulsations from PVC might resonantly couple with acoustic modes, leading to thermoacoustic oscillations which is a significant challenge in the development of rocket engines and gas turbines. The present work focuses on the time-averaged and unsteady flow characteristics of different swirling flow fields in the premixed swirling combustor. The PVC observed in the swirlers adopted presents a single helical structure and the PVC frequency is proportional to the incoming flow velocity. Moreover, another low-frequency helical mode located in the downstream region of the flow fields has been identified and categorized as PVC type II to be distinguished with common PVC mode in this study, which originates from the precession of the central recirculation zone. The experimental results further emphasize the effects of the swirling injector geometry on PVC and the low-frequency helical modes. This indicates that as the diameter of the bluff body or the divergence cup angle increases, the energy of the PVC mode decreases while that of the helical mode increases significantly. In addition, the PVC mode would be gradually suppressed as the tail size of the bluff body increases. These results, especially the coexistence of the PVC mode and the low-frequency helical mode, are rarely reported in previous studies, and they may provide an effective guidance for investigating the mechanism and control of thermoacoustic oscillations.

Generation of inflow turbulence using an improved synthetic eddy method

The generation of turbulent inflow conditions is a key issue in large eddy simulation (LES) or direct numerical simulation (DNS). In this paper, an improved synthetic eddy method (SEM) is proposed to generate inflow turbulence for LES and DNS. The improvements about SEM focus on the eddy radius and eddy distributions. First, the eddy radius is improved to reduce the nonphysical vortex structure on the wall caused by overestimation of the radius. Second, a sampling method using Gaussian distribution is proposed to improve the distribution of eddies, which accurately captures the randomness of turbulent structure size and is close to the actual flow field. The improved method is applied to the direct numerical simulation of the supersonic turbulent boundary layer at Mach 2.7 and the 24° compression ramp. Results indicate that the predictions yielded by the improved method are in good agreement with both DNS and experimental data. Compared to the original method, the improved method exhibits a more rapid recovery of the friction coefficient and effectively shortens the development distances. The improved SEM has enhanced the efficiency and accuracy of generating inlet turbulence, which can provide inlet turbulence boundary conditions for LES and DNS.

Simultaneous Multiscale 4DEnVar With Scale Dependent Localization (SDL) in Hurricane Analysis and Forecast System (HAFS)

AbstractThis study presents the implementation and evaluation of scale‐dependent localization (SDL) in the hurricane analysis and forecast system 4D Ensemble Variational (4DEnVar) Data Assimilation (DA) system. The SDL capability is compared with the traditional Single‐Scale Localization (SSL) method to assess its benefits and necessity for hurricane prediction. The experiments focus on Hurricane Laura (2020) and involve single observation experiments as well as real observation DA cycling experiments. The results indicate that the SDL experiment, which incorporates the Fast Almost Gaussian Filtering approach for scale decomposition, consistently outperforms the corresponding SSL configurations in almost all aspects. Further diagnostics show that due to its multiscale nature, the SDL approach demonstrates better track prediction over small‐scale SSL due to improved environmental analysis and better analyzed vortex position and structure, and superior intensity prediction during the rapid intensification over both the large‐scale SSL and the small‐scale SSL owing to enhanced inner‐core thermodynamic analysis.

Curved vortex surfaces in four-dimensional superfluids. II. Equal-frequency double rotations

As is well known, two-dimensional and three-dimensional superfluids under rotation can support topological excitations such as quantized point vortices and line vortices, respectively. Recently, we have studied how, in a hypothetical four-dimensional (4D) superfluid, such excitations can be generalized to vortex planes and surfaces. In this paper we continue our analysis of skewed and curved vortex surfaces based on the 4D Gross-Pitaevskii equation and show that certain types of such states can be stabilized by equal-frequency double rotations for suitable parameters. This work extends the rich phenomenology of vortex surfaces in four dimensions and raises interesting questions about vortex reconnections and the competition between various vortex structures which have no direct analog in lower dimensions. Published by the American Physical Society 2024

Evolution of Displacement Speed Statistics During Head-On Flame-Wall Interaction within Turbulent Boundary Layers

ABSTRACT The statistical behaviours of the density-weighted displacement speed and its curvature and strain rate dependence during head-on interaction of statistically planar premixed flames have been analysed based on Direct Numerical Simulations. The analysis has been conducted within turbulent boundary layers featuring inert walls, under both isothermal and adiabatic wall boundary conditions. The flame quenches due to heat loss when it reaches close to the wall in the case of isothermal wall boundary condition. By contrast, the flame can propagate all the way to the wall before extinguishing due to the consumption of reactants in the case of adiabatic wall boundary conditions. Thus, the effects of thermal expansion, quantified by dilatation rate, and flame normal acceleration during flame-wall interaction in the case of the isothermal wall are weaker than that in the case of the adiabatic wall case due to flame quenching. This gives rise to significant differences in the curvature and strain rate dependences of the reactive scalar gradient magnitude, which affects the statistical behaviours of the reaction and normal diffusion components of density-weighted displacement speed including their mean values, widths of their probability density functions, and their local strain rate and curvature dependences. It has been found that the interaction of near-wall vortical structures with flame surface increases the range of curvature variation during head-on interaction, which acts to widen the probability density function of the density-weighted displacement speed. This trend is relatively stronger for the isothermal wall boundary condition because of the larger variation of the reaction rate component of the density-weighted displacement speed due to local flame quenching, which also acts to widen the range of the density-weighted displacement speed variation in comparison to that in the case of adiabatic wall boundary conditions. Although the qualitative nature of curvature, strain rate, and stretch rate dependences of the density-weighted displacement speed remain unaffected by the wall boundary condition, the strength of the correlation changes with the progress of head-on interaction. It has been found that the negative correlation between the density-weighted displacement speed and flame curvature weakens with the progress of head-on interaction, which gives rise to a reduction in the strength of the correlation between the density-weighted displacement speed and flame stretch rate. Similarly, the correlation between the tangential strain rate and the density-weighted displacement speed weakens with the progress of head-on interaction. Detailed physical explanations are provided for these behaviours, and their modeling implications are indicated.

Perfect correlation vortices.

We introduce perfect correlation vortices and show that the degree of coherence of any such vortex at the source is nearly statistically homogeneous and independent of the topological charge of the vortex. We demonstrate that while slowly diffracting in free space, perfect correlation vortices maintain their "perfect" vortex structure; they are capable of preserving said structure even in strong atmospheric turbulence. Structural resilience to diffraction and turbulence sets the discovered perfect vortices apart from their coherent cousins and makes them suitable for free-space optical communications.

Recent advancements in flow control using plasma actuators and plasma vortex generators

AbstractFlow‐control techniques have attracted significant attention in many scientific areas due to their ability to improve the effectiveness and regulate the flow of aerodynamic devices. This study explores the latest developments in flow‐control techniques, specifically concentrating on the cutting‐edge technologies of plasma vortex generators (PVGs) and actuators. By taking advantage of the ionization of gases or air, plasma actuators have become a viable method for modifying an object's aerodynamic properties without needing physical moving parts. These actuators create localized plasma discharges that interact with the surrounding flow to provide accurate separation control, boundary‐layer dynamics, and aerodynamic forces on aircraft wings, wind turbine blades, and other surfaces. PVG, which produce controlled vortical structures, offer a novel way to manipulate airflow with plasma actuators. These generators create swirling motions through plasma discharges that can be used in various technical applications, such as automotive, marine, and aviation, to modify boundary layers, reduce drag, and improve lift characteristics. This study offers an overview of recent work, focusing on the theoretical underpinnings, experimental validations, and practical applications of plasma‐based flow‐control technologies. Advances in plasma‐generating techniques, computational modeling approaches, and experimental configurations to optimize and comprehend the intricate fluid–structure interactions are covered in the debate. Moreover, the study delves into incorporating plasma‐based flow management into cars, renewable energy systems, and next‐generation aerospace designs, highlighting the possibility of increased agility, decreased emissions, and efficiency. It also discusses the difficulties and potential paths for developing these technologies further for use in business and industry, highlighting the necessity of dependable, scalable, and durable solutions. Finally, this study summarizes the most recent advancements in vortex generators and plasma actuators for flow control. It demonstrates how they have the power to revolutionize fluid dynamics and aerodynamics in a variety of engineering fields.

Study of vortex dynamics in a solar tower vortex generator

Vortical flows, integral to fluid mechanics, significantly influence the performance and efficiency of various engineering applications. Utilizing a validated three-dimensional computational model, this study characterizes vortex structures and examines the impact of ambient conditions and the strategic positioning of vortex generators (swirler) at critical geometric locations, such as the throat, and regions above and below it, within a converging–diverging solar tower. The research explores the influence of swirler position on recirculation zone characteristics, focusing on center-to-center (C2C) distance, penetration length, and maximum width. When the swirler is positioned above or below the throat, C2C distance shows minimal change. However, placing the swirler exit at the throat significantly reduces C2C distance at the lowest velocity (0.13 m/s), due to a stable recirculation zone. This effect diminishes at higher velocities. Swirler position greatly affects penetration length, resulting in two distinct behaviors: Behavior A, dominated by swirler position, occurs when the swirler is above or below the throat, and Behavior B, which is velocity-dependent, is observed when the swirler exit is at the throat. The study also investigates the interaction between the recirculation zone and swirler exit flow, focusing on restriction and mixing ratios. The restriction ratio is higher below the throat (31 %) compared to the throat (27 %). The mixing ratio shows minimal variation with inlet velocity but significant variation with swirler position. Among the evaluated positions, placing the swirler below the throat results in the lowest swirl strength for most axial distances, except near the exit height, rendering it less suitable for water harvesting applications. These findings provide guidance for optimal swirler placement to achieve desired recirculation zone control and system performance.

Investigation of Reynolds number effects on flow stability of a transonic compressor using a dynamic mode decomposition method

With the continuous increase in aircraft flight altitude, low Reynolds number effects in high-altitude environments significantly impact the stability of compressor operation. In this study, dynamic mode decomposition (DMD) of the flow field at various time instances near stall conditions in a transonic compressor was performed to investigate the influence of Reynolds number (Re) effects on critical flow characteristics and instability mechanisms. The results indicate that under different Reynolds number index (RNI) conditions near stall conditions, the frequency of tip leakage flow varies. Under high RNI condition, the influence of tip leakage flow is concentrated mainly from the leading edge to the mid-chord. The stall mode is primarily caused by the interaction of shock with the Tip Leakage Vortex, resulting in the breakdown of the vortex. The development of vortex structures inside the passage leads to a delay in the flow field response to blade passing. Under low RNI condition, the tip clearance leakage flow is stronger, with a relatively reduced influence from the leading edge to the mid-chord and an increased impact from the mid-chord to the trailing edge. The main cause of the stall mode is the circumferential movement of vortex structures, causing secondary leakage over adjacent blade tip clearances. The blockage formed by vortex structures within the passage is weaker, and the flow field response to blade passing remains consistent with the blade passing frequency. However, as stall develops further, the flow field response significantly decreases.

Theoretical solution of vortex splitting due to islands effect on the vortex

Islands, acting as transitional areas between land and sea, significantly influence their surrounding environments. Studying the changes in vortex structure resulting from the interaction between islands and mesoscale vortices is crucial for understanding the dynamic characteristics and ecological processes of the marine environment. Previous theoretical studies have shown that in a domain without boundaries, due to the conservation of angular momentum, a vortex cannot split by itself. This paper establishes the conditions for the splitting of an anticyclonic vortex when it collides with two square islands. By linking the initial and final states of islands effect on the vortex and utilizing conserved quantities such as angular momentum and mass, along with the slow variation approximation, a nonlinear theoretical solution is constructed. The analysis shows that when the interaction between the vortex and the two islands leads to vortex splitting, the length of the islands, the vortex radius, and the distance between the islands must satisfy a certain condition O(LR)∼O(wR)∼O(1). These results provide support for subsequent analyses of the impact of various parameters on vortex structure when the North Brazil Current (NBC) ring encounters the Lesser Antilles in the tropical western Atlantic.

Investigation of the flow-heat-vortex mechanism in hot, coolant and delivery channels in film cooling under strongly-coupled boundary conditions using direct numerical simulation

The research investigated the flow dynamics, heat transfer, and vortices in film cooling using direct numerical simulation (DNS) with various blowing ratios (Br = 0.63, 0.86, 1.18) and Reynolds numbers (Re = 1000, 1000, 500) at an injection angle of α = 20°. The strongly-coupled (SCP) boundary conditions were applied to solve the transient governing equations including to mass and momentum conservations, thermal convection and heat conduction. Jetting effects in the delivery channel were studied by the DNS method in the environment of both hot and coolant channels. Distinctive vortex structures, including the horseshoe and hairpin vortices inside the delivery channel, were successfully identified with the recirculation and high-velocity stripes inside the channel. The heat-transfer mechanisms resulting from the vortices in hot channel highlighted the presence of kidney-shaped and reverse-kidney-shaped vortices impacting the Nusselt number (Nu) and comprehensive cooling effectiveness, which was enabled by the state-of-the-art VI method of Liutex.

Numerical simulation on the pressure, turbulence, and λ2$$ {\lambda}_2 $$ vortex characteristics within the annular symmetric jet process under different vacuum degrees

AbstractThe jet impingement flash technology represents a paramount research subject in the domain of heat and mass transfer. To augment its commercial potential, the conjunction of annular multi‐aperture jet impingement with negative pressure flash evaporation is introduced in this study. The employment of an annular nozzle array is integral to the enhancement of the heat and mass transfer efficiency between the phases. The Realizable k‐ε model is used in this study. The negative pressure flash vaporization model was also developed by introducing a mass source term and an energy source term based on the Mixture model. The flow characteristics are characterized using numerical simulation. Additionally, the λ2 vortex identification criterion is investigated the vortex structure. The simulation results exhibit good agreement with experimental findings, demonstrating that a higher initial vacuum leads to a stronger flashing effect and a more chaotic movement of the flow group within the flow field. Thus, this study provides a reference method for the structural design and optimization of annular symmetric jet impingement negative pressure deammonia chemical equipment for engineering applications.

Information transfer between tip leakage vortex and blade aerodynamic force of a compressor cascade

The compressor blade exhibits intricate flow patterns near stall conditions, with the tip leakage vortex and the main flow shedding vortex from the blade's trailing edge identified as factors influencing the blade's aerodynamic force oscillation. The tip leakage vortex is recognized as the primary contributor to flow structures during non-synchronous vibration (NSV), although existing compressor NSV reduced order models often attribute the fluid oscillation to the passage flow shedding vortex. A tool capable of distinguishing between flow structures and blade aerodynamic force is crucial for resolving this discrepancy. Detached eddy simulations (DES) in a compressor cascade form the basis for gathering data on vortex structures and blade aerodynamic force. This study employs an information thesis method to elucidate the cause-and-effect relationship between the tip leakage vortex and blade aerodynamic force. Through a series of unsteady single passage simulations and experimental validation, the mechanisms leading to variations in blade aerodynamic force are demonstrated. Notably, this research utilizes Proper Orthogonal Decomposition (POD) mode time coefficients in span and axial directions to represent dynamic information on vortex structures. Additionally, transfer entropy is introduced as a metric to evaluate the causal interaction between turbulent flow of the tip leakage and the blade's aerodynamic force, marking a significant advancement. The analysis of information transfer entropy aids in identifying the vortex structure with a stronger relationship to aerodynamic force, a critical finding. By applying this approach, the study examines tip leakage vortex structures in various tip clearance sizes concerning aerodynamic force. The Tip3C and Tip5C tip geometries exhibit tip leakage vortex features with radial vortex characteristics and backflow, akin to structures associated with non-synchronous vibration. These tip vortex structures in the Tip3C and Tip5C tip geometry cascades demonstrate substantial correlations with aerodynamic force oscillation on the blade surface. Conversely, the Tip1C tip geometry cascade displays distinct tip leakage vortex features compared to Tip3C and Tip5C, with weaker ties to blade aerodynamic force oscillation. As simulation models progress, consideration of vortex structures in the passage and leakage vortex at the leading edge is imperative for accurate predictions of aerodynamic force oscillation response.

A Vortex Particle-on-Mesh Method for Soap Film Simulation

This paper introduces a novel physically-based vortex fluid model for films, aimed at accurately simulating cascading vortical structures on deforming thin films. Central to our approach is a novel mechanism decomposing the film's tangential velocity into circulation and dilatation components. These components are then evolved using a hybrid particle-mesh method, enabling the effective reconstruction of three-dimensional tangential velocities and seamlessly integrating surfactant and thickness dynamics into a unified framework. By coupling with its normal component and surface-tension model, our method is particularly adept at depicting complex interactions between in-plane vortices and out-of-plane physical phenomena, such as gravity, surfactant dynamics, and solid boundary, leading to highly realistic simulations of complex thin-film dynamics, achieving an unprecedented level of vortical details and physical realism.