Vortex Formation Research Articles

Local precursors to anomalous dissipation in Navier-Stokes turbulence: Burgers vortex-type models and simulation analysis

Anomalous dissipation is a mechanism to dissipate kinetic energy which is established by a sufficiently spatially rough velocity field. It implies that the rescaled mean kinetic energy dissipation rate becomes constant with respect to Reynolds number Re, the dimensionless parameter that characterizes the strength of turbulence, given that Re≫1. The present study aims at bridging this statistical behavior of high-Reynolds-number turbulence to one specific structural building block of fluid turbulence—local vortex stretching configurations which take in the simplest case the form of Burgers's classical vortex stretching model from 1948. We discuss the anomalous dissipation in the framework of Duchon and Robert for this analytical solution of the Navier-Stokes equations, apply the same analysis subsequently to a generalized model of randomly oriented Burgers vortices by Kambe and Hatakeyama, and analyze finally direct numerical simulation data of three-dimensional homogeneous, isotropic box turbulence in this respect. We identify local high-vorticity events in fully developed Navier-Stokes turbulence that approximate the analytical models of strong vortex stretching well. They correspond to precursors of enhanced anomalous dissipation. Published by the American Physical Society 2024

Magnetohydrodynamic Enhancement of Biofuel Cell Performance.

Biofuel cells have become an interesting alternative for the design of sustainable energy conversion systems with multiple applications ranging from biosensing and bioelectronics to autonomously moving devices. However, as an electrochemical system, their performance is intimately related to mass transport conditions. In this work, the magnetohydrodynamic (MHD) effect is studied as an easy and straightforward alternative to enhance the performance of a biofuel cell based on the enzymes glucose oxidase (GOx) and bilirubin oxidase (BOD). The synergetic effect between the electric and ionic currents, produced by the enzymatic redox reactions, and a magnetic field orthogonal to the surface of the electrodes, leads to the formation of localized magnetohydrodynamic vortexes. Such an integrated convective regime generates an increase of the bioelectrocatalytic current and its concomitant power output in the presence of the external magnetic field. In addition, by fine-tuning the spatial arrangement of the anode and cathode, it is possible to benefit from the sum of anodic and cathodic MHD vortexes, leading to an enhanced power output of up to 300%.

Numerical simulations of the effects of KC number and entrapped water on the subsea cover hydrodynamic characteristics

ABSTRACT The entrapped water in subsea structures with openings significantly influences hydrodynamic forces and structural stability. This study utilises Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with the k − ω Shear Stress Transport (SST) turbulence model to investigate oscillatory flows around a subsea cover under varying conditions, including different levels of entrapped water (obtained by different opening dimensions) and KC numbers. Numerical simulations are validated through experimental data from oscillatory flows over a subsea cover on a plate. The study examines lift and in-line force coefficients to assess the impact of opening dimensions and KC numbers on hydrodynamic forces. Findings show that opening size primarily affects the lift coefficient due to changes in the behaviour of entrapped water. The influence of entrapped water decreases as both the KC number and opening size increase. A 30% opening yields the smallest lift-to-in-line ratio ( C Lrms / C Drms ) and reduced C Lrms and C Lstd values. Additionally, the flow fields around the cover are also discussed. Smaller openings reduce the amplitude of velocity, vorticity, and pressure inside the cover. Openings mitigate vortex formation and shedding caused by jets between the structure and the wall. The results also reveal a phase shift in lift coefficients for different opening sizes. These findings provide critical insights for the hydrodynamic behaviour and design of similar subsea structures.

Impact of Surface Turbulent Fluxes on the Formation of Roll Vortices in a Mediterranean Windstorm

AbstractRoll vortices can have a significant influence on the exchanges of momentum, sensible heat and moisture throughout the boundary layer. They have been shown to reinforce air‐sea interactions under strong wind conditions. This raises the question of how air‐sea exchanges can, in turn, affect the roll vortices. However, representing surface turbulent fluxes during extreme wind conditions is a major challenge in numerical weather prediction and can lead to large uncertainties in surface wind speed. The sensitivity of rolls to different representations of surface fluxes is investigated using large eddy simulations. The study focuses on Mediterranean windstorm Adrian, where rolls are responsible for the transport of strong winds to the surface. Considering the effects of sea spray increases surface heat fluxes and significantly influences the roll morphology. The rolls become narrower and extend over a greater height, while the downward transport of momentum is intensified and results in higher wind speed near the surface. Roll vortices vanish within a few minutes in the absence of momentum fluxes, which maintain the wind shear necessary for their organization. They also quickly weaken without sensible heat fluxes, which sustain the near‐neutral stratification required for their development. In contrast, latent heat fluxes play a minor role. These findings emphasize the necessity of precisely representing the processes occurring at the air‐sea interface, which do not only affect the thermodynamic surface conditions but also the vertical transport of momentum within the windstorm.

Optimizing vortex-induced vibration suppression in π-shaped girders using wind tunnel testing and CFD analysis

This investigation meticulously explores the vortex-induced vibration (VIV) characteristics in Π-shaped girders within long-span cable-stayed bridges, presenting novel VIV mitigation techniques. Employing detailed wind tunnel evaluations of a 1:50 scale model alongside computational fluid dynamics (CFD) analyses to model the two-dimensional flow around the girder, this study reveals the formation and shedding of vortices behind the railings and main steel girder, which significantly contribute to VIV in such structures. The introduction of horizontal splitter plates, one m in width, in conjunction with two-m-wide V-shaped guide vanes, was found to substantially reduce the VIV amplitude of the Π-shaped girder, minimizing vortex generation on the deck’s sides and dramatically reducing the maximum vertical VIV amplitude from 208.75 mm to only 5.56 mm. CFD simulations confirm the effectiveness of these measures in suppressing Karman vortex street formation, thereby significantly improving downstream airflow characteristics. These findings offer pivotal insights for the advancement of bridge design and the proactive management of VIV, showcasing the potential of integrated aerodynamic modifications to enhance structural resilience and performance.

Thermal and hydrodynamic characteristics of microencapsulated phase change materials slurry flow in wavy microchannels

Microencapsulated phase change material slurry (MPCS) is a novel cooling fluid with promising performance. This study numerically investigates heat transfer and flow characteristics of MPCS within wavy microchannels. MPCS is modeled as a homogeneous, Newtonian fluid. Five wavy geometries were examined across a Reynolds number range of 50 to 250 (laminar flow regime), varying in amplitude and wavelength. The model results show that with increase in Reynolds number and decrease in the amplitude and radius of curvature of wavy microchannels, the pressure drop and Nusselt number also increase. Furthermore, the results reveal that Dean vortices intensify with increasing wave amplitude and Reynolds number. Conversely, these vortices weaken as channel wavelength and radius of curvature increase. The formation of Dean vortices enhances fluid mixing and consequently improves the thermal performance of the slurry. The study concludes that in wavy microchannels employing MPCS, increasing the Reynolds number, increasing the channel amplitude with respect to wavelength, and decreasing the radius of curvature improves the overall performance. Also, the results reveal that in higher Reynolds numbers, the radius of curvature is the most effective parameter on the overall performance of wavy microchannels.

Size Classification and Material Sorting of Fine Powders with a Deflector Wheel Air Classifier and an Electrostatic Separator

In this study, a two-dimensional separation of microparticles based on their settling velocity and triboelectric charge ability is achieved using an air classifier for size fractionation and simultaneous charging, followed by an electrostatic separator. In the first part, considerations for enhancing particle classification with high sharpness and low-pressure drops are discussed through improvements in blade design investigated with CFD simulations and validated experimentally. Blades with extended lengths towards the center of the classifier prevent the formation of high-velocity vortices, thereby minimizing the back-mixing of particles and enhancing separation sharpness. This approach also reduces pressure drops associated with these flow vortices. In the second part of the study, the modified blades within the classifier are utilized for two-dimensional separation. Powders from two different materials are fed into the classification system, where particles become triboelectrically charged, mainly through collisions with the walls of the classification system components. Coarse particles are rejected at the wheel and exit the classifier, while differently charged fine particles of the two materials are directed into an electrostatic separator for material sorting. An enrichment of approximately 25–35% for both materials has been achieved on the electrodes of the separator.

CFD Analysis on Novel Vertical Axis Wind Turbine (VAWT)

The operation of vertical axis wind turbines (VAWTs) to generate low-carbon electricity is growing in popularity. Their advantages over the widely used horizontal axis wind turbine (HAWT) include their low tip speed, which reduces noise, and their cost-effective installation and maintenance. A Farrah turbine equipped with 12 blades was designed to enhance performance and was recently the subject of experimental investigation. However, little research has been focused on turbine configurations with more than three blades. The objective of this study is to employ numerical methods to analyse the performance of the Farrah wind turbine and to validate the findings in comparison with experimental results. The investigated blade pitch angles included both positive and negative angles of 7, 15, 20 and 40 degrees. The k-ω SST model with the sliding mesh technique was used to perform simulations of a 14.4 million element unstructured mesh. Comparable trends of power output results in the experimental investigation were obtained and the assumptions of mechanical losses discussed. Wake recovery was determined at an approximate distance of nine times the turbine diameter. Two large complex quasi-symmetric vortical structures were observed between positive and negative blade pitch angles, located in the near wake region of the turbine and remaining present throughout its rotation. It is demonstrated that a number of recognised vortical structures are transferred towards the wake region, further contributing to its formation. Additional notable vortical formations are examined, along with a recirculation zone located in the turbine’s core, which is described to exhibit quasi-symmetric behaviour between positive and negative rotations.

A magnetohydrodynamic mechanism for the formation of solar polar vortices

Polar vortices are ubiquitous features of planetary atmospheric flows, from the Earth-like rocky planets to Jupiter- and Saturn-like gas giant planets. Very little is known about their existence or dynamics on the Sun. What should be expected near the Sun's pole for the upcoming solar multi-viewpoint and polar missions? Here, we report the magnetohydrodynamic (MHD) nonlinear simulations for the formation and evolution of solar polar vortices using a near-surface MHD shallow-water model. Our findings indicate that the rush to the poles, the migration of magnetic fields toward the pole following the Sun's magnetic cycle, can positively contribute to the formation of polar vortices. The mechanism proposed here for the formation of polar vortices involves the role of magnetic fields and may be relevant to any star with a magnetic cycle. The Sun's polar vortices resulting from this mechanism are predominantly MHD, consisting of a tight pair of cyclonic and anticyclonic swirls. This mechanism is likely to operate during all solar cycle phases except the peak, when the polar field reverses. Polar vortices can impact dynamical evolution of global flows and polar fields, which seed the next activity cycle, hence better knowledge of physics of polar regions may lead to improved solar cycle and space weather forecasts.

Microstructure Evolution and Mechanical Properties of Ta-Cladded Ni-Cr-Mo Low Alloyed Steel via Explosive Welding

The effects of welding variables (stand-off distance and explosive thickness) on the interfacial microstructure evolution and mechanical properties of explosively welded Ta alloy to Ni-Cr-Mo low alloyed steel have been investigated. Regardless of welding conditions (a stand-off distance of 3-5 mm and explosive thickness of 40-80 mm), the Ta/steel interface consistently exhibited a wavy configuration. This wavy interface facilitated the formation of vortex, resulting in strong interlocking. The height of the vortex increased with a larger stand-off distance at a fixed explosive thickness of 60 mm. Similarly, increasing the explosive thickness at a stand-off distance of 3 mm also resulted in a greater vortex height. The explosive weldability window, plotting the collision angle (β) against the collision point velocity (vc), was successfully established for the dissimilar Ta and steel plates. The upper limit prediction with N=0.11, as proposed by Wittman, best matched the experimental results. This guided the determination of the optimal condition, which was a standoff distance of 3 mm and an explosive thickness of 40 mm. A vortex melted zone (VMZ) was identified, which resulted from the dynamic intermixing of Ta and steel, combined with localized melting caused by high-energy collisions and heat accumulation. The VMZ surrounded by a highly deformed Ta alloy, showed the highest hardness. Near the interface on the steel side, a fine recrystallized grain structure was observed. No significant inter-diffusion was detected at the wavy Ta/steel interface. The tension-shear properties of the wavy interface, which was subjected to loading parallel to interface, showed a good balance of strength and ductility, confirming the soundness of Ta/steel interface.

Liquid vortex surface deformation probed by light reflection

Abstract We propose a method that combines an optical system with image processing techniques to scrutinize the dependence of liquid vortex deformation on varying angular velocity based on light reflection on liquid surfaces due to dynamic wettability. In our experiment, a broadened and collimated laser beam is directed onto curved surfaces, providing information on the vortex parameters through the analysis of the reflected beam profile. Additionally, the physical model of the liquid surface in a rotating cylinder before dewetting is examined. We investigate the liquid vortex forms of a saline solution and propylene glycol across various angular velocities, comparing them to the ideal parabolic vortex surface shape. The results show that, under the same experimental conditions, the vortex profiles of both solutions are equivalent. The vortex surface shape at certain angular velocity values, as determined by fitting plots, closely resembles a parabola, with R2 > 99%. The proposed method introduces a new approach for characterizing the dynamics of liquids as well as monitoring natural phenomena.

Self-selection of flow instabilities by acoustic perturbations around rectangular cylinder in cross-flow

This work experimentally investigates the flow structure around a rectangular cylinder with an aspect ratio of 2 under varying incidence angles to examine how acoustic perturbations modify and modulate the unsteady flow instabilities. In the absence of acoustic excitation, the angle of incidence is found to markedly influence the flow topology and the natural shedding pattern altering the vortex formation length and wake dynamics. With acoustic perturbations, it is observed that for incidence angles $\alpha = 0^{\circ }$ and $\alpha = 5^{\circ }$ , the masked impinging leading-edge vortex (ILEV)/trailing-edge vortex shedding (TEVS) instability modes of $n= 1$ and $n= 2$ become evident when their frequencies coincide with the frequencies of the acoustic perturbations (i.e. resonant condition). Both trailing-edge and leading-edge vortices were found to be modulated by the acoustic pressure cycle. These instability modes, which are naturally present under non-resonant conditions for significantly higher aspect ratios, highlight the role of incidence angle and self-excited acoustic resonance in virtually augmenting the streamwise length of the cylinder, thereby facilitating the emergence and sustenance of the ILEV/TEVS shedding pattern.

Study on the characteristics of cavitation and COD removal of an addendum rotational hydrodynamic cavitation reactors

AbstractThis paper presented an addendum rotational hydrodynamic cavitation reactors (ARHCR) that enabled circumferential shear cavitation, axial cavitation, and the Venturi effect, thus increasing cavitation efficiency. The experiments on hydroxyl (·OH) release and chemical oxygen demand (COD) removal demonstrate the feasibility of cavitation performance characterization and COD removal rate. The findings indicate that the COD removal rate can reach a maximum value of 28% or 30% with an increase in flow rate or rotating speed, respectively. Specifically, the maximum values were achieved at a rotation speed of 2,500 rpm or a flow rate of 1.0 m3·h−1. Based on the mechanism of the addendum cavitation, the wedge angles, and the crucial structural parameters, were examined for optimizing cavitation performance. The results revealed that the cavitation number and cavitation bubble were significantly influenced by wedge angles, as well as the efficiency of cavitation energy. The mechanical shear of fluid was enhanced by wedge angles, resulting in constant compression and release of fluid with increased kinetic energy. This led to stronger effects on bulge blocks, and the formation of vortexes that rebounded constantly, resulting in lower pressure areas and expanded regions of cavitation. Among the wedge angles tested, the wedge angle of 15° exhibited the highest cavitation bubbles with a maximum value of 46%. This was 31% higher than the grooves‐type cavitator reported. Furthermore, the cavitation performance was extremely improved with a wedge angle of 15°, as corresponding with the maximum efficiency of cavitation energy. These explorations provide references for the removal of COD in industrial and agricultural wastewater, as well as the development of novel reactors for wastewater treatment.

Unsteady Propeller Wake Interference on Wing in Tractor Configuration at Low-Reynolds-Number Condition

This study investigates the unsteady interaction between the propeller slipstream and NACA0012 airfoil, which causes laminar separation at a moderate angle of attack and nonlinear aerodynamic characteristics at low Reynolds numbers. The Reynolds number based on chord length was set to c, and the propeller advance ratio was set to J=0.8. Wind-tunnel experiments were conducted, including aerodynamic force measurement and time-resolved particle image velocimetry visualization. Regarding time-averaged characteristics, the propeller installed condition exhibits a linear change in lift; reduced drag; and, consequently, an improved lift-to-drag ratio. Consistent with the aerodynamic characteristics, suppressed separation and reduction in wake deficit are confirmed in the time-averaged flowfields. Additionally, vortex formation and shedding synchronized with propeller rotation are observed in unsteady flowfields at the middle and high angles of attack. This periodic and large-scale vortex structure suggests the possibility of further improving the performance of the propeller–wing system in low Reynolds numbers.

Analysis of the interfacial evolution characteristics of hollow droplet impact on a liquid pool

The impact dynamics of a hollow droplet on a liquid pool have significant implications across various industrial applications. This study employs numerical simulations to explore the dynamic evolution of the interface during the impact of a hollow droplet on a liquid pool. The investigation focuses on the effects of varying the hollow ratio Dr and liquid pool depth h* while maintaining a constant volume of liquid within the droplet shell. The findings reveal that both the hollow ratio Dr and pool depth h* critically influence the formation of ejecta + lamella, and vortex rings after the impact of a hollow droplet on a liquid pool. The confinement effect of the pool bottom can influence the evolution of the splashing, while the internal air in the hollow droplet can absorb a part of the impact energy during the collision. Specifically, at shallow pool depths, the interface primarily evolves into ejecta + lamella structures, whereas at greater pool depths, vortex ring formation is predominant. Furthermore, an increase in the hollow ratio leads to a reduction in the critical pool depth hc* at which the transition between these interfacial modes occurs. These findings indicate that, in practical applications involving the impact of hollow droplets on liquid pools, sufficient attention should be given to the pool depth. This enhances our understanding of the bottom pressure, droplet impact, and vortex formation, which is of significant relevance to related industrial technologies.

Energy harvesting from wake-induced vibration of flexible flapper behind a bluff body

Piezoelectric energy harvesting from ambient vibrations offers a promising small-scale energy generation strategy, with wake-induced vibration of flexible structures being an ideal candidate. This study examines a bluff body followed by a fully flexible piezoelectric flapper in a viscous free-stream flow using an in-house discrete forcing immersed boundary method-based fluid-structure-electric energy solver for parametric investigation. Different vortex shedding regimes are identified based on vortex formation around the flexible flapper. The complex and interdependent spatiotemporal dynamics of the wake and flexible body dictated by parameters such as bending rigidity and the gap space between the flapper and bluff body result in various deformation profiles, influencing the strain rate and output power. The study also investigates the independent variation of flapper length and its impact on vortical arrangements and flexibility, introducing different oscillation modes. The present study takes a nuanced view of the overall dynamics and their mutual effect on the power output, unlike most existing studies where enhancing the amplitude and frequency of oscillations for an optimal output was the main concern. Factors such as flapper curvature, its asymmetry, and periodicity have been especially highlighted in the context of the output and the corresponding parametric spaces investigated. Interestingly, the increase in piezo-flapper length has seen a reduction in output, though it was instrumental in bringing symmetry back. The study offers comprehensive insights into ideal harvesting regimes and the underlying dynamical mechanisms and can contribute toward the design of future energy harvesting devices.

Thermal-hydraulic analysis of He-Xe gas mixtures in triangular rod bundle wrapped with different shaped wire wraps

Gas-cooled reactors employing a helium-xenon (He-Xe) gas mixture as the coolant are regarded as one of the most promising technologies for megawatt-scale space energy systems. One technical solution to achieve high power density, reduced mass, and a compact structure in gas-cooled space reactors is the use of fuel arranged in triangular bundles encased with wire wraps. Nevertheless, research on the flow and heat transfer characteristics of He-Xe gas within such configurations remains scarce. In order to enhance the heat transfer performance of He-Xe reactor cores, thermal–hydraulic simulation methods were employed using computational fluid dynamics (CFD) in order to investigate the thermal–hydraulic characteristics within a triangular rod bundle channel equipped with wire wraps. The study examined the impact of wire wrap structures on velocity and temperature fields, inter-subchannel mixing, and the distribution of hot spots. The findings indicate that the wire wrap structure has the effect of promoting transverse flow, inter-channel mixing and vortex formation. Conversely, this also gives rise to the issue of hot spots, particularly at the contact angles between the wire wrap and the fuel rod. At a wall heat flux of 91,477.3 kW m−2, the maximum temperature at the circular wire wrap hot spot can reach 1217 K, while the average wall temperature of the corresponding cross-section is only 770 K. Furthermore, the flow and heat transfer characteristics of three wire wrap geometries—circular, rectangular, and trapezoidal—were evaluated and compared across a Reynolds number range of 2.6 × 104 to 1.4 × 105. The findings indicate that the trapezoidal wire wrap configuration results in the lowest wall hot spot temperature and the smallest temperature inhomogeneity. Furthermore, an analysis of the heat transfer coefficients (Nusselt number) and thermo-hydraulic performance ratios (THPR) demonstrates that the trapezoidal wire wrap exhibits superior heat transfer performance compared to the other two geometries.

Nonlinear saturation of the ion flow driven ion sound instability in a finite length plasma

The saturation mechanism and nonlinear evolution of the ion sound instability driven by the subsonic ion flow in a finite length plasma are studied by a one-dimensional hybrid model considering kinetic ions and Boltzmann electrons. Three regimes of the instability nonlinear behavior are identified as a function of the frequency of the ion-neutral charge exchange (CX) collision fcoll. In the first (collisionless-alike) regime when the CX frequency is low, the instability is saturated by ions trapping in wave potentials leading to the formation of phase space vortexes (PSVs). One of the PSVs subsequently expands and becomes system long in the steady state. The transition to the second (medium) regime occurs when fcoll≳vp/d, where vp is the PSV expansion velocity and d is the system length. In the second regime, CX collisions convert fraction of beam ions into slow ions that can be trapped in potentials of small scale ion sound eigenmodes fluctuations. The trapping of slow ions results in the formation of a chain of small scale PSVs and the disruption of the establishment of the single system long PSV. In the third (collision-dominated) regime when fcoll≳γ (γ is the instability growth rate), CX collisions transform all beam ions into slow ions and the instability is thereby eliminated.

Insights into flame flashback phenomenon utilizing a Strut-Cavity flame holder inside scramjet combustor

Understanding upstream flame propagation in scramjets is challenging, particularly concerning flame flashback in a combustor with a novel strut-cavity flame holder. Two-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) simulations were performed to investigate how Mach number and wall divergence affect flame behavior. The utility of the strut-cavity flame holder was highlighted through a study of its non-reacting flow characteristics. Flow dynamics are significantly altered as the shear layer above the cavity interacts with the downstream hydrogen jet. Shear layer dynamics and fuel-air mixing are improved through key factors such as shock-train behavior, cavity oscillations, and transverse fuel injection. The submerged fuel jet is less exposed to supersonic flow and demonstrates reduced entropy rise, achieving a 16% increase in mixing efficiency compared to standalone struts and a 46% improvement over transverse injection without a flame holder. Thermal choking shifts the shock train upstream, facilitating interactions with the shear layer and enhancing vortex formation, which decreases flow speed and promotes upstream flame propagation. The presence of OH radicals indicates that flame flashback follows a periodic pattern with an initial gradual slope, suggesting effective anchoring. Stability and flashback likelihood are affected by low-speed zones, vortex merging, and wall divergence. At Mach 3, combustion efficiency improves without wall divergence due to increased heat release, while wall divergence prevents flame flashback by sustaining supersonic core flow and managing flow-flame interactions. At higher core flow velocities, flame stabilization occurs at the cavity's separation corner, despite a tendency for upstream propagation, with validation of the URANS results achieved through two-dimensional large eddy simulations.

Effect of drill hole inclination on coal particles-gas flow characteristics in negative pressure sampling process

Negative-pressure fixed-point sampling, based on negative-pressure pneumatic conveying, is used to determine gas content in underground coal seams. The CFD-DEM method was applied to study gas-solid flow characteristics inside drill holes under varying inclination conditions. The results showed that increasing the positive inclination angle enhanced airflow vortex formation, increased particle intake, and raised the proportion of high-velocity particles. In contrast, a greater negative inclination angle shifted the vortex and caused more particle accumulation at the borehole bottom, increasing the proportion of lower-velocity particles. At positive angles of 30° or more, the actual mass flow rate matched the given rate, while negative angles reduced it to 25 %. The optimal positive inclination improved removal rate by up to 26 %, while wind speed had a smaller impact of 9 %.