Large-scale Vortices Research Articles

Inverse energy cascade in turbulent Taylor–Couette flows

The inverse energy cascade in turbulent Taylor–Couette flow is studied in line with the results of the large eddy simulation. The simulation results show that the inverse energy cascade first occurs within the core region of the flow channel of the Taylor–Couette flow at higher Reynolds numbers. It is uncovered that this phenomenon is induced by the pulsed zero shear stress resulting from the singularities of the Navier–Stokes equation. In the core area between the two cylinders, the shear stress is nearly zero at higher Reynolds numbers. The turbulence generated there has high turbulent energy due to discontinuity of the tangential velocity. Since the energy transfer between the fluid layers is inhibited due to the low shear stress, the turbulent energy cannot be transferred along the radial direction, and small-scale vortices with high turbulent energy are produced. These small-scale vortices are located with the large-scale vortices and cannot be dissipated owing to low shear stress. A peak in the energy spectrum at the middle frequency (or wave number) is formed due to the concentration of the small-scale vortices. As the number of the singular points of the Navier–Stokes equation increases with the increasing Reynolds number, the region with zero shear stress expands along the radial direction, intensifying nonlinear instability and energy accumulation. This, in turn, leads to more prominent peaks in the energy spectrum, resulting in a more pronounced inverse energy cascade.

Effect of curvature on the hypersonic turbulent boundary over the curved compression ramp

Direct numerical simulation of the shock wave/turbulent boundary layer interaction on a compression ramp and curved compression ramps with different radii of the curvature at the Mach number Ma=5.0 and Reynolds number Re=16 800/mm is performed, and the purpose of the study is to investigate the impact of different radii of the curvature on the development of the flow. The flow structure and turbulence properties are analyzed. As the curved angle radius increases, the range of flow deceleration and the impact of the shock wave interaction on the turbulent boundary layer gradually decrease, and the peak value of turbulent pulsation amplification in the interaction zone becomes smaller. Mean skin friction decomposition is carried in upstream undisturbed region and reattachment region. The skin friction coefficient in the upstream is primarily composed of the viscous dissipation term Cf,V and the turbulent kinetic energy production term Cf,T. While in the reattachment zone, it is mainly balanced by the term Cf,T and the spatial growth term Cf,G. Bidimensional empirical mode decomposition is applied to further study the contribution of the turbulent motion at different scales to Cf,T, and the result shows that for the compression ramp, Cf,T is mainly contributed by the large-scale vortex structure generated in the interaction zone, while for the curved compression ramp, it is mainly contributed by the rapid amplification of turbulent pulsations caused by the shock wave interaction. This study is not only a new parametric study of the shock wave/turbulent boundary interaction but also provides a reference for the aerodynamic design of hypersonic vehicles.

Phase transitions in anisotropic turbulence.

Turbulence is a widely observed state of fluid flows, characterized by complex, nonlinear interactions between motions across a broad spectrum of length and time scales. While turbulence is ubiquitous, from teacups to planetary atmospheres, oceans, and stars, its manifestations can vary considerably between different physical systems. For instance, three-dimensional turbulent flows display a forward energy cascade from large to small scales, while in two-dimensional turbulence, energy cascades from small to large scales. In a given physical system, a transition between such disparate regimes of turbulence can occur when a control parameter reaches a critical value. The behavior of flows close to such transition points, which separate qualitatively distinct phases of turbulence, has been found to be unexpectedly rich. Here, we survey recent findings on such transitions in highly anisotropic turbulent fluid flows, including turbulence in thin layers and under the influence of rapid rotation. We also review recent work on transitions induced by turbulent fluctuations, such as random reversals and transitions between large-scale vortices and jets, among others. The relevance of these results and their ramifications for future investigations are discussed.

The effect of blade surface grooves on performance of axial fan

This paper explores the effect of blade surface grooves on the axial fan of central air-conditioner outdoor units in detail through experiments and three-dimensional unsteady simulations. The experimental results reveal that the newly designed fan with surface grooves can reduce noise by 1.2 dB(A) without sacrifice of aerodynamic performance. The simulation results demonstrate that the effect of the surface grooves is localized and does not alter the overall load distribution of the axial fan. However, it does affect the tip leakage vortex. The tip leakage vortex is a large-scale vortex that interacts with small-scale vortices shedding from the blade surface grooves, resulting in the weakening of the tip leakage vortex and the secondary tip leakage vortex. Ultimately, this leads to a reduction in the noise of the air-conditioner outdoor unit. This new strategy based on blade surface geometric design provides a novel idea for tip leakage flow control and has significant engineering application value.

On increasing the scour funnel size upstream of a bottom orifice by using a cuboid pile

Scour funnels formed before bottom orifices are of vital importance regarding intaking water with low-concentration and small-sized sediment to protect the water turbine. Via flume experiments, this work investigates the effect of a bottom-mounted cuboid pile on the formation of a scour funnel upstream of a bottom orifice. A phenomenon we call as “reverse sediment collapse” is observed for the first time, which causes a rapid increase in the scour depth upstream of the orifice. Instantaneous velocity measurements show that the turbulence kinetic energy near the cuboid pile is significantly higher compared with that in a case without a pile or with a cylinder under a same waterhead condition, indicating that large-scale vortex motions are enhanced due to the use of a cuboid pile. The equilibrium scour funnel depth upstream of the orifice is found to be increased by as much as 124.6% when a cuboid pile exists compared with that in a case without a pile under a same waterhead condition. The scour process is highly accelerated when a cuboid pile exists, and it is found that the time required when a cuboid pile exists to reach for the first time the same equilibrium scour depth without a pile is dramatically reduced by 97.2%–99.7%. A formula was established to calculate the equilibrium scour depth. The findings provide guidance for future optimization of the form and placement of structures upstream of an orifice to efficiently achieve a larger-sized scour funnel.

PIV Experimental Study of Airflow Structures in a Multi-Slot Ventilation Enclosure with Opposed Jets

The airflow structure of enclosures directly affects the spread of COVID-19 and is also closely related to indoor air quality, the thermal comfort of personnel, and buildings’ energy consumption. A large number of studies on airflow field under mixing and displacement ventilation with a single air inlet in rectangular rooms have been conducted; however, to the best of the authors’ knowledge, only a limited number of studies have dealt with airflow structures in a multi-slot ventilation enclosure with opposed jets. Therefore, this paper uses PIV to study the velocity, turbulence information, and entropy of an unstable airflow field in a multi-slot ventilation enclosure with opposed jets under isothermal and non-isothermal conditions. This paper also presents, due to the collision of the jets to form two large-scale eddies, the airflow field structure being unstable. In the region without air supply inlets and exhaust outlets, a large-scale vortex is formed in the airflow field, resulting in the high information entropy of the flow field. The thermal plume suppresses the large-scale flow field structure and increases the small-scale flow field structure.

Correlation times of velocity and kinetic helicity fluctuations in non-helical hydrodynamic turbulence

Non-helical turbulence within a linear shear flow has demonstrated efficient amplification of large-scale magnetic fields in numerical simulations, but its precise mechanism remains elusive. The incoherent $\alpha$ mechanism proposes that a zero-mean fluctuating transport coefficient $\alpha$ (linked to kinetic helicity) in the shear flow is a candidate driver. Previous renovating-flow models have proposed that the correlation time of helicity fluctuations must be sufficiently extended to overcome turbulent magnetic diffusivity, yet only empirical validation of this concept has been obtained. In this study, we conduct direct numerical simulations of weakly compressible non-helical hydrodynamic turbulence. We scrutinize the correlation times of velocity and kinetic helicity fluctuations in distinct flow configurations, including rotation, shearing and Keplerian flows, as well as the shearing burgulence counterpart. Our findings indicate that rotation contributes to a prolonged correlation time of helicity compared with velocity, particularly notable in auto-correlations of both volume-averaged quantities and individual Fourier modes due to the formation of large-scale vortices. In contrast, moderate shear strength does not exhibit significant scale separation, with shear flows elongating vortices in the shear direction. Shearing burgulence, characterized by shorter helicity correlation times, appears less conducive to hosting the incoherent $\alpha$ effect. Notably, at modest shear rates, only Keplerian flows exhibit sufficiently coherent helicity fluctuations, in contrast to shearing flows. However, the relative strength of helicity fluctuations compared with turbulent diffusivity is significantly lower, raising doubts about the viability of the incoherent $\alpha$ effect as a potential dynamo driver in the subsonic flows examined in this study.

A preliminary investigation on a novel vortex-controlled flameholder for aircraft engine combustor

This study proposes a novel vortex-controlled flameholder (VCF) to enhance the combustion performance, particularly the lean ignition and blowout characteristics of afterburners in advanced aircraft engines across a wide range of operating conditions. Additionally, both experimental and numerical investigations were conducted to explore the effects of the cavity structure on the flow field, lean ignition, lean blowout, flame propagation characteristics, and outlet temperature rise distribution of the flameholder. Two distinct cavity structures designated closed-cavity (case-1) and open-cavity (case-2) were examined, the findings indicating that both case-1 and case-2 can generate large-scale vortex flow structures within the cavity, contributing to achieving excellent combustion stability. Case-2 demonstrated better lean ignition and blowout performance compared to case-1. Furthermore, both case-1 and case-2 exhibited the same ignition process, which comprised four distinct phases: Phase 1 involved the formation of an effective flame kernel; Phase 2 pertained to the ignition of the entire cavity by the flame kernel; Phase 3 represented the full development of the flame downstream of the cavity; Phase 4 described the formation of a fire tornado that anchors the flame front. Interestingly, the outlet temperature rise in case-1 is lower than that in case-2 at low fuel-to-air ratio (FAR) conditions. As the FAR increases, the difference in the outlet temperature rises between the two cases gradually narrows.

PIV experimental study on dynamic and static interference flow field of multi-operating centrifugal pump under the influence of impeller wake

In order to study the influence law of the impeller wake on dynamic and static interference flow field of the centrifugal pump, this paper obtains the dynamic and static interference flow field of the centrifugal pump under different flow conditions (15 m3/h, 50 m3/h, 70 m3/h) based on PIV technology, and analyzes the influence mechanism of the impeller wake change on dynamic and static interference flow field. The results show that rotating stall occurs in the centrifugal pump under low flow condition, but it has little effect on the head loss in the centrifugal pump. In the dynamic and static interference flow field, under the condition of the low flow rate, the impeller wake collides with the baffle tongue, resulting in serious velocity fluctuation, and then there will be the secondary collision with the volute wall, which will eventually cause the wake to dissipate, and the change process of the wake shows the periodic characteristic. In the design condition and the large flow condition, the spacer tongue will have the cutting effect on the wake, and the high-speed accumulation phenomenon will occur in the volute flow path near the spacer tongue. In the side flow path of the volute, under the condition of the low flow, the impeller wake is mainly located at the exit of the impeller, and the end of the impeller wake is easy to fall off and gradually break up under the impact of the main stream. Under the design condition, the flow stability is good, and the wake vortex is close to the trailing edge of the blade, and there is no obvious shedding phenomenon. Under the condition of the large flow rate, the velocity fluctuates sharply, and many large-scale vortex structures appear on the cross section of the flow channel due to the cutting of the wake near the diaphragm tongue. In the impeller passage, the movement and distribution of the wake are significantly affected by changes in flow conditions. The research results provide a basis for optimizing volute channel.

Numerical investigation on the end effects of the flow past a finite rotating circular cylinder with two free ends

This paper studies the impact of the aspect ratio and free end shape on the end effects in the flow past a rotating circular cylinder with two flat, radiused, hemispherical, and conical ends, using the large eddy simulation method at a Reynolds number of 4.6×104. The aspect ratio in the range of 6–30 and the rotation rate in the range of 0–3, are investigated. The results show that the mean drag coefficient initially decreases slightly before rapidly increasing with the rotation rate, with a critical rotation rate that rises from 1 to 1.5 as the aspect ratio increases from 6 to 30. In contrast, the mean lift coefficient increases with both the rotation rate and the aspect ratio. When the rotation rate increases and the aspect ratio decreases, the differences between the aerodynamic coefficients of the four end shapes become more pronounced. The flat end results in the highest mean drag and lift coefficients, while the hemispherical end yields the lowest ones. In addition, when the rotation rate increases, the alternate shedding vortices shift to the opposite side. They even disappear and increase the elongated streamwise vortices. Due to the combined impacts of the rotation and end effects, large-scale tip vortices are formed, significantly altering the wake structure. The intense rotation effect results in expanding the strong influence region of the end effects and shrinking (or even removing) the weak influence region.

Dynamic characterization of the flow field of air classifier disturbed structures and application to efficient separation of fly ash

Classification is crucial for the resource utilization of fly ash, impacting both utilization rates and product quality. This study introduces an enhanced flow field structure in a perturbed rotating centrifugal air classifier by optimizing toothed and tail blade designs for efficient fly ash classification. Using numerical simulations and experiments, we examine the flow field performance and separation efficacy at various speeds, and employ Dynamic Mode Decomposition (DMD) for modal analysis. Results show the toothed blade, angled at 35°, produces small-scale turbulence with high pressure and velocity, while the tail blade’s rightward bend induces large-scale vortices, aiding flow control. Modal analysis highlights that both Drs-types 1 and 2 predominantly exhibit weakly attenuated modes. Optimal classification occurs at Rs = 950 rpm and fv = 0.3 kg/s, with both Drs-types surpassing the original structure in effectiveness and pressure. These findings provide new directions for industrial applications and the design of cyclone air classifiers.

On the Generation of Large-Scale Vortical Flow Structures in Pipe Multifurcations

Abstract In many hydraulic power plants, the water supplied through the penstock needs to be distributed to multiple turbines. The application of pipe multifurcations is motivated by economic and efficiency reasoning. However, flow instabilities and large-scale vortical flow structures can occur for some operating conditions, causing high-amplitude pressure and mass flow rate fluctuations. To enhance the understanding of the flow instability inception, we investigate the unsteady flow in a pipe multifurcation with six branches for different operating conditions. The unsteady flow is simulated employing the large eddy simulation approach, where the numerical mesh is constructed such that y + < 1 and a recycling-type boundary condition is utilised to provide the turbulent inflow conditions. While the general flow pattern are described by the time-averaged results, the work focuses on analysing the unsteady physical phenomena occurring in the pipe multifurcation. At part-load operation, the results clearly show the formation of large-scale vortical flow structures stretching into the active branches. The vortices are observed to move around and eventually burst. When one vortex breaks down, the other large-scale vortices reaching into the other active branches are likely to burst and the flow pattern changes. Such events come with decreased turbulence intensity propagating through the branch line, flow separation at the branch line junction, and mass flow rate fluctuations. After a short time, large-scale vortices were established again.

On the large-scale vortices in an L/D = 2.30 serpentine diffuser with internal bump

In this paper, the generation and spatial evolution of large-scale vortices in an ultra-compact serpentine diffuser (L/D = 2.30) featuring an integrated internal bump are experimentally and numerically investigated. Due to reduction of the local flow area caused by the internal bump, a favorable streamwise pressure gradient is induced first and then followed by an adverse one. Concurrently, a high-pressure region exists on the side of the diffuser as it contracts, yielding a sideward circumferential pressure gradient and then an inward one. Such circumferential pressure gradients divert the boundary layer on the windward side of the internal bump to the side and induce a spanwise U-shaped vortex on the leeward side, which constrains the development of flow separation in this region. The circumferential pressure gradients further induce two pairs of streamwise vortices located at the top and bottom of the diffuser, respectively. These vortex pairs entrain the low-energy fluids and transport them to the aerodynamic interface plane (AIP), resulting in two pairs of low-total-pressure regions and swirling flows. Moreover, in the experimentally studied range of the exit Mach number MAIP from 0.293 to 0.546, the distributions of low-total-pressure regions and swirl angles at the AIP exhibit a small variation, indicating that the internal flow field structures are insensitive to MAIP.

Three-Dimensional Study of Vortico-Acoustic Interaction in a Simulated Solid Rocket Motor

In this study, a three-dimensional numerical simulation of a large length-diameter ratio solid rocket motor is carried out using the large-eddy simulation. Pressure oscillation excited by significant parietal vortex shedding (PVS) is observed. The acoustic feedback model is successfully extended to investigate the interaction between PVS and pressure oscillations, and the result proves that the model has sufficient accuracy (with 2.84% relative error) to predict the PVS frequency in an accelerating flow field. Reduced-order variational mode decomposition (RVMD) is performed to analyze the vorticity field, indicating that the method can identify the PVS frequency accurately (with 3.42% relative error) and capture large-scale vortex structures that remain intact at the rear of the combustion chamber. Based on RVMD and Hilbert spectral analysis, the framework of model-based time-frequency analysis is established and used to verify the stability of PVS frequency.

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.

α effect in three-dimensional vortex of conducting rotating liquid.

We study one-point statistics of helical turbulent pulsations in the background of a three-dimensional large-scale vortex in a rotating fluid. Assuming that the helical flow is created by a statistically axially symmetric random force with broken mirror symmetry, we analytically calculate the velocity-vorticity mean including its magnitude and the anisotropy. For electrically conducting liquid, we examine the α-effect in the system. The found elements of the α-matrix strongly depend on the relation between Rossby Ro and magnetic Prandtl Pr_{m} numbers in the considered region Ro≪1,Pr_{m}≪1. We establish a criterion for the numbers when the α-effect leads to instability of large-scale magnetic field.

Marine heatwaves suppress ocean circulation and large vortices in the Gulf of Alaska

Large-scale anticyclonic vortices forming along the Gulf of Alaska continental slope serve as fertile ecosystems for marine life, significantly shaping the distribution of primary productivity, with 40–80% of the gulf’s open ocean surface chlorophyll-a concentrated in their cores. Between 2013 and 2023, Alaska experienced some of the largest and longest marine heatwaves ever recorded in the world’s oceans, persistently altering its ecosystem and fisheries. Here, using 30 years of satellite and reanalysis data, we find that the coastal upwelling atmospheric forcing conditions associated with the heatwaves have also significantly suppressed the Gulf of Alaska’s ocean circulation and the formation of large anticyclones. Climate model simulations spanning from 1850 to 2100 suggest that future changes in the Aleutian Low pressure system will lead to a 60% increase in upwelling extremes (>2 standard deviations), further weakening the ocean anticyclones. However, large uncertainties remain in the mechanisms controlling the Aleutian Low’s response to climate forcing in the models.

Collective behavior of active filaments with homogeneous and heterogeneous stiffness.

The collective dynamics of active biopolymers is crucial for many processes in life, such as cellular motility, intracellular transport, and division. Recent experiments revealed fascinating self-organized patterns of diverse active filaments, while an explicit parameter control strategy remains an open problem. Moreover, theoretical studies so far mostly dealt with active chains with uniform stiffness, which are inadequate in describing the more complicated class of polymers with varying stiffness along the backbone. Here, using Langevin dynamics simulations, we investigate the collective behavior of active chains with homogeneous and heterogeneous stiffness in a comparative manner. We map a detailed non-equilibrium phase diagram in activity and stiffness parameter space. A wide range of phase states, including melt, cluster, spiral, polar, and vortex, are demonstrated. The appropriate parameter combination for large-scale polar and vortex formation is identified. In addition, we find that stiffness heterogeneity can substantially modulate the phase behaviors of the system. It has an evident destructive effect on the long-ranged polar structure but benefits the stability of the vortex pattern. Intriguingly, we unravel a novel polar-vortex transition in both homogeneous and heterogeneous systems, which is closely related to the local alignment mechanism. Overall, we achieve new insights into how the interplay among activity, stiffness, and heterogeneity affects the collective dynamics of active filament systems.

Insights into turbulent airflow structures in blind headings under different ventilation duct distances

The paper explores the 3D stationary vortex structure of turbulent airflow near the dead-end face of a blind heading, ventilated via a forcing ventilation system. Despite its significance in blind heading ventilation, previous studies primarily focused on temporal dynamics of harmful impurities, overlooking flow structure details crucial for mass transfer processes. Our study delves into the ventilation flow structure across diverse parameters of the auxiliary ventilation system. Alongside standard flow visualization tools, we introduce three dimensionless indicators to comprehensively characterize flow structure, facilitating analysis of its variations with parameter changes and quantitative evaluation of system efficiency. Analysis revealed the formation of a single large-scale vortex within the entire range of considered ventilation system parameters in the heading. This vortex induces a significant increase in air circulation, approximately 2.5–3.5 times greater than airflow emerging from the ventilation duct’s end, thus intensifying mass transfer processes within the heading. We found that ventilation efficiency of the dead-end face zone in a blind heading with a 29.2 m² cross-section decreases linearly with increasing distance between the ventilation duct’s end and the dead-end face. However, compensating for this distance by increasing duct velocity is feasible, bearing significant implications for mine ventilation safety, particularly in ventilating blind headings with distant ducts from the dead-end face.

A new strategy for reducing pressure fluctuation of Francis turbine by bionic modification of local components

Long term operation of hydroelectric units in low load conditions can induce large-scale blade vortices, swirling vortex ropes in the draft tube, and low-frequency high amplitude pressure fluctuation. These phenomena will cause adverse consequences such as excessive vibration of the unit and blade breakage of the runner. Taking inspiration from the protuberances of the leading edge of a humped whale flipper, the present study firstly proposes bionic modifications of guide vanes and draft tube to suppress high-amplitude pressure fluctuations for Francis turbine. Numerical simulations of transient flow characteristics of the prototype unit (PU), the bionic guide vane unit (BG), and the bionic draft tube unit (BDG) under two low load conditions are conducted. Results indicated that the bionic draft tube has a good effect on suppressing pressure fluctuations in the vaneless area and draft tube. Under the Q/QBEF = 0.41 working condition, BDG causes the main frequency amplitude of pressure fluctuation at the center point of the draft tube inlet to change from 8000.2 Pa to 390.9 Pa, a decrease of 95.11 %. Under the Q/QBEF = 0.57 working condition, BDG causes the maximum decrease rate of the main frequency amplitude in the draft tube to be 60.5 %. The reducing effect in BDG of monitoring points in the guide vane area has reached over 43 %. Under low load conditions, the vortices near the wall in the draft tube of BDG are intercepted by bionic structures, reducing the vortex scale and helping to prevent the generation of large swirling vortex ropes. The bionic guide vanes have a significant control effect on pressure pulsation in the the guide vane and vaneless regions, although perform poorly in the draft tube.