Unsteady Structures Research Articles

Refined Simulation Study of Hydrodynamic Properties and Flow Field Characteristics around Tandem Bridge Piers under Ice-Cover Conditions

Ice cover is a common phenomenon in rivers in cold regions during the winter freeze-up period, leading to the formation of unsteady bypass structures around underwater piers. To reveal the variation law of the flow field around a pier under ice, a numerical calculation method is proposed to obtain the spatial and temporal characteristics of the fluid flow environment around the pier. The verification of flow conditions and convergence showed that the numerical model constructed in this study is reliable and can meet research requirements. The simulation results showed that the ice-cover condition considerably impacted the extent of a scour hole, and in the horizontal plane Z of −0.02 m, the lateral influence of the scour hole was approximately 2.6 times the diameter of the pier, which was approximately 42% wider than that of a scour hole under open-flow conditions; in the area on the side of the pier, there was a peak in longitudinal section y/D of −0.6, and the relative turbulence intensity was 0.4 and 0.51 under open-flow and ice-cover conditions, respectively, indicating that ice cover made the peak more significant in the area.

Flow dynamics and turbulent coherent structures around sediment reduction plates of a sewer system

In the management of urban drainage networks, great interest has been generated in the removal of sediments from sewer systems. The unsteady three-dimensional (3D) flow and turbulent coherent structures surrounding sediment reduction plates in a sewer system are investigated by means of the detached-eddy simulation (DES). Particular emphasis is given to detailing the instantaneous velocity and vorticity fields within the grooves, along with an examination of the three-dimensional, long-term, average flow structure at a Reynolds number of approximately 105. Velocity vectors demonstrate continuous flapping of the flow on the groove wall, periodically interacting with ejections of positive and negative vorticity originating from the grooves. The interaction between the three-dimensional groove flow and the shear flow leads to the downstream transport of patches of positive and negative vorticity, which significantly influence sediment transport. The high-velocity shear flows and strong vortices generated in undulating topography, as identified by the Q-criteria, are the key factors contributing to the efficient sediment reduction capabilities of the sediment reduction plates. The sediment reduction plates with partially enclosed structures exhibit low sedimentation rates in grooves on the plate, a broader acceleration region, and a lesser impact on the flow capacity. The results improve the understanding of the hydrodynamics and turbulent coherent structures surrounding the sediment reduction plates while elucidating the driving factors behind the enhancement of sediment scouring and suspension capacities. These results indicate that the redesign of the plates as partially enclosed structures contributes to further improving their sediment reduction performance.

Rotationally induced ingress in rotor–stator systems

The presence of a rotating disk adjacent to a stationary disk forms a rotor–stator cavity known as a wheel-space. It is necessary for gas turbine wheel-spaces to be purged with sealing flow bled from the compressor to counteract the harmful effects of ingress. This paper presents a combined experimental, theoretical, and computational study of rotationally induced ingress in rotor–stator systems. Measurements were made in a wheel-space with an axial clearance rim seal under axisymmetric conditions in the absence of a mainstream annulus through-flow. Ingress was quantified using a gas concentration technique and the flow structure in the cavity was explored with static and total pressure measurements to determine the swirl ratio. A low-order theoretical model was developed based on the boundary layer momentum-integral equations. The theory gave excellent results when predicting the effects of ingress and purge flows on the radial pressure and swirl gradients. Unsteady Reynolds-Averaged Navier–Stokes computations were conducted to provide greater fluid dynamic insight into the wheel-space flow structure and ingress through the rim seal. The computational results demonstrated some of the closest agreement with experimental measurements of ingress available in the literature, showing that rotationally induced ingress is dominated by unsteady large-scale structures in the rim seal gap instead of the previously ascribed disk-pumping effect. The study serves as an important validation case for investigations of ingress in rotor–stator systems in more complex environments.

Scale-resolving simulations of turbulent flows with coherent structures: Toward cut-off dependent data-driven closure modeling

Complex turbulent flows with large-scale instabilities and coherent structures pose challenges to both traditional and data-driven Reynolds-averaged Navier–Stokes methods. The difficulty arises due to the strong flow-dependence (the non-universality) of the unsteady coherent structures, which translates to poor generalizability of data-driven models. It is well-accepted that the dynamically active coherent structures reside in the larger scales, while the smaller scales of turbulence exhibit more “universal” (generalizable) characteristics. In such flows, it is prudent to separate the treatment of the flow-dependent aspects from the universal features of the turbulence field. Scale resolving simulations (SRS), such as the partially averaged Navier–Stokes (PANS) method, seek to resolve the flow-dependent coherent scales of motion and model only the universal stochastic features. Such an approach requires the development of scale-sensitive turbulence closures that not only allow for generalizability but also exhibit appropriate dependence on the cut-off length scale. The objectives of this work are to (i) establish the physical characteristics of cut-off dependent closures in stochastic turbulence; (ii) develop a procedure for subfilter stress neural network development at different cut-offs using high-fidelity data; and (iii) examine the optimal approach for the incorporation of the unsteady features in the network for consistent a posteriori use. The scale-dependent closure physics analysis is performed in the context of the PANS approach, but the technique can be extended to other SRS methods. The benchmark “flow past periodic hills” case is considered for proof of concept. The appropriate self-similarity parameters for incorporating unsteady features are identified. The study demonstrates that when the subfilter data are suitably normalized, the machine learning based SRS model is indeed insensitive to the cut-off scale.

LES of the upper plenum of the liquid metal fast reactor under the forced flow conditions

A Large Eddy Simulation (LES) of the upper plenum of the E-SCAPE (European SCAled Pool Experiment) facility under the normal operations has been conducted to gain deeper insights into the thermal hydraulics phenomena in liquid metal fast reactors (LMFRs). The results unravel the overall flow features in the upper plenum, the thermal instability in the above core structure region and the impact of the jets from the barrel holes on the large flow circulation and thermal mixing.The above core structure region is represented using a homogeneous porous model. It is shown that during the normal operations, most of the hot stream of the lead bismuth eutectic (LBE) from the core center rises to the top and disperses from the upper barrel holes. Conversely, the cold LBE spread out from the lower barrel holes. Mixing between the two streams only affect the fluids leaving from the middle heights demonstrating limited mixing. These jets help to create a large circulation in the upper plenum region which prevents any thermal stratification. At the interface between the hotter and colder streams, there are unsteady large-scale structures resulting in a mixing layer in the thermal field. This strong mixing is not conventional turbulence and potentially cannot be captured by Reynolds Averaged Navier Stokes average (RANS) modelling.The flow in the upper plenum is dominated by the influences of the different types of jets, which include some free jets issued horizontally or angled upwards and some jets impinging onto the structures in the plenum. The jets overall behave similar to ‘standard’ jets, though significant interactions between the top and bottom jets and the background circulations have strong influences at later stages of the jets, typically after four jet diameters leading to the jets not reaching self-similarity. In addition, the turbulence that is observed in the upper plenum is largely generated by the jet flows and hence the distribution of turbulence is very non-uniform. Away from the jets, turbulence is minimal with the mixing largely driven by the large-scale circulation.

Event-triggered background-oriented schlieren: high-frequency visualization of a heated jet flow.

This Letter reports a novel, to our knowledge, event-triggered background-oriented schlieren (EBOS) technique using a combination of an event-triggered camera and pulsed laser speckle projection. The BOS images are reconstructed using the event data generated by the pulsed laser speckle projection and then processed to obtain the density and temperature distribution of the flow. This technique enables continuous visualization and recording of flows at kFPS frame rates with a very low cost, breaking through the short operating times of existing high-frame-rate BOSs. To examine the event-triggered BOS technique, tests are conducted on a hot air gun. The measured temperature distribution coincides with the thermocouple data with an error of no more than 10.8%. Measurements taken during the start-up of the hot air gun demonstrate that the presented technique can measure the evolution of the jet temperature for at least 150 s, as well as capture the localized unsteady turbulent structure in the heated jet flow.

Unsteady large-scale wake structure behind levitated free-stream-aligned circular cylinder

The relationships between characteristic large-scale wake structures appearing behind a free-stream-aligned circular cylinder are investigated and discussed from the velocity field obtained by wind tunnel tests. The tests were conducted under a supportless condition using a magnetic suspension and balance system and stereo PIV measurements at a Reynolds number of $3.46\times 10^4$ . The velocity fields were analysed with a modal decomposition combining azimuthal Fourier decomposition and proper orthogonal decomposition. The wake behind the free-stream-aligned circular cylinder with three different fineness ratios of 1.0, 1.5 and 2.0 was investigated, and the wake structures in a non-reattaching flow formed by the cylinder at a fineness ratio of 1.0 are mainly discussed in the present study. Four characteristic large-scale wake structures of the recirculation bubble pumping, azimuthal shear mode, large-scale vortex shedding and streaks are identified and mainly focused on in the present study. The state of the vortex shedding is classified into three: anticlockwise/clockwise circular and flapping patterns. Each state has a relationship with the azimuthal shear mode and it tends to appear when the state is circular. Furthermore, from the analysis of the relationship between modes, the recirculation bubble pumping is found to be related to the vortex shedding position in the radial direction and the strength of the streaks. Particularly, analysis of causality shows that the recirculation bubble pumping is affected by them in the low-frequency range.

Facilitating Layered Oxide Cathodes Based on Orbital Hybridization for Sodium-Ion Batteries: Marvelous Air Stability, Controllable High Voltage, and Anion Redox Chemistry.

Layered oxides have become the research focus of cathode materials for sodium-ion batteries (SIBs) due to the low cost, simple synthesis process, and high specific capacity. However, the poor air stability, unstable phase structure under high voltage, and slow anionic redox kinetics hinder their commercial application. In recent years, the concept of manipulating orbital hybridization has been proposed to simultaneously regulate the microelectronic structure and modify the surface chemistry environment intrinsically. In this review, the hybridization modes between atoms in 3d/4d transition metal (TM) orbitals and O 2p orbitals near the region of the Fermi energy level (EF) are summarized based on orbital hybridization theory and first-principles calculations as well as various sophisticated characterizations. Furthermore, the underlying mechanisms are explored from macro-scale to micro-scale, including enhancing air stability, modulating high working voltage, and stabilizing anionic redox chemistry. Meanwhile, the origin, formation conditions, and different types of orbital hybridization, as well as its application in layered oxide cathodes are presented, which provide insights into the design and preparation of cathode materials. Ultimately, the main challenges in the development of orbital hybridization and its potential for the production application are also discussed, pointing out the route for high-performance practical sodium layered oxide cathodes.

Numerical study of motorbike aerodynamic wing kit

Abstract This study aims to design the configuration of an aerodynamic wing kit (AWK) on a racing motorbike to achieve the highest downforce-to-drag ratio. The numerical study involves a motorbike traveling in a straight line, where the AWK improves performance and safety by generating downforce to prevent lift. The geometry of the AWK uses a NACA 4412 airfoil with a span of 0.6 m. The computational mesh is generated using SnappyHexMesh and installed on a simplified motorbike to minimize the mesh skewness. The Navier–Stokes equations are solved with OpenFOAM computational fluid dynamics using the Reynolds-averaged Navier–Stokes k-ω SST and large eddy simulation (LES) turbulence models. Case 1 compares a motorbike with and without a dummy, both equipped with the AWK, varying the angle of attack (AoA) from 0 to −41°. Case 2 studies the single wing at different wind speeds (i.e. 20, 60 and 100 m/s) to determine the highest downforce-to-drag ratio at an AoA of −37°. These results serve as the basis for Case 3, which investigates non-parallel wing configurations with a fixed upper wing and a rotating lower wing. In Case 4, where both upper and lower wings rotate simultaneously as parallel wings, the peak downforce-to-drag ratio occurs at an AoA of −41°. Finally, Case 5 modifies the AoA of −41° of the parallel wing of Case 4 to a closed-wing version to comply with Fédération Internationale de Motocyclisme safety regulations. With the LES turbulence model, unsteady and complex turbulence structures can be visualized using the Q-criterion. A comparison of the time-averaged lift coefficient between the wingless and closed-wing configurations shows an increase in downforce of approximately 360%. Subsequently, the popularity of AWK will contribute to the safety of racing motorbike driving.

Influence of tip clearance on internal energy loss characteristics of axial flow pumps under different operating conditions

Axial flow pumps possess a unique structure where there must be clearances between the impeller and the piping wall, usually not exceeding 0.1% of the impeller diameter. Despite the small size of the clearance, the internal micro-vortex structures have a non-negligible impact on the main flow field of the impeller. Under the action of the pressure difference between the suction and pressure surfaces of blades, some fluids form high-energy jets in the tip clearance area, known as tip leakage vortices (TLVs). TLV interacts with the flow of the main flow field, exerting a significant impact on the internal flow state, energy loss, and hydraulic performance of the pump. To identify the influence of TLVs on the internal flow field and energy loss of axial flow pumps, this work uses a modified partially averaged Navier–Stokes (PANS) model to perform full flow field numerical calculations for a certain axial flow pump and conducts a comparative analysis of the internal flow field energy dissipation, unsteady vortex structures, energy loss, and other characteristics under three different tip clearances: 0.2 mm (0.05%D), 0.6 mm (0.15%D), and 1.0 mm (0.25%D) based on the energy transport theory. The results indicate that at optimal operating conditions, the internal energy distribution of the fluid in each flow passage is uniform, and the energy loss is primarily caused by axial backflow in the tip area; under critical rotating stall conditions, clearance size affects the distribution state of enstrophy in the guide vane flow passage, leading to average enstrophy being highest at the rim area and the most uneven distribution of enstrophy, inducing larger energy loss in the impeller; during deep stall conditions, the unevenness of internal energy distribution is stronger than that under critical stall conditions, but the overall energy loss within the impeller flow area is lower than that under critical stall conditions, while energy unevenness is mitigated as the tip clearance size increases.

Numerical Study on Unsteady Flow Field Structure over Wing within Propeller Slipstream at Low-Reynolds-Number

Numerical Study on Unsteady Flow Field Structure over Wing within Propeller Slipstream at Low-Reynolds-Number

Unsteady Flow Structure of Corner Separation in a Highly Loaded Compressor Cascade

Abstract Corner separation is an inherently unsteady flow feature in an axial compressor cascade, and it significantly affects the aerodynamic performance of compressors. The flow field of a highly loaded compressor cascade at the Mach number of 0.59 under the moderate separation condition is simulated based on delayed detached eddy simulation. Comparisons of averaged flow field and transient flow field show that the three-dimensional corner separation flow is highly unsteady and composed of fine-scale vortex structures. The classical recognition of corner separation structures is a consequence of time-averaging. To better understand the contribution of unsteady structures to the averaged flow structures, the evolutions of flow fields in time series and the power spectrums are analyzed. A dominant periodic flow fluctuation is caused by the development of separating vortices with a characteristic frequency around 3500 Hz or at a Strouhal number of 0.75. Further, energy scales and spatiotemporal features of these dominant unsteady behaviors are analyzed using proper orthogonal decomposition and dynamic mode decomposition methods. Results show that the low-frequency behaviors mainly caused by the passage vortex at lower-span regions govern large-scale changes of separation flow in size and intensity and act with certain intermittency. The vortex developing mode around 3500 Hz prevails at higher regions affected by the concentrated shedding vortex. As the separating vortices dissipate approaching the midspan, the effect of the vortex developing mode on axial velocity fluctuation is reduced, although it dominates the pressure fluctuation with good stability in the whole passage.

Insights into CFD Modelling of Water Hammer

A problem with 1-D water hammer modelling is in the application of accurate unsteady friction. Moreover, investigating the time response of fluid dynamics and unsteady turbulence structures during the water hammer is not possible with a 1-D model. This review article provides a summary of 1-D modelling using the recent finite volume approach and the discussion extends to a quasi-2-D model and historical developments as well as recent advancements in 3-D CFD simulations of water hammer. The eddy viscosity model is excellent in capturing pressure profiles but it is computationally intensive and requires more computational time. This article reviews 3-D CFD simulations with sliding mesh, an immersed solid approach, and dynamic mesh approaches for modelling valve closures. Despite prediction accuracy, a huge computational time and high computer resources are required to execute 3-D flow simulations with advanced valve modelling techniques. Experimental validation shows that a 3-D CFD simulation with a flow rate reduction curve as a boundary condition predicted accurate pressure variation results. Finally, a brief overview of the transient flow turbulence structures for a rapidly accelerated and decelerated pipe flow using DNS (Direct numerical simulation) data sets is presented. Overall, this paper summarises past developments and future scope in the field of water hammer modelling using CFD.

Experimental Investigation of Transient Flow Phenomena in Rotating Compressor Cavities

Abstract The clearance of compressor blade tips during aero-engine accelerations is an important design issue for next-generation engine architectures. The transient clearance depends on the radial expansion of the compressor discs, which is directly coupled to conjugate heat transfer in co-rotating discs governed by unsteady and unstable buoyancy-induced flow. This paper discusses an experimental and modeling study using the Bath Compressor Cavity Rig, which simulates a generic axial compressor at fluid-dynamically scaled conditions. The rig was specifically designed to generate heat transfer of practical interest to the engine designer and validate computational codes. This work presents the first study of the fundamental fluid dynamic and heat transfer phenomena under transient conditions. The rotating flow structure was seen to be characterized by coherent pairs of cyclonic/anticyclonic vortex pairs; the strength, rotational frequency, stability, and number of these unsteady structures changed with changing rotational Reynolds and Grashof numbers during the transients. These structures, measured by unsteady pressure transducers in the rotating frame of reference, were only present when the flow in the rotating cavity was dominated by buoyancy. Experimental correlations of both Nusselt number and radial mass flowrate in the rotating core were correlated against Grashof number. Remarkably, the experiments revealed a consistent correlation for both steady-state and transient conditions over a wide range of Gr. The results have a practical application to thermo-mechanical models for engine design.

Coherent structure modification by a shear acting at the surface of a turbulent open channel

Recent experiments in the river Seine have revealed the presence of persistent large rolls. This finding has motivated the present numerical study of a turbulent open channel flow. Our pseudo-spectral direct numerical simulations are aimed at understanding how the shear at the surface, for instance produced by an external wind, can change vortical structures in the bulk. Simulations are run at a Reynolds number in the range [5times 10^3div 10^4], in line with similar laboratory experiments. We investigate how flow structures located near the bottom wall, near the surface or inside the bulk are modified by a surface shear. Statistical signatures are extracted through Fourier analysis of the simulated fields, and typical wavelengths of vortices or streaks are computed. Moreover, instantaneous fields are used to show the existence of upwelling and downwelling motions. These motions can be hindered by the presence of a large enough surface shear. This condition turns out to be necessary for the existence of streaks at the surface as well. Our investigation clarifies the dynamics of vortices in an open channel flow at moderate Reynolds number, indicating that unsteady vortex structures are indeed present, but the existence of long coherent rolls cannot be accounted for by the sole presence of shear at the surface. Numerical studies with a much longer domain and possibly at higher Reynolds numbers are needed to provide a firm answer to the question.

Investigation of unsteady flow behavior of cryogen-spray coupled with cold air jet

Cryogen spray cooling (CSC) coupled with cold air jet (CAJ) was used to mitigate Cryogen consumption. However, the CAJ interaction could affect the flow structure and spray's unsteadiness, permitting coolant intermittency. In this study, the unsteady behavior and flow structure of CSC coupled with CAJ have been qualified. During the experiment, CAJ at an inclined angle of 30°, temperature of 266 K, flowrate of 0.0, 3.0, 6.0 m3/h, and axial distance of 20 mm from the spray-nozzle was injected into a typical R-134a spray system. A high-speed camera was used to capture the unsteady spray through the Mie-scattering technique, while large-eddy simulation (LES) coupled with discrete-phase model (DPM) was used to predict the vortical structure. The spatial correlation was used to identify the coherent structures, while the proper orthogonal decomposition (POD) was used to demonstrate their unsteady signatures. The results showed a significant impact of the CAJ on the spray-pattern and associated unsteadiness, providing an asymmetric spray-plume at the CAJ windward side. The CAJ increased the fluctuating energies of the unsteady coherent structures by 32 % through CAJ-to-spray interactions. It contributed to the unsteady spray through the counter-rotating vortex-pairs (CRVP) at the lateral sides and hairpin vortices along the injection line. The CRVP clustered the spray in recirculating regions, introducing unsteady CAJ entrainment at the leeward and thermal field at the windward. This work could insight into our understanding of unsteady multiphase flow, promoting advanced cooling technology by uncovering their causes and effects.

Effect of Aspect Ratio on Swept-Wing Dynamic Stall

The role of aspect ratio on the dynamic stall process of swept finite wings is investigated using high-fidelity implicit large-eddy simulations. Two aspect ratios ( and 8) are explored for a 30 deg swept wing (NACA 0012) pitching sinusoidally from an initial incidence of 4 deg to a maximum angle of attack of 22 deg with a reduced frequency of over one pitching cycle. The flow is simulated at a chord Reynolds number of and a freestream Mach number of . The unsteady three-dimensional flowfield for the higher-aspect-ratio wing showed similarity with the lower-aspect-ratio wing through the initial flow separation at the leading edge. Motion-induced effects promoted earlier initiation of the unsteady vortical structures at higher aspect ratios. The vortex tube at the larger span underwent significant distortion, which contrasted with the vortex observed at the lower span. The vortical structure eventually interacted with the trailing-edge vortex, which was not observed at . Examination of the unsteady loads detailed a larger lift slope, mean values, peak values, and earlier stall as the aspect ratio increased. Analysis of the aerodynamic pitch damping suggests the wing is less susceptible to local torsional instabilities than the wing.

Experimental study of a self-excited jet precession in a sudden expansion flow

This study presents Stereoscopic particle image velocimetry measurements of a precessing motion resulting from a sudden expansion of an axisymmetric jet into a coaxial chamber. The sudden expansion has an expansion ratio of 4.9 and an aspect ratio of 2.03. The velocity profile in the inlet tube is measured upstream of the expansion using Laser doppler anemometry. The Reynolds number (Re) is varied from 3160 to 63,870. The time-averaged flow field exhibits an axisymmetric behavior, and the velocity profile near the expansion resembles that of a jet. The half-width and the opening angle of the jet are similar for all Reynolds numbers measured, with values of $$3.73\pm 0.04^{\circ }$$ and $$8.73\pm 0.19^{\circ }$$ , respectively. Despite its axisymmetry, the flow field contains unsteady coherent structures in nature, which are analyzed using both spectral proper orthogonal decomposition and phase averaging. These structures make the instantaneous flow field asymmetric and create a precession around the central axis of the geometry. The structure of this precession is similar for $$8300 \le \hbox {Re} \le$$ 63,870. For $$3160 \le \hbox {Re} \le 8300$$ , the precession is very weak and hard to extract from the turbulent flow fields. The Strouhal number (St) increases linearly for $$8300 \le \hbox {Re} \le$$ 23,600, after which it reaches a value of around 0.003. Precessing structures of this kind were previously only reported for swirling sudden expansion flows, fluidic precessing jet nozzles or annular free jets. However, this study shows that they also exist in confined, non-swirling sudden expansion flows, without exiting to an ambient fluid.

Evolution of unsteady vortex structures in the tip region of an axial compressor rotor

The evolution of unsteady vortex structures in the tip region of an axial compressor rotor is investigated based on delayed detached eddy simulation. The vortex structures are identified by the LTcri method, and the velocity fields are visualized by the particle tracing method. The results show that the evolution of the tip leakage vortex (TLV) can be divided into three phases: the generation phase, the development phase, and the dissipation phase. The unsteadiness of the flow field mainly appears in the dissipation phase as a consequence of the unsteady secondary tip leakage. There are three primary unsteady vortex structures caused by the tip leakage flow (TLF), and these vortex structures are related to each other as a feedback loop. The intermittent formation of the vortex ropes leads to the breakdown of the TLV and thus results in the roll-up of the backflow vortex (BFV) due to the radial velocity gradient. The secondary leakage of the BFV locally enhances the TLF jet and affects the formation of the vortex ropes in turn. This feedback loop causes the unsteady behavior of the TLF and has great impacts on the performance and stability of the compressors.

Experimental Observation of the Spontaneous Emission of a Space–Time Wavepacket in a Multimode Optical Fiber

We provide a complete analysis, from theory to experiment, of the spontaneous emergence of a discretized conical wave of X-type (i.e., a localized 2D+1 space-time wavepacket) when an intense ultrashort pulse nonlinearly propagates in a multimode fiber. In particular, we reveal that this spatiotemporal phenomenon corresponds to broadband intermodal dispersive wave emission from an unsteady localized wave structure formed during nonlinear propagation. Theoretical phase-matching predictions are experimentally and numerically confirmed in a commercially available step-index multimode fiber. Our results provide a general understanding of phase-matched radiations emitted by nonlinear waves in multidimensional dispersive optical system.