Coherent Flow Research Articles

Experimental study on acoustic resonance of subsonic and slightly underexpanded impinging jets

The aeroacoustic feedback loops in high-speed circular jets that impinge on a large flat plate are investigated via acoustic measurements and schlieren visualizations. In the present experiments, the nozzle pressure ratio ranges from 1.39 to 2.20, the corresponding ideally expanded jet Mach number $M_j$ is from 0.70 to 1.12 and the nozzle-to-plate distance ( $H$ ) is from 4.0 $D$ to 6.0 $D$ , where $D$ is the nozzle exit diameter. The results of acoustic measurements show that the strongest tones are generated in a limited frequency band. The empirical dispersion relations obtained from the fluctuating greyscales along the jet centreline of time-resolved schlieren images have good agreement with the dispersion relations from the vortex-sheet model. The coherent flow structures at tonal frequencies are extracted by spectral proper orthogonal decomposition and are analysed in detail. For the $M_j<0.82$ jets, the upstream-propagating guided jet mode is progressively confined to the potential core of jets with increasing tonal frequency, which provides the first direct experimental support for theoretical results. The evolution in the structures of acoustic resonance loops is studied along a single frequency stage of axisymmetric impinging tones. When the acoustic resonance between the upstream- and downstream-propagating guided jet modes is formed at tonal frequencies, the impinging tones are intenser. Slightly underexpanded impinging jets can simultaneously produce impingement tones and screech tones. Shock-cell structures have modulatory effects on the downstream-propagating Kelvin–Helmholtz wavepacket and the upstream- and downstream-propagating guided jet modes. Due to the interaction between the flow structures at the frequencies of impinging and screech tones, tones of axisymmetric modes can be produced outside the frequency ranges in which the axisymmetric upstream-propagating guided jet modes are supported by jets.

Multiobjective optimization of Vortex Generators for heat transfer enhancement in turbulent flows

Convective heat transfer can be enhanced by streamwise vortices and coherent flow structures produced downstream Vortex Generators (VGs). The thermal performance depends on the VG shape which controls the intensity and topology of the vortices. Thus, VG shape optimization is a major challenge in designing sustainable and efficient heat exchangers. In the present study, a multi-objective optimization is performed through large space exploration design to find optimal VG shapes that adapt to different flow regimes. Simulations are carried out in parallel plate turbulent flow for three different Reynolds numbers; Re1=4600,Re2=9920, and Re3=30050. The present numerical results are first validated against numerical and experimental data from the open literature. Then, and for the first time, seven design parameters are investigated, based on VG shape dimensions. The new optimized VG show a thermal enhancement factor between 11 and 35%, relative to a smooth channel case, and between 3 and 14%, relative to different VG shapes from the recent open literature. The present results are analyzed in details by focusing on the physics of the flow structure and its correlation to the convective heat transfer enhancement using local criteria for flow visualization and vortex intensity quantification.

Transition to fully developed turbulence in liquid-metal convection facilitated by spatial confinement

Using thermal convection in liquid metal, we show that strong spatial confinement not only delays the onset Rayleigh number $Ra_c$ of Rayleigh–Bénard instability but also postpones the various flow-state transitions. The $Ra_c$ and the transition to fully developed turbulence Rayleigh number $Ra_f$ depend on the aspect ratio $\varGamma$ with $Ra_c\sim \varGamma ^{-4.05}$ and $Ra_f\sim \varGamma ^{-3.01}$ , implying that the stabilization effects caused by the strong spatial confinement are weaker on the transition to fully developed turbulence when compared with that on the onset. When the flow state is characterized by the supercritical Rayleigh number $Ra/Ra_{c}$ ( $Ra$ is the Rayleigh number), our study shows that the transition to fully developed turbulence in strongly confined geometries is advanced. For example, while the flow becomes fully developed turbulence at $Ra\approx 200Ra_c$ in a $\varGamma =1$ cell, the same transition in a $\varGamma =1/20$ cell only requires $Ra\approx 3Ra_c$ . Direct numerical simulation and linear stability analysis show that in the strongly confined regime, multiple vertically stacked roll structures appear just above the onset of convection. With an increase of the driving strength, the flow switches between different-roll states stochastically, resulting in no well-defined large-scale coherent flow. Owing to this new mechanism that only exists in systems with $\varGamma <1$ , the flow becomes turbulent in a much earlier stage. These findings shed new light on how turbulence is generated in strongly confined geometries.

Connecting transonic buffet with incompressible low-frequency oscillations on aerofoils

Self-sustained, low-frequency, coherent flow unsteadiness over rigid, stationary aerofoils in the transonic regime is referred to as transonic buffet. This study examines the role of shock waves in sustaining this transonic phenomenon and its relation to low-frequency oscillations (LFO) that occur in flow over aerofoils in the incompressible regime (Zaman et al., J. Fluid Mech., vol. 202, 1989, pp. 403–442). This is investigated by performing large-eddy simulations of the flow over a NACA0012 profile for a wide range of flow conditions under free-transition conditions. At low Reynolds numbers, zero incidence angle and sufficiently high free-stream Mach numbers, $M$ , transonic buffet occurs with shock waves present in the flow. However, when $M$ alone is lowered, self-sustained, periodic oscillations at a low frequency are observed even though shock waves are absent and the entire flow field remains subsonic at all times. At higher incidence angles, the oscillations are sustained at progressively lower $M$ and are present even at $M=0.3$ , where compressibility effects are low. A spectral proper orthogonal decomposition (SPOD) shows that the spatial structure of these oscillations is consistent for all cases. The SPOD modes are topologically similar, suggesting a connection between transonic buffet and LFO in the incompressible regime. Comparisons with other studies examining transonic buffet on various aerofoils, under forced-transition and fully turbulent conditions support this hypothesis. Future studies using tools of global linear stability analysis, especially at high free-stream Reynolds numbers are required to examine whether the underlying mechanisms of transonic buffet and incompressible LFO are the same.

Time-averaged flow characterisations in a bimodal gravel-bed stream and relative role of sediment depositions on near-bed coherent flow structures

Abstract The present study aims to quantify experimentally the relative role of sediment depositions on near-bed flows and turbulence in a gravel-bed stream. Time-averaged velocity was measured over a sand-filled gravel-bed stream with four cases of sediment depositions and compared with those over a gravel-bed stream without sediment depositions. An acoustic Doppler velocimeter was used to measure the instantaneous velocity of flows. The progressive infilling of void spaces in the gravel-bed stream forms distinct bimodal depositions that alter the mean flows characterised by increasing zero-velocity levels and massive damping in the bed shear stresses. The data plots of turbulent intensity depict enhancement of streamwise turbulent intensity in the near-bed flow region with increasing sand depositions. Moreover, the opposite nature of streamwise and vertical turbulent kinetic energy fluxes in the interfacial sublayer leads to slowing down the time-average Reynolds shear stresses at the vicinity of the gravel-bed surface. At the vicinity of the crest, the ejection and sweep events contributed approximately 86 and 56%, respectively, to the total Reynolds shear stress production in the case of gravel bed under clear-water flow conditions. By contrast, the contributions of turbulent sweep events increased over the sand-filled gravel bed at the same location.

Experimental study of the upstream bathymetry effects on a ducted twin vertical axis turbine

Tidal turbines at sea are subject to complex flow conditions. Among that complexity, wide bathymetry obstacles can generate powerful coherent flow structures whose impact on horizontal axis tidal turbines was proved to be highly detrimental. Consequently, the present paper aims at studying the effects of such coherent flow structures on the response of another tidal turbine geometry, namely a bottom-mounted ducted twin vertical axis tidal turbine (2-VATT). We tested a 1/20 scale model of such a 2-VATT in Ifremer’s flume tank either with a flat bed or downstream of a wide bathymetry obstacle at a constant far upstream velocity and we measured the turbine response simultaneously with the flow velocity. These flow measurements make it possible to compute cross-correlations in order to analyse the influence of the velocity fluctuation on the turbine response. The results reveal a strong drop in the average power and loads coefficients of the 2-VATT combined with significantly larger fluctuation. The velocity deficit and the high level of turbulence in the obstacle wake is responsible for a 40% lower average power coefficient and more than 3 times higher standard deviation compared to the flat bed configuration. The loads standard deviations are multiplied by 2 for the drag and by 10 for the lift when the 2-VATT is downstream of the bathymetry obstacle. This behaviour strongly increases the risks of structural fatigue failure, but the turbine drifting or overturning risks under those conditions remain lower than with the flat bed. The power fluctuation increase appears to be mostly due to the flow shear in the obstacle wake whereas the load fluctuation is mostly due to the periodical passing of the coherent flow structures. Thus, a proper characterisation of the flow at each precise turbine locations prior deployment at sea is highly recommended to design their structure accordingly.

The Chimera Revisited: Wall- and Magnetically-Bounded Turbulent Flows

This review is a first attempt at bringing together various concepts from research on wall- and magnetically-bounded turbulent flows. Brief reviews of both fields are provided: The main similarities identified are coherent (turbulent) structures, flow generation, and transport barriers. Examples are provided and discussed.

Anisotropic Diffusion of Elongated Particles in Active Coherent Flows.

The study of particle diffusion, a classical conundrum in scientific inquiry, holds manifold implications for various real-world applications. Particularly within the domain of active flows, where the motion of self-propelled particles instigates fluid movement, extensive research has been dedicated to unraveling the dynamics of passive spherical particles. This scrutiny has unearthed intriguing phenomena, such as superdiffusion at brief temporal scales and conventional diffusion at longer intervals. In contrast to the spherical counterparts, anisotropic particles, which manifest directional variations, are prevalent in nature. Although anisotropic behavior in passive fluids has been subject to exploration, enigmatic aspects persist in comprehending the interplay of anisotropic particles within active flows. This research delves into the intricacies of anisotropic passive particle diffusion, exposing a notable escalation in translational and rotational diffusion coefficients, as well as the superdiffusion index, contingent upon bacterial concentration. Through a detailed examination of particle coordinates, the directional preference of particle diffusion is not solely dependent on the particle length, but rather determined by the ratio of the particle length to the associated length scale of the background flow field. These revelations accentuate the paramount importance of unraveling the nuances of anisotropic particle diffusion within the context of active flows. Such insights not only contribute to the fundamental understanding of particle dynamics, but also have potential implications for a spectrum of applications.

Flow investigation of two-stand ultrasonic flow meters in a wide dynamic range by numerical and experimental methods

The enhancement of two-stand ultrasonic flow meters relies upon obtaining a precise understanding and prediction of their complex flow physics throughout their entire dynamic range of operation. This study provides a comprehensive numerical and experimental investigation of the flow physics of a typical two-stand ultrasonic flow meter by industry standards. Predictions based on computational fluid dynamics simulations are employed to obtain numerical results, which are validated through experiments based on laser Doppler velocimetry and static pressure drop. Results indicate that no qualitative changes occur beyond an inflow Reynolds number of 104 in terms of coherent structures and flow dynamics. Analysis of the static pressure distribution across cross-sections reveals that the stands are the most influential areas contributing to pressure drop. In cases with turbulent inflow, there is a noticeable recovery of static pressure following significant pressure gradients across the stands, while such recovery is absent in scenarios with laminar inflow. Both numerical and experimental approaches yield excellent agreement in outcomes, accurately estimating the axial velocity within the flow meter’s measurement volume and the pressure drop across it, with deviations within experimental uncertainty ranges of 2 standard deviations. The developed numerical methodology demonstrates its potential to accurately evaluate complex internal-flow systems with similar flow features and Reynolds number ranges. The flow dynamics for a wide dynamic range of operation in two-stand ultrasonic flow meters are shown in detail in both laminar and turbulent flow regimes, displaying rolling vortices, detached flow, and recirculation zones.

Active nematics in corrugated channels.

Active nematic fluids exhibit complex dynamics in both bulk and in simple confining geometries. However, complex confining geometries could have substantial impact on active spontaneous flows. Using multiparticle collision dynamics simulations adapted for active nematic particles, we study the dynamic behaviour of an active nematic fluid confined in a corrugated channel. The transition from a quiescent state to a spontaneous flow state occurs from a weak swirling flow to a strong coherent flow due to the presence of curved-wall induced active flows. We show that the active nematic fluid flows in corrugated channels can be understood in two different ways: (i) as the result of an early or delayed flow transition when compared with that in a flat-walled channel of appropriate width and (ii) boundary-induced active flows in the corrugations providing an effective slip velocity to the coherent flows in the bulk. Thus, our work illustrates the crucial role of corrugations of the confining boundary in dictating the flow transition and flow states of active fluids.

Frequency jump of a flickering buoyant jet diffusion flame influenced by ambient coflow

Flickering of laminar diffusion flames is known to be caused by the buoyancy-induced toroidal vortices attached to the outer flame surface, making a classical model for understanding flame-vortex interactions. This work experimentally investigates the impacts of ambient coflow on the flickering behaviors of a buoyant jet diffusion flame. Utilizing a simultaneous high-speed flame imaging/OH⁎ chemiluminescence imaging/particle image velocimetry measurement system, the evolutions and interactions of coherent flame and flow structures are resolved. The results demonstrate that the periodic deformation of the flame surface (i.e., flame flickering) arises from the periodic formation, growth, and shedding of the buoyancy-induced outer vortex rings (OVRs), manifesting as a hydrodynamic instability developing along the outer shear layer (OSL). Upon applying a weak-to-moderate coflow, the flame bulge's size gradually reduces, and the flickering frequency slightly increases with increasing coflow velocity. The notable frequency jump phenomenon is confirmed as the coflow velocity rises to a threshold value such that the flickering frequency undergoes a sudden increase of 3–4 Hz; this is also accompanied by a significant reduction in the flame bulge's size and a delay in flame pinch-off. Moreover, a second frequency jump of 2–3 Hz occurs at a higher coflow velocity. Based on quantitative analysis of the OVR core's evolution, we find that the frequency jumps result from a sudden downstream shift of the initial OVR core, essentially indicating the jump of the instability onset point of the OSL. From a hydrodynamic instability perspective, this phenomenon can be explained as a stabilization effect of the coflow on the OSL, resulting in the re-establishment of the instability condition in a downstream location.

Influence of dual purging jets on interferometric measurement of optical path difference

Incompressible large eddy simulations of dual purging jets at three Reynolds numbers of ReL = 1500, 1000, and 500 are performed, providing the database for simulated interferometric measurements subjected to the spatiotemporally varying temperature fields. The temperature is modeled as a passive scalar, and the index-of-refraction field is calculated from the air temperature and pressure using the modified Edlén equation. Based on wave optics, wavefront distortions of laser beams traversing the flow field are computed, and the optical path difference measurement is simulated. Statistical results of the velocity and temperature indicate distinctly different flow patterns at three Reynolds numbers. Spectral proper orthogonal decomposition is adopted to analyze the relationship between coherent flow structures and temperature wavepackets. There is a dominant frequency (St = 0.29) at ReL = 1500 and 1000, which is attributed to the Kelvin–Helmholtz instability, whereas the two jets have notable flapping behaviors at ReL = 500. The wavefront distortions are found to be insensitive to the Reynolds number. The effects of the flow region and tracing distance are examined, and the optical performances of different beam pairs for interferometry are evaluated. The optical beams passing through the outer shear layers of the dual jets and wall jets are distorted severely, resulting in large systematic and random errors in the interferometric measurement.

Lagrangian investigation of the correlation of helicity with coherent flow characteristics for turbulent transport

The correlation between helicity and turbulent transport in turbulent flows is probed with the use of direct numerical simulation and Lagrangian scalar tracking. Channel flow and plane Couette flow at friction Reynolds number 300 and Lagrangian data along the trajectories of fluid particles and passive particles with Schmidt numbers 0.7 and 6 are used. The goal is to identify characteristics of the flow that enhance turbulent transport from the wall, and how flow regions that exhibit these characteristics are related to helicity. The relationship between vorticity and relative helicity along particle trajectories is probed, and the relationship between the distribution of helicity conditioned on Reynolds stress quadrants is also evaluated. More importantly, the correlation between relative helicity density and the alignment of vorticity with velocity vectors and eigenvectors of the rate of strain tensor is presented. Separate computations for particles that disperse the farthest into the flow field and those that disperse the least are conducted to determine the flow structures that contribute to turbulent dispersion. The joint distribution of helicity and vertical velocity, and helicity and vertical vorticity depends on the location of particle release and the Schmidt number. The trajectories of particles that disperse the least are characterized by a correlation between the absolute value of the relative helicity density and the absolute value of the cosine between the vorticity vector and the eigenvectors of the rate of strain tensor, while the value of this correlation approaches zero for the particles that disperse the most.

SUPPLY FLOW OPTIMIZATION IN THE PROCESS OF ACQUISITION OF WORK UNIFORMS: A CASE STUDY IN A NETWORK OF DEPARTMENT STORES IN JOINVILLE

With the growing need to serve customers with greater quality, seeking competitive advantages, it is essential that service employees remain properly uniformed. For this need to be met, the company must deliver these uniforms, maintaining a coherent and well-defined process flow in the logistics of this material. This article aims to optimize the supply flow in the process of acquiring work uniforms in a chain of department stores in Joinville through lean thinking. To meet this objective, it was necessary to analyze the current process flow and apply purchasing management, inventory management and process management tools. These applications resulted in the elimination of waste, increased process quality and a linear flow, without bifurcations, looping’s and low complexity.

Sensitivity of turbulence parameters to tidal energy converter loads in BEM simulations

Renewable energy is playing an increasingly central role in the global energy supply due to decarbonisation and energy security aims. A vital aspect of renewable energy systems will be the predictability of the energy source, something that tidal stream energy can provide. The tidal sites suitable for energy extraction are by their nature turbulent, creating variations in the tidal energy converter (TEC) loads and affecting device durability. Developers use Blade Element Momentum (BEM) models to predict loading and improve designs of TECs. To simulate turbulence effects within these models, a synthetic flow field is generated using a combination of measured and assumed parameters. Inaccuracies in these parameters can lead to uncertainties in the simulated loads. This study investigates the sensitivity of turbulence characteristics to loads using a commercial BEM software. Variability in parameters shows a profound impact on the loads. Varying turbulence intensity resulted in a 90%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$90\\%$$\\end{document} change in fatigue loads for intensities ranging 2–24%. Length-scales showed a 49%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$49\\%$$\\end{document} decrease in loads across the range tested (5–70 m). A coherent flow field increased loads by 45%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$45\\%$$\\end{document} compared to a non-coherent flow. Hub-bending loads varied by 30%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$30\\%$$\\end{document} between different shear profiles, however varying the standard deviation profiles did not show notable effects. The results from this study emphasise the necessity for accurate turbulence parameter inputs to reduce uncertainty in device load modelling. It also highlights the importance of using realistic shear profiles as well as appropriate coherence models.

Heat transport and flow morphology of geostrophic rotating Rayleigh–Bénard convection in the presence of boundary flow

Using direct numerical simulations, we investigate the heat transport in bulk and boundary flows separately in rotating Rayleigh–Bénard convection in cylindrical cells. In the bulk we observe a steep scaling relationship between the Nusselt number ( $Nu$ ) and the Rayleigh number ( $Ra$ ), which is consistent with the results from simulations using periodic boundary conditions. For the boundary flow, we observe a power law $Nu_{BF}\sim (Ra/Ra_w)^1$ at the leading order, where $Nu_{BF}$ is the local Nusselt number of the boundary flow and $Ra_w$ is the onset Rayleigh number of the wall mode. We develop a model using the boundary layer marginal stability theory to explain this power law, and further show that a more precise description of the data can be obtained if a higher-order correction is introduced. A striking finding of our study is the observation of a sharp transition in flow state, manifested by a sudden drop in $Nu_{BF}$ with a corresponding collapse of the boundary flow coherency. After the transition, the boundary flow breaks into vortices, leading to a reduction in flow coherency and heat transport efficiency. As the physical properties of the vortices should not depend on the aspect ratio, $Nu_{BF}$ for all aspect ratios collapse together after the transition. Moreover, the centrifugal force helps trigger the breakdown of the coherent boundary flow state. For this reason, $Nu_{BF}$ for the cases with non-zero centrifugal force collapse together. We further develop a method that enables us to separate the contributions from the bulk and boundary flows in the global Nusselt number using only the global $Nu$ and it does not require the centrifugal force to be absent.

Dynamics and scaling of particle streaks in high-Reynolds-number turbulent boundary layers

Inertial particles in wall-bounded turbulence are known to form streaks, but experimental evidence and predictive understanding of this phenomenon is lacking, especially in regimes relevant to atmospheric flows. We carry out wind tunnel measurements to investigate this process, characterizing the transport of microscopic particles suspended in turbulent boundary layers. The friction Reynolds number$Re_\tau = {O}(10^4)$allows for significant scale separation and the emergence of large-scale motions, while the range of viscous Stokes number$St^+=18$–870 is relevant to the transport of dust and fine sand in the atmospheric surface layer. We perform simultaneous imaging of both carrier and dispersed phases along wall-parallel planes in the logarithmic layer, demonstrating that streamwise particle streaks largely overlap with large-scale low-speed flow regions. The fluid–particle slip velocity indicates that with increasing inertia, the particle streaks outlive the low-speed fluid streaks. Moreover, two-point statistics show that the width of the particle streaks increases linearly with Stokes number, bounded by the size of the coherent flow structures. Finally, the particle-sampled flow topology suggests that particle streaks reside between the legs of hairpin packets. From these observations, we infer a conceptual view of the formation of particle streaks in the frame of the attached eddy model. A scaling for the particle streaks’ width is derived as a function of$Re_\tau$and$St^+$, which reproduces the measured trends and predicts widths${O}(0.1)$m in the atmospheric surface layer, comparable to aeolian streamers observed in the field.

Flow Instability Control in a Model Swirl-Stabilized Combustor with Central Jet Injection

Swirling flows often occur in nature and industrial applications. With an increase in swirl intensity, such rotating flows are known to become unstable and undergo a sudden breakdown of the vortex core, resulting in unsteady flow dynamics with intensive pressure fluctuations. In particular, swirling flows are organized in combustion chambers to stabilize the flame around the central recirculation zone, formed due to the vortex core breakdown. However, the impact of large-scale vortex structures, including the precessing vortex core and secondary helical vortices, on unsteady combustion regimes is still unclear. The present paper demonstrates experimentally that for the swirling flow of a model swirl combustor, the injection of a central jet may be used to alter the configuration of coherent flow structures, including helical vortices. In particular, the asymmetric hydrodynamics mode, associated with the precessing vortex core, is suppressed, whereas the symmetrical one becomes dominant. This effect demonstrates the importance of central jet injection to control the dominant mode of flow instability for the design of swirl combustors.

Significant influence of fluid viscoelasticity on flow dynamics past an oscillating cylinder

This study focuses on numerically investigating the impact of fluid viscoelasticity on the flow dynamics around a transversely forced oscillating cylinder operating in the laminar vortex shedding regime at a fixed Reynolds number of $Re = 100$ . Specifically, we explore how fluid viscoelasticity affects the boundary between the lock-in and no lock-in regions and the corresponding wake characteristics compared with a simple Newtonian fluid. Our findings reveal that fluid viscoelasticity enables the synchronization of the vortex street with the cylinder motion at lower oscillation frequencies than those required for a Newtonian fluid. Consequently, the lock-in region boundary for a viscoelastic fluid differs from that of a Newtonian fluid and expands in the non-dimensional cylinder oscillation amplitude and frequency parameter space. In the primary synchronization region, the wake of a Newtonian fluid exhibits ‘2S’ (two single vortices) and ‘P+S’ (a pair of vortices and a single vortex) shedding modes. In contrast, a ‘2P’ (two pairs of vortices) vortex mode is observed for a viscoelastic fluid within the same region. To gain a deeper understanding of the differences in the coherent flow structures and their associated frequencies between the two fluids, we employ the data-driven reduced-order modelling technique, known as the dynamic mode decomposition (DMD) technique. Utilizing this technique, we successfully extract and visualize the two competing fundamental frequencies (cylinder oscillation and natural vortex shedding frequencies) and their associated flow structures in the case of the no lock-in state, whereas only the dominant cylinder oscillation frequency and associated flow structure in the case of the lock-in state. Furthermore, we propose that the presence of excess strain resulting from the stretching of polymer molecules in viscoelastic fluids leads to a distinct difference in the wake structure compared with Newtonian fluids. This observation aligns with the findings obtained from the $Q$ -criterion and vorticity transport analysis of the wake.

Tomographic particle image velocimetry measurement on three-dimensional swirling flow in dual-stage counter-rotating swirler

Three-Dimensional (3D) swirling flow structures, generated by a counter-rotating dual-stage swirler in a confined chamber with a confinement ratio of 1.53, were experimentally investigated at Re = 2.3 × 105 using Tomographic Particle Image Velocimetry (Tomo-PIV) and planar Particle Image Velocimetry (PIV). Based on the analysis of the 3D time-averaged swirling flow structures and 3D Proper Orthogonal Decomposition (POD) of the Tomo-PIV data, typical coherent flow structures, including the Corner Recirculation Zone (CRZ), Central Recirculation Zone (CTRZ), and Lip Recirculation Zone (LRZ), were extracted. The counter-rotating dual-stage swirler with a Venturi flare generates the independence process of vortex breakdown from the main stage and pilot stage, leading to the formation of an LRZ and a smaller CTRZ near the nozzle outlet. The confinement squeezes the CRZ to the corner and causes a reverse rotation flow to limit the shape of the CTRZ. A large-scale flow structure caused by the main stage features an explosive breakup, flapping, and Precessing Vortex Core (PVC). The explosive breakup mode dominates the swirling flow structures owing to the expansion and construction of the main jet, whereas the flapping mode is related to the wake perturbation. Confinement limits the expansion of PVC and causes it to contract after the impacting area.