Evolution Of Flow Structures Research Articles

Some observations concerning "laminarization" in heated vertical tubes

For low "turbulent" Reynolds numbers in strongly-heated vertical gas flow, fundamental results of existing direct numerical simulation (DNS) databases have been examined to deduce differences between cases which "revert" to turbulent and those that yield integral parameters which correspond to laminar flow (henceforth laminarizing or laminarized). Objectives are (1) to examine the streamwise evolution of near-wall flow structures and (2) to determine which, if any, proposed laminarization parameters could be used to discriminate between turbulent and laminarizing flows resulting. Views of streamlines in r-θ cross sections showed evidence of a ring of irregular-shaped streamwise vortices near the wall at all locations. The transverse advection of these vortices is hypothesized to provide a path of least resistance to the transport of streamwise momentum in the wall-normal direction. It is demonstrated that a steady streamwise laminar vortex can provide apparent turbulent momentum transport (as a consequence of the Reynolds decomposition) such that a reasonable mean velocity profile is predicted near the wall. For the present cases, the values of the non-dimensional radius (yc,w+ = Reτ,w) and of the modified wall Reynolds number did discriminate whether turbulent or laminarizing flow resulted.

Numerical simulation of a reacting porous char particle in supercritical water with structural evolution

A deep understanding of the particle conversion behavior is necessary for reactor-scale simulation and design, and the unqiue properties of supercritical water make it different from traditional gasification/combustion process. A 2-D transient numerical model coupling flow, heat transfer, species transport and porous structure evolution was therefore developed in this paper for supercritical water gasification of a single porous char particle. The gasification behavior was studied from 873 to 1023 K for particles of 0.1 and 0.5 mm, and the effects of convection and structural parameters were discussed. The results showed that the gasification shifted from kinetically controlled regime to diffusion controlled regime as temperature and particle size increased, and the conversion characteristics in each regime were quite different with the comprehensive effects of temperature and species distribution and porous structure. Under convective conditions, both heat and mass transfer was enhanced which significantly promoted char gasification, and asymmetric conversion was observed. Besides, the increase of the structural parameter ψ in random pore model and initial porosity shortened the char conversion time and showed more significant impact in kinetically controlled regime.

Numerical study of the slip line instabilities in shock-wavywall reflection

The study of shock-induced flow instabilities and turbulent mixing is of great significance in the field of astrophysics and inertial confinement fusion. The reflection of a planar shock on a wavy wall is simulated with the Gas Kinetic Scheme (GKS). The time evolution of shock waves and multi-scale flow structures is accurately captured at different incident Mach number for the wavy walls with different wave-height and wavenumbers. Due to the disturbance from the wavy wall, the triple points spontaneously formed and traverse on the front of the wave. The trajectory of the triple points forms a typical cellular pattern. After each collision of the triple points, the slip lines will shed, and the vortex structure will appear from the baroclinic deposition of vorticity. This baroclinic deposition of vorticity is a different mechanism that includes both Richtmyer-Meshkov process and Kelvin-Helmholtz instability. The analysis of the vorticity transport equations is presented to understand this shock driven slip line instability. Finally, this study provides some references for the future investigation of turbulent mixing.

Methodology for the large-eddy simulation and particle image velocimetry analysis of large-scale flow structures on TCC-III engine under motored condition

Large-eddy simulation has been increasingly applied to internal combustion engine flows because of their improved potential to capture the spatial and temporal evolution of turbulent flow structures compared with Reynolds-averaged Navier Stokes simulation. Furthermore, large-eddy simulation is universally recognized as capable of simulating highly unsteady and random phenomena, which drive cycle-to-cycle variability and cycle-resolved events such as knocks and misfires. To identify large-scale structure fluctuations, many methods have been proposed in the literature. This article describes the application of several analysis methods for the comparison between different datasets (experimental or numerical) and the identification of large-structure fluctuations. The reference engine is the well-known TCC-III single-cylinder optical unit from the University of Michigan and GM Global R&D center; the analyses were carried out under motored engine conditions. A deep analysis of in-cylinder gas dynamics and flow structure evolution was performed by comparing the experimental results (particle image velocimetry of the velocity fields) with a dataset of consecutive large-eddy simulation cycles on four different cutting planes at engine-relevant crank angle positions. Phase-dependent proper orthogonal decomposition was used to obtain further conclusions regarding the accuracy of the simulation results and to apply conditional averaging methods. A two-point correlation and an analysis of the tumble center are proposed. Finally, conclusions are drawn to be used as guidelines in future large-eddy simulation analyses of internal combustion engines.

Investigation of large-scale coherent structures and momentum interference of twin rectangular jets

An experimental study was performed to investigate the effects of nozzle spacing between twin rectangular jets on the non-linear momentum interference and spatial evolution of large-scale flow structures. Five test cases of different spacing ratios (S/de = 1.8, 2.8, 3.7, 5.5 and 7.3) were examined. Two-point correlation, joint probability density function and swirling strength were used to characterize the large-scale structures. Turbulent/non-turbulent interface along the inner and outer shear layers were investigated. The results showed that due to the interactions in the merging region, the spatial correlation of streamwise velocity fluctuations was considerably lower along the inner shear layer than along the outer shear layer. As a result of the mixing process, the swirling strength of retrograde vortices generated along the inner shear layer was also lower than that of the prograde vortices. Along the symmetry line, longitudinal and transverse integral length scales increased linearly as the downstream distance from the jets increases. For the longitudinal length scale, the slope of the linear region was independent of the spacing ratio, while for the transverse length scale, the slope increases as the spacing ratio increases. For cases of S/de < 3.7, due to the suppression effects of the adjacent jet, the jump in the conditionally-averaged mean streamwise velocity profile around the turbulent/non-turbulent interface was lower for the inner shear layer compared to the outer shear layer.

Consistent Flow Structure Evolution in Accelerating Flow Through Hexagonal Sphere Pack

Direct numerical simulation based on incompressible Navier–Stokes equations with an immersed boundary method is used to simulate accelerating porous media flow through a bed of uniform spheres arranged in hexagonal close packing order. The transient flow is realised by driving initially resting fluid by a constant pressure gradient. A wide spectrum of Reynolds number based on the sphere diameter and volume-averaged velocity is considered, which ranges from creeping flow up to a Reynolds number of approximately 350, where turbulent flow structures are evident inside the pores. It is found that nonlinear dependence of the volume-averaged velocity with respect to the applied pressure gradient is the consequence of emergence of streamwise jets and the accompanying streamwise vortices, as previously observed for other sphere pack arrangements. Furthermore, two distinct flow modes are identified in the steady flow regime which satisfy full geometric symmetries. The flow then becomes unsteady around Reynolds number of 90 which coincides with a partial breaking of the symmetries, and pore-scale turbulence emerges once all the symmetries vanish when Reynolds number is larger than 200. For all the considered unsteady flow, independent of being turbulent or not, we observe a consistent sequence of flow structure evolution during the flow development with progressively broken symmetries albeit at widely varying instantaneous Reynolds numbers. Moreover, we show that the symmetry breaking takes place in larger pore spaces first, then propagate into smaller pores located in downstream.

Analysis of the ground effect on development of flow structures around an inclined solar panel

The complex three-dimensional flow that develops around an inclined flat solar panel near the ground is investigated using Computational Fluid Dynamics. The early stage evolution of the flow and the interaction of the shear layers emanating from the sides of the panel, the large separation region behind the panel and the boundary layers on the panel and ground are captured using Delayed Detached-Eddy Simulation to model the turbulence. The mean analysis shows that a small clearance produces a wall-jet like flow in the gap region between the panel and the ground, which tends to elongate the wake region in the downstream direction. On the other hand, a strong upwash is observed for a larger gap, reducing the length of the wake. Transient three-dimensional flow structures are captured using vorticity contours and the λ2-criterion. The early stage development of flow around the panel shows inverted hairpin-like vortices that are shed from the leading edge, touch down on the ground, generate a counter-rotating sheared vortex and a pair of vertical vortex tubes that extend from the ground and curl up into the wake. This pair of vortex tubes appears to be the source of the meandering structures reported in the literature. When the flow reaches a quasi-steady state, there is an asymmetric distorted flow for the smaller gap, whereas there is a nearly symmetric wake pattern for the larger gap.

Feedback-free microfluidic oscillator with impinging jets

Microscale pulsatile liquid flows which self-oscillate in laminar flow conditions and are based on facing impinging jets were studied both experimentally and using direct numerical simulations. The two facing jets first bifurcate in opposite directions and later approach one another, collide, and switch sides, with a regular periodicity. Experimentally, the evolution of the self-oscillation frequency was shown to be dependent on the jet separation distance and average velocity. Simulated velocity fields provide information on vortical flow structure evolution at the self-oscillation onset.

Numerical simulation of the downwash flow field and droplet movement from an unmanned helicopter for crop spraying

Unmanned helicopter-based spray systems are developing rapidly in Asia. However, droplet movement during aerial spraying remains poorly understood, owing to the complexity of downwash flow-structure development. Therefore, a computational fluid-dynamics method was used to investigate the downwash flow field of a full-scale unmanned agricultural helicopter (AF-25B; Copterworks) and the resultant movements of the spray droplets. A large-eddy simulation was performed and the lattice Boltzmann method was used to accurately capture the development of the rotor-tip vortex. To assess model performance, a hovering case with the height of 2.7 m was simulated and compared with previous field- and indoor-test results. The simulated instantaneous flow structures are qualitatively comparable with the indoor test results, supporting the reliability of the simulation method. Then, forward flight was simulated at heights of 1.5, 2.5, and 3.5 m above ground. In the forward-flight case, the flow speed on the right side was approximately 15% lower than that on the left side. The iso-surface of the vorticity was used to represent the asymmetric flow structures produced by the rotor. This flow structure was asymmetric, and the yaw angle, θ, was approximately 3–12° inclined to the right of the helicopter. When the application height was 1.5, 2.5, and 3.5 m, the angle of the edge of the flow structure, α, was about 105, 80, and 44°, respectively, and the entire flow structure width was 13, 9, and 6 m, respectively, and inversely proportional to the application heights. The temporal developments of the flow structures and droplet movements after the helicopter flew by were also simulated. The droplet expansion speed was found to have decreased when the application height increased. The droplet distribution was observed to become more asymmetric, with fewer droplets on the left side of the helicopter, when the application height was decreased. The coefficient of variation of deposition could, therefore, get reduced by the increase in the application height and cause the droplet deposit to be more uniform. However, the deposition quality may also get rapidly decreased as a consequence. A potential method to predict aerial-spray drift and deposition under complex conditions is presented in this study.

Interaction of metastable shock waves with perturbations

The shock wave metastable behavior in equilibrium media is detected in numerical experiments. This phenomenon is directly related to the ambiguous representation of a shock wave discontinuity when the shock belongs to the S-shaped fragment of the pressure-velocity Hugoniot. After interaction with weak perturbations the metastable shock wave recovers its original form and parameters (like stable shock). If the amplitude of the perturbations exceeds some critical value, the shock wave instability is developed. The instability is accompanied by formation of the transverse secondary waves switching local post-shock parameters between two Hugoniot segments separated from each other by the absolute instability region. The problem of the metastable shock wave interaction with local entropy perturbation passing through the shock front is studied in the framework of three-dimensional problem formulation. The shock wave transition to the limiting self-oscillating mode is described. Some features of this mode are completely determined by the shape of the Hugoniot and isentropes and does not depend on the shock wave structure. Namely, the shape of the shock front corrugations and flow structure evolution are analyzed. These features may be served as marker of the corresponding thermodynamic anomalies.

Analysis of flow characteristics downstream delta-winglet vortex generator using stereoscopic particle image velocimetry for laminar, transitional, and turbulent channel flow regimes

The evolution of flow structures downstream a single pair of delta-winglet vortex generators (VGs) is investigated experimentally using stereoscopic particle image velocimetry. In addition, the laser Doppler anemometer technique is performed to characterize the upstream flow. Experiments are conducted in a bounded channel flow (height H) for the Reynolds numbers (ReDH, based on the hydraulic diameter height) ranging from 400 to 12 000. The purpose of this study is to provide detail insight into the generation and the dissipation of longitudinal vortices over a wide flow regime range including the laminar–turbulent transition. With a focus on transverse sections, the flow field is detailed. For all flow regimes, the main flow topology shows that the two main counter-rotating vortices are generated at a certain streamwise distance downstream the VG and then are advected gradually toward the channel lateral-walls. A secondary vortex pair is induced closer to the wall. Our results show that close to the VGs, local regions (1 &amp;gt; z/H &amp;gt; −1) are strongly defined as the inception of the turbulence production. The intensity of this latter is shown to vanish beyond a certain distance far from the origin of the perturbation (when x/H is greater than 3). The instantaneous flow structure describes the mechanism of vortex generation, relying on the intermittence of the flow organization and the sweep and ejection event balance. Detailed analysis on the turbulence properties and wall shear stress has been assessed and revealed that the flow transition induced by the perturbation of the VG is achieved at a Reynolds number no greater than 1500.

Numerical studies on convective mass transfer augmentation in high-speed flows with lateral sweep vortex generator and dimple cavity

A numerical study on convective mass transfer enhancement by a lateral sweep vortex generator (LSVG)-dimple combination is presented. Surface features that can generate secondary flow structures are widely used for overcoming the difficulties in the transport process encountered in high-speed flows. Unsteady, three dimensional, turbulent fluid flow and mass transfer has been simulated using an Advection Upstream Splitting Method (AUSM) based finite volume solver. An improvised Temperature-Dependent Mass Efflux (TDME) condition is invoked to simulate mass evolution from a liquefying boundary as a function of adjoining fluid temperature. The computational procedure has been validated using experimental data of heat transfer coefficients obtained for a similar vortex generator. Extensive numerical simulations have been performed for various geometry and flow conditions to analyze the nature of the evolution of secondary flow structures and their role in promotion of mass advection. Q-Criterion estimates have been used for identifying regions of intense vortex fluid motion and its role in the promotion of mass transfer. The present vortex generator dimple combination can enhance mass transfer for a wider expanse of the domain. Correlations for the estimation of spatiotemporal convective mass transfer effects have also been derived from unsteady numerical computations.

Structure evolution at early stage of boundary-layer transition: simulation and experiment

The beginning of laminar–turbulent transition is usually associated with a wave-like disturbance, but its evolution and role in precipitating the development of other flow structures are not well understood from a structure-based view. Nonlinear parabolized stability equations (NPSE) were solved numerically to simulate the transition of K-regime, N-regime and O-regime. However, only the K-regime transition was examined experimentally using both hydrogen bubble visualization and time-resolved tomographic particle image velocimetry (tomo-PIV). Based on the ‘NPSE visualization’ and ‘tomographic visualization’, at least four common characteristics of the generic transition process were identified: (i) inflectional regions representing high-shear layers (HSL) that develop in vertical velocity profiles, accompanied by ejection–sweep behaviours; (ii) low-speed streak (LSS) patterns, manifested in horizontal timelines, that seem to consist of several three-dimensional (3-D) waves; (iii) a warped wave front (WWF) pattern, displaying multiple folding processes, which develops adjacent to the LSS in the near-wall region, prior to the appearance of 𝛬-vortices; (iv) a coherent 3-D wave front, similar to a soliton, in the upper boundary layer, accompanied by regions of depression along the flanks of the wave. It was determined that the amplification and lift-up of a 3-D wave causes the development of the HSL, WWF and multiple folding behaviour of material surfaces, that all contribute to the development of a 𝛬-vortex. The amplified 3-D wave is hypothesized as a soliton-like coherent structure. Based on our results, a path to transition is proposed, which hypothesizes the function of the WWF in boundary-layer transition.

Transient large-scale two-phase flow structures in a 3D bubble column reactor

This paper analyses the local and time-dependent behavior of large-scale structures responsible for liquid circulation in gas-water flow of a 3-D cylindrical bubble column of high aspect ratio with a multiple orifice for uniform aeration. The large-scale flow structures play an important role in the mixing and the mass transfer while coherent structures dominate hydrodynamic characteristics of the turbulent flow field. A three-dimensional Euler-Euler large eddy simulation (LES) model was used to calculate large-scale structures and their interaction with bubbles at inlet superficial gas velocities of UG= 6 and 8.4 cm/s where vortical-spiral and turbulent flow regimes occur. The two-phase model gives good agreements with experimental measurements. We use a conditional sampling procedure of liquid velocity and gas hold-up time series to identify and educe the development of coherent flow structures which consists in a pair of counter-rotating vortices convected in a staggered pattern along the column in both the vortical-spiral and central plume regions. On average, the detected instantaneous events for each template account for about 12–15% of the data recorded and may appear simultaneously. These events produce important fluctuations in the axial liquid velocity and gas void fraction. The sampling procedure yielded the averaged topology of the three-dimensional large-scale structures which was visualized using isosurfaces of the vorticity for different gas flow rates. The structures have spiral tube-shaped topology rotating along the column near the walls with a pair of counter-rotating cells sustained through the flow. This work provides deep insights into turbulent flow field in gas-liquid bubble column by LES and pattern recognition.

DDES analysis of unsteady flow evolution and pressure pulsation at off-design condition of a centrifugal pump

In the present paper, to investigate unsteady flow structures and pressure pulsations at off-design conditions, a low specific speed centrifugal pump is investigated by the DDES (Delayed Detached Eddy Simulation) method. Emphasis is laid on the unsteady evolution of flow structure and pressure pulsation at the stalled condition. Results show that the jet-wake flow pattern could be captured by the current numerical simulation method, and it shows a high calculation precision for the jet flow. Relative velocity decreases rapidly in the wake region towards the blade suction side. Significant inflection point exists between the jet and wake flow regions, and it almost keeps unchanged at the identical impeller-volute position under various flow rates. At the stalled status, the inflection point moves towards the blade pressure side with the blade passing the volute tongue, and evident non-uniform flow distributions are formed in the blade channels characterized by the multiple vortices. The blade channel is gradually blocked by the corresponding stall cell when the blade approaches the volute tongue, and it indicates that the volute tongue has a significant effect on the evolution process of the stall structure. By combination analysis of the unsteady evolution of the stall structure and pressure spectrum, the stall frequency at 0.25fn (being fn the impeller rotating frequency) is captured for the current model pump.

Time-resolved aerodynamic loads on high-speed trains during running on a tunnel–bridge–tunnel infrastructure under crosswind

A high-speed train will likely experience hazardous driving environments because of the transient effect of complex airflow condition when the train runs on the infrastructure scenario consisting of the tunnel–bridge–tunnel under crosswind. The evolution of aerodynamic loads and flow structure is investigated by numerical simulations. In these simulations, the arrangement of an infrastructural scenario refers to a real prototype under construction. Results indicate the flow structure and pressure distribution around a train change from a symmetrical state in the tunnel to a remarkable asymmetry state in the bridge within 0.1 s. This phenomenon causes a sharp aerodynamic shock on the train body. The coupling effect of the crosswind environment and the transformation of the train running scenario will likely affect the transient aerodynamic response. The maximum values of different aerodynamic loads exhibit time-resolved discrepancy when a train runs in ISTBT. The transient characteristics of the aerodynamic loads strengthen with the increase in wind speed but weaken with the increase in train speed. Such characteristics will likely deteriorate further if the wind direction is in the opposite direction relative to that of a moving train.

Time-Resolved Measurements of Turbulent Mixing in Shock-Driven Variable-Density Flows

Recent developments of burst-mode lasers and imaging systems have opened new realms of simultaneous diagnostics for velocity and density fields at a rate of 1 kHz–1 MHz. These enable the exploration of previously unimaginable shock-driven turbulent flow fields that are of significant importance to problems in high-energy density physics. The current work presents novel measurements using simultaneous measurements of velocity and scalar fields at 60 kHz to investigate Richtmyer-Meshkov instability (RMI) in a spatio-temporal approach. The evolution of scalar fields and the vorticity dynamics responsible for the same are shown, including the interaction of shock with the interface. This temporal information is used to validate two vorticity-deposition models commonly used for initiation of large scale simulations, and have been previously validated only via simulations or integral measures of circulation. Additionally, these measurements also enable tracking the evolution and mode merging of individual flow structures that were previously not possible owing to inherently random variations in the interface at the smallest scales. A temporal evolution of symmetric vortex merging and the induced mixing prevalent in these problems is presented, with implications for the vortex paradigms in accelerated inhomogenous flows.

Three-dimensional vortical structures and wall shear stress in a curved artery model

We numerically investigate spatial and temporal evolution of multiple three-dimensional vortex pairs in a curved artery model under a fully developed pulsatile inflow of a Newtonian blood-analog fluid. We discuss the connection along the axial direction between regions of organized vorticity observed at various cross sections of the model, extending previous two-dimensional analysis. We model a human artery with a simple, rigid 180° curved pipe with circular cross section and constant curvature, neglecting effects of taper, torsion, and elasticity. Numerical results are computed from a discontinuous high-order spectral element flow solver using the flux reconstruction scheme and compared to experimental results obtained using particle image velocimetry. The flow rate used in both the simulation and the experiment is physiological. Vortical structures resulting from secondary flow are observed in various cross sections of the curved pipe, in particular, during the deceleration phase of the physiological waveform. We provide side-by-side comparisons of the numerical and experimental velocity and vorticity fields during acceleration and deceleration, the latter during which multiple vortical structures of both Dean-type and Lyne-type coexist. Correlations and quantitative comparisons of the data at these cross sections are computed along with trajectories of Dean-type vortices. Comparing cross-sectional flow fields and vortices provides a means to validate wall shear stress values computed from these numerical simulations, since the evolution of interior flow structures is heavily dependent upon geometry curvature and inflow and boundary conditions.

Experimental Study of Solids Motion in an 18 m Gas–Solids Circulating Fluidized Bed with High Solids Flux

A comprehensive experimental study of solids motion is conducted in an 18-m high circulating fluidized bed with extremely high solids circulating rate (Gs) up to 1000 kg/(m2 s), which is never achieved in such a tall riser in previous researches. The axial distributions of particle velocity show exponential types and slight changes in the fully-developed zone with Gs higher than 400 kg/(m2 s). The radial profiles of particle velocity are the parabolic shapes and become steeper with increasing Gs. The development of gas-solids flow structure becomes slower with increasing Gs or decreasing Ug. When Gs reaches 800 kg/(m2 s), the full development is achieved until to the axial height higher than 12 m. Better correlations between the local particle velocity and solids holdup and less down-flowing particles indicate better reactor performance with the reducing solids back-mixing and uniform residence times at extremely high Gs.

Flow structure of bubbly to slug transition flow in a small pipe

To develop a mechanistic constitutive model to characterize the flow structure development in more than one flow regime, the detailed measurement of the flow structure is indispensable. In this study, the void fraction, interfacial area concentration, and bubble velocity profiles of upward air-water two-phase flow has been measured using the four-sensor conductivity probe in a 25.4 mm ID pipe. The test matrix has two features that distinguish it from previous studies, which is the void fraction-based flow condition selection and the wide range. The test matrix consists of 24 flow conditions with 3 different superficial liquid velocities (0.5, 1.0, 2.0 m/s) and 8 different void fractions (0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.45, 0.60). The results indicate that the flow characteristics and flow structures not only depend on the void fraction but also significantly depend on the superficial mixture velocity. The velocity effects on various flow characteristics are discussed such as void distribution, cap bubble generation, interfacial area. Besides, a velocity correlation has been developed to predict the bubble velocity radial distribution, with the mean absolute percentage error of 10.05% for the current data.