Three-dimensional Flow Structures Research Articles

Dynamics of three-dimensional vortical structures behind a barchan dune based on tomographic particle image velocimetry

In this study, tomographic particle image velocimetry (PIV) was used to measure three-dimensional (3D) flow structures behind a fixed-bed barchan dune model at a Reynolds number of 2528 in a circulation water tunnel. The topological evolution of the 3D instantaneous vortex structures and their dynamic characteristics in the dune wake were analyzed. 3D instantaneous arch-shaped vortex “chains” shedding from the barchan and typical quasi-streamwise vortex structures induced by the development of an internal boundary layer were found to be located both before and after the reattachment region, which differs a little from the results of previous studies. Nevertheless, both conditionally averaged and typical instantaneous 3D flow fields revealed that the arch-shaped vortex system dominates the barchan dune wake, and the inclination angle of these spanwise-oriented structures was found to change as it propagates downstream. Additionally, the dynamic characteristics of the dune wake were found to be similar to a flapping phenomenon such as oscillation of the recirculation in a backward-facing step flow. To provide complementary insight into this phenomenon, the separated and reattached shear flow were also examined using two-dimensional time-resolved planar PIV. It was found that the basic unsteady flapping motion, vortex roll-up, and complex vortex interactions can all be analyzed with ease in a Lagrangian framework.

THE DECORATION FORM OF CERAMIC REVOLVING CORE BOTTLES IN THE QING DYNASTY

Chinese ceramic art in the Qing Dynasty, it has won a lot of praise for its unique shape and exquisite, wonderful decoration form. The ceramic revolving core bottle（CRCB） has an obvious unique decorative form. From the perspective of formal analysis, through the concrete analysis of the decoration form of the CRCB, this paper puts forward three characteristics of the CRCB in the Qing Dynasty: rich color expression, three-dimensional space expression, flow and continuous picture structure. It is believed that the CRCB is a unique decorative form based on the ceramic decoration form of the past dynasties in China, which integrates the formal elements in Chinese classical gardens and Chinese paintings. The author holds that the performance of this form reflects the authority of the ruler, the popularity of Chinese classical gardens in the Qing Dynasty, and the revival and re-creation of Chinese classical painting forms in the Qing Dynasty, which is the witness of the exchange between Chinese ceramic art and Chinese traditional culture.

Contactless generation and trapping of hydrodynamic knots in sessile droplets by acoustic screw dislocations

Helicity is an important quantity in fluid mechanics that indicates the presence of linked or knotted hydrodynamic vortex filaments. Such flow structures are not only promising elementary structures to study mass and momentum transfer in turbulent flows but also potent analogs for other topological problems arising in particle physics, liquid crystals, and plasma physics. However, experimental studies of knots and links are highly challenging due to the limited control over helicity generation and difficult observation of the resulting fast-paced multiscale flow evolution. In this paper, we propose using acoustic streaming to link hydrodynamic filaments in fluids. The method is contactless, almost instantaneous, and relatively insensitive to viscosity. Importantly, it allows starting from quite arbitrary three-dimensional flow structures without relying on external boundary conditions. We demonstrate our approach by using an acoustic screw dislocation to link two hydrodynamic vortex filaments in a sessile droplet. We observe an inversion of the flow chirality (measured by the hydrodynamic helicity) as the topological charge of the screw dislocation is increased. Combined with recent progress in acoustic field synthesis, this work opens a window to study more complex hydrodynamic knots and links topology at a broader range of space and time scales.

Turbulence characteristics in a weir-orifice-slot combined fishway with an identical layout

A combined fishway containing notched weir, central orifice and vertical slot with an identical layout was designed, and its turbulence characteristics were experimentally studied. An acoustic Doppler velocimetry (ADV) was used to measure three-dimensional velocity. Flow regime, average velocity, turbulence intensity, Reynolds stress, power frequency-spectrum, correlation function, turbulence scale were analysed. The experimental results showed the combined fishway was of remarkable three-dimensional flow structures. Longitudinal velocity exhibited obvious extreme value region on the horizontal plane. Extreme values of longitudinal turbulence intensity were concentrated mainly on the mixing region between weir flow and orifice jet, as well as the vertical slot wall jet region, extreme value region of transverse turbulence intensity occurred on the central plane of orifice jet, and extreme values of vertical turbulence intensity were scattered. Reynolds stress extreme value occurred mainly in the convergent zone of multiple jets. Dominant frequency of power frequency-spectrum was highest for notched weir and lowest for vertical slot. Longitudinal velocity fluctuation exhibited a correlation with time, which was characterised by dominant frequency. Turbulence scales showed eddy structures were related to flow zones.

Fluid structure interaction of a subaqueous pendulum: Analyzing the effect of wake correction via large eddy simulations

The dynamic behavior of a subaqueous cylindrical pendulum and corresponding flow dynamics are investigated. The objectives were twofold: (i) to examine whether the two-dimensional model equations sufficiently capture the three-dimensional dynamics and (ii) to investigate the emerging three-dimensional vortical flow structures. Large eddy simulations with two-way coupling fluid structure interaction were carried out using the immersed boundary method to simulate the motion of the pendulum and its interactions with the initially stagnant water. The resulting pendulum motion is compared against measured data obtained in a series of experimental tests to validate the simulation results and the model equations with and without wake corrections. An analysis of the flow vorticity revealed the development of a vortex ring during the first swing and the formation of tip vortices. The evolution of the vortex rings emerging from the motion of the subaqueous cylindrical pendulum was visualized using Q-criteria showing a reasonable agreement with vortical structures observed in the experiment using particle imaging velocimetry. The hydrodynamic moments acting on the simulated pendulum and the moments calculated from the model equations are analyzed. Using the insights from these numerical simulations, a modification of the wake correction is proposed to enhance the accuracy of the rate of decay and period. The transient effect of coherent flow on pendulum dynamics, especially the added mass effect, is discussed.

Pitch/Plunge Equivalence for Dynamic Stall of Swept Finite Wings

A high-fidelity numerical study is undertaken to expand the notion of pitch/plunge equivalence to finite, swept wings undergoing deep-dynamic stall and builds upon the analysis of straight wings in another paper by the authors [“Pitch/Plunge Equivalence for Dynamic Stall of Unswept Finite Wings,” AIAA Journal (submitted for publication)]. The wing has an aspect ratio of with a NACA 0012 section, a rounded tip, and a sweep angle of ; it operates at a Reynolds number of and a Mach number of . Equivalent pure-pitch and pure-plunge motions at two different reduced frequencies are considered to assess pitch-induced apparent camber on the wing and its effects on the three-dimensional flow structure, surface topology, and aerodynamic loading. Regardless of the motion type or rate, the wing undergoes dynamic tip stall that is initiated through the outboard separation of a dynamic stall vortex as it lifts off the surface and forms into an arch vortex before ejecting into the wake. The pitching cases exhibit an advanced development and evolution in time of these features that are observed as variable amplifications of the aerodynamic loading, which is due predominantly to an apparent camber effect. Corrections to the total and sectional lift and moment coefficients are explored by extending steady thin-airfoil theory to pitching swept wings. Remarkably, the corrected plunge-equivalent histories provide a full collapse of the force and moment responses between the pitching and plunging cases, despite the delayed behavior of the flow structure. This equivalence and the similar process of dynamic stall should facilitate the comparison of experiments and/or simulations employing different motion types, even for finite-aspect-ratio and swept wings.

Pitch/Plunge Equivalence for Dynamic Stall of Unswept Finite Wings

Pitch/plunge equivalence for deep dynamic stall of a straight finite wing is considered by employing large-eddy simulations. The flowfields are computed with a well-established time-implicit high-fidelity large-eddy simulation approach. The wing has a moderate aspect ratio of and a NACA 0012 section incorporating a rounded tip. The flow parameters are a freestream Mach number of and chord Reynolds numbers of . Pure pitching and equivalent plunging maneuvers are considered with a reduced frequency of and minimum and maximum effective angles of attack of 4 and 22 deg, respectively. The pitch and plunge dynamic stall cases are compared in detail in terms of the unsteady three-dimensional flow structure, surface pressure, and loading. Differences in the lift and moment coefficient are shown to be correctable, employing a steady thin-airfoil theory for a cambered airfoil that accounts for the rotation-induced apparent camber present in the pitching case. This pitch/plunge equivalence is extendable to the dynamic stall of a swept wing, as demonstrated in another paper by the authors [“Pitch–Plunge Equivalence for Dynamic Stall of Swept Finite Wings,” AIAA Journal (submitted for publication)].

Numerical investigation on the endwall fences with varying geometrical parameters on secondary flow control

This study investigates the effect of the fences with varying height and chord length on the secondary flow reduction. Three-dimensional flow structure on the endwall and streamline on the blade surface are analyzed to determine the effect of fence geometry on the secondary flow reduction. The results demonstrate that the pressure leg of the horseshoe vortex induced by the fences can prevent the passage vortex of the main blade. In this work, a fence geometry is found to reduce the secondary flow loss and decrease the total pressure loss coefficient of the main blade by about 4%. However, the fences with large chord length are found to bring a greater additional flow loss, which is induced by the flow separation of the fences. Thus, the fences could be applied only when the fences induced additional loss is lower than the loss due to the secondary flow that they prevent.

Experimental Investigation of the Near-Surface Flow Dynamics in Downburst-like Impinging Jets Immersed in ABL-like Winds

Downburst winds are strong downdrafts of cold air that embed into the atmospheric boundary layer (ABL) and produce intense horizontal outflow upon impingement on the ground. They are highly transient and three-dimensional extreme wind phenomena with a limited spatiotemporal structure that often makes the anemometric measurements in nature inadequate for reconstructing their complex flow fields. In the framework of the project THUNDERR, an experimental campaign on downburst outflows has been carried out at the WindEEE Dome at Western University, Canada. The present study analyzes the three-dimensional interaction between downburst (DB) outflows produced as large-scale impinging jets and ABL winds. Most experimental, numerical and analytical models in the literature neglect this flow interplay or treat it in an oversimplistic manner through a vector superposition. We found that the generated near-surface outflow is asymmetric, and a high-intensity wind zone develops at the interface between DB and ABL winds. The time variability of the leading edge of the outflow was investigated by synchronizing all wind measurements across the testing chamber. The three-dimensional flow structure was studied using a refined grid of Cobra probes that sampled the flow at high frequencies. The passage of the primary vortex produced a significant decrease in the height of maximum radial wind speed, predominantly in the ABL-streamwise direction. The turbulence intensity was the highest in the region where DB propagates into oppositely directed ABL winds.

Numerical Simulation of Thermocapillary Convection in a Half-Zone Liquid Bridge Model with Large Aspect Ratio under Microgravity

The coupled momenta induced by thermal effects near interfaces cause complex three-dimensional flow structures, called thermocapillary flow or Marangoni convection. Thermocapillary convection is crucial for crystal growth quality, and the mainstream method used to study thermocapillary convection is the half-zone liquid bridge model. This paper designs a gas–liquid two-phase system and reports the numerical results on the instability and associated roll structures of thermocapillary convection in half-zone liquid bridge under microgravity environment. The gas and liquid transferred momentum and energy through the free surface. The geometry of interest is high aspect ratio (AR) silicone oil suspended between coaxial disks heated differentially. It was found that with the increase in AR, the vortex of thermocapillary convection gradually moves to the upper disk at the steady state. In the range of 2 < AR < 2.5, the vortex cell splits from 1 to 2, and the distance between the vortex center increases with the increase in AR. The flow field after the onset of instability exhibits a traveling wave with wave number m = 1 when AR ≤ 3 and exhibits a standing wave with wave number m = 1 when AR ≥ 3.5.

Three-dimensional flow structures and heat transfer characteristics of compressible flow over a cylindrical cavity

High-speed flows over a cavity produce remarkably local heat augments. Heat transfer characteristics of compressible flow over a cylindrical cavity were revealed using wind tunnel experiments and computational analysis. Heating augmentation induced by cylindrical cavities is significant near the trailing edge of the cavity and distributed in a finite width along the spanwise direction. The heating augmentation of the cylindrical cavity decreases with the decrease in the ratio of cavity diameter (D) to cavity depth (H). Relationships between heat transfer and flow structures of the cylindrical cavity are observed. 3D geometric features of the cylindrical cavity lead to the formation of slant recirculating eddy and the existence of symmetric open flow regions in both spanwise sides of the closed cavity.

Three-Dimensional Numerical Simulation of a Flat Plate Perpendicularly Submitted to Current With a Blockage Ratio of 0.214: URANS and Detached Eddy Simulation

Abstract The uniform flow over a nominally two-dimensional normal thin flat plate with blockage ratio 0.214 was numerically investigated in three dimensions by three methods: unsteady Reynolds-averaged Navier–Stokes (URANS) based on the realizable k–epsilon (RKE) turbulence model, URANS based on the k–omega shear stress transport (SST) turbulence model, and detached eddy simulation (DES). The Reynolds number based on the inlet flow velocity and the chord width of the plate was 117,000. A comprehensive comparison against earlier experimental results showed that URANS-SST method only could give a correct Strouhal number but overestimated the mean base pressure distribution and mean drag coefficient, while URANS-RKE and DES methods succeeded in giving accurate predictions of all. Moreover, by comparing the instantaneous vorticity contours and three-dimensional (3D) turbulent flow structures, it is found that DES is better suited for the present case because it can capture irregular small-scale structures and reproduce the three-dimensionality and low-frequency unsteadiness of the vortex shedding. Finally, through the volume-of-fluid (VOF)-based simulation of the free surface, it is demonstrated that the free surface has no significant effect on the mean drag coefficient and Strouhal number.

Evaluation of Pool-Type Fish Passage with Labyrinth Weirs

Due to the construction of dams and sluices, habitat fragmentation of riverine species is an issue of concern in hydropower projects. An artificial fishway aims to restore flow connectivity and facilitate migration. As an alternative to the conventional layout with straight walls, labyrinth layouts are introduced, consisting of either triangular, trapezoidal, or rectangular weirs. Three-dimensional CFD simulations are performed to examine their influences on the fishway hydraulics. The labyrinth layouts exhibit an appreciable three-dimensional flow structure in a pool, featuring flow fluctuations in both cross- and along-channel directions, which is indicative of the potential for multiple species with distinct swimming preferences. The streamwise reduction in mean flow velocity is correlated with the Froude number, with the labyrinth weirs leading to a somewhat lower reduction than the conventional one. The dimensionless pool water depth declines with specific flow discharge. The turbulence kinetic energy is the lowest in the triangular layout and the vorticity in the conventional one. Among the labyrinth layouts, the differences in volumetric power dissipation are small. An equation is developed for dissipation estimation. All the labyrinth layouts generate higher energy loss than the conventional one. With optional weir configurations, the findings provide reference for engineering design.

Prediction of river discharges at confluences based on Entropy theory and surface-velocity measurements

Hydrodynamic features of the confluence zone of large rivers are complicated because of their three-dimensional flow structure. The confluence between the Rio Negro and the Rio Solimões, characterised by black and white waters, respectively, ranks among the largest river junctions on Earth. An Entropy-based investigation was carried out to assess the discharge and analyse the 2D structure of velocity distribution for large river flows relying on monitoring of near-surface velocity only. The estimated flow data where compared with in-situ ADCP data gathered across some transects of the Negro and Solimões rivers during both low and relatively high flow conditions. Results are illustrated through some transects at the confluence zone, upstream and downstream of the zone and in the Careiro Channel. Comparisons highlight that the Entropy-based flow velocity in terms of depth-averaged velocity, cross-sectional mean flow velocity and vertical velocity distributions, starting from the measured surface velocity, is in agreement with those determined by the ADCP measurements, with an error in mean flow velocity and discharge lower than 15%. The research highlights the potential of the Entropy method to estimate the discharge and velocity in very large rivers just relying on near-surface velocities monitoring.

Numerical analysis of the flow in a cylindrical container injected air from a slit nozzle

This paper describes the flow characteristics of air injected from a slit nozzle in a cylindrical container. The numerical simulation was performed by the large-eddy simulation (LES) using ANSYS Fluent 19.0 based on the finite-volume method. The non-dimensional nozzle offset distance was set to yn/D = 0.4, where D is the diameter of the cylindrical container. The Reynolds number of the plane jet flow was settled at 2,000. The velocity profiles by the numerical simulation agreed with the experimental results. The jet reached near the bottom of container, and the jet flowed out from the left side of container. Although the flow in the cylindrical container showed a three-dimensional turbulent flow structure, the mean velocity field was generated symmetrically with respect to the central cross-section of the container perpendicular to the slit nozzle. The large recirculating flow around the z-axis occurred near the central cross-section of the container by the jet. In addition, the weak recirculated flows around the y-axis were observed near the upper side wall in the container.

Numerical model and hydrodynamic performance of tuna finlets

Finlets, a series of small individual triangular fins located along the dorsal and ventral midlines of the body, are remarkable specializations of tuna and other scombrid fishes capable of high-speed swimming. In this study, a symmetric model containing nine finlets of tuna is proposed to overcome the limitation of measurement without losing authenticity. Hydrodynamic performance along with three-dimensional flow structures obtained by direct numerical simulation are demonstrated to disclose the underlying hydrodynamics mechanism of finlets. Complex interactions of leading-edge vortices (LEVs), trialing-edge vortices (TEVs), tip vortices (TVs) and root vortices (RVs) are observed from the three-dimensional vortical structures around the finlets. Two more cases consisting of the 3rd to 9th (without the first two) and the 3rd to 7th (without the first two and the last two) finlets are also simulated to examine the effects of the first two and the last two finlets.

Numerical investigation of atmospherelike flows in a spherical geometry.

Buoyancy-driven convection flows play a crucial role in global heat and momentum transport in atmosphere. Simplified planetary and stellar atmospheres can be described by a spherical gap geometry with special boundary conditions for the temperature. In the spherical gap, the dielectrophoretic effect is used to synthesize the radial gravity field. Lateral thermal boundary conditions are used to model solar radiation at the equator and at the poles. The temperature reaches a maximum value at the equator and becomes colder near the poles. In the case of a rotating gap, the influence of the Coriolis and centrifugal forces are taken into account. Different regimes of the two-dimensional steady basic flow are discussed in dependence on the Taylor number and Rayleigh number and for the radii ratio η=R_{in}/R_{out}, where R_{in},R_{out} are the radii of the inner and outer surfaces, respectively. Linear instability theory is used to study when the basic flow becomes unstable. The critical Rayleigh number at which the steady axisymmetric basic flow becomes time-dependently axisymmetric or three dimensional is found to be a function of the Taylor number. Furthermore, the critical azimuthal wave number m_{c}, which is responsible for the structure of the supercritical three-dimensional flow, and the critical frequency of the perturbation ω_{c} were found. The spatial location of the perturbation helps to understand the origin of the instability.

Flow and sediment transport in a sharp river bend using a 3D-RANS model

Flow dynamics and sediment transport in a river bend have recently been studied using experimental and numerical investigations. A three-dimensional numerical modeling model named NaysCUBE was used in this study to describe the flow pattern and process of sediment transport in a sharp river bend as a complement to the prior work of the physical hydraulic model. The model uses the RANS equation to simulate flow where a fully complex 3D flow is governed. Despite the limitations of the RANS model, NaysCUBE well reproduces the flow pattern and turbulence phenomena in a movable bed channel with sharp curvature. Compared with data from a prior experiment, the morphological adjustment is simulated sufficiently. The three-dimensional flow structures are useful for determining the appropriate countermeasures for local scouring and riverbank protection.

Investigation on steady regimes in a X-shaped micromixer fed with water and ethanol

Microreactors are very attractive for intensifying chemical and biochemical reactions, as they enable a large heat transfer with high control of the operating conditions. These features allow improving the achievable reaction yield and selectivity. However, the mixing of reactants should be ensured, especially in simple designs, as the flow regime is laminar. This work aims at characterizing the flow regimes and degree of mixing in a X-shaped micromixer with two opposite inlets and at investigating how these regimes are affected by fluid properties. To this purpose we carry out numerical simulations of two different mixtures, i.e. Water-Water (W-W) and Water-Ethanol (W-E), for Reynolds numbers Re⩽200, to gain insight into the complex three-dimensional flow structures stemming from the impinging jets at the confluence. Besides, experimental flow visualizations from the W-W case provide a validation of the numerical model. The X-shaped reactor is observed to enable a superior mixing performance compared to T-, Y-, and Arrow-shaped microreactors, which are extensively employed.

Measurement of velocity in swirling flows of liquid metals in the presence of a magnetic field

Studies of the dynamics of a round submerged jet of liquid metal in a transverse magnetic field. The experiments were performed in the range of Reynolds numbers up to 1000 and Hartmann numbers up to 1300 using a probe technique for measuring velocity using potential sensors. Detailed three-dimensional flow structures are visualized using a direct numerical simulation method independent of the experiment. The paper presents data on the averaged velocity field and statistical characteristics of the flow, reproducing the complex nature of a substantially unsteady flow, in which zones of reverse fluid flows are detected and a significant heterogeneity of the velocity field is observed.