Arch-shaped Vortex Research Articles

Direct numerical simulation of 45° oblique flow past surface-mounted square cylinder

Comprehensive coherent structures around a surface-mounted low aspect ratio square cylinder in uniform flow with an oblique angle of $45^{\circ }$ were investigated for cylinder-width-based Reynolds numbers of 3000 and 10 000 by direct numerical simulation based on a topology-confined mesh refinement framework. High-resolution simulations and the critical-point concept were scrutinized to reveal for the first time the reasonable and compatible topologies of flow separation and complete near-wall structures, due to their extensive impact on various engineering applications. Large-scale horseshoe vortices are observed at two notable foci in the viscous sublayer. Within this layer, a wall-parallel jet is formed by downflow intruding into the bottom surface at the half-saddle point, then deflecting in the upstream direction and finally penetrating the bottom surface until the half-saddle point. A pair of conical vortices on the cylinder's top surface switch themselves on two sides of the square cylinder, where the switching frequency is identical with that of the sway of the side shear layer. The undulation of the Kelvin–Helmholtz instability is identified in the instantaneous development of a conical vortex and side shear layer, where the scaling of the ratio of the Kelvin–Helmholtz and von Kármán frequencies follows the power-law relation obtained by Lander et al. (J. Fluid Mech., vol. 849, 2018, pp. 1096–1119). Large-scale arch-shaped vortex is often detected in the intermediate wake region of a square cylinder, involving two interconnected portions, such as the leg portion separated from leeward surfaces and head portion rolled up from the top surface. The leg portion of the arch-shaped vortex was rooted by two foci near the bottom-surface plane.

Analysis of coherent structures over interacting barchan dunes based on tomographic particle image velocimetry

The influence of barchan dune interaction upon unsteady flow separation and wake dynamics around the fixed-bed downstream barchan dune (DBD) model are experimentally investigated at a Reynolds number of 2640 based on the tomographic particle image velocimetry (PIV) system. The time-averaged statistics including the mean velocities, recirculation area, vortex spatial topology, Reynolds stress, and turbulent kinetic energy were used to characterize the flow field and large-scale anisotropy. It was found that arch-shaped vortex “chains” with strong spanwise coherence shedding from isolated barchan crestline populate the whole wake region, while elongated rod-shaped vortex structures with strong streamwise coherence induced by the up-downwash flow around the DBD were found to fill the whole measurement range, which is closely related to “sheltering” effect on the incoming flow acting at DBD due to the presence of upstream barchan dune (UBD). Additionally, in order to study the complex dynamic features of these predominated vortex structure transformations, time-resolved planar particle image velocimetry was applied. This technique allows for providing complementary insights into the temporal behavior of the unsteady coherent flow structures populating the wake field in different experimental configurations. It was found that the basic unsteady flapping motion, vortex roll-up, and complex vortex interactions including vortex pairing, merging, and breaking up can all be analyzed by dividing into certain scales with ease in a combination wavelet and Lagrangian framework.

Flow Dynamics and Pollutant Transport at an Artificial Right-Angled Open-Channel Junction with a Deformed Bed

Artificial open-channel junctions have sharp corners and a smaller width-to-depth ratio. Despite numerous studies on channel junctions, knowledge on turbulent structures and their role in mixing at artificial junctions is lacking. To fill this research gap, the present study uses a large-eddy simulation (LES) model to investigate the three-dimensional (3D) turbulent structures and their role in pollutant transport at a right-angled laboratory open-channel junction with a deformed bed. The deformed-bed junction represents a quasi-equilibrium condition and consists of a scour zone and deposition bar. The numerical model is validated against experimental data of the velocity field and turbulent kinetic energy. Comparisons of the flow field between the flat-bed condition and quasi-equilibrium deformed-bed condition showed a reduced flow separation zone and less developed recirculating gyre in the latter case because of the strong secondary currents. The coherence of the turbulent structures was drastically disrupted at the deformed-bed junction because the Kelvin-Helmholtz (KH) instability results in more randomly oriented residuals. In contrast, the breakdown of the shear layer at a flat-bed junction displayed the trail of the arch-shaped vortices. The role of turbulent structures in pollutant transport is elucidated by using a neutrally buoyant conservative tracer. Large turbulent structures associated with the KH instability help the tracer evolve faster at the deformed-bed junction.

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.

Two-dimensional characterization of three-dimensional magnetic bubbles in Fe3Sn2 nanostructures.

We report differential phase contrast scanning transmission electron microscopy (TEM) of nanoscale magnetic objects in Kagome ferromagnet Fe3Sn2 nanostructures. This technique can directly detect the deflection angle of a focused electron beam, thus allowing clear identification of the real magnetic structures of two magnetic objects including three-ring and complex arch-shaped vortices in Fe3Sn2 by Lorentz-TEM imaging. Numerical calculations based on real material-specific parameters well reproduced the experimental results, showing that the magnetic objects can be attributed to integral magnetizations of two types of complex three-dimensional (3D) magnetic bubbles with depth-modulated spin twisting. Magnetic configurations obtained using the high-resolution TEM are generally considered as two-dimensional (2D) magnetic objects previously. Our results imply the importance of the integral magnetizations of underestimated 3D magnetic structures in 2D TEM magnetic characterizations.

Experimental investigation of turbulent wake flows in a helically wrapped rod bundle in presence of localized blockages

In nuclear sodium fast reactors, bundles of rods are tightly packed into a triangular lattice, enclosed in a hexagonal duct, and each pin is spirally wrapped with a thin wire. Flow blockages can potentially impact the local flow characteristics and heat transfer mechanisms in the bundle due to its small subchannel size. The effects of the blockage on the flow structures and heat transfer mechanisms are important aspects that require an accurate investigation. In this study, the flow-field characteristics in the vicinity of a blockage located in the exterior subchannel of rod bundles with helically wrapped wires were experimentally investigated. The velocity fields in the exterior subchannel were acquired by applying matched-index-of-refraction and time-resolved particle image velocimetry (TR-PIV) techniques for Reynolds numbers of Re1 = 4000 and Re2 = 17 000, i.e., equivalent to Rew1 = 19 600 and Rew2 = 83 200, respectively, based on the blockage width. The results from the TR-PIV measurements revealed an arch-shaped vortex with a large flow recirculation and a pair of counter-rotating vortices in the wake region downstream of the blockage, which is commonly observed in the wake flow of bluff bodies. The relative lateral distance and angle between the two vortices decreased when the Reynolds numbers increased. Profiles of maximum turbulence intensity along the shear layers illustrated the transition process including the growth, peak, and decay along the flow direction. From the spectral analysis of the turbulent velocities extracted at points along the shear layer, the Strouhal numbers (St) representing the vortex shedding frequency were found to be St = 0.25 and St = 0.56 for the left and right shear layers, respectively. Characteristics of shear layers generated by the blockage in the exterior subchannel were investigated via the two-point cross correlation of fluctuating velocities. The spatiotemporal cross correlations of turbulent velocities, computed at points in the region where the left shear layer exhibited rolling effects and vortex breakdowns, were considerably wider and longer. The convection velocity Uc was estimated to be ∼0.82Um to 0.93Um. Proper orthogonal decomposition (POD) analysis was applied to the instantaneous velocity fields to extract the statistically dominant flow structures. It was found that POD modes 2–3 and 4–5 formed the pair modes when the corresponding POD temporal coefficients depicted sinusoidal shapes and exhibited nearly circular orbits in the phase space. Spectral analysis of the POD temporal coefficients confirmed the vortex shedding frequencies detected in the analysis of turbulent velocities.

Effect of lattice structures on heat transfer deterioration of supercritical CO2 in rectangle channels

In view of the heat transfer deterioration of supercritical CO2 under the condition of low mass flow rate and high heat flux, the influence of Body-Centred Cubic (BCC) lattice, Kagome lattice and Pin-fin structure on the heat transfer of supercritical CO2 in rectangular channels were investigated in this article. The results showed that all the three structures can generate arch-shaped vortices near the wall, which increases the wall shear stress and the fluid turbulence intensity, promotes the full mixing of the mainstream and the near-wall fluid, and weakens the heat transfer deterioration. In addition, there was obvious longitudinal vortices distributed between adjacent lattices, which increases the turbulence intensity of the mainstream, facilitates the mixing of high and low speed fluids, so as to improve the heat transfer rate of supercritical CO2 channel and reduce the heat transfer deterioration. In the vertical upward channel, the buoyancy reduces the turbulence intensity of the near-wall fluid and the shear stress of wall, and results in the heat transfer deterioration. In the horizontal channel, the buoyancy leads high-density fluid to settle, thus enhances the turbulence intensity of the fluid near the down-wall surface, and intensifies the heat transfer of the down-wall surface.

Numerical simulation of turbulent thermal boundary layer and generation mechanisms of hairpin vortex

Three-dimensional flow fields are computed along a ribbed flat-plate by direct numerical simulation. Inlet Reynolds number defined by the rib height is at Re=1200 and Mach number is at Ma=0.2. The detailed bypass transition process and its generation mechanisms are discussed and analyzed. Results present the relatively larger skin-friction coefficients in the bypass transition region. Moreover, the Reynolds stresses and turbulent kinetic energy are stronger in the bypass transition region, giving rise to a larger Nusselt number distribution. Various large-scale vortices are found downstream, where the momentum thickness Reynolds number is in the range of 250<Reθ<438. These large-scale vortices include the streamwise vortices, arch-shaped vortices, hairpin vortices, etc. The hairpin vortex can be evolved from a spanwise vortex. Besides, younger hairpin vortex can be generated from a primary hairpin vortex, a phenomenon captured by the current simulation. More importantly, the present research suggests a new mechanism that hairpin vortex can be evolved from a pair of separated arch-shaped vortices. The study presents the convincing proofs for this particular generation mechanism based on the third-generation vortex identification method. Furthermore, the dynamic evolution process is quantified by the spanwise component of rotational strength. The inherent mechanisms of this phenomenon are analyzed based on the Biot-Savart law. In addition, the strongly-coupled correlations between the velocity and temperature streaks are discovered and quantified for the first time.

End-wall heat transfer of a rectangular bluff body at different heights: Temperature-sensitive paint measurement and computational fluid dynamics

Complementary methods — temperature sensitive paint measurement and computational fluid dynamics — are used to elucidate the end-wall heat transfer of a rectangular body in a turbulent channel flow. Four systems with bluff body-height-to-channel-height ratio Hb/H=0.25, 0.5, 0.75, and 1.0 are compared. For Hb/H=1.0, an extremely high heat transfer rate is observed in the upstream area, attributing to the highly unstable corner vortex; the absence of pair vortices entails a distinctly different flow pattern and heat transfer in the downstream region. For Hb/H=0.75, the presence of the paired vortices together with Karman vortex shedding leads to the arch-shaped vortex and heat transfer augmentation. Ongoing from Hb/H=0.75 to 0.25, the growth of the arch vortex is highly weakened. At Hb/H=0.25, the heat transfer measurement reflects the absence of the Karman vortex in the wake region. Augmentation of the convective heat transfer pertinent to the shear layer’s reattachment is shown to be sensitive to Hb/H. As Hb/H decreases, the reduced reattachment length results in attenuated heat transfer in the wake region; the strength of the horseshoe vortex structure, characterized by the heat transfer rate, is also progressively weakened.

Volumetric Velocity Measurements in the Wake of a Hemispherical Roughness Element

Plenoptic particle image velocimetry was used to perform instantaneous three-dimensional velocity measurements in the near wake of a wall-mounted hemispherical roughness element at a Reynolds number (based on roughness height) of and boundary layer to roughness height ratio of 2.4. The experiment was performed in a refractive index matched flow facility to mitigate laser reflections from the hemispherical surface. Data gathered from this experiment represented one of the first applications of plenoptic particle image velocimetry. The ensemble-averaged flow is characterized by a separated shear layer and a symmetric recirculation region. In the instantaneous three-dimensional velocity fields, a separated shear layer and recirculation region, both with asymmetric characteristics, are present. Additionally, arch vortices are found that are detached to the hemispherical surface. The proper orthogonal decomposition was applied to both the three-dimensional velocity and three-dimensional vorticity fields with the goal of identifying the structures created by the hemisphere. The velocity modes showed features associated with the overall flow, whereas the vorticity modes were largely associated with the near wake. The most energetic proper orthogonal decomposition modes showed fluctuations in the boundary layer and recirculation region, as well as suggested the existence of shed arch-shaped vortices.

Unstable flow structure around partially buried objects on a simulated river bed

The unsteady flow characteristics around two partially buried objects, a short cylinder and a truncated cone, were examined with a three-dimensional, non-hydrostatic hydrodynamic model under similar steady unidirectional currents with flow Reynolds numbers, Re, of 86,061 and 76,209, respectively. Model simulations were conducted with the two objects partially buried in a simulated rippled river bed. A Reynolds-averaged Navier–Stokes (RANS) equation model coupled with a κ-ε turbulence closure was used to validate the experimental velocity measurements. A large eddy simulation (LES) turbulence model was subsequently used to characterize the unsteady flow structure around the objects. The LES closure allowed for the characterization of highly unsteady coherent turbulent structures such as the horse-shoe vortex, the arch-shaped vortex, as well as vortex shedding in the wake of the object.

Vortex development on pitching plates with lunate and truncate planforms

AbstractThe three-dimensional flow field and instantaneous forces are measured on pitching rectangular, lunate and truncate planforms of aspect-ratio four. The leading-edge vortex on the rectangular planform is compressed as it grows, and subsequently forms an arch-shaped vortex. For the lunate and truncate planforms, which both have identical spanwise leading-edge curvature but differ in planform area, outboard-directed convection of vorticity, rather than vortex stretching, mitigates arch-vortex formation. The vortical near wake that is formed by the planforms with spanwise leading-edge curvature is found to be strongly correlated with a favourable lift-to-drag ratio during the force-relaxation phase.

A Study on Behaviors of Separated Flow under the Adverse Pressure Gradient (2nd Report, Evaluation of Three-Dimensional Unsteady Flow Structures with Cross-Correlation Coefficient)

Three-dimensional measurements of cross-correlation coefficient of velocity fluctuations are carried out to investigate flow behaviors of two-dimensional separated flows over a flat plate under an adverse pressure gradient. The velocity fluctuations are obtained with two I-type hotwire probes, one of which is fixed near the flat plate and the other traverses the flow region. The results show that regions with positive and negative cross-correlation distribute checkerwise in a vertical section parallel to the main flow and three-dimensionality of the region with positive correlation near the flat plate grows as the region flows down, whereas spanwise two-dimensionality of the negative region above it holds. In addition, time sequences of the correlation distributions tell that advection velocities of the separated flow structures are about 0.3 times as large as the inflow velocity. Simulations with simplified potential vortices are also made to specify the three-dimentional flow structure of the separation. They illustrate that alternate rows of arch-shaped vortices formed immediately downstream of the separation can reproduce well the characteristics of the experimental results.

Contribution of vortex structures and flow separation to local and overall pressure and heat transfer characteristics in an ultralightweight lattice material

Ultralightweight lattice-frame materials (LFMs) with open, periodic microstructures are attractive multifunctional systems that can perform structural, thermal, actuation, power storage and other functions [A.G. Evans, J.W. Hutchinson, M.F. Ashby, Multifunctionality of cellular metal systems, Prog. Mater. Sci. 43 (1999) 171–221]. This paper presents experimental and numerical studies of local fluid flow behaviour and its contribution to local and overall pressure and heat transfer characteristics of such a lattice material with tetrahedral unit cells. A single layer of the LFM with porosity of 0.938 is sandwiched between impermeable endwalls that receive uniform heat flux and the heat transfer is subjected to forced air convection. Experimental measurements with particle image velocity (PIV) and thermochromic liquid crystal (TLC), backed by computational fluid mechanics (CFD) simulations, revealed two dominant local flow features in the LFM. Distinctive vortex structures near the vertices where the LFM meets the endwalls and flow separation on the surface of LFM struts were observed. The vortex structures formed around the vertices include horseshoe vortices and arch-shaped vortices. The horseshoe vortex increases local heat transfer on the endwall region up to 180% more than that in regions where the least influence of the horseshoe vortex is present. The arch-shaped vortex behind the vertices creates regions of flow recirculation and reattachment, leading to relatively high heat transfer. The location of flow separation along the struts varies with the spanwise position due to the presence of vertices (or endwalls). The regions on the strut surface before flow separation contribute approximately 40% of the total heat transfer in the LFM. The delay of the flow separation leads to an increase in the overall heat transfer. Comparisons with foams and other heat dissipation media such as packed beds, louvered fins and microtruss materials suggest that the LFMs compete favourably with the best available heat dissipation media.

Heat transfer on the base surface of threedimensional protruding elements

Five basic geometries (cylinder, cube, diamond, pyramid and hemisphere) are comparatively examined at a Reynolds number of 1.7 × 10 4 to determine the effect of a single roughness element on surface heat (mass) transfer. Local mass transfer measurements obtained using the naphthalene sublimation technique are substantiated by flow visualization. Results show that the upstream horseshoe vortex system and the inverted arch-shaped vortex immediately behind the elements are the dominating effects in elementendwall interaction. Based on information concerning the extent of spanwise influence of each element, an inter-element spacing may be proposed for array configurations to achieve optimal heat transfer performance.

Effect of Flow Angle-of-Attack on the Local Heat/Mass Transfer From a Wall-Mounted Cube

An experimental study of the local mass transfer over the entire surface of a wall-mounted cube is performed with a particular emphasis on the effects of flow angles-of-attack (0 deg ≤ α ≤ 45 deg). Invoking an analogy between heat transfer and mass transfer, the presently obtained mass transfer results can be transformed into their heat transfer counterparts. Reynolds number based on the cube height and mean free-stream velocity varies between 3.1 × 104 and 1.1 × 105. To substantiate the mass transfer results, streakline patterns are visualized on the cube surfaces as well as the endwall using the oil-graphite technique. Significantly different flow regimes and local mass transfer characteristics are identified as the angle-of-attack varies. The overall convective transport is dominated by three-dimensional flow separation that includes multiple horseshoe vortex systems and an arch-shaped vortex wrapping around the rear portion of the cube. In addition to the local study, power correlations between the surface-resolved mass transfer Sherwood number and the Reynolds number are presented for all α values studied. Mass transfer averaged over the entire cube is compared with that of its two-dimensional counterpart with crossflow around a tall prism.

Promotion of Sediment Suspension Utilizing Coherent Vortices around Obstacle

The promotion of sediment suspension in a laboratory flume is discussed. Due to strong upward flow behind a spherical obstacle fixed at the bottom, bed materials are transported as suspended load. Horse-shoe vortices in front of the obstacle generate scour hole around it, and arch-shaped vortices behind the obstacle produce ripples in its near downstream area. Sand particles are entrained by the coherent vortices even after the scour hole has developed, which is detected by the distributions of suspended sediment. The obstacle placed in front of a weir can increase the amount of the suspended sediment passing over the weir.

Smoke Observation on Boundary Layer Transition Caused by a Spherical Roughness Element

Observations were made in the boundary layer along a flat plate in a wind tunnel. A sphere of various diameters was placed on the plate as an isolated roughness element and the resulting flow patterns were examined by the smoke emitted from various heights and positions relative to the sphere. The patterns were stereographically photographed from upper and lateral sides, in order to make clear the mechanism of the change of patterns with varying velocities, namely Reynolds numbers. At low velocities, peculiar vortex filaments are formed. With increasing velocity, they begin to be deformed periodically and a characteristic row of arch-shaped vortices is formed. As the velocity is further increased, the wedge-shaped turbulent region appears downstream and gradually approaches the sphere, encroaching upon the laminer part. Vortex filaments parallel to the direction of the main stream appear with a constant spacing with their heads along the sides of the wedge. These observations were compared with measurements by hot-wire and with the results of visualization reported by hot-wire and with the results of visualization reported by other investigators.