Concentrated Vortex Research Articles

Secondary flow control using endwall jet fence in a high-speed compressor cascade

This paper proposes a secondary flow control concept using Endwall jet fence (EJF). A parametric investigation concerning the variations of the jet location along the axial and pitch-wise direction as well as the skew angle is conducted numerically to validate the potential of EJF in a high-speed compressor cascade with an inlet Mach number of 0.67. And then the interaction mechanisms between the EJF and the endwall secondary flow are discussed in detail. The results show that the EJF could reduce the corner separation and losses significantly by inputting transverse momentum component, inducing a concentrated jet vortex to block the pitch-wise migration of the passage vortex as well as enhancing the energy exchange between the endwall boundary layer and the mainstream. The jet location and the skew angle are important for the influence of EJF on the cascade performance. In this work, a maximum total pressure loss reduction of 11.6 % is obtained by the EJF located at 30 % of the axial chord and 10 % of the pitch with a skew angle of β = 40°, whereas the jet-to-inflow mass flow ratio is only about 0.4 %, validating the high efficiency of this flow control concept. For the off-design points, the EJF also shows appreciable potential on the endwall secondary flow control and loss reduction.

PIV Measurements of a Turbulent Boundary Layer Perturbed by a Wall-Mounted Transverse Circular Cylinder Element

The perturbation induced by a wall-mounted transverse circular cylinder element (TCCE) to a turbulent boundary layer (TBL) is studied using particle image velocimetry (PIV). Measurements are also performed on a smooth wall, which is used as baseline for comparison. The modification of the TBL caused by the sudden perturbation, in a statistical sense, is quantified by comparing the distributions of mean velocity, Reynolds stresses and shape factor in the two geometrical configurations. The impact of the TCCE on the TBL is found to extend more than 9 boundary-layer thickness downstream. Combining the side- and top-view PIV data indicates that the packet-like structures are stretched along the streamwise direction within a flat inclination angle when climbing over the TCCE, and retain their large spatial scales. The separated vortices induced downstream of the TCCE, are quickly lifted up to form the hairpin vortices within strong swirling strength. Then, the robust hairpin packets are generated downstream through the auto-generation scenario, which occupies the low-speed regions beneath the residual hairpin packet’s heads and impairs the process that the low-momentum fluids feed the concentrated vorticity into the residual hairpin heads. Meanwhile, the streamwise-stretched low-momentum regions (LMRs) are redeveloped in the inner layer underneath the new-generated hairpin packets. Further downstream, even though the residual hairpin packets’ heads are merged by the robust hairpin packets, they are not completely dissipated, and still interact with the new-generated hairpin packets in the outer layer to the end of the field of view.

Hybrid method based on embedded coupled simulation of vortex particles in grid based solution

The paper presents a novel hybrid approach developed to improve the resolution of concentrated vortices in computational fluid mechanics. The method is based on combination of a grid based and the grid free computational vortex (CVM) methods. The large scale flow structures are simulated on the grid whereas the concentrated structures are modeled using CVM. Due to this combination the advantages of both methods are strengthened whereas the disadvantages are diminished. The procedure of the separation of small concentrated vortices from the large scale ones is based on LES filtering idea. The flow dynamics is governed by two coupled transport equations taking two-way interaction between large and fine structures into account. The fine structures are mapped back to the grid if their size grows due to diffusion. Algorithmic aspects of the hybrid method are discussed. Advantages of the new approach are illustrated on some simple two dimensional canonical flows containing concentrated vortices.

Large‐eddy simulation of vortex streets and pollutant dispersion behind high‐rise buildings

Understanding turbulent flow and pollutant dispersion in urban areas is one of the important problems in urban meteorology and environmental fluid mechanics. In this study, we examine turbulent flow and pollutant dispersion in a densely built‐up area in Seoul, South Korea, using the parallelized large‐eddy simulation model (PALM). In particular, we focus on vortex streets and associated pollutant dispersion behind high‐rise buildings. The turbulence recycling method is used to produce inflow profiles. Vortices are generated near the high‐rise buildings and propagate downstream forming vortex streets behind the high‐rise buildings. To investigate characteristics of the vortex streets, spectral and correlation analyses are performed. The spectral analysis reveals that vortices have a non‐dimensional vortex shedding frequency of 0.1–0.2, and this periodicity is weakened due to the influence of other buildings. The correlation analysis shows that vortices appear frequently in regions of negative pressure perturbation. The vertical turbulent momentum fluxes induced by ejections and sweeps largely contribute to the total vertical turbulent momentum flux downstream of the high‐rise buildings. Especially, ejections in the wake region are stronger compared to other regions because ejections are induced by vortices near the top of the high‐rise buildings. It is found that pollutant dispersion is interrupted by both low‐rise and high‐rise buildings. Strong updraughts behind the high‐rise buildings transport pollutant upward and increase the mean pollutant concentration at upper levels. Vortices forming the vortex streets play a role in pollutant mixing in such a way that the vortices eject air of high pollutant concentration from the wake region behind the high‐rise buildings and entrain air of low pollutant concentration into the wake region. The mixing by vortices is verified by the correlation between vorticity and pollutant concentration.

Hovering hummingbird wing aerodynamics during the annual cycle. I. Complete wing.

The diverse hummingbird family (Trochilidae) has unique adaptations for nectarivory, among which is the ability to sustain hover-feeding. As hummingbirds mainly feed while hovering, it is crucial to maintain this ability throughout the annual cycle—especially during flight-feather moult, in which wing area is reduced. To quantify the aerodynamic characteristics and flow mechanisms of a hummingbird wing throughout the annual cycle, time-accurate aerodynamic loads and flow field measurements were correlated over a dynamically scaled wing model of Anna’s hummingbird (Calypte anna). We present measurements recorded over a model of a complete wing to evaluate the baseline aerodynamic characteristics and flow mechanisms. We found that the vorticity concentration that had developed from the wing’s leading-edge differs from the attached vorticity structure that was typically found over insects’ wings; firstly, it is more elongated along the wing chord, and secondly, it encounters high levels of fluctuations rather than a steady vortex. Lift characteristics resemble those of insects; however, a 20% increase in the lift-to-torque ratio was obtained for the hummingbird wing model. Time-accurate aerodynamic loads were also used to evaluate the time-evolution of the specific power required from the flight muscles, and the overall wingbeat power requirements nicely matched previous studies.

Concentrated separation vortex induced by non-uniform suction flow and its role in performance breakdown of a water-jet pump

Many water-jet pumps have a suction velocity profile, which is non-uniform and unfavorable for the pump performance. This article applies the numerical method to investigate the generation mechanism of performance breakdown in a water-jet pump under non-uniform suction flow. The mechanism dictating flow details in the impeller, including the inlet velocity, flow angle, and boundary layer separation, is systematically studied at the design point. Observations show that the nature of non-uniform inflow is a circumferential distortion upstream of the impeller shroud, which is proved to be a stationary stall cell associated with a velocity deficit near the top sector. Meanwhile, a detailed analysis allows the identification of flow separation and vortex in the impeller. The circumferential distortion gives rise to a high incidence, the occurrence of suction surface separation, and the consequent formation of a concentrated separation vortex near the blade shroud. Furthermore, particular attentions are devoted...

Investigation of the role of non-uniform suction flow in the performance of water-jet pump

Insufficient understanding of performance deviation between the uniform and non-uniform suction flows is a major problem encountered in the design and application of water-jet pumps. In this study, numerical investigation is conducted to illustrate the performance deviation and explain its generating mechanism. Steady simulations are performed to compare the pump performance with two different computational domains. One is a single water-jet pump under uniform inflow and the other is a complete water-jet propulsion system with non-uniform inflow. Results indicate that large non-uniformity of suction flow causes a substantial drop in the pump head. Flow details in the system are comprehensively discussed to describe the non-uniform inflow. Its primary feature is a distinct swirl distortion near the top which evolves into a circumferential vortex. Vortices and blade loadings are also explored. Consequently, the internal correlation between the non-uniform inflow and performance deviation is revealed. Due to non-uniform disturbances, a spanwise vortex shedding from the hub triggers a sharp downward peak of blade loading in the hub section. Moreover, a concentrated vortex deviates from the suction surface and induces an entire reduction in blade loading of the shroud section. These remarkable reductions in the blade loading eventually generate a head deviation.

Wandering of a Wing-Tip Vortex: Rapid Scanning and Correction of Fixed-Point Measurements

Oscillations of a wing-tip vortex over planes orthogonal to the freestream direction are typically ascribed to a specific fluid dynamic phenomenon, which is referred to as vortex wandering or meandering. Vortex wandering affects noticeably fixed-point measurements, producing measured vortices with larger size and weaker intensity than the actual ones, and thus methods for correction of wandering smoothing effects are needed. For this survey, the tip vortex generated from a tapered NACA 0012 half-wing is measured through the rapid scanning technique, which consists of quickly traversing a five-hole pressure probe through the vortex core to obtain velocity signals practically not affected by wandering. Rapid scanning allows evaluating probability distributions of the vortex center positions, which are found to be characterized by bivariate normal probability density functions. Estimate of wandering smoothing effects on fixed-point measurements is carried out by comparing rapid scanning data not affected by wandering with fixed-point three-sensor hot-film measurements. The vortex strength seems to be the main vortex parameter controlling wandering; wandering amplitude is reduced by increasing wing angle of attack or freestream velocity or by reducing streamwise distance from the wing. However, a sufficiently strong or concentrated vortex can be weakly affected by wandering.

Influences of Larger-Scale Vortex Variability on Tornado Pressure Minima in an Outer-Flow Region: Explorations Using a Parametric Tangential Wind Model

Abstract Previous studies have neglected to distinguish between a central pressure deficit due to a tornado itself and due to a parent mesocyclone in which the tornado is embedded. To obtain improved understanding of the influences of larger-scale vortex variability on smaller-scale tornado pressure deficits, a parametric tangential wind model supplemented with a cyclostrophic speed equation was used to explore the role that the variability plays in influencing radial pressure deficits by deducing radial pressure deficit distributions from radial profiles of hypothetically superpositioned, dual-maxima tangential velocities in the free atmosphere, where a dominant swirling flow was in approximate cyclostrophic balance. The cyclostrophic approximation was partitioned into two separate components, allowing one to scrutinize and determine which of the concentric vortices contributes most significantly to the tornado pressure minima. The model parametrically constructed a smaller-scale, stronger vortex (rapidly swirling flow) that was centered within a larger-scale, weaker vortex (slowly swirling flow) to represent a tornado centered within a supercell, low-level, parent mesocyclone above a tornado boundary layer. The radial pressure deficit fluctuations were varied by changing one of five key velocity-controlling parameters assigned to one vortex to represent a variety of vortex strengths. Based on eight experiments, the larger-scale, weaker (smaller scale, stronger) vortex contributed less (more) to the total pressure deficit than the smaller-scale, stronger (larger scale, weaker) vortex. The stronger vortex centered within the larger-scale, weaker vortex has a larger central pressure minimum than it does in the absence of the larger-scale vortex.

Concentric Perfect Optical Vortex Beam Generated by a Digital Micromirrors Device

We propose a kind of perfect optical vortex beam (POV) generated via Fourier transformation of Bessel-Gauss (BG) beams through encoding of the amplitude of the optical field with binary amplitude digital micromirrors device. We call them concentric POVs which consist of several POVs and where every POV has the same center and independent characters, and we confirm the correct phase patterns of the concentric POVs with the method of Mach–Zehnder interference and self-interference. This special property could be considered as a potential of implement to increase the orbital angular momentum communication capacity vastly.

The wall-free non-stationary fire whirls generation by axisymmetric burning of solid fuel pellets

This paper is aimed to demonstrate an opportunity of the generation of wall-free non-stationary fire whirls under laboratory conditions without using mechanical swirling devices and estimation of their integral parameters. A simple experimental facility making possible the generation of concentrated fire vortex structures by means of combustion of solid fuel (urotropine) arranged symmetrically on a metallic underlying surface is described. With the use of photography, some data on the generation conditions and integral parameters (lifetime, height, and diameter) of fire whirls have been determined.

Flow pattern similarities in the near wake of three bird species suggest a common role for unsteady aerodynamic effects in lift generation.

Analysis of the aerodynamics of flapping wings has yielded a general understanding of how birds generate lift and thrust during flight. However, the role of unsteady aerodynamics in avian flight due to the flapping motion still holds open questions in respect to performance and efficiency. We studied the flight of three distinctive bird species: western sandpiper (Calidris mauri), European starling (Sturnus vulgaris) and American robin (Turdus migratorius) using long-duration, time-resolved particle image velocimetry, to better characterize and advance our understanding of how birds use unsteady flow features to enhance their aerodynamic performances during flapping flight. We show that during transitions between downstroke and upstroke phases of the wing cycle, the near wake-flow structures vary and generate unique sets of vortices. These structures appear as quadruple layers of concentrated vorticity aligned at an angle with respect to the horizon (named 'double branch'). They occur where the circulation gradient changes sign, which implies that the forces exerted by the flapping wings of birds are modified during the transition phases. The flow patterns are similar in (non-dimensional) size and magnitude for the different birds suggesting that there are common mechanisms operating during flapping flight across species. These flow patterns occur at the same phase where drag reduction of about 5% per cycle and lift enhancement were observed in our prior studies. We propose that these flow structures should be considered in wake flow models that seek to account for the contribution of unsteady flow to lift and drag.

Edge effects in flow around a plasma actuator

We have studied the structure of flow formed in the boundary layer at the lateral edge of the discharge zone of a dielectric barrier discharge plasma actuator. It is established that a region with nonzero component of tangential velocity exists near the plasma layer boundary. At some distance downstream of the actuator, a concentrated vortex is formed with the axis aligned with the flow direction. In the presence of two closely spaced boundaries of the plasma region, a pair of counter-rotating vortices is formed. Separate microdischarges in the plasma layer also appear to be a source of similar longitudinal vortices with smaller amplitude.

Stereoscopic PIV measurements of the flow field in a turbine cascade

This paper presents experimental measurements of the flow field in a Low-speed Turbine Cascade using a stereoscopic particle-image velocimetry (SPIV). During the measurements, a pair of frame-straddling-based CCD cameras were configured at different sides of the laser light sheet, and appropriate tracing particles (DEHS) were employed. The measurements were conducted at the incidence angle of 0 degree and exit Reynolds number of 1.7 × 105 with the tip clearance 1.18% of blade chord. The tip flow features, such as the evolution and breakdown of tip leakage vortex, the horseshoe vortex, turbulence characteristics of tip leakage flow, were studied for the flow field analysis. The results showed that the tip leakage flow/vortex mainly dominate flow fields in the tip region. The tip leakage vortex performs as a concentrated vortex before its breaking down and splitting into small vortices. The highest turbulence intensity mainly occurs in the tip region along with the trajectory of tip leakage vortex, and when the vortex breaks down, the turbulence intensity reduces rapidly. Additionally, the SPIV with this configuration also shows an advantage in investigating the flow structures and mechanism inside the turbine cascade.

Investigation of Unsteady Flow Phenomena in First Vane Caused by Combustor Flow With Swirl

The flow at the combustor turbine interface of power generation gas turbines with can combustors is characterized by high and nonuniform turbulence levels, lengthscales, and residual swirl. These complexities have a significant impact on the first vanes aerothermal performance and lead to challenges for an effective turbine design. To date, this design philosophy mostly assumed steady flow and thus largely disregards the intrinsic unsteadiness. This paper investigates the steady and unsteady effects of the combustor flow with swirl on the turbines first vanes. Experimental measurements are conducted on a high-speed linear cascade that comprises two can combustors and four nozzle guide vanes (NGVs). The experimental results are supported by a large eddy simulation (LES) performed with the inhouse computational fluid dynamics (CFD) flow solver TBLOCK. The study reveals the highly unsteady nature of the flow in the first vane and its effect on the heat transfer. A persistent flow structure of concentrated vorticity is observed. It wraps around the unshielded vane's leading edge (LE) at midspan and periodically oscillates in spanwise direction due to the interaction of the residual low-pressure swirl core and the vane's potential field. Moreover, the transient behavior of the horseshoe-vortex system due to large fluctuations in incidence is demonstrated.

Sign-changing tower of bubbles for a sinh-Poisson equation with asymmetric exponents

Motivated by the statistical mechanics description of stationary 2D-turbulence, for a sinh-Poisson type equation with asymmetric nonlinearity, we construct a concentrating solution sequence in the form of a tower of singular Liouville bubbles, each of which has a different degeneracy exponent. The asymmetry parameter \begin{document} $γ∈(0, 1]$ \end{document} corresponds to the ratio between the intensity of the negatively rotating vortices and the intensity of the positively rotating vortices. Our solutions correspond to a superposition of highly concentrated vortex configurations of alternating orientation; they extend in a nontrivial way some known results for \begin{document} $\gamma=1$ \end{document} . Thus, by analyzing the case \begin{document} $\gamma≠1$ \end{document} we emphasize specific properties of the physically relevant parameter \begin{document} $\gamma$ \end{document} in the vortex concentration phenomena.

Development of the instability of an axisymmetric vortex flow in a circular cylinder

A vortex structure consisting of two concentric vortex rings located in a circular cylinder is studied. The rings touch each other and have a uniform vorticity. The geometric and dynamic parameters of the rings satisfy the condition of zero total intensity. A linear stability analysis of this structure is performed, including the limiting cases when the inner ring transforms into a circle or the outer ring is adjacent to the boundary of the cylinder. The results of numerical simulation of the evolution of unsteady flows are presented for a wide range of variation of governing parameters of the problem. It is established that the evolution of unstable vortex flows results in the formation of several types of quasistationary structures, the topology of which depends mainly on the dominant disturbance mode.

Slowly-growing gap-opening planets trigger weaker vortices

The presence of a giant planet in a low-viscosity disc can create a gap edge in the disc's radial density profile sharp enough to excite the Rossby Wave Instability. This instability may evolve into dust-trapping vortices that might explain the "banana-shaped" features in recently observed asymmetric transition discs with inner cavities. Previous hydrodynamical simulations of planet-induced vortices have neglected the timescale of hundreds to thousands of orbits to grow a massive planet to Jupiter-size. In this work, we study the effect of a giant planet's runaway growth timescale on the lifetime and characteristics of the resulting vortex. For two different planet masses (1 and 5 Jupiter masses) and two different disc viscosities ($\alpha$=3$\times 10^{-4}$ and 3$\times10^{-5}$), we compare the vortices induced by planets with several different growth timescales between 10 and 4000 planet orbits. In general, we find that slowly-growing planets create significantly weaker vortices with lifetimes and surface densities reduced by more than $50\%$. For the higher disc viscosity, the longest growth timescales in our study inhibit vortex formation altogether. Additionally, slowly-growing planets produce vortices that are up to twice as elongated, with azimuthal extents well above $180^{\circ}$ in some cases. These unique, elongated vortices likely create a distinct signature in the dust observations that differentiates them from the more concentrated vortices that correspond to planets with faster growth timescales. Lastly, we find that the low viscosities necessary for vortex formation likely prevent planets from growing quickly enough to trigger the instability in self-consistent models.

Vortical Sheet Behavior in the Wake of a Rotor in Ground Effect

Particle image velocimetry measurements with high spatial resolution were conducted on a hovering two-bladed rotor system operating in ground effect above a planar surface. The objective was to gain a better understanding of the fluid dynamics of the wake sheets trailed behind rotating blades, with specific focus on their contribution to the development of the near-wall flowfield at the ground plane. Two blade sets were tested: 1) a baseline untwisted blade and 2) a twisted blade with 17 deg of nose-down linear twist. Planar particle image velocimetry measurements were made for multiple regions of interest in the rotor wake to understand the temporal development of the wake sheets as they convected from their origin at the trailing edges of the blades until they interacted with the ground. The wake sheets were shown to contain relatively large amounts of concentrated vorticity, which persisted much longer in the wake than for the corresponding out of ground effect case. The wake sheet substantially influenced the nature of the developing flow over the ground, resulting in relatively large and higher transient excursions in the near-wall flow velocities further inboard and closer to the rotor. Performance measurements were also taken for both blade sets at various heights above the ground, with the untwisted blades showing the most significant increase in performance.

Free space propagation of concentric vortices through underwater turbid environments

Concentric optical vortex beams of 3-petal, 5-petal, and 6-petal spatial profiles are generated at 450 nm using a single diffractive optical element. The spatial and temporal propagation characteristics of these beams are then studied in a scattering underwater environment. Experimental results demonstrate a less than 5% reduction in the spatial pattern for turbidities in excess of 10 attenuation lengths. The temporal properties of concentric vortex beams are studied by temporally encoding an on-off keyed, non-return-to-zero (OOK-NRZ) data stream at 1.5 GHz.