Concentrated Vortex Research Articles

Self-sustained oscillations of shallow flow past sequential cavities

AbstractShallow flow past successive cavities is characterized by means of high-image-density particle image velocimetry (PIV). Highly coherent, self-sustained oscillations arise due to coupling between the inherent instability of the separated shear layer along the opening of each sequential cavity; and a gravity standing wave mode within each cavity. The globally coupled nature of the flow structure is evident through dominance of the same spectral component in the undulating vorticity layers along each of the successive cavities and the wall pressure fluctuations within the cavities. Unlike coupled phenomena associated with flow past a single cavity, optimal coupling for successive cavities requires a defined phase shift between the gravity standing wave patterns in adjacent cavities and, furthermore, an overall phase shift of the undulating shear layer along the cavity openings. The magnitudes of these phase shifts depend on the mode of the gravity standing wave in each cavity, i.e. longitudinal or transverse mode, which is respectively aligned with or normal to the main stream. Such phase shifts result in corresponding displacements of patterns of phase-referenced vorticity concentrations along the cavity openings and changes in the timing of impingement of these concentrations upon the downstream corners of successive cavities. All of the foregoing aspects are related to the unsteady recirculation flow within the cavity, the time-dependent streamline topology, and concentrations of Reynolds stress along the cavity opening.

Clusterization of particles in turbulent and vortex two-phase flows

Experimental investigations and theoretical calculations devoted to studying different aspects of the formation of clusters in two-phase flows are reviewed. The physical mechanisms of the formation of the regions of increased concentrations of particles in turbulent flows and under flowing bodies by two-phase streams, as well as in free concentrated vortices, are considered.

The genesis of Typhoon Nuri as observed during the Tropical Cyclone Structure 2008 (TCS-08) field experiment – Part 3: Dynamics of low-level spin-up during the genesis

Abstract. Aircraft reconnaissance data collected during the Tropical Cyclone Structure 2008 field campaign are used to examine further kinematical, dynamical, and thermodynamical aspects of the genesis of Typhoon Nuri. Data from the first two missions into the pre-Nuri disturbance document the transition from a tropical wave to a tropical depression. Dropwindsonde-derived tangential wind profiles at several radii from the low-level circulation center indicate that the magnitude of low-level circulation increases and that the corresponding tangential velocity maximum moves inward from the first to second reconnaissance mission. To compliment these findings, a three-dimensional variational analysis incorporating both dropwindsonde and aircraft Doppler radar data is conducted. These data are used to perform circulation tendency calculations at multiple distances from the low-level circulation center. The results demonstrate a net spin-up of the system-scale circulation in the low levels near the center and in the outer regions of the recirculating Kelvin cat's eye circulation. In these regions, the spin-up tendency due to the influx of cyclonic absolute vorticity exceeds the frictional spin-down tendency for both Nuri missions. The system-scale spin-up is found to be accompanied by areas of low-level vorticity concentration through vortex-tube stretching associated with cumulus convection. The areal coverage and intensity of these high-vorticity regions increase between the first and second Nuri missions. The findings of this study are consistent in some respects to the Nuri observational analysis carried out by Raymond and López-Carrillo (2011), but differ in their suggested key results and related scientific implications that the pre-Nuri disturbance was spinning down in the planetary boundary layer on the first day of observations. The findings herein strongly support a recent tropical cyclogenesis model positing that the Kelvin cat's eye circulation of the parent wave-like disturbance provides a favorable environment for convective vorticity organization and low-level spin-up on the mesoscale.

About choice of particle parameters for visualization and diagnostics of free concentrated air vortices

The problem of the choice of parameters (density, size) of particle inertia for visualization and diagnostics of free vortices of various intensities (characteristic velocity, vortex funnel diameter) is considered. The dimensionless criterion (Stokes number) is used, which determines the peculiarities of solid particles behavior in concentrated vortex structures. An example of the choice of particle parameters for visualization of a laboratory-simulated air vortex funnel and debris cloud is described.

Tracking stages of transition in Couette flow analytically

AbstractThe current study focuses on a transition scenario in which the linear transient growth mechanism is initiated by four decaying normal modes. It is shown that the four modes, the initial structure of which corresponds to counter-rotating vortex pairs, are sufficient to capture the transient growth mechanism. More importantly, it is demonstrated that the kinetic energy growth of the initial disturbance is not the key parameter in this transition mechanism. Rather, it is the ability of the transient growth process to generate an inflection point in the wall-normal direction and consequently to make the flow susceptible to a three-dimensional disturbance leading to transition to turbulence. Because of the minimal number of modes participating in the transition process, it is possible to follow its earlier key stages analytically and to compare them with the results of direct numerical simulation. This procedure reveals the role of various flow parameters during the transition, such as the difference between symmetric and antisymmetric transient growth scenarios. Moreover, it is shown that the resulting modified base flow of the linear process is not sufficient to produce a significant localized maximum of the base-flow vorticity (i.e. a ‘strong’ inflection point), and it is only due to nonlinear effects that the base flow becomes unstable with respect to an infinitesimal three-dimensional disturbance. Finally, the physical mechanism during key stages of transition is well captured by the analytical expressions. Furthermore, the vortex dynamics during these stages is very similar to the model proposed by Cohen, Karp & Mehta (J. Fluid Mech., vol. 747, 2014, pp. 30–43) according to which streamwise variation of the initial counter-rotating vortex pair is required to generate concentrated spanwise vorticity, which together with the lift-up by the induced velocity and shear of the base flow generates packets of hairpins.

The possibility of generation of concentrated fire vortices without forced swirling

The possibility in principle of physical simulation of concentrated fire vortices in laboratory conditions without using mechanical swirling devices is shown.

Two-Dimensional Potential Method Simulations of a Body–Strake Configuration

The lee-side flow over a cruciform wing–body configuration at supersonic speeds in the “” orientation up to angles of 25 deg have been simulated using the single concentrated vortex and discrete vortex model methods. The wings or strakes are of very low aspect ratio of order 0.025 with taper ratio with a length of mounted on a tangent ogive body. The simulations revealed that the single concentrated vortex model is only suitable at low angles, while the discrete vortex model can predict both the flowfield and normal force reasonably when the lee-side boundary-layer separation characteristics are modeled. The resulting secondary vortex due to the wing–body junction has been identified to significantly influence the lee-side flow topology, and its exclusion results in higher normal forces and incorrect prediction of vortex positions.

Vortex formation of a finite-span synthetic jet: High Reynolds numbers

The formation and evolution of flow structures of a high-speed, finite-span synthetic jet issued into a quiescent flow were investigated experimentally using Stereoscopic Particle Image Velocimetry for jet peak velocities up to 150 m/s. The effect of high jet Reynolds number (Re○ = 1150, 3450, and 5750, with corresponding Strouhal numbers of St = 0.215, 0.046, and 0.028, respectively) on a finite-span synthetic jet with aspect ratios of 18 and 24 were explored. It was found that the velocity and vorticity fields were greatly affected by the Reynolds number, with the lowest Reynolds number case producing the highest normalized peak velocities and vorticity. Moreover, for all three Reynolds numbers, axis switching was observed and its streamwise location increased as the Reynolds number increased. The Q-criterion was utilized for decoupling vortices from the vorticity concentrations in the flow field. This enabled the identification of vortices in the flow field and the reconstruction of the 3D vortex ring that was formed near the orifice, and to track its evolution and advection. At this range of high Reynolds numbers, secondary flow structures were observed, where these structures were formed either immediately downstream of the orifice or farther downstream, depending on the Reynolds number and the aspect ratio. It is apparent from this work that the effect of the Reynolds number has a pronounced influence on the evolution of the flow structures and, as a result, may alter the effectiveness of flow control utilizing these jets. Furthermore, investigation of the behavior of synthetic jets with low aspect ratios (AR < 25), at high Reynolds numbers (Re○ > 1000), with peak velocities exceeding 100 m/s, addresses a critical gap in the understanding the finite-span synthetic jet flow field for practical applications.

Fire Whirl due to Interaction between Line Fire and Cross Wind

During the wildland fire of Brazil in 2010 (http://www.dailymail.co.uk/sciencetech/article-1306088/Braziltornado-Whirling-column-flames-sweeps-burning-fields.html), a special fire whirl occurred over a narrow but long fire front and moved due to the wind effect. This paper presents an elementary study on such a moving fire whirl by conducting line fire experiments with cross wind. Experimental analysis indicates that a line fire near the ground, a reasonable attack angle between the line fire and the cross wind, and wind speed within a critical range are the three essential conditions for the formation of fire whirl in a line fire. By examining the advection and bending of vorticity, it is also deduced that the concentrated vortex of fire whirl results from the coupling of the line fire plume and the horizontal vortex line near the ground surface. By assuming the solid-body rotation of fire whirl flame, a possible mechanism of moving fire whirl is proposed, which states that the flame moving is mainly controlled by the drag force, lift force and ground friction. Accurate experimental measurements are needed to testify or verify this mechanism in the future work.

Vortices at nanoscale: Still some room at the bottom

Vortices as topological defects existon all scales of Nature from cosmicstrings to a dense quark matter.In macroscopic quantum systemslike superconductors, superfluidsand Bose-Einstein condensatesthe vortices are of special interest.There, they are quantized having“normal” cores where the superfluidorder parameter is zero and itsphase usually changes by 2

Spin-up of a magnetically driven tornado-like vortex

AbstractThe spin-up of a concentrated vortex in a liquid metal cylinder with a free surface is considered experimentally and numerically. The vortex is driven by two flow-independent magnetic body forces. A continuously applied rotating magnetic field provides the source of the angular momentum. A pulse of about one order of magnitude stronger travelling magnetic field drives a converging flow that temporarily focuses this angular momentum towards the axis of the container. A highly concentrated vortex forms that produces a funnel-shaped surface depression. We explore experimentally the duration, the depth and the conditions of formation of this funnel. Additionally, we measure the axial velocity and calculate the axisymmetric flow field of this transient vortex at a lower force magnitude. The spin-up vortex is similar to the corresponding developed time-averaged turbulent vortex driven by the same magnetic forces (Grants et al., J. Fluid Mech., vol. 616, 2008, pp. 135–152). There are two main differences. First, the maximum swirl concentration condition cannot be expressed as a constant ratio of the two driving forces. Second, a much higher degree of swirl concentration is feasible. We explain these differences as due to a much lower turbulence during the spin-up.

Relationship between the flow blockage of tip leakage vortex and its evolutionary procedures inside the rotor passage of a subsonic axial compressor

The near casing flow fields inside the rotor passage of a 1.5 stage axial compressor with different blade-loading levels and tip gap sizes were measured by using stereoscopic particle image velocimetry (SPIV). Based on a carefully defined blockage extracting method, the variations of blockage parameter inside the blade passage were analyzed. It was found that the variation of blockage parameter appeared as a non-monotonic behavior inside the blade passage in most cases. This non-monotonic behavior became much more remarkable as the blade loading increases or mass flow rate decreases.The variations of the blockage parameter inside the blade passage had close relation to the evolutionary procedures of the tip leakage vortex (TLV). The destabilization of the TLV caused a rapid increasing of the blockage parameter. After the TLV lost the features of a concentrated streamwise vortex, the blockage parameter usually got a peak value. And then, because of the intense turbulent mixing between the TLV low momentum flow and its surrounding flows, the flow deficit inside the TLV recovered.

Parabolized Navier–Stokes model for study the interaction between roughness structures and concentrated vortices

We perform a combined analytical-numerical study of hydrodynamic sensitivity of a concentrated vortex in collision with a roughness wall. Models of the hydrodynamic instability and wall-bounded flow sensitivity are provided analytically by means of parabolization of the Navier-Stokes equations. The differential quadrature method was originally applied as a numerical technique to solve sets of linear algebraic equations by use of a uniform grid. We extend this technique to multidimensional clustered grids and we develop a two-level differential quadrature algorithm to compute the fluctuating flow and the sensitivity field. Extensive numerical results are presented for a two-dimensional Lamb–Oseen problem and both the solution accuracy and convergence rate are assessed. A variational flow control method is also investigated, that proved to be useful for mitigating the flow instabilities by setting the base flow to optimal parameters.

Vortex control and aerodynamic performance improvement of a highly loaded compressor cascade via inlet boundary layer suction

Effects of inlet boundary layer suction on the vortex structure and cascade loss in a highly loaded compressor cascade were investigated experimentally. Ink-track visualization was undertaken on cascade endwall and the blade surface. Ten traverse planes from upstream to downstream of the cascade in a rectangular wind tunnel were measured by an L-shaped five-hole probe. These tested planes revealed the process of emergence, development and decline of several principal vortices as well as the corresponding additional losses. Details of ink-track visualization displaying the secondary flow behavior of boundary layer upon endwall and blade surface assist to make judgment on vortex evolution. Inspection of the vortex structure revealed that highly loaded compressor was characterized by large-scale vortices in the endwall region. After suction, these vortices are all well organized and under control. Among all of them, passage vortex is most sensitive to the variation of the inlet boundary layer, and its main function is to spread low-energy fluid rather than to produce loss. On the other hand, a wall vortex and a concentrated shedding vortex take place inside and after the cascade, respectively, and engender considerable accompanying loss as they dissipate. The effects of inlet boundary layer suction on them are correspondingly weaker. About one forth of the total loss in the baseline cascade was eliminated when boundary layer suction flow rate reaches 2.5 % of the inlet mass flow. The feasibility of simplifying the suction system is also verified through this work.

WAVELET MULTI-RESOLUTION ANALYSIS ON LARGE-EDDY SIMULATION OF TURBURLENT FLOW BEHIND A VEHICLE EXERNAL MIRROR

The three-dimensional orthogonal wavelet multi-resolution technique was applied to analyze flow structures of various scales around an externally mounted vehicle mirror. Firstly, the three-dimensional flow of mirror wake was numerically analyzed at a Reynolds number of 105 by using the large-eddy simulation (LES). Then the instantaneous velocity and vorticity were decomposed into the large-, intermediate- and relatively small-scale components by the wavelet multi-resolution technique. It was found that a three-dimensional large-scale vertical vortex dominates the mirror wake flow and makes a main contribution to vorticity concentration. Some intermediate- and relatively small-scale vortices were extracted from the LES and were clearly identifiable.

Phase-resolved and time-averaged puff motions of an excited stack-issued transverse jet

The dynamics of puff motions in an excited stack-issued transverse jet were studied experimentally in a wind tunnel. The temporal and spatial evolution processes of the puffs induced by acoustic excitation were examined using the smoke flow visualization method and high-speed particle image velocimetry. The temporal and spatial evolutions of the puffs were examined using phase-resolved ensemble-averaged velocity fields and the velocity, length scales, and vorticity characteristics of the puffs were studied. The time-averaged velocity fields were calculated to analyze the velocity distributions and vorticity contours. The results show that a puff consists of a pair of counter-rotating vortex rings. An initial vortex ring was formed due to a concentration of vorticity at the lee side of the issuing jet at the instant of the mid-oscillation cycle. A vortex ring rotating in the opposite direction to that of the initial vortex ring was subsequently formed at the upwind side of the issuing jet. These two counter-rotating vortex rings formed a “mushroom” vortex pair, which was deflected by the crossflow and traveled downstream along a time-averaged trajectory of zero vorticity. The trajectory was situated far above the time-averaged streamline evolving from the leading edge of the tube. The velocity magnitudes of the vortex rings at the upwind and the lee side decreased with time evolution as the puffs traveled downstream due to momentum dissipation and entrainment effects. The puffs traveling along the trajectory of zero vorticity caused large velocities to appear above the leading-edge streamline.

Controlled transitory stall on a pitching airfoil using pulsed actuation

Transitory separation control of a static and pitching 2-D airfoil is investigated in wind tunnel experiments using pulsed actuation on time scales that are an order of magnitude shorter than the characteristic convective time scale Tconv. Actuation is provided by momentary [O(0.05Tconv)] pulsed jets that are generated by a spanwise array of combustion-based actuators integrated in the center segment of the airfoil. The flow field in the center plane above the airfoil and in its near wake is computed from high-resolution PIV measurements in multiple overlapping cross-stream frames that are obtained phase-locked to the actuation and allow for tracking of vorticity concentrations. A single actuation pulse leads to a strong transitory increase in the circulation about the airfoil that is manifested by a partial collapse of the separated flow domain and is accompanied by the shedding of a large-scale clockwise vortex, and the attachment and accumulation of the surface vorticity layer behind it. The slow relaxation of the flow following termination of pulsed actuation returns the airfoil to full stall within 10Tconv. It is shown that repetition of actuation pulses within Tconv can increase the streamwise extent of the attached flow domain, and the trapped vorticity leads to a substantial increase in the peak transitory circulation before the flow separates again when the actuation is terminated. The coupling of the pulsed actuation to the airfoil’s motion enhances the actuation’s control authority. Single pulse can significantly increase the lift over most of the oscillation cycle both at post-stall and at angles of attack that are below stall. Several actuation pulses distributed during the pitch oscillation cycle can momentarily extend the accumulation of vorticity and thus increase the transitory and cycle-averaged lift, and improve the airfoil’s pitch stability.

Investigation of the flow characteristics in a multirow finned-tube heat exchanger model by means of PIV measurements

Multirow plain fin-and-tube heat exchangers (PFTHEs) are widely encountered in industrial processes. The airflow between the interfin spaces generates three-dimensional vortical structures at fin–tube junctions which increase local flow mixing. In this paper, two-components PIV velocity measurements were used to investigate the flow characteristics in a model of a four-row staggered PFTHE at Reynolds number ReD=2000. For each row, the flow structure was analyzed in radial planes at the fin–tube junction in order to characterize the spatial evolution of the vortical structures around the tube. These vortical structures generated in the vicinity of fin–tube junctions were visualized by analyzing the velocity gradient tensor and their spatial evolution and dissipation was characterized. This study highlights the complexity of both row-by-row and angular evolutions of the flow structure. Significant vorticity concentration was found in the vicinity of fin–tube junctions, in the flow core where horseshoe vortices develop. The maximum primary vortex strength throughout the heat exchanger model is observed at the second fin–tube junction.

Creation of diffraction-limited non-Airy multifocal arrays using a spatially shifted vortex beam

Diffraction-limited non-Airy multifocal arrays are created by focusing a phase-modulated vortex beam through a high numerical-aperture objective. The modulated phase at the back aperture of the objective resulting from the superposition of two concentric phase-modulated vortex beams allows for the generation of a multifocal array of cylindrically polarized non-Airy patterns. Furthermore, we shift the spatial positions of the phase vortices to manipulate the intensity distribution at each focal spot, leading to the creation of a multifocal array of split-ring patterns. Our method is experimentally validated by generating the predicted phase modulation through a spatial light modulator. Consequently, the spatially shifted circularly polarized vortex beam adopted in a dynamic laser direct writing system facilitates the fabrication of a split-ring microstructure array in a polymer material by a single exposure of a femtosecond laser beam.

Influences of boundary layer scale separation on the vorticity transport contribution to turbulent inertia

In the flows of interest, the mean effect of turbulent inertia can be expressed as the difference of two velocity vorticity correlations. This difference must be sufficiently non-zero if turbulent inertia is to have a net influence on the mean dynamics. One of the correlations is physically related to change of scale effects, while the other is related to advective vorticity transport. The vorticity transport mechanism is studied under the influence of increasing scale separation. Through the use of both laboratory and field data, the scale separation between the fluctuations of wall-normal velocity, v, and spanwise vorticity ωz, are shown to increase with distance from the wall and Reynolds number. Time-delayed correlations between these quantities reveal that only slight variations in their average phase would cause significant variations in the mean transport of momentum. Spectra are used to explore previous observations of scale selection between v and ωz. The wavelengths corresponding to the peaks in the v and ωz spectra are used to describe scale separation effects. The variations in the wavelength ratios are shown to correlate with the scaling properties that follow from the magnitude ordering of terms in the mean momentum equation. Scale separation is seen to result from two mechanisms: spatial confinement and spatial dispersion. The influence of vorticity stretching apparently generates motions bearing concentrated vorticity that, with increasing Reynolds number, are confined to a smaller fraction of the region where the mean viscous force is of leading order. Where the mean dynamics are inertially dominated, the characteristic vortical motions are advectively dispersed. The width of this domain asymptotically grows like the boundary layer thickness. In the region y+ ≲ 40, the streamwise correlation lengths of v and ωz are shown to scale with the square root of the Reynolds number. This is consistent with inner-outer interactions suggested by the scaling structure of the mean momentum equation.