Onset Of Vortex Breakdown Research Articles

Influence of coaxial cylinders on the vortex breakdown in a closed flow

The effect of stationary cylindrical rods located at the centerline axis on the vortex breakdown (VB) in a closed flow is studied experimentally and numerically. The flow takes place in a cylindrical container with a rotating end wall. The dimensionless numbers characterizing the dynamics of the system are the Reynolds number, based on the angular velocity of the rotating wall, the aspect ratio of the container and dimensionless radius of the rod. We find that the onset of VB, as a function of the Reynolds number, is anticipated for very small values of the rod radius R r , while it is delayed for values of R r beyond a critical value. In order to characterize this effect, the critical Reynolds number for the appearance of the vortex breakdown as a function of the radius of the stationary rods and the different aspect ratios was accurately determined, using digital particle image velocimetry and numerically using a three-dimensional Navier–Stokes solver. The numerical and experimental results are compared showing an excellent agreement.

A sensitivity study of vortex breakdown onset to upstream boundary conditions

This paper theoretically investigates the influence of the upstream boundary conditions on the bifurcation structure leading to vortex breakdown. The axisymmetric flow of an inviscid fluid in a pipe of constant cross-section and finite axial length is considered. Solutions bifurcating from the columnar solution at criticality are analysed via a weakly nonlinear expansion and computed in the fully nonlinear regime using numerical continuation, until a centreline recirculation is found at the pipe outlet. Bifurcation diagrams are determined for a parametric family of inflows describing a wide range of axial and azimuthal profiles, the third inlet condition being chosen either as a fixed azimuthal vorticity or as a vanishing radial velocity. Including the traditional picture given by Wang & Rusak (J. Fluid Mech., vol. 340, 1997a, p. 177), six different diagrams are found to be possible. In particular, a scenario of smooth transition to breakdown may exist as the swirl is increased, with no loss of stability and no hysteresis, breakdown appearing for swirl levels larger than the critical swirl in a pipe. This transition involves a new type of flow akin to a pre-breakdown flow. Our results, furthermore, suggest that rigidly rotating Poiseuille flow could correspond to the limit for which breakdown is impossible because it is predicted at infinitely large swirl numbers. We finally find that flows with a large rotational core are particularly sensitive to an accurate modelling of the upstream boundary conditions, weakly confined vortices being much more robust.

Dye Visualization of the Flow Structure over a Yawed Nonslender Delta Wing

S TUDIES of aerodynamic structures and behaviors of the nonslender delta wings are invariably essential to develop a method to control the development of the vortex breakdown as well as the development of vortices. Unsteady aerodynamics of nonslender delta wings, consisting of shear layer instabilities, the structure of vortices, the occurrence of breakdown, and fluid/structure interactions were extensively reviewed by Gursul et al. [1]. They emphasized the sensitivity of the vortical flow structures varying the angle of attack of the deltawing.Yavuz et al. [2] studied thevortical flow structure on a plane immediately adjacent to the surface of nonslender delta wing, 38:7 deg. Yaniktepe and Rockwell [3] performed experimental investigations on the flow structures at trailing-edge regions of diamondand lambda-type wings. In both wings, vortical flow structures in the crossflowplanes of trailing edge vary rapidly with the angles of attack . Sohn et al. [4] visually investigated the development and interaction of vortices in crossflow planes at various locations on the delta wing with leading edge extension (LEX) using micro water droplets and a laser beam sheet. The range of angle of attack was taken as 12 24 deg at yaw angles of 0, 5, and 10 deg. It was indicated that, by introducing yaw angle , the coiling, merging, and diffusion of thewing and LEX vortices increased on the windward side, whereas they became delayed significantly on the leeward side. Their study confirmed that the yaw angle had a profound effect on the vortex structures. Taylor and Gursul [5] visualized leading-edge vortices of a 50 deg sweep angle, having angles of attack as low as 2:5 deg. Gursul et al. [6] report that combat air vehicles (UCAVs) and micro air vehicles have particularly dominant vortical flows having low sweep angles (25–55 deg), and future UCAVs are expected to be highly maneuverable and highly flexible. Yaniktepe and Rockwell [7] aimed at investigating the unresolved concepts, which included averaged structure of shear layer from the leading edge of the wing, unsteady features of separated layer adjacent to the surface of the wing, and control of flow structure by leading-edge perturbations. Elkhoury and Rockwell [8] have investigated to provide various measurements of the visualized dye patterns, including the degree of interaction of vortices, the onset of vortex breakdown, and effective sweep angle of the wing root vortex, as a function of both Reynolds number and angle of attack . Elkhoury et al. [9] had investigated the Reynolds number dependence of the near-surface flow structure and topology on a representative UCAV planform. The present investigation focuses on the formation and development of leading-edge vortices, vortex breakdown, and threedimensional separationandstallof thecomplexanddisorganizedflow structure over the delta wing. The leading-edge sweep angle was 40 deg. The angle of attack was varied within the range of 7 17 deg and the yaw angle was varied within the range of 0 15 deg.

A numerical technique for laminar swirling flow at the interface between porous and homogenous fluid domains

AbstractThere have been a few recent numerical implementations of the stress‐jump condition at the interface of conjugate flows, which couple the governing equations for flows in the porous and homogenous fluid domains. These previous demonstration cases were for two‐dimensional, planar flows with simple geometries, for example, flow over a porous layer or flow through a porous plug. The present study implements the interfacial stress‐jump condition for a non‐planar flow with three velocity components, which is more realistic in terms of practical flow applications. The steady, laminar, Newtonian flow in a stirred micro‐bioreactor with a porous scaffold inside was investigated. It is shown how to implement the interfacial jump condition on the radial, axial, and swirling velocity components. To avoid a full three‐dimensional simulation, the flow is assumed to be independent of the azimuthal direction, which makes it an axisymmetric flow with a swirling velocity. The present interface treatment is suitable for non‐flat surfaces, which is achieved by applying the finite volume method based on body‐fitted and multi‐block grids. The numerical simulations show that a vortex breakdown bubble, attached to the free surface, occurs above a certain Reynolds number. The presence of the porous scaffold delays the onset of vortex breakdown and confines it to a region above the scaffold. Copyright © 2008 John Wiley & Sons, Ltd.

Vortex Breakdown in an Enclosed Cylinder With a Partially Rotating Bottom-Wall

A numerical study of the axisymmetric flow in a cylindrical chamber of height H is presented, which is driven by a bottom disk rotating at angular velocity Ω. However, unlike most previous studies, the present rotating disk is of smaller radius than the bottom-wall. The boundary curves for the onset of vortex breakdown are presented using different definitions of the nondimensional parameters, depending on whether the cylinder radius R or the disk radius rd is used as the length scale. The study shows that the boundary curves are best correlated when presented in terms of the Reynolds number Ωrd2∕υ, aspect ratio H∕R, and cylinder-to-disk ratio R∕rd. The cylinder-to-disk ratio R∕rd up to 1.6 is found to have noticeable effect on vortex breakdown; this is attributed to the change of effective aspect ratio. The contours of streamline, angular momentum, and azimuthal vorticity are presented and compared with those of whole bottom-wall rotation.

Steady axisymmetric flow in an enclosed conical frustum chamber with a rotating bottom wall

The swirling flow in an enclosed conical frustum chamber with a rotating bottom wall was studied by a numerical model based on the steady, axisymmetric Navier–Stokes equations. The flow behavior was investigated over a wide range of parameters, that is, the Reynolds number up to 2500, the aspect ratio up to 3.5, and the slope angle of the sidewall from 60° to 180°. The vortex breakdown boundary curves were summarized. It was found that vortex breakdown is delayed or even suppressed in the convergent chamber (chamber section narrowing upward). However, for the divergent chamber, the onset of vortex breakdown is precipitated by an initial increase in the slope angle while delayed by a further increase in the slope angle. The flow separation along the inclined sidewall may occur in the divergent chamber with a big slope angle. For an even bigger slope angle, this separation bubble directly combined with the corner vortex at the upper right corner, forming a large recirculation region. The present results suggested that, although vortex breakdown can occur in the divergent chamber with any slope angle, it cannot occur in a convergent chamber with a small slope angle as well as in a conical chamber.

Simulation of wing-body junction flows with hybrid RANS/LES methods

In this paper, flows past two wing-body junctions, the Rood at zero angle of attack and NASA TN D-712 at 12.5° angle of attack, are investigated with two Reynolds-Averaged Navier-Stokes (RANS) and large eddy simulation (LES) hybrid methods. One is detached eddy simulation (DES) and the other is delayed-DES, both are based on a weakly nonlinear two-equation k– ω model. While the RANS method can predict the mean flow behaviours reasonably accurately, its performance for the turbulent kinetic energy and shear stress, as compared with available experimental data, is not satisfactory. DES, through introducing a length scale in the dissipation terms of the turbulent kinetic energy equation, delivers flow separation, a vortex or the onset of vortex breakdown too early. DDES, with its delayed effect, shows a great improvement in flow structures and turbulence characteristics, and agrees well with measurements.

Characterization of flow behavior in an enclosed cylinder with a partially rotating end wall

The vortex breakdown phenomena in an enclosed cylindrical chamber with a rotating disk whose radius is smaller than that of the chamber were investigated by a numerical model based on the steady, axisymmetric Navier-Stokes equations. The flow behaviors were investigated over a wide range of parameters. The main recirculation region remains unchanged if the cylinder-to-disk ratio, the ratio of the chamber radius to the disk radius, is beyond the recirculation-invariance ratio. The recirculation-invariance ratio displays a generally linear relationship with the vertical ratio, the ratio of the chamber height to disk radius. The trends of the vortex breakdown boundary curves at different cylinder-to-disk ratios suggest that three regions, namely the quasi whole end-wall rotating region, the vortex-breakdown boundary invariance region, and the mixed region, can be used to characterize the occurrence of vortex breakdown. Depending on the specific conditions, the presence of the stationary end wall either serves to precipitate or delay the onset of vortex breakdown. The stationary end wall has the effect of reducing the physical aspect ratio but it also dissipates the fluid’s angular momentum along its surface. These two opposite effects lead to different trends of the vortex-breakdown boundary curves in different regions, depending on which effect is dominant.

Effects of conical lids on vortex breakdown in an enclosed cylindrical chamber

The effects of the conical lids on vortex breakdown have been investigated numerically. The boundaries for the onset of vortex breakdown are plotted in terms of the Reynolds number, the aspect ratio H1∕R, where H1 is the height of the side wall, and the height ratio H2∕H1, where H2 is the height of the axis. The concave-cone lid (H2∕H1>1) delays or even completely suppresses vortex breakdown, while the convex-cone lid (H2∕H1<1) precipitates the onset of vortex breakdown. The attached bubbles were found in the chamber with the convex-cone lid at a smaller height ratio. The critical aspect ratio, below which vortex breakdown cannot occur, decreases with the increase of the height ratio. The results indicate that a conical lid can enlarge the vortex breakdown boundaries and this might be of practical interest when using this device for mixing. The results also suggest that the actual shape of the lid has an influence on suppressing or intensifying vortex breakdown.

Numerical Simulation of Swirling Flow in Complex Hydroturbine Draft Tube Using Unsteady Statistical Turbulence Models

A numerical method is developed for carrying out unsteady Reynolds-averaged Navier-Stokes (URANS) simulations and detached-eddy simulations (DESs) in complex 3D geometries. The method is applied to simulate incompressible swirling flow in a typical hydroturbine draft tube, which consists of a strongly curved 90° elbow and two piers. The governing equations are solved with a second-order-accurate, finite-volume, dual-time-stepping artificial compressibility approach for a Reynolds number of 1.1 million on a mesh with 1.8 million nodes. The geometrical complexities of the draft tube are handled using domain decomposition with overset (chimera) grids. Numerical simulations show that unsteady statistical turbulence models can capture very complex 3D flow phenomena dominated by geometry-induced, large-scale instabilities and unsteady coherent structures such as the onset of vortex breakdown and the formation of the unsteady rope vortex downstream of the turbine runner. Both URANS and DES appear to yield the general shape and magnitude of mean velocity profiles in reasonable agreement with measurements. Significant discrepancies among the DES and URANS predictions of the turbulence statistics are also observed in the straight downstream diffuser.

High-Speed Digital-Particle-Image-Velocimetry Study of Vortex Breakdown

Time-resolved digital particle image velocimetry is employed to investigate the flowfield along a plane passing through the axis of a delta-wing leading-edge vortex. A sampling frequency of 500 Hz allows the detailed documentation of the temporal evolution of the field. Spiral vortex breakdown is studied in terms of velocity and vorticity fields along a plane through the vortex axis. The sense of vorticity is reversed at the onset of vortex breakdown, in agreement with earlier experimental and numerical results. Evidence is now provided that periodic oscillations observed earlier are attributed to a rotating and expanding spiral of the helical vortex breakdown.

Confined flow vortex breakdown control using a small rotating disk

As part of a program to investigate the effectiveness of vortex breakdown in bioreactors for cell and tissue growth, a nonintrusive method of flow control is presented. A small rotating disk flush-mounted opposite to the rotating lid in a confined cylindrical vessel is found to precipitate or delay the onset of vortex breakdown depending on whether it is corotating or counterrotating, respectively. Furthermore, corotation increases the bubble radial and axial dimensions while shifting the bubble in the upstream direction. By contrast, counterrotation tends to reduce the size of the bubble, or completely suppress it, while shifting the bubble in the downstream direction. It has also been shown that corotating swirl addition using the small disk is orders of magnitude more energy efficient in manipulating the vortex breakdown bubble than using end wall rotation.

Suppression of Wing Rock Using Artificial Neural Networks and Fuzzy Logic Controller

further increases in Reynolds number are likely to yield only small decreases in xb/C. In the foregoing, emphasis has been on the onset of pronounced undulations/breakdown for the vortices emanating from the apex of the planform. It is, however, possible to discern the onset of vortex breakdown of the wing root vortex at higher values of Reynolds number. For example, see α = 7d eg,Re = 3 × 10 4 . This onset of breakdown at α = 7d eg appears to move closer to the wing root for an increase of Reynolds number Re to 4 × 10 4 . VI. Angles of Inclination of Wing Root and Apex Vortices Another indicator of the effect of Reynolds number is the angle of inclination of the vortex emanating from the wing root. It is pos- sible to determine an effective sweep anglerv of this vortex core, as defined in the schematic of Fig. 5. As indicated, it is the angle be- tween a line drawn through the center of the vortex core (or, in cases where the core exhibits a spiral, through the center of the spiral) and a line that is perpendicular to the plane of symmetry of the wing. In other words, this effective sweep angle is equivalent to the defi- nition of sweep angle of the leading edge of a delta wing. The dye images of Figs. 2a and 2b indicate that, at lower values of Reynolds number extending up to Re = 2 × 10 4 , the value of the effective sweep anglerv varies substantially, whereas at Re = 3 × 10 4 and 4 × 10 4 little change is evident, indicating that an asymptotic value has been attained. The plot of Fig. 5 clarifies this trend, and em- phasizes that the anglerv is a mild function of angle of attack α. This observation contrasts with the strong effect of α on the onset of pronounced undulations/breakdown xb/C indicated in Fig. 4. Re- garding the values of sweep angle of the vortices from the apex of the wing, it is predominantly a function of Reynolds number. At a given Reynolds number, over the values of α = 4-13 deg, the range of values of sweep angleav of the apex vortex are as follows. For Re = 10,000, 14 ◦ ≤ � av ≤ 17 ◦ ; for Re = 15,000, 15 ◦ ≤ � av ≤ 20 ◦ ; for Re = 20,000, 17.5 ◦ ≤ � av ≤ 20 ◦ ; for Re = 30,000, 21 ◦ ≤ � av ≤ 23 ◦ ; and for Re = 40,000, 27.5 ◦ ≤ � av ≤ 32 ◦ .

The quasi-cylindrical description of submerged laminar swirling jets

The quasi-cylindrical approximation is used to describe numerically the structure of a submerged swirling jet for subcritical values of the swirl ratio S⩽Sc. The emerging flow structure is affected by the swirling motion, which enhances the entrainment rate of the jet and induces an adverse pressure gradient that reduces its momentum flux. The effect is more pronounced as the swirl ratio S is increased, yielding for sufficiently large values of S a jet with an annular structure. The integration describes the smooth transition towards the far-field self-similar solution for all values of S smaller than a critical value S=Sc, at which the numerical integration fails to converge at a given downstream location. The comparisons with previous experimental results confirm the correspondence between the onset of vortex breakdown and the failure of the quasi-cylindrical approximation.

Large eddy simulations of turbulent swirling flows in a dump combustor: a sensitivity study

Large eddy simulations (LES) of confined turbulent swirling flows in a model dump combustor are carried out. The simulations are based on a high-order finite difference method on a Cartesian grid, with the sub-grid scale stress tensor modelled using a scale-similarity model. The aims of this work are to study the physics of the flow and to evaluate the performance of LES method for simulation of the major features of turbulent swirling flows-the vortex breakdown, the highly anisotropic and fast-decaying turbulence structure. Influences of inflow/outflow conditions, combustor geometry, inlet swirl profile and Reynolds numbers on the vortex breakdown and turbulence structures are investigated. At very high swirl levels, the influence of the outflow conditions and the outlet geometry is fairly significant, not only at downstream near the outlet, but also at far upstream. At low Reynolds numbers, the onset of vortex breakdown is fairly sensitive to the change of Reynolds number; however, at high Reynolds numbers it is rather insensitive to the Reynolds number. Comparisons of LES results with experimental data are made. The LES results are shown to be in reasonably good agreement with the experimental data if appropriate inflow and outflow boundary conditions are imposed.

Characteristics of F/A-18 vertical tail buffeting

A time-accurate computational analysis of vertical tail buffeting of full F/A-18 aircraft is conducted at typical flight conditions to identify the buffet characteristics of fighter aircraft. The F/A-18 aircraft is pitched at wide range of high angles of attack at Mach number of 0.243 and Reynolds number of 11 millions. Strong coupling between the fluid and structure is considered in this investigation. Strong coupling occurs when the inertial effect of the motion of the vertical tail is fed back into the flow field. The aerodynamic flow field around the F/A-18 aircraft is computed using the Reynolds-averaged full Navier–Stokes equations. The dynamical structural response of the vertical tail is predicted using direct finite-element analysis. The interface between the fluid and structure is applied using conservative and consistent interfacing methodology. The motion of the computational grid due to the deflection of the vertical tail is computed using transfinite interpolation module. The investigation revealed that the vertical tail is subject to bending and torsional responses, mainly in the first modes of vibrations. The buffet loads increase significantly as the onset of vortex breakdown moves upstream of the vertical tails. The inboard surface of the vertical tail has more significant contribution in the buffet excitation than the outboard surface. In addition, the pressure on the outboard surface of the vertical tail is less sensitive to the angle of attack than the pressure on the inboard surface. The buffet excitation peaks shift to lower frequency as the angle of attack increases. The computational results are compared, and they are in close agreement, with several flight and experimental data.

Recirculation and flowfield regimes of unconfined non-reacting swirling flows

This paper focuses on unconfined swirling flows of air surrounding a bluff-body having a central jet of air. Only one co-flowing primary annular air stream is swirled. The flow conditions investigated here cover a range of swirl numbers and streamwise annular velocities. Emphasis is placed on discerning typical flow structures and on discovering the influence of controlling parameters on the downstream regions of the jet and the recirculation within. The paper is part of a larger program aimed at providing an improved understanding of swirling flows. Laser doppler velocimetry is used to map three components of velocity 〈 u〉, 〈 v〉 and 〈 w〉. In the flows investigated, an upstream recirculation zone is established above the burner’s exit plane. This zone is typical of that found behind bluff-bodies placed in strong axial streams. Additionally, it is shown that the formation of a downstream recirculation zone, hence the onset of vortex breakdown, depends not only on the swirl number but on other flow parameters such as the axial velocity of the primary swirling air, or its Reynolds number. Together, these two parameters appear to control the radial spread of the flow. Spatially, the progression towards downstream recirculation with changes in flow parameters is seen to occur around a fixed location in the streamwise direction. Beyond this axial position, the flowfield seems to be less affected by increases in swirl number. Non-recirculating flow structures are resolved in these flows and are found to cause elevated 〈 u ′ w ′〉 shear stresses.

Control of vortex breakdown by a coaxial wire

A very small diameter wire is tethered from the apex of a delta wing and nominally aligned with the centerline of the leading-edge vortex. The wire can alter both the onset and structure of vortex breakdown. A technique of high-image-density particle image velocimetry allows acquisition of patterns of instantaneous and averaged vorticity and velocity, which reveal the relationship between: Advancement of vortex breakdown towards the apex of the wing; and corresponding changes of patterns of vorticity and velocity contours. The diameter of the wire is one percent of the core diameter of the pre-breakdown vortex. It is possible to alter the onset of vortex breakdown by as much as approximately one chord length of the wing. A critical parameter is the length of the wire, which is normalized by: The chord of the wing; or the distance to onset of vortex breakdown in absence of the wire. Once a critical length of the wire is attained, further increases in length have no effect on the onset of breakdown. This effect is interpreted in terms of abrupt changes in patterns of vorticity and streamwise gradients of velocity along the central region of the vortex. It is possible to attain a switch in sign of azimuthal vorticity and a wakelike region of the vortex, in absence of a stagnation point.

VORTEX BREAKDOWN FROM A PITCHING DELTA WING INCIDENT UPON A PLATE: FLOW STRUCTURE AS THE ORIGIN OF BUFFET LOADING

A delta wing is subjected to both static and dynamic variations of angle-of-attack; the vortices from the wing impinge upon a stationary plate. A technique of high-image-density particle image velocimetry is employed to compare the patterns of vortex development with and without the impingement plate. For the limiting case of static variations of angle-of-attack, the presence of the plate exerts a large influence on the onset of vortex breakdown. In contrast, dynamic (unsteady) variation of angle-of-attack yields changes of breakdown location that are generally similar for cases with and without the impingement plate. The detailed structure of the vortex breakdown-plate interaction is represented by patterns of instantaneous velocity and vorticity, which serve as the origin of buffet-induced loading of the surface of the plate.

Vortex Structure on a Delta Wing at High Angle of Attack

The structure of the leading-edge vortex from a delta wing at high angle of attack is addressed using a scanning laserversion of high-image-density particleimagevelocimetry.Emphasisis ontheglobal patterns of instantaneous vorticity.Thesepatternsarerelated to distributions ofaveraged and e uctuating velocity and vorticity. At low angle ofattack,intheabsenceofvortex breakdown,itispossibleto detecta totalofe vedistinctlayersofvorticity;theyall exhibit small-scale concentrations of azimuthal vorticity. Immediately downstream of thetrailing edgeof the wing, larger-scale vorticity concentrations appear in the outermost vorticity layers. At sufe ciently high angle of attack, vortex breakdown evolves from the innermost two vorticity layers. For all of these classes of vortical structures, the values of dimensionless wavelength and circulation are assessed. Moreover, the onset of vortex breakdown is interpreted in terms of both instantaneous and averaged patterns of velocity and vorticity. These considerations lead to a direct comparison of vorticity-based and stagnation criteria for breakdown. In turn, these criteria are linked to the onset of regions of e uctuating vorticity in the initial region of breakdown.