Structure Of Vortex Breakdown Research Articles

Vortex breakdown in time-dependent electromagnetically driven flow between concentric spheres

The electromagnetically driven flow in the wide gap of a concentric sphere system is studied experimentally and numerically in the laminar regime (Re≤1540). The azimuthal driving Lorentz force is primarily promoted by the interaction of a direct current and a dipolar magnetic field. The current is injected through two ring-shaped copper electrodes located at the equatorial zone of each sphere, and the magnetic field is produced by a permanent magnet located inside the inner sphere. Velocity profiles for the azimuthal component in the equatorial plane were obtained with particle image velocimetry, and the radial velocity component of the flow was recorded using ultrasonic Doppler velocimetry. Laser-fluorescein technique was used for flow visualization. It was found that for a critical electric current (Re = 1140), an instability occurs and the flow becomes time-dependent. We found, theoretically and experimentally, a vortex breakdown structure at each of the polar zones of the spherical gap, and to the best knowledge of the authors, this is the first time it is reported with electromagnetic forcing. A full three-dimensional numerical simulation reproduces the experimental observations qualitatively and quantitatively.

Flame investigations of a laboratory-scale CECOST swirl burner at atmospheric pressure conditions

Experimental and numerical studies were performed to understand the stabilization of lean premixed natural gas/air flames in a gas turbine model combustor which was equipped with a swirl burner, known as the CECOST burner, designed to replicate the flow and flame structures in an industrial gas turbine engine. The operability range, flame stabilization, and flashback were investigated employing simultaneous OH– and CH2O-PLIF, and high-speed chemiluminescence imaging. Large eddy simulation (LES) was carried out for analysis of the vortex breakdown structures under non-reacting conditions. It was found that the vortex breakdown structures under isothermal conditions were insensitive to the Reynolds number (Re) for Re ≥ 10000; however, the stability of the flames and operability range of the burner were highly sensitive to Re as well as to equivalence ratio (ϕ). The equivalence ratio was varied at various Reynolds numbers to observe different regimes of the flame ranging from the lean blowout (LBO) limit to the flashback limit. The LBO limit was found to be mainly a function of equivalence ratio while being nearly independent of the Reynolds number, whereas the occurrence of flashback showed distinct characteristics for different ranges of the Reynolds number. At low and moderate Reynolds numbers, (Re ≤ 17000), flashback occurred when increasing ϕ from lean towards stoichiometric conditions. The coupling between the flow field and heat release induces vortex breakdown in the mixing tube and initiates flashback. In contrast, at higher Reynolds numbers (Re > 17000) no flashback was observed even when ϕ was increased to stoichiometric conditions. At these conditions with high Re, the increase in the bulk flow velocity affects the vortex breakdown structure, pushing the vortex breakdown downstream, which in turn prevents the flame from flashing back into the mixing tube.

Coupling of vortex breakdown and stability in a swirling flow

Experiments and simulations are combined to study flows in a T-mixer with offset inlets, whose stability is coupled to the vortex breakdown structure in the system. This leads to an unexpected flow regime in which increasing the flow rate can re-stabilize steady-state solutions of the flow.

Frequency Response of Turbulent Partially Premixed Flame Stabilized in Free-Standing Vortex Breakdown

ABSTRACTThe response of the heat release rate (flame transfer function, FTF) to a harmonic perturbation in air mass flow rate is studied with large eddy simulation in the case of partially premixed turbulent flames stabilized at the central recirculation zone (CRZ) generated via the mechanism of vortex breakdown in high swirl flows. The analysis is based on a previously developed methodology which, making use of the proper orthogonal decomposition (POD), can separate the FTF into three main components: the first driven by the equivalence ratio fluctuations, the second and the third by axial and tangential velocity fluctuations released at the swirler trailing edge. In the particular case of a free-standing CRZ, i.e., not anchored to a bluff body, it is found that the shape of the CRZ and the position of its apex are largely sensitive to small variations in dynamic head of the approach swirl flow at the centerline. Two main structures are found: type A) CRZ radially narrow with apex positioned inside the burner and type B) CRZ radially wide with apex positioned in the combustor. It is shown that the mechanism driving the flame response largely depends on the structure of the CRZ. More specifically, the amplitude of the FTF is found in the case of type B flow structure to be significantly larger than in the case of type A. On the other hand, FTFs with type A flow structure are characterized by phase distribution with higher sensitivity to variations in the equivalence ratio. This trend and behavior of the FTFs, which are also qualitatively identified in the experimental FTFs, are interpreted in terms of the coupling existing between flame and CRZ which is significantly different between the two vortex breakdown structures.

Effect of geometrical contraction on vortex breakdown of swirling turbulent flow in a model combustor

Large Eddy Simulation (LES) studies of isothermal and incompressible turbulent swirling flows in a model gas turbine combustion chamber geometry have been carried out. The focus is on the effect of outlet geometry contraction on the vortex breakdown structure and the precessing vortex core in the chamber. Nine different outlet geometries with different contraction ratio Cr are considered. The results from a baseline case are compared with experimental data in the literature. The swirling flow is generated using a swirler with fifteen guide vanes similar to an existing industrial gas turbine burner. In all cases the swirler and the main chamber geometry are kept the same. The detailed swirler geometry is considered in the simulation using unstructured grids. Sensitivity tests on the influence of the grid resolution and the subgrid scale models are carried out. The mean flow field shows different vortex breakdown structures when the contraction ratio changes from 0.325 to 1.0. In particular, along the axis of the chamber the flow is shown to switch its direction when the contraction increases as a result of the change of the structure of the center recirculation zone. The underlying flow physics is analyzed by comparing the budget terms in the momentum equations, and by performing a global instability analysis.

Experimental investigation of the vortex breakdown in a lean premixing prevaporizing burner

The spiral vortex breakdown has been experimentally studied in the flow field generated by a lean premixing prevaporizing (LPP) combustor model. The vortex breakdown structures are characterized by a solid-body rotation. Hence, phase-locked laser Doppler velocimeter measurements, performed in a meridional plane, have allowed reconstruction of the periodic velocity and Reynolds stress fields within a time period and in a relative cylindrical frame of reference. Then, the turbulent kinetic energy (TKE) production term has been computed. The velocity distribution analysis indicates that a spiral vortex core wraps around an inner reverse flow zone. Both are characterized by a precessing motion. Then, the main contributions to the TKE production have been identified together with their spatial locations. Their distribution indicates that the maxima are placed in two regions: one located between the inner reverse flow and the spiral vortex and the second between the spiral vortex core and the external nearly quiescent fluid. In the latter, the TKE production term distributions have the features of a Kelvin–Helmholtz-like instability. In the former, the TKE production seems to be related to a Kelvin–Helmholtz-like instability only at the interface between the bubble and the spiral vortex core.

Acoustic response of vortex breakdown modes in a coaxial isothermal unconfined swirling jet

The present experimental work is concerned with the study of amplitude dependent acoustic response of an isothermal coaxial swirling jet. The excitation amplitude is increased in five distinct steps at the burner’s Helmholtz resonator mode (i.e., 100 Hz). Two flow states are compared, namely, sub-critical and super-critical vortex breakdown (VB) that occur before and after the critical conical sheet breakdown, respectively. The geometric swirl number is varied in the range 2.14-4.03. Under the influence of external pulsing, global response characteristics are studied based on the topological changes observed in time-averaged 2D flow field. These are obtained from high resolution 2D PIV (particle image velocimetry) in the longitudinal-mid plane. PIV results also illustrate the changes in the normalized vortex core coordinates (rvcc/(rvcc)0 Hz, yvcc/(yvcc)0 Hz) of internal recirculation zone (IRZ). A strong forced response is observed at 100 Hz (excitation frequency) in the convectively unstable region which get amplified based on the magnitude of external forcing. The radial extent of this forced response region at a given excitation amplitude is represented by the acoustic response region (b). The topological placement of the responsive convectively unstable region is a function of both the intensity of imparted swirl (characterized by swirl number) and forcing amplitude. It is observed that for sub-critical VB mode, an increase in the excitation amplitude till a critical value shifts the vortex core centre (particularly, the vortex core moves downstream and radially outwards) leading to drastic fanning-out/widening of the IRZ. This is accompanied by ∼30% reduction in the recirculation velocity of the IRZ. It is also observed that b < R (R: radial distance from central axis to outer shear layer-OSL). At super-critical amplitudes, the sub-critical IRZ topology transits back (the vortex core retracts upstream and radially inwards) and finally undergoes a transverse shrinkage (rvcc/rvcc0Hz decreases by ∼20%) when b ≥ R. In contrast, the vortex core of super-critical breakdown mode consistently spreads radially outwards and is displaced further downstream. Finally, the IRZ fans-out at the threshold excitation amplitude. However, the acoustic response region b is still less than R. This is explained based on the characteristic geometric swirl number (SG) of the flow regimes. The super-critical flow mode with higher SG (hence, higher radial pressure drop due to rotational effect which scales as ΔP ∼ ρuθ2 and acts inwards towards the center line) compared to sub-critical state imposes a greater resistance to the radial outward spread of b. As a result, the acoustic energy supplied to the super-critical flow mode increases the degree of acoustic response at the pulsing frequency and energizes its harmonics (evident from power spectra). As a disturbance amplifier, the stronger convective instability mode within the flow structure of super-critical VB causes the topology to widen/fan-out severely at threshold excitation amplitude.

Detached-eddy simulations on massively separated flows over a 76/40° double-delta wing

We investigated the flow past a 76/40° double-delta wing at angles of attack from 10° to 35° using SA-DES and a hybrid fourth-order scheme. Comparisons were made with the surface separation pattern, the pressure distribution and the location of vortex breakdown with experimental data, and acceptable agreement was obtained. Further discussions were made under the scenario of massively separated flows on topics such as the characteristics of conical flow, the structure of vortex breakdown and vortical substructures.

Swirling turbulent flows in a combustion chamber with and without heat release

This paper presents experiments and numerical simulations of swirling turbulent flows with and without combustion in a laboratory gas turbine combustor. Three approaches, particle image velocimetry (PIV), laser Doppler velocimetry (LDV) and large eddy simulation (LES) are employed to minimize the uncertainties of the results. The aim is to characterize the main flow structures and turbulence in a combustor that is relevant to gas turbines. Isothermal flows with different outlet geometry and lean premixed preheated natural gas/air flames with equivalence ratio of 0.47 are considered to demonstrate the effect of heat release and combustor geometry on the flows. At the combustor inlet the swirl numbers are about 1.4 and the Reynolds numbers are about 20,000 for all cases. The PIV, LDV and LES show consistent agreement on the mean flow field, whereas for the velocity variances the results from LDV and LES agree each other but the PIV data show considerably lower turbulence level. The mean flow field is characterized by three large-scale recirculation zones resulted from sudden expansion of the combustor geometry and vortex breakdown due to inflow swirl. Combustor outlet geometry exhibits great impact on the vortex breakdown structure. Heat release enhances the production of turbulence near the axis of the combustor, but it does not alter the fundamental vortex breakdown structure. A mechanistic explanation of the underlying flow physics is presented to describe the effect of swirl, heat release and outlet contraction on the flow field.

A weakly nonlinear mechanism for mode selection in swirling jets

AbstractGlobal linear and nonlinear bifurcation analysis is used to revisit the spiral vortex breakdown of nominally axisymmetric swirling jets. For the parameters considered herein, stability analyses single out two unstable linear modes of azimuthal wavenumber $m= \ensuremath{-} 1$ and $m= \ensuremath{-} 2$, bifurcating from the axisymmetric breakdown solution. These modes are interpreted in terms of spiral perturbations wrapped around and behind the axisymmetric bubble, rotating in time in the same direction as the swirling flow but winding in space in the opposite direction. Issues are addressed regarding the role of these modes with respect to the existence, mode selection and internal structure of vortex breakdown, as assessed from the three-dimensional direct numerical simulations of Ruith et al. (J. Fluid Mech., vol. 486, 2003, pp. 331–378). The normal form describing the leading-order nonlinear interaction between modes is computed and analysed. It admits two stable solutions corresponding to pure single and double helices. At large swirl, the axisymmetric solution bifurcates to the double helix which remains the only stable solution. At low and moderate swirl, it bifurcates first to the single helix, and subsequently to the double helix through a series of subcritical bifurcations yielding hysteresis over a finite range of Reynolds numbers, the estimated bifurcation threshold being in good agreement with that observed in the direct numerical simulations. Evidence is provided that this selection is not to be ascribed to classical mean flow corrections induced by the existence of the unstable modes, but to a non-trivial competition between harmonics. Because the frequencies of the leading modes approach a strong $2$:$1$ resonance, an alternative normal form allowing interactions between the $m= \ensuremath{-} 2$ mode and the first harmonics of the $m= \ensuremath{-} 1$ mode is computed and analysed. It admits two stable solutions, the double helix already identified in the non-resonant case, and a single helix differing from that observed in the non-resonant case only by the presence of a slaved, phase-locked harmonic deformation. On behalf of the finite departure from the $2$:$1$ resonance, the amplitude of the slaved harmonic is however low, and the effect of the resonance on the bifurcation structure is merely limited to a reduction of the hysteresis range.

Yaw Angle Effect on Flow Structure over the Nonslender Diamond Wing

Yaw Angle Effect on Flow Structure over the Nonslender Diamond Wing

Experimental vortex breakdown topology in a cylinder with a free surface

The free surface flow in a circular cylinder driven by a rotating bottom disk is studied experimentally using particle image velocimetry. Results are compared with computational results assuming a stress-free surface. A dye visualization study by Spohn et al. [“Observations of vortex breakdown in an open cylindrical container with a rotating bottom,” Exp. Fluids 14, 70 (1993)], as well as several numerical computations, has found a range of different vortex breakdown structures in this flow. We confirm the existence of a transition where the top of the breakdown bubble crosses from the axis to the surface, which has previously only been found numerically. We employ a technique by Brøns et al. [“Topology of vortex breakdown bubbles in a cylinder with rotating bottom and free surface,” J. Fluid Mech. 428, 133 (2001)] to find the corresponding bifurcation curve in the parameter plane, which has hitherto only been used on numerical data. The bifurcation curve located here agrees well with previous numerical simulations. For low values of the Reynolds number we find a regime with vortex breakdown that has not been previously identified. Experiments deviate substantially from computations, indicating the importance of surface effects in this regime.

On the axisymmetric vortex breakdown of a swirling jet entering a sudden expansion pipe

This Brief Communication investigates the onset and bifurcation structure of vortex breakdown in a laminar sudden expansion flow. Bifurcation diagrams for variable inlet swirl levels are determined by integration of the time dependent, incompressible, axisymmetric, Navier–Stokes equations in a sudden expansion with expansion ratio λ=2.0 and for moderately large values of the Reynolds number. As in the case of swirling flows in pipes, vortex breakdown is seen to occur associated with the emergence of a stagnation point when the Reynolds number is sufficiently small (Re≲125 for λ=2), while for larger values it appears as a fold in the solution space. The standing wave analysis for the stream tube emerging from the inlet orifice rim indicates that the local criticality of the flow is a necessary condition for the appearance of the vortex breakdown. The axisymmetric computations also reveal the unsteadiness of the nonslender solutions above Re≳400.

Three-dimensional vortex breakdown in swirling jets and wakes: direct numerical simulation

Vortex breakdown of nominally axisymmetric, swirling incompressible flows with jet- and wake-like axial velocity distributions issuing into a semi-infinite domain is studied by means of direct numerical simulations. By selecting a two-parametric velocity profile for which the steady axisymmetric breakdown is well-studied (Grabowski & Berger 1976), issues are addressed regarding the role of three-dimensionality and unsteadiness with respect to the existence, mode selection, and internal structure of vortex breakdown, in terms of the two governing parameters and the Reynolds number. Low Reynolds numbers are found to yield flow fields lacking breakdown bubbles or helical breakdown modes even for high swirl. In contrast, highly swirling flows at large Reynolds numbers exhibit bubble, helical or double-helical breakdown modes, where the axisymmetric mode is promoted by a jet-like axial velocity profile, while a wake-like profile renders the flow helically unstable and ultimately yields non-axisymmetric breakdown modes. It is shown that a transition from super- to subcritical flow, as defined by Benjamin (1962), accurately predicts the parameter combination yielding breakdown, if applied locally to flows with supercritical inflow profiles. Thus the basic form of breakdown is axisymmetric, and a transition to helical breakdown modes is shown to be caused by a sufficiently large pocket of absolute instability in the wake of the bubble, giving rise to a self-excited global mode. Two distinct eigenfunctions corresponding to azimuthal wavenumbers have been found to yield a helical or double-helical breakdown mode, respectively. Here the minus sign represents the fact that the winding sense of the spiral is opposite to that of the flow.

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.

Visualization of impingement of broken-down vortex on tail

The impingement of a streamwise vortex on a fin is investigated experimentally with high-image-density particle image velocimetry (PIV) and dynamic pressure transducers in order to relate the structure of vortex breakdown to the spectra of surface pressure along the fin. The instantaneous and ensemble-averaged distributions of vorticity from PIV show strong concentrations of vorticity along the edges of the breakdown region. Correspondingly, the amplitude response of the pressure spectra is the strongest when the fin is located near the edge of vortex breakdown. The dominant frequency of the surface pressure is insensitive to cross-stream displacements of the vortex axis relative to the leading edge of the fin. Spectra of surface pressure at crucial locations on the fin are related to these features of the instantaneous and averaged flow structure.

Confined vortex breakdown generated by a rotating cone

Confined vortex breakdown generated by a rotating cone within a closed cylindrical container has been studied both by numerical simulation and by experimental techniques. A comprehensive investigation of the various flow regimes has been carried out by flow visualization. From laser–Doppler measurements of the entire flow field (three velocity components) detailed maps of the time-averaged flow structures for single and double breakdown have been constructed. Three-dimensional time-dependent simulations of steady and unsteady breakdown have been performed. Steady numerical and experimental flow fields obtained at Reynolds number 2200 for a gap ratio of 2 show notable agreement. At critical Reynolds numbers of approximately 3095, for a gap ratio of 2, and 2435, for a gap ratio of 3, the flow was observed becoming unsteady. The periodic behaviour exhibited by the unsteady flow suggested the occurrence of a supercritical Hopf bifurcation. This conjecture was confirmed by the evolution of the oscillation amplitude as a function of criticality, measured for a gap ratio of 3. The dynamical behaviour of unsteady vortex breakdown structures is depicted by numerical simulation of two distinct oscillatory regimes, at Reynolds numbers 2700 and 3100. A thorough analysis of the numerical results has shown that whereas the former regime is characterized by the steady oscillation of closely axisymmetric breakdowns, the latter displays precession of breakdown structures about the central axis. Additionally, it was observed that the mode bringing about the Hopf bifurcation is non-axisymmetric, with azimuthal periodicity of π/2 radians. From examination of measured velocity power spectra at higher Reynolds numbers, a transition scenario was also educed. In the present case, the Ruelle–Takens–Newhouse theorem has been shown to apply.

B105 デルタ翼の前縁剥離渦崩壊が誘起する垂直尾翼のバフェット現象 (PIVによる渦度測定と表面圧力変動の相関)

The impingement of a streamwise vortex on a fin is investigated experimentally with high-image-density particle image velocimetry (PIV) and dynamic pressure transducers in order to relate the structure of vortex breakdown to the spectra of surface pressure along the fin. The instantaneous and ensemble-averaged distributions of vorticity from PIV show strong concentrations of vorticity along the edges of the breakdown region. Correspondingly, the amplitude response of the pressure spectra is the strongest when the fin is located near the edge of vortex breakdown. The dominant frequency of the surface pressure is insensitive to cross-stream displacements of the vortex axis relative to the leading-edge of the fin. Spectra of surface pressure at crucial locations on the fin are related to these features of the instantaneous and averaged flow structure.

On the structure of vortex breakdown on a delta wing

This paper presents a detailed description of the vortex breakdown phenomenon on a delta wing. While there have been a number of earlier studies which have given us a broad, overall picture of these interesting and important flow fields, none has so far yielded the kind of fine structure considered here, in spite of using very fine grids. The present work has been made possible by the use of a new type of grid, called the embedded conical grid, which exploits the special nature of the flow field. Based on the Euler equations, the flow field past a 70 degrees sharp–edged delta wing in a subsonic free–stream is resolved here using this grid. The breakdown is found to begin as a rapid deceleration of the inner core due to a sudden increase in the adverse axial pressure gradient. This leads to a sudden broadening of the core and the formation of a deficit velocity region which becomes intensified causing axial flow reversal. The near–axis vorticity lines get tilted away from the vortex axis and their spiralling direction is reversed. The effect of artificial dissipation and grid coarseness on the computed vortex breakdown has been examined. The results presented here, though obtained for the delta wing geometry, are expected to add to our understanding of vortex breakdown in other flow situations as well.

Instantaneous structure of vortex breakdown on a delta wing via particle image velocimetry

Instantaneous structure of vortex breakdown on a delta wing via particle image velocimetry