Vortex Breakdown Flow Research Articles

Surface roughness effects in a transonic axial flow compressor operating at near-stall conditions

Surface roughness is a major contributor to performance degradation in gas turbine engines. The fan and the compressor, as the first components in the engine's air path, are especially vulnerable to the effects of surface roughness. Debris ingestion, accumulation of grime, dust, or insect remnants, typically at low atmospheric conditions, over several cycles of operation are some major causes of surface roughness over the blade surfaces. The flow in compressor rotors is inherently highly complex. From the perspective of the component designers, it is, thus, important to study the effect of surface roughness on the performance and flow physics, especially at near-stall conditions. In this study, we examine the effect of surface roughness on flow physics such as shock-boundary layer interactions, tip and hub flow separations, the formation and changes in the critical points, and tip leakage vortices among other phenomena. Steady and unsteady Reynolds Averaged Navier Stokes calculations are conducted at near-stall conditions for smooth and rough National Aeronautics and Space Administration rotor 67 blades. Surface streamlines, Q-criterion, and entropy contours aid in analyzing the flow physics qualitatively and quantitatively. It is observed that from the onset of stall, to fully stalled conditions, the blockage varies from 21.7% to 59.6% from 90% span to the tip in the smooth case, and from 40.5% to 75.2% in the rough case. This significant blockage, caused by vortex breakdown and chaotic flow structures, leads to the onset of full rotor stall.

Uncertainty quantification analysis of Reynolds-averaged Navier–Stokes simulation of spray swirling jets undergoing vortex breakdown

The computational fluid dynamics-based design of next-generation aeronautical combustion chambers is challenging due to many geometrical and operational parameters to be optimized and several sources of uncertainty that arise from numerical modeling. The present work highlights the potential benefits of exploiting Bayesian uncertainty quantification at the preliminary design stage. A prototypical configuration of an acetone/air spray swirling jet is investigated through an Eulerian–Lagrangian method under non-reactive conditions. Two direct numerical simulations (DNSs) provide reference data, coping with different vortex breakdown states. Consequently, a set of Reynolds-averaged Navier–Stokes simulations is conducted. Polynomial chaos expansion (PCE) is adopted to propagate the uncertainty associated with the spray dispersion model and the turbulent Schmidt number, delivering confidence intervals and the sensitivity of the output variance to each uncertain input. Consequently, the most significant sources of modeling uncertainty may be identified and eventually removed via a calibration procedure, thus making it possible to carry out a combustion chamber optimization process that is no longer affected by numerical biases. The uncertainty quantification analysis in the current study demonstrates that the spray dispersion model slightly affects the fuel vapor spatial distribution under vortex breakdown flow conditions, compared with the output variance induced by the selection of the turbulent Schmidt number. As a result, additional high-fidelity experimental and numerical campaigns should exclusively address the development of an ad hoc model characterizing the spatial distribution of the latter in the presence of vortex breakdown phenomenology, discarding any effort to improve the spray dispersion formulation.

Combined control of variant clearance and casing aspiration on tip flow field and instability of a transonic compressor

To enhance the transonic compressor performance, the axially non-uniform variant tip clearance schemes and combined schemes with casing aspiration were applied and their effects on the tip flow field and flow instability were studied numerically. The results show that the expanding tip clearance can increase the stall margin and peak efficiency by reducing the tip leakage flow intensity within the first half chord, and cause the interface between the leakage flow and mainstream to move downstream. The effect of the shrinking clearance is on the contrary. The compressor performance can be further improved by the combination of expanding clearance and casing aspiration. When the circumferential suction groove is located at 35% of the axial chord length corresponding to the low-velocity region due to the leakage vortex breakdown, the stability enhancement is the largest, which is about 7.2% higher than that under the uniform clearance scheme. Casing aspiration decreases the blade tip loading and attenuates the tip leakage vortex, tip secondary vortex, and their interaction, which results in the suppression of vortex breakdown and tip flow blockage. Moreover, the oscillations and instability of the flow field are weakened. Therefore, the tip flow field is modified and the compressor performance is improved.

Prediction of Vertical Tail Buffet Using CFD/CSD Coupling Method

The vertical tail buffet induced by the vortex breakdown flow is numerically investigated. The unsteady flow is calculated by solving the RANS equations. The structural dynamic equations are decoupled in the modal coordinates. The radial basis functions (RBFs) are employed to generate the deformation mesh. The buffet response of the flexible tail is predicted by coupling the three sets of equations. The results show that the presence of asymmetry flow on the inner and outer surface of the tail forced the structural deflection offsetting the outboard. The frequency of the 2nd bending mode of the tail structure meets the peak frequency of the pressure fluctuation upon the tail surface, and the resonance phenomenon was observed. Therefore, the 2nd bending responses govern the flow field surrounding the vertical tail. Finally, the displacement of the vertical tail is small, while the acceleration with a large quantitation forces the vertical tail undergoing severe addition inertial loads.

Numerical simulation of the unsteady aerodynamic loads on the tail fin in the vortex breakdown flow

Numerical simulation of the unsteady aerodynamic loads on the tail fin in the vortex breakdown flow

Numerical simulation of the unsteady aerodynamic loads on the tail fin in the vortex breakdown flow

This work presents numerical simulation results of the flow around an airbrake-tail fin system of an aircraft. In this research, airbrake location and deflection angles effects on the unsteady aero...

Control of vortex breakdown in confined two-fluid flows

This paper describes the first experimental evidence of how vortex breakdown develops and disappears in both upper and lower fluids in a sealed vertical cylindrical container, where two immiscible fluids circulate driven by the rotating lid while other walls are stationary. The rotating lid generates both swirling and meridional circulations of the upper and lower fluids. The most intriguing and practically important flow phenomenon is the formation of local circulation cells (vortex breakdown). Our experimental study reveals that vortex breakdown can occur in (a) upper, (b) lower, and (c) both fluids. The kind of flow pattern depends on properties of liquids. The vortex breakdown flows can intensify heat and mass transfer.

Correlations of unsteady vortex burst point and dynamic stability over a pitching double-delta wing

Pitching dynamic stability is significant for the aerodynamic performance of an aircraft at a large angle of attack (AoA). Commonly, the dynamic behaviour of vortex breakdown flows over an aircraft with a delta or double-delta wing (DW or DDW) configuration is closely related to the pitching dynamic stability. An 80°/65° DDW subjected to sinusoidal pitching motions around the stall AoA was numerically investigated using the delayed detached eddy simulation method [1]. This study is focused on the correlation between the unsteady behaviour of the vortex burst point (BP) and pitching dynamic stability. The reverse of the pitching dynamic stability is closely related to the inconsistent movement of the BP during the upstroke and downstroke in one pitching period. When the upstream speed of the BP is higher than the downstream speed, the variation in the pitching moment is ahead of the AoA, which leads to dynamic instability. Conversely, when the downstream speed is higher than the upstream speed, the DDW is dynamically stable. This correlation can be interpreted through the energy transfer relationship between the vortex breakdown flows and DDW model.

Bistability of bubble and conical forms of vortex breakdown in laminar swirling jets

Vortex breakdown (VB) in swirling jets can be classified as either a bubble (BVB) or a conical (CVB) form based on the shape of its recirculation zone. The present study investigates the hysteresis features of these forms in laminar swirling jets using direct numerical simulations. It is established here that BVB and CVB are bistable forms in a large swirl range and for a Reynolds number of 200 (based on jet radius and centreline velocity). Considerable differences were observed in the length scales associated with the two, with the approximate recirculation zone diameters of the BVB and CVB being 1 and 15 jet diameters, respectively. Additionally, two types of BVB were observed, identified as a two-celled BVB with spiral tail and an asymmetric BVB. The former is characterized by an almost steady bubble with a two-celled structure. By contrast, the entire bubble envelope oscillated in a non-axisymmetric fashion for the latter. These two types of BVB themselves were found to coexist in a small swirl range. A global linear stability analysis was used to show that two different unstable single helical modes are associated with these two types. In comparison to using the base flow, a stability analysis performed on the mean flow was found to predict the coherent features of asymmetric BVB observed in the simulations more precisely. This study highlights the rich variety of VB flow states that coexist in various ranges of swirl strengths and the significance of hysteresis effects in laminar swirling jets.

Experimental Investigation of Asymmetric Vortex Breakdown Flow Control by Microperturbation over Highly Swept Wings

AbstractAn experimental investigation on asymmetric vortex breakdown flow over three delta wings with sweep angle of 85°, 80°, and 70° was carried out in a wind tunnel at high angles of attack and ...

Dynamic response of vortex breakdown flows to a pitching double-delta wing

A finite volume-based solver with rigid moving mesh and delayed detached eddy simulation (DDES) techniques is implemented to investigate the unsteady flows around an 80°/65° double-delta wing (DDW) subjected to sinusoidal pitching motions. The focus concentrates on understanding the behaviour of the burst point (BP), helical mode instability, pressure fluctuations, and dynamic pitching stability. The role of the reduced frequency (RF) in the response of the above features is analyzed in detail. It is in consistence with the previous experiments that the movement of the BP is nearly a simple harmonic motion and locked with the frequency of the pitching motion accompanied with a phase lag that is strongly affected by the RF. It is found that the time-averaged flow in the post-breakdown regions is approximately a conical flow, whose cone angle also depends on the RF. The natural frequency of BP oscillation at the stationary state of AOA = 36° is the critical frequency, which determines the sign of dynamic pitching derivative. A pair of critical frequencies is found, by which the features of the BP, cone angle of the helical structures, and dynamic pitching stability are divided into several linear sections. Two simplified 1st and 2nd order differential models are proposed and applied for predicting the dynamic behaviour of the BP, and the 2nd order model can give coincident hysteresis loops with the present computational fluid dynamics (CFD) results in a certain range.

Performance of Transition-Sensitive Models in Predicting Flow Structures over Delta Wings

Laminar-turbulent transition may have considerable influence on the performance of moderate and high-speed aerodynamic vehicles. The two recently developed transition-sensitive models, SST-TR and , are employed to assess their abilities in resolving the complex underlying flow physics associated with flow structures over delta wings. In particular, their capabilities in capturing the aerodynamic characteristics of three-dimensional flow associated with vortex breakdown, shear-layer instabilities, and boundary-layer transition are assessed. These closures are employed to simulate vortex breakdown and near-wall flow structures over a delta wing planform with a moderate sweep angle of 50 deg at a Reynolds number of , where a combination of laminar, transitional, and turbulent flowfields coexist. Accuracy of these models in predicting boundary-layer transition adjacent to a delta wing surface with a high-sweep angle of 70 deg and Reynolds numbers of and is investigated. Large-eddy simulation with dynamic Smagorinsky subgrid scale model is also employed to assess its capability in predicting transition on delta wing planforms. All numerical simulations are compared with available experimental data found in the literature.

Mixing in a vortex breakdown flow

AbstractIn this paper we present experimental and theoretical results on the mixing inside a cylinder with a rotating lid. The helical flow that is created by the rotation of the disc is well known to exhibit a vortex breakdown bubble over a finite range of Reynolds numbers. The mixing properties of the flow are analysed quantitatively by measuring the exponential decay of the variance as a function of time. This homogenization time is extremely sensitive to the asymmetries of the flow, which are introduced by tilting the rotating or the stationary disc and accurately measured using particle image velocimetry (PIV). In the absence of vortex breakdown, the homogenization time is strongly decreased (by a factor of 10) with only a moderate tilt angle of the rotating lid (of the order of $1{5}^{\circ } $). This phenomenon can be explained by the presence of small radial jets at the periphery which create a strong convective mixing. A simple model of exchange flow between the periphery and the bulk correctly predicts the scaling laws for the homogenization time. In the presence of vortex breakdown, the scalar is trapped inside the vortex breakdown bubble, and thus increases substantially the time needed for homogenization. Curiously, the tilt of the rotating lid has a weak effect on the mixing, but a small tilt of the stationary disc (of the order of ${2}^{\circ } $) strongly decreases (by a factor of 10) the homogenization time. Even more surprising is that the homogenization time diverges when the size of the bubble vanishes. All of these features are recovered by applying the Melnikov theory to calculate the volume of the lobes that exit the bubble. It is the first time that this technique has been applied to a three-dimensional stationary flow with a non-axisymmetric perturbation and compared with experimental results, although it has been applied often to two-dimensional flows with a periodic perturbation.

Blade Loading Effects on Axial Turbine Tip Leakage Vortex Dynamics and Loss

Abstract Numerical simulations have been carried out to define the loss generation mechanisms associated with tip leakage in unshrouded axial turbines. Tip clearance vortex dynamics are a dominant feature of two mechanisms important in determining this loss: (i) decreased swirl velocity due to vortex line contraction in regions of decreasing axial velocity, i.e., adverse pressure gradient, and (ii) vortex breakdown and reverse flow in the vortex core. The mixing losses behave differently from the conventional view of flow exiting a turbine tip clearance. More specifically, it is shown through control volume arguments and computations that as a swirling leakage flow passes through a pressure rise, such as in the aft portion of the suction side of a turbine blade, the mixed-out loss can either decrease or increase. For turbines, the latter typically occurs if the deceleration is large enough to initiate vortex breakdown, and it is demonstrated that this can occur in modern turbines. The effect of blade pressure distribution on clearance losses is illustrated through computational examination of turbine blades with forward loading at the tip and with aft loading. A 15% difference in leakage loss is found between the two due to lower clearance vortex deceleration (lower core static pressure rise) with forward loading and, hence, lower vortex breakdown loss. Additional computational experiments, carried out to define the effects of blade loading, incidence, and solidity, are found to be consistent with the proposed ideas linking blade pressure distribution, vortex breakdown, and turbine tip leakage loss.

Flow structure over the yawed nonslender diamond wing

The aim of this study is to qualitatively and quantitatively investigate the flow characteristics of a nonslender diamond wing with a sweep angle of Λ=40° by using dye visualization and a stereoscopic Particle Image Velocimetry (stereo-PIV) technique. The flow structure near the stationary diamond wing surface and the formation of the vortex breakdown were studied by varying the yaw angle within the range of 0°⩽θ⩽15° and varying the angle of attack within the range of 10°⩽α⩽17°. Experimental analysis is composed of the time-averaged patterns of streamlines, contours of vorticity distributions, Reynolds stress correlations, transverse and streamwise velocity components, distribution of fluctuating velocities, and turbulent kinetic energy for interpreting the flow physics. It is found that the formation of the vortex breakdown and vortical flow structures are sensitive to the yaw angle; the distributions of the time-averaged flow data over the surface of the diamond wing alter profoundly when the yaw angle of the diamond wing is increased by over 6°. The magnitude of the turbulent kinetic energy decreases on the leeward side of the wing surface because the formation of the vortex breakdown occurs farther downstream than on the windward side, causing less concentrated velocity fluctuations over the wing surface on the leeward side.

Yaw Angle Effect on Flow Structure over the Nonslender Diamond Wing

Yaw Angle Effect on Flow Structure over the Nonslender Diamond Wing

Dynamics of Premixed H2/CH4 Flames Under Near Blowoff Conditions

Swirling flows are widely used in industrial burners and gas turbine combustors for flame stabilization. Several prior studies have shown that these flames exhibit complex dynamics under near blowoff conditions, associated with local flamelet extinction and alteration in the vortex breakdown flow structure. These extinction events are apparently due to the local strain rate irregularly oscillating above and below the extinction strain rate values near the attachment point. In this work, global temporally resolved and detailed spatial measurements were obtained of hydrogen/methane flames. Supporting calculations of extinction strain rates were also performed using detailed kinetics. It is shown that flames become unsteady (or local extinctions happen) at a nearly constant extinction strain rate for different hydrogen/methane mixtures. Based upon analysis of these results, it is suggested that classic Damköhler number correlations of blowoff are, in fact, correlations for the onset of local extinction events, not blowoff itself. Corresponding Mie scattering imaging of near blowoff flames also was used to characterize the spatio-temporal dynamics of holes along the flame that are associated with local extinction.

A coaxial vortex ring model for vortex breakdown

A simple–yet plausible–model for B-type vortex breakdown flows is postulated; one that is based on the immersion of a pair of slender coaxial vortex rings in a swirling flow of an ideal fluid rotating around the axis of symmetry of the rings. It is shown that this model exhibits in the advection of passive fluid particles (kinematics) just about all of the characteristics that have been observed in what is now a substantial body of published research on the phenomenon of vortex breakdown. Moreover, it is demonstrated how the very nature of the fluid dynamics in axisymmetric breakdown flows can be predicted and controlled by the choice of the initial ring configurations and their vortex strengths. The dynamic intricacies produced by the two ring + swirl model are illustrated with several numerical experiments.

Target-free Stereo PIV: a novel technique with inherent error estimation and improved accuracy

A novel, accurate and simple stereo particle image velocimetry (SPIV) technique utilising three cameras is presented. The key feature of the new technique is that there is no need of a separate calibration phase. The calibration data are measured concurrently with the PIV data by a third paraxial camera. This has the benefit of improving ease of use and reducing the time taken to obtain data. This third camera also provides useful velocity information, considerably improving the accuracy of the resolved 3D vectors. The additional redundancy provided by this third perspective in the stereo reconstruction equations suggests a least-squares approach to their solution. The least-squares process further improves the utility of the technique by means of the reconstruction residual. Detailed error analysis shows that this residual is an accurate predictor of resolved vector errors. The new technique is rigorously validated using both pure translation and rotation test cases. However, while this kind of validation is standard, it is shown that such validation is substantially flawed. The case of the well-known confined vortex breakdown flow is offered as an alternative validation. This flow is readily evaluated using CFD methods, allowing a detailed comparison of the data and evaluation of PIV errors in their entirety for this technique.

Direct numerical simulation of passive scalars mixing in an axisymmetric swirl induced vortex breakdown flow

A swirling jet is comprised of axial momentum superimposed on angular momentum. It is a complicated flow whose characteristics and dynamics are not fully understood, although it is widely used in a variety of combusting situations. It is known, for example, that swirling jets have a larger spreading rate and higher mass entrainment than non-swirling jets (Panda & McLaughlin, 1994), however the mechanisms responsible for these characteristics are not known. In combusting flows, swirl is often a desirable commodity as it enhances mixing, stabilises the high intensity combustion process and promotes efficient and clean combustion in a variety of applications, including petrol and diesel engines, gas turbines and industrial furnaces. In determining the rate of combustion, a first approximation is to assume equal diffusivity of all reactants, and then the rate of mixing is the controlling factor determining the rate of combustion. In reality the turbulence will preferentially move a scalar with high molecular diffusivity relative to one with a lower molecular diffusivity. This process, called differential diffusion, is a potential complication of the mixing process and will influence the speed and efficiency of combustion.