Vortex Breakdown Research Articles

South Pole Station ozonesondes: variability and trends in the springtime Antarctic ozone hole 1986–2021

Abstract. Balloon-borne ozonesondes launched weekly from South Pole Station (1986–2021) measure high-vertical-resolution profiles of ozone and temperature from the surface to 30–35 km altitude. The launch frequency is increased in late winter before the onset of rapid stratospheric ozone loss in September. Ozone hole metrics show that the yearly total column ozone and 14–21 km partial column ozone minimum values and September loss rate trends have been improving (less severe) since 2001. The 36-year record also shows interannual variability, especially in recent years (2019–2021). Here we show additional details of these 3 years by comparing annual minimum profiles observed on the date when the lowest integrated total column ozone occurs. We also compare the July–December time series of the 14–21 km partial column ozone values to the 36-year median with percentile intervals. The 2019 anomalous vortex breakdown showed stratospheric temperatures began warming in early September followed by reduced ozone loss. The minimum total column ozone of 180 Dobson units (DU) was observed on 24 September. This was followed by two stable and cold polar vortex years during 2020 and 2021 with total column ozone minimums at 104 DU (1 October) and 102 DU (7 October), respectively. These years also showed broad near-zero-ozone (loss saturation) regions within the 14–21 km layer by the end of September which persisted into October. Validation of the ozonesonde observations is conducted through the ongoing comparison of total column ozone measurements with the South Pole ground-based Dobson spectrophotometer. The ozonesondes show a more positive bias of 2 ± 3 % (higher) than the Dobson following a thorough evaluation and homogenization of the long-term ozonesonde record completed in 2018.

Investigation of the unsteady flow in a transonic axial compressor adopted in the compressed air energy storage system

During operation, compressed air energy storage systems should respond rapidly to variations in power network demand, requiring that the compression system should always be in changeable off-design conditions. Compression systems with low flow rates confront difficulties such as diminished aerodynamic performance and increased flow losses. Given that the emergence of compressor instability is a process of the accumulation and intensification of flow unsteadiness. In this study, a numerical simulation was conducted for a transonic compressor to investigate the unsteady evolution characteristics of the tip leakage flow field, and it focused on the flow field of a condition where the flow unsteadiness was stimulated and further investigated the origin of flow unsteadiness, which was supposed to provide insights in developing flow control strategies and enable the application in a compressed air energy storage system under low mass flow rate conditions. The Fourier fast transformation (FFT) signal analysis showed that characteristic frequency of the unsteady tip flow field was 1863.3 Hz. Unsteady disturbances were predominantly distributed in the following three regions: the leakage vortex region of the blade suction surface, the location of the epitaxially detached shock wave at the entrance of passage, and the 5 %–30 % chord of the blade pressure surface. The high disturbance region of leakage vortex was generated by the periodic shift in leakage vortex intensity over time, the interaction between the main flow and various types of secondary flow due to leakage vortex breaking processes was the primary cause for the disturbance region of the epitaxially detached shock wave at the entrance of passage, and the high disturbance region of blade pressure surface was caused by the adverse pressure gradient effect and flow direction deflection of the airflow in the passage, as well as the impacted on the pressure surface of the adjacent blade. It was speculated that leakage vortex breakdown due to vortex wave interference was the key factor to induce the unsteadiness of the flow field. A new vortex structure was formed at the leading edge of adjacent blades after leakage vortex breakdown, which was defined as a “leading edge vortex”, and its series of evolutionary processes occurring timely were the main reasons for the unsteady flow in the tip flow field. This study laid a theoretical foundation for the research of the flow field control strategy of axial compressor operating in off-design conditions in a large-scale compressed-air energy storage system.

Resonator-like behavior of a wall-bounded precessing vortex core in a diffuser with wall asymmetries

This paper reports a detailed investigation of the interaction between a wall-bounded precessing vortex core (PVC) occurring in swirling flows after vortex breakdown and a wall asymmetry. Experiments are carried out in an axisymmetric diffuser downstream of an axial swirl generator inducing a swirling flow with a swirl number of S = 1.1. Wall pressure measurements and two-component particle image velocimetry (PIV) are conducted for Reynolds numbers (Re) ranging from 20 000 to 76 000 in the initial axisymmetric configuration and several asymmetric configurations, with an additional cylindrical protrusion placed on the diffuser wall at different streamwise and circumferential positions. It is first confirmed that synchronous pressure fluctuations at the PVC frequency are only produced in asymmetric configurations. Furthermore, the analysis of the pressure data in several asymmetric configurations revealed for the first time a resonator-like behavior of a wall-bounded PVC. While a change of the protrusion circumferential position in a given cross section of the diffuser only affects the phase of the synchronous pressure fluctuations, the amplitude of the latter features successive minima (pressure node) and maxima (pressure anti-node) as the protrusion is moved along the diffuser in the streamwise direction. In addition, as the protrusion is moved closer to a pressure node, the phase of the synchronous pressure fluctuations exhibits a sudden variation of ±π. Similar results are observed for all tested values of Reynolds number, whereas the PVC frequency linearly increases with Re. A reconstruction of the PVC helical structure based on PIV measurements showed that these consecutive pressure nodes are spaced by a distance equal to approximately one third of the PVC helical pitch. Finally, it also revealed that two different states are observed, depending on the position of the protrusion along the diffuser: the synchronous pressure component reaches its maximum value as the PVC center is approaching either its closest or farthest angular position with respect to the protrusion. The transition from one state to another one depends on the streamwise position of the protrusion with respect to the pressure nodes. These unprecedented experimental observations pave the way to novel theoretical developments for a better understanding and modeling of synchronous pressure fluctuations induced by wall-bounded PVC in asymmetric geometries.

Characterizing Swirl Strength and Recirculation Zone Formation in Tangentially Injected Isothermal Flows

This paper presents a computational study characterizing the swirl intensity distribution and Internal Recirculation Zone (IRZ) formed in a cylindrical domain with tangential injections and isothermal flow. The range of inlet boundary conditions investigated is 5o to 25o for the injection angle and 7190 to 100711 for the bulk flow Reynolds number. The evolution of swirl intensity is presented with and without incorporating effects of the accompanying pressure variations. The Shear Stress Transport (SST) k-ω is used to model turbulence. Results show that the Swirl strength created by such tangential injections strongly depends on the injection angle but does not vary with bulk flow Reynolds number (Re), except for low Re values. The swirling flow is shown to result in IRZ formation at injection angles 6o and above or when asymptotic value of the maximum Swirl Number in the domain exceeds approximately 0.6, same as the transition value of inlet Swirl Number in swirling flows with axial injections. The IRZ length increases with injection angle and varies with Re for lower values of Re at a given injection angle but asymptotes for higher values above 40000. The conventional Swirl Number rises rapidly downstream of the injection plane followed by a slow decline. On the other hand, an alternative Swirl Number, which incorporates the gauge pressure variation, shows slow and consistent decay all the way downstream of the injection plane. The Swirl Number incorporated with gauge pressure term subsumes interconversions between the axial momentum and pressure in the regions of vortex breakdown and IRZ formation, thereby presenting an alternative picture of swirl intensity evolution in swirling flows.

Wall temperature effects on the hypersonic boundary-layer transition over an inclined, blunt cone

Wall temperature is a crucial factor for the transition of hypersonic boundary layers. This paper investigates the wall temperature effects on the hypersonic boundary-layer transition over an inclined blunt cone with two wall temperatures by direct numerical simulations and instability analysis. The breakdown processes of leeward streamwise vortices and crossflow vortices are both studied. Inner modes with lower phase velocities and outer modes with higher phase velocities are responsible for the breakdown of leeward streamwise vortices, respectively, in cooled- and heated-wall cases. A higher wall temperature will promote the boundary layer transition in the vicinity of the leeward centerline due to the collision of nearby crossflow vortices with the leeward mushroom structure, instead of by destabilizing inner and outer modes. In the crossflow-dominated region, the transition is triggered by the interaction between second modes and crossflow vortices for the cooled-wall case. Increasing the wall temperature will promote the transition in this region even though second modes are suppressed.

Buoyancy force and magnetic field effects on laminar vortex breakdown and fluid layers

In this study, the Generalized Integral Transformation Technique (GITT) is used to describe the effect of buoyancy force and magnetic field on the vortex breakdown process generated by the rotation of an electrically conductive fluid. A magnetic field is positioned vertically to stabilize the swirling flow caused by the rotation of the bottom disc of a cylindrical recipient. Three fluids were compared in this study where the range of Richardson number is 0 ≤Ri ≤2.0. When the temperature difference is greater than Ri = 0.1, many layers become visible. These stratified flu id layers act as thermal insulators. In the case of stratification, the increased magnetic field reduces the total number of layers formed in the fluid. The influence of gradient temperature on the distribution of the layers generated is discussed. The limitations between the multilayer structure and the monolayer structure for three fluids are calculated as a function of the flow parameters.

DNS Study on Turbulent Transition Induced by an Interaction between Freestream Turbulence and Cylindrical Roughness in Swept Flat-Plate Boundary Layer

Laminar-to-turbulent transition in a swept flat-plate boundary layer is caused by the breakdown of the crossflow vortex via high-frequency secondary instability and is promoted by the wall-surface roughness and the freestream turbulence (FST). Although the FST is characterized by its intensity and wavelength, it is not clear how the wavelength affects turbulent transitions and interacts with the roughness-induced transition. The wavelength of the FST depends on the wind tunnel or in-flight conditions, and its arbitrary control is practically difficult in experiments. By means of direct numerical simulation, we performed a parametric study on the interaction between the roughness-induced disturbance and FST in the Falkner–Skan–Cooke boundary layer. One of our aims is to determine the critical roughness height and its dependence on the turbulent intensity and peak wavelength of FST. We found a suppression and promotion in the transition process as a result of the interaction. In particular, the immediate transition behind the roughness was delayed by the long-wavelength FST, where the presence of FST suppressed the high-frequency disturbance emanating from the roughness edge. Even below the criticality, the short-wavelength FST promoted a secondary instability in the form of the hairpin vortex and triggered an early transition before the crossflow-vortex breakdown with the finger vortex. Thresholds for the FST wavelengths that promote or suppress the early transition were also discussed to provide a practically important indicator in the prediction and control of turbulent transitions due to FST and/or roughness on the swept wing.

A Study of the Velocity Field During Mitigation of Vortex Breakdown in Model Francis Turbine at High Load

Abstract Off-design operation leads to the development of flow instabilities like vortex breakdown phenomenon which manifests as an enlarged vortex core in the draft tube at high load operating conditions. These flow instabilities are known to potentially have detrimental effects on turbine performance necessitating investigations on their formative and mitigation mechanisms. This work clarifies the evolving velocity maps characterizing vortex breakdown seen in a model Francis turbine draft tube during the transition from high load to best efficiency point. Velocity measurements have been performed inside a draft tube cone using a 2D particle image velocimetry system. Results show a wake-like velocity profile characterizing the vortex core in the draft tube cone at high load condition. The vortex core is a centrally located flow feature embodying a quasi-stagnant flow with recirculation regions. Surrounding the core, an axial outflow is seen with shear layers arising at the interface of core and outflow due to a substantial velocity gradient. Mitigation of this vortex core through a load rejection operation was further investigated. It is seen that as the flowrate approaches the best efficiency point, the shear layers between the outflow and central stagnation region break. The breakup leads to an axially dominated and streamlined flow. This is enabled by the reduction of the swirl until no central flow separation at the stagnation point occurs. The flow at the best efficiency point is thus devoid of the vortex core due to the absence of flow stagnation, the primary instability causing the core development.

Multi-fidelity vortex simulations of rotor flows: Validation against detailed wake measurements

Two flow models with different fidelity of the DTU vortex solver MIRAS have been used to simulate the wake generated by a model wind turbine with various levels of asymmetry. Predictions are validated against experimental Particle Image Velocimetry measurements and dye visualizations. The experiments were conducted in a recirculating free-surface water channel with an immersed two-bladed rotor mounted on a shaft. The blades were designed to approximate a Joukowsky rotor. A detailed comparison between the measurements and the simulations is first performed for an unperturbed baseline case at different tip speed ratios. The analysis consists of a tip vortex characterization, including the vortex core profile, and a comparison of the instantaneous and mean velocity and vorticity fields, along with the mean wake profile at different downstream locations. Overall, good agreement is obtained between measurements and simulations, especially with the higher-fidelity particle-mesh model, which is capable of very closely predicting many of the flow features observed in the experiments. Rotor asymmetry triggers a vortex instability, commonly known as leapfrogging, which accelerates the breakdown of tip vortices, enhancing the mixing of wake structures and promoting a faster wake recovery. The prediction accuracy of this instability by the solvers is analyzed for different tip speed ratios and perturbation amplitudes. This work aims at setting the groundwork for future flow instability studies with the MIRAS solver.

Global modes of variable-viscosity two-phase swirling flows and their triadic resonance

The linear and nonlinear dynamics of two-phase swirling flows produced by the Grabowski–Berger profile under the influence of a viscosity stratification are investigated. We perform axisymmetric nonlinear simulations and fully three-dimensional linear global stability analysis of the flow for several swirl numbers $S$ and viscosity ratios $\tilde {\mu }$ . They are accompanied by nonlinear three-dimensional simulations and subsequent modal analysis using the bispectral mode decomposition, recently introduced by Schmidt (Nonlinear Dyn., vol. 102, issue 4, 2020, pp. 2479–2501). We find a pronounced destabilising effect of the viscosity stratification on both the onset of axisymmetric vortex breakdown and helical instability that is linked to the required shear stress continuity across the interface. Consequently, destabilisation is shifted to lower $S$ as compared with an equivalent flow with uniform viscosity. Further, the stability analysis reveals the simultaneous destabilisation of two global modes with wavenumbers $m=1$ and $m=2$ that have harmonic frequencies. The analysis of the nonlinear flow reveals a strong triadic resonance between these modes that governs the nonlinear dynamics and leads to a rapid departure from the linear dynamics. At larger swirl, the bifurcation of additional modes initiates an interaction cascade by means of triadic resonance which is elucidated by the bispectral analysis. It leads to the emergence of a variety of additional modes in the nonlinear flow. This study contributes to an improved understanding of the influence of viscosity stratification on the onset of vortex breakdown and the destabilisation of global modes. Further, it provides a clear picture of the dynamics of swirling flows with a codimension-two point and related triadic interaction of two global modes at harmonic frequencies and wavenumbers.

Evolution of tornado-like vortices in three-dimensional compressible rectangular cavity flows

The spatial structure and time evolution of tornado-like vortices in a three-dimensional cavity are studied by topological analysis and numerical simulation. The topology theory of the unsteady vortex in the rectangular coordinate system (Zhang, Zhang & Shu, J. Fluid Mech., vol. 639, 2009, pp. 343–372) is generalized to the curvilinear coordinate system. Two functions $\lambda (q_1,t)$ and $q(q_1,t)$ are obtained to determine the topology structure of the sectional streamline pattern in the cross-section perpendicular to the vortex axis and the meridional plane, respectively. The spiral direction of the sectional streamlines in the cross-section perpendicular to the vortex axis depends on the sign of $\lambda (q_1,t)$ . The types of critical points in the meridional plane depend on the sign of $q(q_1,t)$ . The relation between the critical points of the streamline pattern in the meridional plane and that in the cross-section perpendicular to the vortex axis is set up. The flow in a three-dimensional rectangular cavity is numerically simulated by solving the three-dimensional Navier–Stokes equations using high-order numerical methods. The spatial structures and the time evolutions of the tornado-like vortices in the cavity are analysed with our topology theory. Both the bubble type and spiral type of vortex breakdown are observed. They have a close relationship with the vortex structure in the cross-section perpendicular to the vortex axis. The bubble-type breakdown has a conical core and the core is non-axisymmetric in the sense of topology. A criterion for the bubble type and the spiral type based on the spatial structure characteristic of the two breakdown types is provided.

The Role of Hydrodynamic Instabilities on Near-Lean Blowout Flame Shapes in a Swirl-Stabilized Spray Combustor

Abstract This study investigates the role of hydrodynamic instabilities on near-lean blowout (LBO) flame shapes in a swirl-stabilized spray combustor. Hydrodynamic instabilities often manifest themselves in swirling flows as a helical vortex that winds around the vortex breakdown bubble. However, the heat released from combustion tends to suppress coherent vortex structures, which can limit the helical vortex to certain combustor geometries and operating conditions. Flame shape changes often accompany changes in hydrodynamic stability because they reposition the heat release and consequently modify the degree of coherent vortex suppression. In this study, laser diagnostics measurements were used to characterize the flow fields and spray patterns corresponding to different flame shapes that were observed in the Air Force Research Laboratory (AFRL) referee combustor. In particular, the flame fluctuated between its original shape, FS1, and a new flame shape, FS2, when the combustor operated on the threshold of LBO. Proper orthogonal decomposition (POD) was used to analyze the measurements. POD showed that the appearance of FS2 coincided with coherent vortex structures that resembled those in the hydrodynamically unstable nonreacting flow field. Furthermore, fuel Mie scattering measurements and phase-averages of the velocity field provided evidence that the FS2 spray was periodically disturbed by a helical vortex. Near the swirler exit, this helical vortex structure involved both outer and inner shear layer vortices that appeared to be synchronized with each other. However, the inner shear layer vortices decayed as the flow progressed downstream and only the outer shear layer vortices remained throughout the measurements' field of view. In contrast, there was no indication of a helical vortex structure in either the flow field or fuel spray measurements corresponding to FS1.

A Tale of Two Vortex Evolutions: Using a High-Resolution Ensemble to Assess the Impacts of Ventilation on a Tropical Cyclone Rapid Intensification Event

Abstract The multiscale nature of tropical cyclone (TC) intensity change under moderate vertical wind shear was explored through an ensemble of high-resolution simulations of Hurricane Gonzalo (2014). Ensemble intensity forecasts were characterized by large short-term (36-h) uncertainty, with a forecast intensity spread of over 20 m s−1, due to differences in the timing of rapid intensification (RI) onset. Two subsets of ensemble members were examined, referred to as early-RI and late-RI members. The two ensemble groups displayed significantly different vortex evolutions under the influence of a nearby upper-tropospheric trough and an associated dry-air intrusion. Mid-to-upper-tropospheric ventilation in late-RI members was linked to a disruption of inner-core diabatic heating, a more tilted vortex, and vortex breakdown, as the simulated TCs transitioned from a vorticity annulus toward a monopole structure. A column-integrated moist static energy (MSE) budget revealed the important role of horizontal advection in depleting MSE from the TC core, while mesoscale subsidence beneath the dry-air intrusion acted to dry a deep layer of the troposphere. Eventually, the dry-air intrusion retreated from late-RI members as vertical wind shear weakened, the magnitude of vortex tilt decreased, and late-RI members began to rapidly intensify, ultimately reaching a similar intensity as early-RI members. Conversely, the vortex structures of early-RI members were shown to exhibit greater intrinsic resilience to tilting from vertical wind shear, and early-RI members were able to fend off the dry-air intrusion relatively unscathed. The different TC intensity evolutions can be traced back to differences in the initial TC vortex structure and intensity. Significance Statement Despite recent advances, tropical cyclone intensity forecasts struggle to accurately predict episodes of rapid intensification. Such forecasts become increasingly challenging when a storm is embedded within an environment of moderate vertical wind shear. This study uses an ensemble of high-resolution simulations to examine how environmental influences can affect the tropical cyclone vortex and precipitation structure, which, in turn, modulate the intensity of the storm and the onset of rapid intensification. We propose a feedback that exists where slightly weaker and less resilient vortices are more susceptible to ventilation from dry, environmental air, aided in part by differential advection from the tilted circulation, resulting in a degradation of vortex organization and a delayed onset of rapid intensification.

Experimental and numerical studies of the effects of the contraction ratios on the swirling flow characteristics of the model combustor outlet in lean gas turbines

Large Eddy Simulation (LES) is utilized in this work to study isothermal turbulent swirling flow characteristics for a lean partially premixed gas turbine model combustor. The goal is to investigate the influence of contraction ratio (CR) variation on the vortex breakdown phenomena features and the turbulence structure in these combustors. Particle image velocimetry (2D-PIV) measurements were carried out on a single-case model combustor using (air-air) phase to perform the mean flow behavior and the validation process under atmospheric conditions with CR equal to 0 (largest diameter, Dinlet = Doutlet), the equivalence ratio (φ) of 0.3, swirl number of (Sn) = 0.66 and Reynolds number of (Re) = 42,000. While five test cases with various CRs were applied in LES under the same conditions using (air-CH4) phase to predict the vortical structure and the precession vortex core (PVC). The CRs ranged as follows: 0, 50 %, 60 %, 70 % and 80 % (smallest diameter). Experimental and LES results show that there was a reasonable agreement between the PIV and LES results with maximum deviations of 2.8 % and 4.65 % for the normalized mean axial and radial velocities, respectively. As the CR was increased, the pressure drop and the mean axial velocity increased linearly at the contraction part. In addition, the downstream stagnation point and the size of the central toroidal recirculation zone (CTRZ) stabilized when CR = 60 %, 70 %, and 80 %. The swirl level was further enhanced by 40 % when CR = 60 % and 80 %. The presence of CR successfully suppressed the pressure and axial velocity fluctuations generated in the shear layer zones by 35 %. LES data was analyzed by Fast Fourier Transformation (FFT) and Proper Orthogonal Decomposition (POD) to investigate the PVC dynamics. The FFT showed four dominant frequencies as a result of the appearance of the PVC and vortex breakdown oscillation. The frequency of PVC (fpvc) decreased linearly with an increase in the CR where the fpvc was 20.8 Hz when CR = 0 and became 12.5 Hz at CR = 80 %. In contrast, the other frequencies were still constant even with increased CR at 187 Hz, 562 Hz, and 979 Hz. The POD analysis showed the first mode was more energetic and had 40 % of the total pressure fluctuations.

Simplified model for helical vortex dynamics in the wake of an asymmetric rotor

Helical vortex systems, such as those found in the wakes of wind turbines, helicopter rotors and propellers, are subject to instabilities that lead to pairing between adjacent vortex loops. Certain modes of these instabilities can be triggered by an asymmetry in the rotor generating the vortices. In three-vortex systems, like those formed by many industrial rotors, the nonlinear vortex interactions are highly complex, introducing the need for a simple model to predict their dynamics. The current study presents a model for helical vortex systems based on an infinite strip of periodically repeating point vortices, whose motion can be computed using a single equation. This highly simplified model is shown to accurately reproduce the helical vortex dynamics predicted by a more sophisticated filament model and observed in water channel experiments on model rotors. The model is then used to investigate different types of vortex perturbations. Perturbation direction is found to have an important effect on the evolution of the instability, and displacements are observed to induce vortex pairing more quickly than circulation changes. These findings can be used to design asymmetric rotors that induce vortex breakdown more effectively, mitigating detrimental wake effects such as increased fatigue loading on downstream structures.

NUMERICAL INVESTIGATION OF FLOW CHARACTERISTICS OF A NON-SLENDER LAMBDA WING UNMANNED AERIAL VEHICLE

In this paper, the low-speed aerodynamic performance of lambda wing with a chord length of c=210 mm and sweep angle of Λ =51°, thickness 3 mm, and beveled leading edges on the windward side with an angle of 58° is investigated numerically. Reynolds Average Navier Stokes (RANS) theorem equations with Spalart-Allmaras turbulence model were solved up to an angle of attack 45° for incompressible flow around the wing surface and, are compared to experiment to corralete simulation precision of computational fluid dynamic approaches. Detail about the aerodynamic performance of lambda wing including development and formation of the leading-edge vortex (LEV), the interaction of flow with the surface, flow separations, and stall are studied, presented, and discussed. LEV was started at 5°, vortex breakdown was observed at halfway along the leading edge at the angle of 20°, finally, by the time angle is 30°, bursting vortex gives a way to stall stage.

Experimental and numerical study on an enhanced swirl cooling with convergent tube wall and local dimple arrangements

This paper experimentally and numerically investigates an enhanced swirl cooling structure, featuring multiple connected convergent swirl tubes and dimpled wall. The dimpled swirl tube adopts a novel local dimpled wall scheme, only arranging dimple arrays under the tangential jets, which takes advantage of the heat transfer potential of the individual dimple. A counterpart multistage convergent swirl tube without dimples is also studied as a comparison baseline to clarify the action of local dimple arrays on swirling flow. The spatially resolved heat transfer data are measured by applying transient thermochromic liquid crystal (TLC) technique. Four channel Reynolds numbers (Re = 20,000 to 50,000) are tested based on the maximum tube parameters. The results show that the enhanced dimpled swirl tube performs a synergistic reinforcement of the high-speed swirling flow over the dimple arrays, which produces many local high Nusselt number patterns. Under the individual injection slot, the upstream dimples have a better enhanced heat transfer performance. For the studied working conditions, the globally averaged Nusselt number of the dimpled swirl tube can be increased by 6.0%–11.5% in comparison with the baseline convergent swirl tube. Besides, the dimpled wall also suppresses the swirl intensity, which leads to the weakening of wall-fluid shear stress action and the disappearance of downstream vortex breakdown, and consequently reduces the pressure loss by 2.0%–10.2%. Based on the well-validated CFD results, the dimpled wall under incoming jets redistributes the contribution of the near-wall turbulence and the forced convection due to the near-wall flow motion to the heat transfer enhancement in the swirl tube, which achieve an ideal cooling performance of high heat transfer in combination with low pressure loss. This provides a new inspiration for the design and optimization of swirl cooling device.

Identification and dynamics of coherent structures in a Coanda swirling jet flow

Swirl stabilised flows are the most common technology for gas turbine combustion stabilisation. Although success on the characterisation of these flows is tangible, there are several structures that appear in such flows that are still a concern for fundamental and applied research. One of these structures is the Precessing Vortex Core (PVC), whose ambiguous characteristics make it an adequate mechanism for chemical mixing whilst being a detrimental component for thermoacoustic stabilisation. The PVC has been extensively characterised in swirling jet flows exhibiting vortex breakdown (in this study named Open Jet Flow or OJF). However this has not been the case for swirling Coanda (i.e. wall attached) Jet Flows (CoJF). Therefore, this research presents experimental work detailing the identification and visualisation of coherent structures in both types of jet flows. A generic swirl burner was fitted with a base plate in a position so that any of these two flows, OJF and CoJF, could be obtained within the same burner geometry. High speed stereo particle image velocimetry (S-PIV) was used to obtain a time-resolved flow field of each flow state at different flow rates. Two PVCs were clearly identified in both flows, one in the inner shear layer between the vortex breakdown bubble and the jet and one between the jet and the ambient. Precessing frequencies for the CoJF where ∼8.5% higher (St ∼0.9-1.0) than those of the OJF (St ∼0.8-0.9). Spectral proper orthogonal decomposition was used to identify the coherent structures in the flow. Swirl strength (λci2) analysis was conducted on the coherent modes to establish quantitative and qualitative results and to produce a 3D reconstruction of the flow field. The results indicate that precessing coherent flow structures are present in wall attached Coanda swirling flows and these are similar to the structures found in swirling jet flows undergoing vortex breakdown.

Control of separated flow transition over a highly loaded compressor blade via dynamic surface deformation

The laminar separation and transition process seriously deteriorate the aerodynamic performance of a compressor blade at a low Reynolds number (Re). To suppress the laminar separation without reattachment over a highly loaded compressor blade at Re=3.5 × 105, an active flow control strategy based on dynamic surface deformation (DSD) is numerically investigated by large eddy simulations (LESs). The control mechanism of DSD is analyzed, and the vortex dynamics in the transition process are resolved. Furthermore, the loss mechanism is clarified based on the turbulent kinetic energy (TKE) budget analysis. For the uncontrolled case, the rolling-up and breakdown of large-scale hairpin vortices accelerate the generation of turbulent fluctuations, which distinctly extend the region of high-level TKE production term and thus cause large total pressure loss. As the DSD is activated at the oscillation frequency f0 (f0 is the vortex shedding frequency in separated shear layers), periodic pressure fluctuation waves are induced with the appearance of small-scale vortices. The convection of these vortices enlarges the local vorticity and makes the boundary layer velocity profile fuller, which successfully eliminates the large-scale laminar separation on the compressor blade. As such, the region of high-level TKE production term shrinks distinctly due to the weakened vortex dynamics, and the total pressure loss is reduced by 78.9%. With increasing the oscillation frequency from f0 to 2f0 or 4f0, the loss is slightly enlarged due to the rapid growth of turbulent boundary layers, but it is still lower than that of the uncontrolled case. The results demonstrate that DSD is feasible in controlling large-scale laminar separation, and there exists an optimal oscillation frequency.

Numerical Investigation of a Coupled Blow-Off/Flashback Process in a High-Pressure Lean-Burn Combustor

Abstract Large eddy simulation is used to investigate the flashback mechanism caused by the combustion-induced vortex breakdown (CIVB) in a high-pressure lean-burn annular combustor with lean direct injection of kerosene. A single sector of the geometry, including a central pilot flame surrounded by a main flame, is simulated at takeoff conditions. A previously developed flamelet-based approach is used to model turbulence–combustion interactions due to its relatively low cost, allowing to simulate a sufficiently long time window. In stable operations, the flame stabilizes in an M-shape configuration and a periodic movement of the pilot jet, with the corresponding formation of a small recirculation bubble, is observed. Flashback is then observed, with the flame accelerating upstream toward the injector as already described in other studies. This large eddy simulation (LES), however, reveals a precursor partial blow-out of the main flame induced by a cluster of vortices appearing in the outer recirculation region. The combined effect of vortices and sudden quenching alters the mixing level close to the injector, causing first the main, then the pilot flame, to accelerate upstream, and initiate the CIVB cycle before the quenched region can re-ignite. Main and pilot flames partly extinguish as they cross their respective fuel injection point, and re-ignition follows due to the remnants of the reaction in the pilot stream. The process is investigated in detail, discussing the causes of CIVB-driven flashback in realistic lean-burn systems.