Swirling Jet Flow Research Articles

Mean and fluctuating helicity in swirling jet flows

Mean and fluctuating helicity in swirling jet flows

Reacting Flow Prediction of the Low-Swirl Lifted Flame in an Aeronautical Combustor With Angular Air Supply

Abstract The development of lean-burn combustion systems is of paramount importance for reducing the pollutant emissions of future aero engine generations. By tilting the burners of an annular combustor in circumferential direction relative to the rotational axis of the engine, the potential of increased combustion stability is opened up due to an enhanced exhaust gas recirculation between adjacent flames. The innovative gas turbine combustor concept, called the short helical combustor (SHC), allows the main reaction zone to be operated at low equivalence ratios. To exploit the higher stability of the fuel-lean combustion, a low-swirl lifted flame is implemented in the staggered SHC burner arrangement. The objective is to reach ultralow NOx emissions by complete evaporation and extensive premixing of fuel and air upstream of the lean reaction zone. In this work, a modeling approach is developed to investigate the characteristics of the lifted flame in an enclosed single-burner configuration, using the gaseous fuel methane. It is demonstrated that by using the large eddy simulation method, the shape and liftoff height of the flame are adequately reproduced by means of the finite-rate chemistry approach. For the numerical prediction of the lean lifted flame in the SHC arrangement, the focus is on the interaction of adjacent burners. It is shown that the swirling jet flow is deflected toward the sidewall of the staggered combustor dome, which is attributed to the asymmetrical confinement. Since the stabilization mechanism of the low-swirl flame relies on outer recirculation zones, the upstream transport of hot combustion products back to the flame base is studied by the variation of the combustor confinement ratio. It turns out that increasing the combustor size amplifies the exhaust gas recirculation along the sidewall, and increases the temperature of recirculating burned gases. This study emphasizes the capability of the proposed lean-burn combustor concept for future aero engine applications.

Mitigation of PWR fuel assembly vibrations using bio-inspired nozzles

Jet flows injected in a transverse flow need rapid and effective mixing for various applications ranging from medicine injection into bloodstreams to nuclear pressurized water reactors (PWRs). Inspired by marine organisms, spiral snails, and sharks, bio-inspired nozzles are proposed and experimentally investigated to explore their advantages in suppressing nuclear fuel assembly vibrations. It has been observed that a combination of axial flow and jet cross-flow causes vibrations of fuel rods and potential wear at spacer grid supports. Marine biomimetics is used to improve the mixing between the jet flow and surrounding fluid flows. Inspired by the structure of gastropod shells, a variable whorl spacing nozzle is proposed to induce a swirling jet flow to enhance the mixing rate with the flow inside the reactor cores. In addition, the smooth maneuverability of the sharks highlights the importance to include gill slits structure into nozzles. This work focuses on mitigating PWR fuel assembly vibrations using two biomimetic nozzles, a snail nozzle and a shark nozzle. These two nozzles are proposed to improve the mixing rate between the injected flow and the primary coolant flow, resulting in a reduced jet flow effect on fuel rods. A single-span mock-up PWR array is designed, fabricated, and instrumented to mimic the real nuclear fuel assembly. The array is experimentally tested under combined axial flow and jet cross-flow to investigate its dynamical behavior. Three different nozzles, a basic circular nozzle, a snail nozzle, and a shark nozzle, are tested. The research investigates the ability of the proposed marine biomimetic nozzles to suppress the vibration of the rod bundle by comparing the results from the three tested nozzles. The obtained results suggest that the proposed snail-inspired biomimetic nozzle is significantly better than the circular nozzle since it reduces rod bundle vibration by increasing flow mixing. A 50% reduction was achieved by implementing it instead of the circular nozzle. More importantly, the shark-inspired nozzle delays the critical jet flow rate, at which the unstable vibration occurs in the rod bundle, by 20%. In addition to delaying instability, a vibration amplitude reduction of 87.5% was obtained using the proposed shark-inspired nozzle compared to the circular nozzle. The results are promising for various applications including gas burners, combustion chambers, and chemical reactors for providing efficient and rapid mixing between two fluid streams.

Investigation on the flow characteristics near the duct of a pump-jet propulsion under different advance coefficient conditions

The flow near the duct is very important for the efficiency and stable operation of a pump-jet propulsion. To study the flow near the outer surface and outlet of a duct, the larger-eddy simulation (LES) method is used to analyze the flow characteristics under different operating conditions. Cv on different lines in the swirling-jet flow obtained by the LES method fits well with that of the laser Doppler velocimeter experiment. Based on the reliable results of numerical simulation, it was found that lower inflow velocity delays the generation of separation vortex on the duct's outer surface. When the developing separation vortex interacts with the swirling-jet flow, the strong interaction may destroy the structure of the tip leakage vortex and has a huge influence on the swirling-jet flow with the increasing advance coefficient. As a result, the stronger interaction makes the downstream swirling-jet flow even more complicated. In addition, the strongest intensity of the hub vortex occurs when the advance coefficient is 1.4. ui′uj′‾ is very important for the reallocation of the time-average fluctuation kinetic energy. Therefore, it is selected for further flow analysis. Compared with others terms, uz′uz′‾ has larger magnitude and is consistent with the distribution of Ck.

Comparison of turbulence models for solving problems of swirling jet flows

There are several turbulence models that can be used to solve the swirling jet problems, which are typical turbulence problems. A comparison of these models allows us to determine which one is better suited for a given task. When comparing turbulence models for the swirling jet problem, the most important criteria are the accuracy and stability of the solution. Accuracy is assessed by comparison with experimental data or other numerical methods, and stability is assessed by the absence of oscillations and convergence of the solution. In addition, it is also important to consider the computational complexity of each model and its applicability to a specific problem. The swirling flow in a nozzle is studied in the article using various turbulence models (SA, k-ε, k-ω, L-VEL, v2-f, yPlus, SST). To evaluate the performance of different CFD (Computational Fluid Dynamics) turbulence models, their numerical results for velocity profiles are compared with known experimental data.

DES AND RANS MODELING OF PRIMARY ATOMIZATION IN A COAXIAL SWIRLING LIQUID-GAS JET

This study assesses different turbulence modeling approaches for simulation of two-phase coaxial annular swirling jet flows. The problem selected from literature for comparison involves an analytical inlet profile for an annular liquid sheet sandwiched between two coaxial annular gaseous jets. The liquid-gas interface is resolved using the volume-of-fluid model with continuum surface force approximation. Reynolds-averaged Navier-Stokes simulations and detached eddy simulations (DES) are conducted to obtain transient multiphase numerical solutions. Different turbulence models explored include the <i>k-ε</i> renormalization group (RNG) with swirl modification, the Reynolds stress model (RSM), RSM with scale-adaptive simulation (RSM-SAS), and DES. Comparisons with the direct numerical results from literature suggest that the <i>k-ε</i> RNG and RSM approaches simulate only the streamwise shear of the liquid jet and are inadequate in capturing the swirling aspect of the jet flow and expected instabilities. DES can predict several expected features such as radial asymmetry, surrounding gas vortices causing jet instabilities, and eventual jet breakup with reasonable accuracy. While RSM-SAS predicts radial asymmetry, some jet instability, and is much more accurate than <i>k-ε</i> RNG and RSM, it fails to predict instabilities as good as DES and does not predict a complete jet breakup. RSM-based methods are found to be computationally very expensive compared to the <i>k-ε</i> RNG model, suggesting DES as the better alternative than RSM methods for such applications if resources are available.

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.

Detached eddy simulation on the structure of swirling jet flow field

The improved delayed detached eddy simulation method with shear stress transport model was used to analyze the evolution of vortex structure, velocity and pressure fields of swirling jet. The influence of nozzle pressure drop on vortex structure development and turbulence pulsation was investigated. The development of vortex structure could be divided into three stages: Kelvin-Helmholtz (K-H) instability, transition stage and swirling flow instability. Swirling flow could significantly enhance radial turbulence pulsation and increase diffusion angle. At the downstream of the jet flow, turbulence pulsation dissipation was the main reason for jet velocity attenuation. With the increase of pressure drop, the jet velocity, pulsation amplitude and the symmetry of velocity distribution increased correspondingly. Meanwhile the pressure pulsation along with the axis and vortex transport intensity also increased significantly. When the jet distance exceeded about 9 times the dimensionless jet distance, the impact distance of swirling jet could not be improved effectively by increasing the pressure drop. However, it could effectively increase the swirl intensity and jet diffusion angle. The swirling jet is more suitable for radial horizontal drilling with large hole size, coalbed methane horizontal well cavity completion and roadway drilling and pressure relief, etc.

Extensional study of optical-flow enhanced hybrid PIV method for dual-plane stereoscopic PIV measurement

High-speed dual-plane stereoscopic particle image velocimetry (DP-s-PIV) can capture time-resolved three-component velocity and vorticity vectors in the measurement region, but its spatial resolvability of tiny vortices is blocked by the intrinsic limitation of the cross-correlation evaluation. To overcome this limitation, we propose a scheme to extend the original two-component optical-flow enhanced hybrid PIV method to DP-s-PIV through 3D reconstruction, with its feasibility and resolvability validated through comparison with the cross-correlation evaluation used in DaVis 8.4. The comparisons suggest that the optical-flow enhanced hybrid PIV scheme can greatly improve the spatial resolvability of the velocity field for DP-s-PIV. Moreover, the hybrid scheme can also provide better visualization of vortical structures and enhanced precision in evaluating the vorticity distribution, allowing for further identification of vortex types in a stratified swirling jet flow through the combined criteria (vorticity component and Q criterion).

Identification and Dynamics of Coherent Structures in a Coanda Swirling Jet Flow

Identification and Dynamics of Coherent Structures in a Coanda Swirling Jet Flow

Flame flow field interaction in non-premixed CH4/H2 swirling flames

Alternative fuels and stocks like biomass or chemical and refinery waste, may potentially be used in gas turbines and industrial applications after gasification. Thus, understanding the role of hydrogen in these fuels is critical to the broader aim of utilising alternative fuels for power generation. In this work, the interaction between the flame and the flow field was studied in a quarl-stabilised swirl non-premixed flame burning CH4 and H2–enriched CH4. Simultaneous high-speed OH-PLIF/PIV imaging at 5 kHz was carried out on these flames to explore the flame-flow interaction. The instantaneous flow fields in the CH4 or CH4+H2 flames showed a small scale vortical structure near the shear layers, which were not apparent in the time-averaged flow fields. Increasing H2% in the fuel jet was observed to dampen the velocity fluctuations. The fuel composition affected the spatial location of the reaction zone; in the CH4 flames, the axial position of the reaction zone is seen to track the relatively large-magnitude axial velocity fluctuations while remaining in locally low-speed regions of the flow. In contrast, in H2-enriched flames, where the flame is more robust, the reaction zone was able to survive longer, in terms of axial distance, in the vicinity of high swirling jet velocity, with less sensitivity to velocity fluctuations. With increasing the H2%, the reaction zone steadily leaves the IRZ towards the swirling jet flow and localised between its outer and inner vortices. This acts as a stabilisation factor where the internal vortices convect hot product towards the fresh mixture. Moreover, the flame curvatures, the vorticity and compressive strain fields interactions with the reaction zone are presented and discussed. This article outlines results that yield more in-depth insight into hydrogen-enriched hydrocarbon non-premixed swirling flames' combustion, which is essential to accelerate the fuel switching from hydrocarbons to hydrogen.

Detection of vortical structures in sparse Lagrangian data using coherent-structure colouring

In this study, vortical structures are detected on sparse Shake-The-Box data sets using the Coherent-Structure Colouring (CSC) algorithm. The performance of this Lagrangian approach is assessed by comparing the CSC-coloured tracks with the baseline vorticity field. The ability to extract vortical structures from sparse data is accessed on two Lagrangian particle tracking data sets: the flow past an Ahmed body and a swirling jet flow. The effects of two normalized parameters on the identification of vortical structures were defined and studied: the mean track length and the mean inter-particle distance. The accuracy of the vortical-structure detection problem through CSC is shown to improve with decreasing inter-particle distance values, whereas little dependence on the mean track length is observed at all. Overall, the CSC algorithm showed to yield accurate detection of coherent structures for inter-particle distances smaller than 15% of the characteristic dimension of the structure. However, the results quickly deteriorate for sparser Lagrangian data.

Determination of single and double helical structures in a swirling jet by spectral proper orthogonal decomposition

This paper studies the coherent structures found in an annular swirling jet flow undergoing vortex breakdown with control parameters, the Reynolds number Re = 8500 and the swirl number Sw = 0.38. The flow field is simulated using the large eddy simulation method with a dynamic k model. The first- and second-order statistics of the velocity fields are compared to tomographic particle image velocimetry measurements of the same flow configuration to validate the numerical simulation. The fast Fourier transform of the sampled velocity and pressure signals indicates a precessing vortex core with a frequency of 22 Hz. This frequency is in line with the one detected by spectral proper orthogonal decomposition, which is utilized to identify the coherent structures in the near-field region of the swirling flow in the present work. In detail, apart from the single helical structure usually found in swirling flows, a double helix, rarely observed in turbulent swirling jets, is also identified. This structure is not a second-order harmonic mode of the single one, as shown by statistical analysis of the mode temporal coefficients. Moreover, the calculation of energy production shows that this coherent precessing motion extracts energy from the mean flow field in the wake behind the bluff-body and in the breakup region of the vortex.

Study of integrated vortex ventilation and dust removal system in mechanized excavation face

The aerosolized dust produced during coal mining operations pollutes underground air environment and increases the risk of lung diseases in coal miners, such as pneumoconiosis. Aiming to solve the dust pollution in mechanized excavation face, the integrated vortex ventilation and dust removal system is designed based on swirling jet flow theory. The design principles and technological parameters of the major components of integrated vortex ventilation and dust removal (IVVDR) system, namely vortex ventilation device and wet dust collector are described systematically. Through numerical simulation and field experiments, the dust control and dust removal effects of IVVDR system were analyzed and tested. The numerical simulation results show that the swirling jet flows produced by wall-attached duct can control most of total and respirable dust in the area between working face and exhausting air outlet. In addition, the IVVDR system provides better control and removal effect of smaller respirable dust. In field application, the IVVDR system has better performance compared with the long-compression short-suction (LCSS) ventilation. In conclusion, the concentration of total and respirable dust in purified air are below 10 mg/m3, and in accordance with the mine safety regulations.

Propagation of uncertainty in a rotating pipe mechanism to generate an impinging swirling jet flow for heat transfer from a flat plate

In Computational Fluid Dynamics (CFD) studies composed of the coupling of different simulations, the uncertainty in one stage may be propagated to the following stage and affect the accuracy of the prediction. In this paper, a framework for uncertainty quantification is applied to the two-step simulation of the mechanical design of a swirling jet flow generated by a rotating pipe (\textit{Simulation 1}) impinging on a flat plate to provide convective heat transfer (\textit{Simulation 2}). The first approach is the Stochastic Collocation Method (SCM) with Clenshaw-Curtis sparse grids. The conclusion drawn from the analysis is that the simulated system does not exhibit a significant sensitivity to stochastic variations of model input parameters, over the tested uncertainty ranges. Additionally, a set of non-linear regression models for the stochastic velocity and turbulent profiles for the pipe nozzle are created and tested, since impinging jets at Reynolds number of $Re=23000$ are very frequent in the literature, but stochastic inlet conditions have never been provided. Numerical results demonstrate a negligible difference in the predicted convective heat transfer with respect to the use of the profiles simulated via CFD. These suggested surrogate models can be directly embedded onto other CFD applications (e.g arrays of jets or jet flows impinging on plates with different shapes) in which a realistic swirling flow under uncertainty can be of interest.

Experimental investigation of lean-dome high-airflow airblast pilot mixers' operability, emissions, and dynamics

In order to further improve operability and low-power combustion efficiency of single-venturi twin concentric swirlers Lean Direct Injection (LDI)-based lean dome combustion systems, the authors' previous work investigated the effect of outer swirler vane angle on LDI reacting flow characteristics. A typical single-element LDI injector comprises of a fuel atomization device (pressure injector or airblast), surrounded by high airflow twin swirlers with converging passages feeding a single venturi followed by a single diverging passage called flare. LDI injectors without flare can be classified as high airflow airblast or air-assist pressure atomizers, in contrast with conventional air atomizing devices where air-to-fuel ratio rarely exceeds 3 or 4. In the present investigation, the impact of the flare section on the reacting flow field and flame structure is systematically explored. Specifically, the lean blowout limits, flame shapes, reacting flow dynamics, and NOx emissions from swirl-venturi LDI and airblast injectors are experimentally characterized using a suite of optical diagnostics, including time-resolved particle image velocimetry, OH⁎/CH⁎/NO2⁎ chemiluminescence, and broadband flame imaging. Results reveal that the inclusion of a flare significantly increases the overall swirl strength of the injector. Conversely, by removing the flare, swirl strength is reduced, leading to flow pattern changes (such as a transition from recirculating to swirling jet flow), higher lean blowout limits, and higher peak oscillation frequency of the flow field, while NOx emissions decrease due to reduced characteristic residence times. Such results demonstrate an inherent design tradeoff between lowering NOx emissions and maintaining/enhancing operability for a single swirl injector.

Two-step numerical simulation of the heat transfer from a flat plate to a swirling jet flow from a rotating pipe

PurposeImpinging jets have been widely studied, and the addition of swirl has been found to be beneficial to heat transfer. As there is no literature on Reynolds-averaged Navier Stokes equations (RANS) nor experimental data of swirling jet flows generated by a rotating pipe, the purpose of this study is to fill such gap by providing results on the performance of this type of design.Design/methodology/approachAs the flow has a different behaviour at different parts of the design, the same turbulent model cannot be used for the full domain. To overcome this complexity, the simulation is split into two coupled stages. This is an alternative to use the costly Reynold stress model (RSM) for the rotating pipe simulation and the SST k-ω model for the impingement.FindingsThe addition of swirl by means of a rotating pipe with a swirl intensity ranging from 0 up to 0.5 affects the velocity profiles, but has no remarkable effect on the spreading angle. The heat transfer is increased with respect to a non-swirling flow only at short nozzle-to-plate distances H/D < 6, where H is the distance and D is the diameter of the pipe. For the impinging zone, the highest average heat transfer is achieved at H/D = 5 with swirl intensity S = 0.5. This is the highest swirl studied in this work.Research limitations/implicationsHigh-fidelity simulations or experimental analysis may provide reliable data for higher swirl intensities, which are not covered in this work.Practical implicationsThis two-step approach and the data provided is of interest to other related investigations (e.g. using arrays of jets or other surfaces than flat plates).Originality/valueThis paper is the first of its kind RANS simulation of the heat transfer from a flat plate to a swirling impinging jet flow issuing from a rotating pipe. An extensive study of these computational fluid dynamics (CFD) simulations has been carried out with the emphasis of splitting the large domain into two parts to facilitate the use of different turbulent models and periodic boundary conditions for the flow confined in the pipe.

Single- and double-helix vortex breakdown as two dominant global modes in turbulent swirling jet flow

In this paper, we study the shape and dynamics of helical coherent structures found in the flow field of an annular swirling jet undergoing vortex breakdown. The flow field is studied by means of time-resolved tomographic particle image velocimetry measurements. The obtained flow fields are analysed using both classic and spectral proper orthogonal decomposition. Despite the simple geometrical set-up of the annular jet, the flow field is very complex. Two distinct large-scale helical flow structures are identified: a single and a double helix, both co-rotating with the swirl direction, and it is revealed that these structures are not higher harmonics of each other. The structures have a relatively low energy content which makes it hard to separate them from other dynamics of the flow field, notably turbulent motions. Because of this, classic proper orthogonal decomposition fails to identify both structures properly. Spectral proper orthogonal decomposition, on the other hand, allows them to be identified accurately when the filter size is set at around eight times the precession period. The precession frequencies of the single and double helices correspond to Strouhal numbers of 0.273 and , respectively. A global stability analysis of the mean flow field shows that these structures correspond to two separate global modes. The precessing frequencies obtained by the stability analysis and the related spatial structures match very well with the experimental observations. The current work extends our knowledge on turbulent vortex breakdown and on mean field global stability theory in general. It leads to the following conclusions. Firstly, single- and double-helix vortex breakdown are both manifestations of global modes. Previous studies have shown that both and modes can coexist in swirling jets. However, the mode has been identified as a second harmonic of the first mode, while this study identifies both as two independent global modes. Secondly, this work shows that the simultaneous occurrence of multiple helical global modes is possible within a turbulent flow and their shapes and frequencies are very well predicted by mean field stability analysis. The latter finding is of general interest as it applies to a wide class of fluid problems dominated by multiple oscillatory structures.

The impact of swirling flow strength on lean-dome LDI pilot mixers’ operability and emissions

In an effort to reduce the environmental impact of aviation, lean-dome combustion concepts such as lean direct injection (LDI) are being pursued for their potential to achieve very low emissions. However, low-emissions potentials may be accompanied by operability challenges. In the present effort – part of a series of fundamental investigations into LDI operability challenges – systematic experiments of lean-dome-relevant pilot combustor devices are conducted in a single-element methane-fueled combustor test rig with two different LDI configurations by using a suite of optical diagnostics. While maintaining all other design features constant, the clockwise vane angle of the outer air swirler is altered between 60° (LDI-60-CW configuration) and 45° (LDI-45-CW configuration), representing strong and weak swirling flows, respectively. Measurements of the non-reacting/reacting flow fields and flame characteristics are performed as a function of overall equivalence ratio via time-resolved particle image velocimetry, OH*/CH*/NO2* chemiluminescence, OH planar laser-induced fluorescence, and broadband flame imaging. It is found that a large center recirculation zone exists only with the LDI-60-CW configuration, while the LDI-45-CW flow is dominated by a swirling jet flow. In addition, it is shown that the center recirculation zone in the LDI-60-CW configuration results in noticeably lower lean blowout limits than the LDI-45-CW configuration, but at the cost of much higher NOx production for vigorously-burning equivalence ratios. At lower equivalence ratios near lean blowout conditions, both configurations result in similar NOx production. These results are discussed in terms of identifying possible research directions for optimizing LDI designs for reduced NOx emissions while maintaining or improving operability relative to current rich-dome combustion technology.

Experimental investigation of the precessing vortex core in annular swirling jet flows in the transitional regime

In this paper, we study the flow structures and associated dynamics of an annular swirling jet in the transitional flow regime at Re = 520. In literature, studies on fully laminar or turbulent jets are plentiful, but detailed quantitative data for transitional jets are very scarce. Time-resolved stereoscopic particle image velocimetry measurements are employed to study the flow dynamics in combination with a spectral proper orthogonal decomposition analysis to extract the large-scale motions. Three different swirl numbers Sw are measured: Sw = 0.38, 0.47 and 0.57, with the lowest one very close to the critical swirl number for vortex breakdown to occur. Swirl has a significant impact on the flow field. For the lowest swirl number (Sw = 0.38), a vortex breakdown bubble appears/disappears periodically corresponding to a Strouhal number St = 0.023. If the swirl is increased to Sw = 0.47, the breakdown bubble remains permanently present in the flow field, accompanied with a large-scale vortical structure called the Precessing Vortex Core (PVC). The PVC precesses around the central axis of the jet at a Strouhal number of 0.23. It was observed that the precessing frequency of this spiral vortex core decreases to St = 0.146 as the swirl is increased to Sw = 0.57. This decrease of precession frequency finds its origin in the increased radial spreading of the jet, reducing the rotation rate of the flow near the centre. The 3D structure of the PVC is reconstructed from the planar stereo-PIV measurements by rotating the measurement plane around the central axis of the jet geometry in synchronisation with the precession. It is shown that the PVC is wrapped around the central vortex breakdown bubble and the windings are directed opposite to the swirl direction. This geometrical structure is also found in fully laminar or turbulent swirling jets and this paper shows it is the same for transitional jets.