Experimental investigations of a swirling jet in both stationary and rotating surroundings

The ‘plug’ flow emerging from a long rotating tube into a large stationary reservoir was used in the experimental investigation of swirling jets with Reynolds numbers, Re = 600, 1,000 and 2,000, and swirl numbers, S = ΩR/U, in the range 0–1.1, to cover flow regimes from the non-rotating jet to vortex breakdown. Here Ω is the nozzle rotation rate, R is the radius of the nozzle exit, and U is the mean mass axial velocity. The jet was more turbulent and eddies shed faster at larger Re. However the flow criticality and shear layer morphology remained unchanged with Re. After the introduction of sufficient rotation, co-rotating and counter-winding helical waves replaced vortex rings to become the dominant vortex structure. The winding direction of the vortex lines suggests that Kelvin–Helmholtz and generalized centrifugal instability dominated the shear layer. A quantitative visualization study has been carried out for cases where the reservoir was rotating independently with S _a  = Ω _a R/U = ±0.35, ±0.51 and ±0.70 at Re = 1,000 and 2000, where Ω _a is the rotation rate of the reservoir. The criterion for breakdown was found to be mainly dependent on the absolute swirl number of the jet, S. This critical swirl number was slightly different in stationary and counter-swirl surroundings but obviously smaller when the reservoir co-rotated, i.e. S _c  = 0.88, 0.85 and 0.70, respectively. These results suggest that the flow criticality depends mainly on the velocity distributions of the vortex core, while instabilities resulting from the swirl difference between the jet and its ambient seem to have only a secondary effect.     

Swirl number and nozzle confinement effects in a flat-vane axial swirler

This paper presents the results of a parametric experimental study of free swirling flow at the exit of a flat-vane axial swirler. A total of 16 data sets were acquired by combining four swirler vane angles (22°, 29°, 50.5°, and 58.3°) and four exit nozzles of different diameters (30, 40, 52, and 76 mm). Sophisticated pressure probes consisting of precise microphones and a two-component LDV system were used to investigate the effect of these geometrical parameters on swirling flow regimes characterized by the swirl number. Particular attention was paid to the precessing vortex core (PVC) phenomenon observed at the exit of the swirler nozzle. It has been shown that by varying the vane angle and the diameter of the exit nozzle, it is possible to independently control the swirl number value and the occurrence of a PVC. A distinct correlation has been found between the PVC-induced pressure pulsations detected by acoustic probes and the tangential velocity fluctuations measured by LDV. The use of microphones provides a quick way to measure the frequency response of swirl flow in a wide range of geometries and flow configurations. The PVC effect does not occur at low subcritical values of the integral swirl number (S < 0.5) and in the case of strong swirl flow (S_g = 0.9 and 1.2) in the absence of constriction by the nozzle (D_e/D₀ = 1). The disappearance of the PVC effect for strong swirl flow without constriction is due to the extreme displacement of the flow to the nozzle walls. The absence of a PVC in the flow was inferred not only from measurements of the frequency response of the flow over a wide range of Re numbers, but also from the absence of specific markers in velocity RMS distributions. Measurement results are used to derive an empirical correlation of the integral swirl number and the Strouhal number with a modified geometric swirl number. This allows a generalization of the frequency characteristics of swirling flows with a PVC for flat-vane axial swirlers, which are widely used in engineering.     

Near-ground tornado-like vortex structure resolved by particle image velocimetry (PIV)

The near-ground flow structure of tornadoes is of utmost interest because it determines how and to what extent civil structures could get damaged in tornado events. We simulated tornado-like vortex flow at the swirl ratios of S = 0.03–0.3 (vane angle θ_v = 15°–60°), using a laboratory tornado simulator and investigated the near-ground-vortex structure by particle imaging velocimetry. Complicated near-ground flow was measured in two orthogonal views: horizontal planes at various elevations (z = 11, 26 and 53 mm above the ground) and the meridian plane. We observed two distinct vortex structures: a single-celled vortex at the lowest swirl ratio (S = 0.03, θ_v = 15°) and multiple suction vortices rotating around the primary vortex (two-celled vortex) at higher swirl ratios (S = 0.1–0.3, θ_v = 30°–60°). We quantified the effects of vortex wandering on the mean flow and found that vortex wandering was important and should be taken into account in the low swirl ratio case. The tangential velocity, as the dominant velocity component, has the peak value about three times that of the maximum radial velocity regardless of the swirl ratio. The maximum velocity variance is about twice at the high swirl ratio (θ_v = 45°) that at the low swirl ratio (θ_v = 15°), which is contributed significantly by the multiple small-scale secondary vortices. Here, the results show that not only the intensified mean flow but greatly enhanced turbulence occurs near the surface in the tornado-like vortex flow. The intensified mean flow and enhanced turbulence at the ground level, correlated with the ground-vortex interaction, may cause dramatic damage of the civil structures in tornadoes. This work provides detailed characterization of the tornado-like vortex structure, which has not been fully revealed in previous field studies and laboratory simulations. It would be helpful in improving the understanding of the interaction between the tornado-like vortex structure and the ground surface, ultimately leading to better predictions of tornado-induced wind loads on civil structures.     

Coherent structures in unsteady swirling jet flow

An LDA technique and phase-averaging analysis were used to study unsteady precessing flow in a model vortex burner. Detailed measurements were made for Re=15,000 and S=1.01. On the basis of the analysis of phase-averaged data and vortex detection by the λ₂-technique of Joeng and Hussain (1995), three precessing spiral vortex structures were identified: primary vortex (PV), inner secondary vortex (ISV), and outer secondary vortex (OSV). The PV is the primary and most powerful structure as it includes primary vorticity generated by the swirler; the ISV and OSV are considered here as secondary vortical structures. The jet breakdown zone is the conjunction of a pair of co-rotating co-winding spiral vortices, PV and ISV. The interesting new feature described is that the secondary vortices form a three-dimensional vortex dipole with a helical geometry. The effect of coupling of secondary vortices was suggested as a mechanism of enhanced stability reflected in their increased axial extent.     

Laminar co-rotating Batchelor vortex merging

The dynamics of laminar co-rotating vortex pairs without axial flow have been recently thoroughly studied through theoretical, experimental and numerical studies, which revealed different instabilities contributing to the decay of the vortices. In this paper, the objective is to extend the analysis to the case of co-rotating vortices with axial flow at low Reynolds numbers. A high-order incompressible Navier–Stokes flow solver is used. The momentum equations are spatially discretized on a staggered mesh by finite differences and all derivatives are evaluated with 10th order compact finite difference schemes with RK-4 temporal discretization. The initial condition is a linear superposition of two co-rotating circular Batchelor vortices with q = 1. It is found that there is an initial evolution that resembles the evolution that single q = 1 vortices go through. Azimuthal disturbances grow and result in the appearance of large-scale helical sheets of vorticity. With the development of these instability waves, the axial velocity deficit is weakened. The redistribution of both angular and axial momentum between the core and the surroundings drives the vortex core to a more stable configuration, with a higher q value. After these processes, the evolution is somewhat similar to a pair of co-rotating Lamb–Oseen vortices. A three-dimensional instability develops, with a large band of unstable modes, with the most amplified mode corresponding scaling with the vortex initial separation distance. P. J. S. A. Ferreira de Sousa wishes to acknowledge the support of FCT—SFRH/BD/1129/2000 and SFRH/BPD/21778/2005.     

Study on the instability in separating–reattaching flow over a surface-mounted rib

A numerical study on the flow structure and instability in the separated–reattached flow over a surface-mounted rib at Re = 1000 is performed using large eddy simulation. It is found that the phenomenon of vortex pairing, which has been extensively observed in similar flows, exists in the separation zone. Based on the spectral analysis, the Kelvin–Helmholtz (K-H) instability of shear layer at St ≈ 0.361 (St ≡ fh/U₀) and its subharmonic at St ≈ 0.18 are found. It is assumed that the K-H instability reduces to its subharmonic through the vortex pairing. This process is further confirmed by the flow visualisation. The two-dimensional (2D) structures are subjected to sinusoidal undulation along the spanwise and observed to undergo helical pairing process, which is attributed to the transformation of 2D structures into 3D. However, the low frequency due to flapping of the shear layer is not found.     

Inviscid evolution of incompressible swirling flows in pipes: The dependence of the flow structure upon the inlet velocity field

This paper analyses the influence of the inlet swirl on the structure of incompressible inviscid flows in pipes. To that end, the inviscid evolution along a pipe of varying radius with a central body situated inside the pipe is studied for three different inlet swirling flows by solving the Bragg–Hawthorne equation both asymptotically and numerically. The downstream structure of the flow changes abruptly above certain threshold values of the swirl parameter (L). In particular, there exist a value L_r above which a near-wall region of flow reversal is formed downstream, and a critical value L_f above which the axial vortex flow breaks down. It is shown that the dependence upon the pipe geometry of these critical values of the swirl parameter varies strongly with the inlet azimuthal velocity profile considered. An excellent agreement between asymptotic and numerical results is found.     

Obstacle-induced spiral vortex breakdown

An experimental investigation on vortex breakdown dynamics is performed. An adverse pressure gradient is created along the axis of a wing-tip vortex by introducing a sphere downstream of an elliptical hydrofoil. The instrumentation involves high-speed visualizations with air bubbles used as tracers and 2D Laser Doppler Velocimeter (LDV). Two key parameters are identified and varied to control the onset of vortex breakdown: the swirl number, defined as the maximum azimuthal velocity divided by the free-stream velocity, and the adverse pressure gradient. They were controlled through the incidence angle of the elliptical hydrofoil, the free-stream velocity and the sphere diameter. A single helical breakdown of the vortex was systematically observed over a wide range of experimental parameters. The helical breakdown coiled around the sphere in the direction opposite to the vortex but rotated along the vortex direction. We have observed that the location of vortex breakdown moved upstream as the swirl number or the sphere diameter was increased. LDV measurements were corrected using a reconstruction procedure taking into account the so-called vortex wandering and the size of the LDV measurement volume. This allows us to investigate the spatio-temporal linear stability properties of the flow and demonstrate that the flow transition from columnar to single helical shape is due to a transition from convective to absolute instability.     

Coherent structures in countercurrent axisymmetric shear flows

The dynamical behaviors of coherent structures in countercurrent axisymmetric shear flows are experimentally studied.The forward velocity U1 and the velocity ratio R=(U1-U2)/(U1+U2),where U2 denotes the suction velocity,are consldered as the control parameters.Two kinds of vortex structures,i.e.,axisymmetric and helical structures,were discovered with respect to different reginmes in the R versus U1 diagram .In the case of U1 rangjing from 3 to 20m/s and R from 1 to 3,the axisymmetric structures plan an important role.Based on the dynamical behaviors of axisymmetric structures,a critical forward velocity U1cr=6.8m/s was defined,subsequently,the subcritical velocity regime:U1&gt;U1cr and the supercritical velocity regime:U1&lt;U1er,In the subcritical velocity regine,the flow system contains shear layer self-excited oscillations in a certain range of the velocity ratio with respect to any forward velocity.In the supercritical velocity regime,the effect of the velocity ratio could be explained by the relative movement and the spatial evolution of the axisymmetric structure undergoes the following stages:(1) Kelvin-Helmholtz instability leading to vortex rolling up,(2) first time vortex agglomeration.(3) jet colunn self-excited oscillation,(4) shear layer self-excited oscillation,(5)“ordered tearing“,(6) turbulence in the case of U1&lt;4m/s (the “ordered tearing“ does not exist when U1&gt;4m/s),correspondingly,the spatial evolution of the temporal asymptotic behavior of a dynamical system can be described as follows:(1) Hopt bifurcation,(5) chaos(“weak turbulence“)in the case of U1&lt;4m/s(superharmonic bifurcation does not exist when U1&gt;4m/s).The proposed new terms,superharmonic and reversed superbarmonic bifurcations,are characterized of the frequency doubling rather than the period doubling.A kind of unfamiliar vortices referred to as the helical structure was discovered experimentally when the forward velocity around 2m/s and the velocity range from 1.1 to 2.3,There are two base frequencies contained in the flow system and they could coexist as indicated by the Wigner-Ville-Distribution and the temporal asymptotic behavior of the dynamical system corresponding to the helical vortex could be described as 2-torus as indicted by the 3D reconstructed phase trajectory and correlation dimension.The scenario of the spatial evolution of helical structures could be described as follows:the jet column is separated into two parts at a certain spatial location and they entangle each other to form the helical vortex until the occurrence of those separated jet columns to reconnect further downstream with the result that the flow system evolves into turbulence in a catastrophic form.Correspondingly,the dynamical system evolves directly into 2-tiorus through the supercritical Hopf bifurcation followed by a transition from a quasi-periodic attractor to a strange attractor.In the case of U1=2m/s,the parametric evolution of the temporal asymptotic behavior of the dynamical system as the velocity ratio increases from 1 to 3 could be described as follows:(1)2-torus(Hopf bifurcation),(2) limit cycle(reversed Hopf bifurcation),(3) strange attractor (subbarmonic bifurcation).     

Observations of large-scale structures in wakes behind axisymmetric bodies

Wakes behind disk-shaped axisymmetric bodies of varying solidity are studied using flow visualization and two-dimensional Fourier decomposition of velocity measurements. Evidence of a reverse flow region behind some of the bodies is observed to coincide with the presence of large-scale structures in the near and far wake. Fourier analysis shows that these large-scale structures are predominately helical (m= ±1) and occur at a characteristic frequency which corresponds to their wavelength as observed from flow visualization. Our measured value for this characteristic frequency agrees with vortex shedding frequencies observed for these types of wakes.     

Experimental and Numerical Investigation of Swirl Induced Self-Excited Instabilities at the Vicinity of an Airblast Nozzle

We report on the experimental and numerical investigation of swirl induced self-excited instabilities in the form of precessing helical structures at the vicinity of an Airblast Atomiser. Within the scope of this work, the increase of the knowledge of the fundamental factors governing the precessing vortex core phenomenon (PVC) (Gupta et al. 1984) by applying dual air-flow Airblast nozzles is aimed. This study concentrates on the experimental investigation of the impact of important parameters of a combustor system on the performance characteristics of this instability. In particular, in terms of this work the properties of the PVC are determined by applying two different Airblast Atomisers, one of them producing an attached swirl flame, and another producing a lifted swirl flame. Measurements are also performed for a confined and a non confined flame, aiming to determine the impact of the confining duct on the performance of the PVC. In order to gain some further knowledge, regarding the impact of this aerodynamic instability on combustion of gaseous fuel, the features of PVC are experimentally identified under reacting conditions, by employing a laser light sheet (LLS) measurement technique. By applying the LLS measurement technique, further investigation on the nature of the PVC is also attempted. The power spectral density (PSD) function of the flow field was determined on the basis of raw data provided by 3D Laser Doppler Anemometry (3D-LDA). In order to validate the measurement technique as well as the nature of the instability, the planar Mie-Scattering of the flow was evaluated by employing a high speed camera performing at 12 kHz. The precessing character of the flow was also confirmed by means of a numerical simulation using the 3D Reynolds Stress Model (3D-RSM). The results of the numerical investigation provided some useful information concerning the onset of the instability within the primary swirler as well as its size and amplitude. According to this analysis, a high frequency instability was confirmed within a region of about one burner diameter downstream of the burner exit. Finally an evaluation of the (LDA) method in terms of providing accurate (PSD) was performed for the case of a swirl flow field.     

Numerical and experimental study of an axially induced swirling pipe flow

In-line flow segregators based on axial induction of swirling flow have important applications in chemical, process and petroleum production industries. In the later, the segregation of gas bubbles and/or water droplets dispersed into viscous oil by swirling pipe flow may be beneficial by either providing a pre-separation mechanism (bubble and/or drop coalescer) or, in the case of water-in-oil dispersions, by causing a water-lubricated flow pattern to establish in the pipe (friction reduction). Works addressing these applications are rare in the literature. In this paper, the features and capabilities of swirling pipe flow axially induced by a vane-type swirl generator were investigated both numerically and experimentally. The numerical analysis has been carried out using a commercial CFD package for axial Reynolds numbers less than 2000. Pressure drop, tangential and axial velocity components as well as swirl intensity along a 5 cm i.d. size and 3 m long pipe were computed. Single phase flow experiments have been performed using a water–glycerin solution of 54 mPa s viscosity and 1210 kg/m³ density as working fluid. The numerical predictions of the pressure drop were compared with the experimental data and agreement could be observed within the range of experimental conditions. The experiments confirmed that swirl flow leads to much higher friction factors compared with theoretical values for non-swirl (i.e. purely axial) flow. Furthermore, the addition of a conical trailing edge reduces vortex breakdown. Visualization of the two-phase swirling flow pattern was achieved by adding different amounts of air to the water–glycerin solution upstream the swirl generator.     

An experimental investigation of pneumatic swirl flow induced by a three lobed helical pipe

This paper presents a discussion of the results and conclusions drawn from a series of experiments conducted to investigate the swirl flow that are generated by a three lobed helical pipe mounted within a laboratory scale pneumatic conveying rig. The experiments employed Laser Doppler Anemometry (LDA) to quantify the strength of the induced vortex formations and the decay rates of the observed downstream swirl flows over a range of Reynolds number in the turbulent regime. Instantaneous point velocity measurements were resolved in three directions across regular measurement grids transcribed across parallel planes located at four distances downstream of the swirl inducing pipe section. The equivalent axial, radial and tangential velocities were subsequently computed at these grids points. The degree of swirl measured across each measurement plane was expressed in terms of a defined swirl number.It was concluded that the three lobed helical pipe gave rise to a wall jet type of swirl whose rate of observed downstream decay is related to the Reynolds number of the upstream flow and the distance downstream of the swirl pipe. The decay rates for the swirl flows were found to be inversely proportional to the Reynolds number of the upstream flow. The swirl pipe was observed to create a redistribution of the downstream velocity field from axial to tangential, accompanied by a transfer of axial to angular momentum. The findings of this paper are believed to improve understanding to assist the selective use of swirl flow within lean phase particles pneumatic transport systems.     

Influence of swirl on the stability of a rod in annular leakage flow

The influence of swirl (flow rotation) on the stability of a rod in annular leakage flow is investigated. Under the assumption of laminar flow and plane vibrations (no whirling), it is shown that the swirl acts, in effect, as an elastic foundation with negative foundation stiffness, the magnitude being proportional to the mean circumferential flow rate squared. Consequently, swirl always lowers the critical axial flow speed in case of divergence instability of a rod of finite length. Numerical analysis is needed to predict the effect of swirl in case of flutter instability of a finite rod; this is not performed here. However, for the flutter-like instability of travelling waves in an infinite rod-channel system, it is shown analytically that swirl again always lowers the critical axial flow speed. Finally, it is found that by circumferential flow alone, the travelling waves are extinguished at a certain flow rate, followed by a divergence-like instability.     

Numerical investigation of a perturbed swirling annular two-phase jet

A swirling annular gas–liquid two-phase jet flow system has been investigated by solving the compressible, time-dependent, non-dimensional Navier–Stokes equations using highly accurate numerical methods. The mathematical formulation for the flow system is based on an Eulerian approach with mixed-fluid treatment while an adjusted volume of fluid method is utilised to account for the gas compressibility. Surface tension effects are captured by a continuum surface force model. Swirling motion is applied at the inlet while a small helical perturbation is also applied to initiate the instability. Three-dimensional spatial direct numerical simulation has been performed with parallelisation of the code based on domain decomposition. The results show that the flow is characterised by a geometrical recirculation zone adjacent to the nozzle exit and by a central recirculation zone further downstream. Swirl enhances the flow instability and vorticity and promotes liquid dispersion in the cross-streamwise directions. A dynamic precessing vortex core is developed demonstrating that the growth of such a vortex in annular configurations can be initiated even at low swirl numbers, in agreement with experimental findings. Analysis of the averaged results revealed the existence of a geometrical recirculation zone and a swirl induced central recirculation zone in the flow field.     

Volumetric reconstruction of the mean flow around circular cylinders fitted with strakes

The volumetric reconstruction technique presented in this paper employs a two-camera stereoscopic particle image velocimetry (SPIV) system in order to reconstruct the mean flow behind a fixed cylinder fitted with helical strakes, which are commonly used to suppress vortex-induced vibrations (VIV). The technique is based on the measurement of velocity fields at equivalent adjacent planes that results in pseudo volumetric fields. The main advantage over proper volumetric techniques is the avoidance of additional equipment and complexity. The averaged velocity fields behind the straked cylinders and the geometrical periodicity of the three-start configuration are used to further simplify the reconstruction process. Two straked cylindrical models with the same pitch (p = 10d) and two different heights (h = 0.1 and 0.2d) are tested. The reconstructed flow shows that the strakes introduce in the wake flow a well-defined wavelength of one-third of the pitch. Measurements of hydrodynamic forces, fluctuating velocity, vortex formation length, and vortex shedding frequency show the interdependence of the wake parameters. The vortex formation length is increased by the strakes, which is an important effect for the suppression of vortex-induced vibrations. The results presented complement previous investigations concerning the effectiveness of strakes as VIV suppressors and provide a basis of comparison to numerical simulations.     

An automated vortex detection scheme using the wavelet transform of the<Emphasis Type="Italic"> d</Emphasis><Subscript>2</Subscript> field

A vortex detection scheme using the wavelet transform of the discriminant of the eigenvalues of the velocity gradient matrix (d ₂) is presented in this paper. The use of d ₂ field results in better eduction capability over the previously used test fields such as enstrophy since it automatically distinguishes shear layers from vortices. Level sets are used to refine the shape of the vortex without causing a huge computational penalty. Further, the algorithm is easily automated to aid batch processing. The detection scheme was applied to swirl flow fields, successfully estimating the vortex location, the core radius and the vortex shape.     

Application of the Dorodnitsyn finite element method to swirling boundary layer flow

The Dorodnitsyn finite element method for turbulent boundary layer flow with surface mass transfer is extended to include axisymmetric swirling internal boundary layer flow. Turbulence effects are represented by the two-layer eddy viscosity model of Cebeci and Smith¹ with extensions to allow for the effect of swirl. The method is applied to duct entry flow and a 10 degree included-angle conical diffuser, and produces results in close agreement with experimental measurements with only 11 grid points across the boundary layer. The introduction of swirl (w_e/u_e = 0.4) is found to have little effect on the axial skin friction in either a slightly favourable or adverse pressure gradient, but does cause an increase in the displacement area for an adverse pressure gradient. Surface mass transfer (blowing or suction) causes a substantial reduction (blowing) in axial skin friction and an increase in the displacement area. Both suction and the adverse pressure gradient have little influence on the circumferential velocity and shear stress components. Consequently in an adverse pressure gradient the flow direction adjacent to the wall is expected to approach the circumferential direction at some downstream location.     

Near-field tip vortex behind a swept wing model

The near-field flow structure of a tip vortex behind a sweptback and tapered NACA 0015 wing was investigated and compared with a rectangular wing at the same lift force and Re=1.81×10⁵. The tangential velocity decreased with the downstream distance while increased with the airfoil incidence. The core radius was about 3% of the root chord c _r, regardless of the downstream distance and α for α<8°. The core axial velocity was always wake-like. The core Γ_c and total Γ_o circulation of the tip vortex remained nearly constant up to x/c _r=3.5 and had a Γ_c/Γ_o ratio of 0.63. The total circulation of the tip vortex accounted for only about 40% of the bound root circulation Γ_b. For a rectangular wing, the axial flow exhibited islands of wake- and jet-like velocity distributions with Γ_c/Γ_o=0.75 and Γ_o/Γ_b=0.70. For the sweptback and tapered wing tested, the inner region of the tip vortex flow exhibited a self-similar behavior for x/c _r≥1.0. The lift force computed from the spanwise circulation distributions agreed well with the force-balance data. A large difference in the lift-induced drag was, however, observed between the wake integral method and the inviscid lifting-line theory.     

Numerical study of self-sustained oscillatory flows and heat transfer in channels with a tandem of transverse vortex generators

A numerical investigation was conducted into channel flows with a tandem of transverse vortex generators in the form of rectangular cylinders. The oscillatory behavior of the flow is studied. Data for heat transfer and flow losses are presented for 100≤Re≤400 and cylinder separation distances 1≤S/H≤4. The results are obtained by numerical solution of the full Navier-Stokes equations and the energy equation. Self-sustained flow oscillations are found for Re>100. Alternate and dynamic shedding of large vortex structures from the cylinders is observed by visualization of the numerically determined flow field. A heat transfer enhancement up to a factor 1.78 compared to plane channel flow is observed. Received on 16 July 1997