Hydrodynamics During the Transient Evolution of Open Jet Flows from/to Wall Attached Jets (original) (raw)

Coherent structures in unsteady swirling jet flow

Experiments in Fluids, 2005

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 k 2-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.

Structure of a swirling jet with vortex breakdown and combustion

Journal of Physics: Conference Series

An experimental investigation is performed in order to compare the time-averaged spatial structure of low-and high-swirl turbulent premixed lean flames by using the particle image velocimetry and spontaneous Raman scattering techniques. Distributions of the timeaverage velocity, density and concentration of the main components of the gas mixture are measured for turbulent premixed swirling propane/air flames at atmospheric pressure for the equivalence ratio Φ = 0.7 and Reynolds number Re = 5000 for low-and high-swirl reacting jets. For the low-swirl jet (S = 0.41), the local minimum of the axial mean velocity is observed within the jet center. The positive value of the mean axial velocity indicates the absence of a permanent recirculation zone, and no clear vortex breakdown could be determined from the average velocity field. For the high-swirl jet (S = 1.0), a pronounced vortex breakdown took place with a bubble-type central recirculation zone. In both cases, the flames are stabilized in the inner mixing layer of the jet around the central wake, containing hot combustion products. O 2 and CO 2 concentrations in the wake of the low-swirl jet are found to be approximately two times smaller and greater than those in the recirculation zone of the high-swirl jet, respectively.

Characteristics of swirling and precessing flows generated by multiple confined jets

Physics of Fluids, 2019

An experimental study is reported of the interaction between multiple iso-thermal jets within a cylindrical chamber under conditions relevant to a wide range of engineering applications, including the confined swirl combustors, industrial mixers and concentrated solar thermal devices. The Particle Image Velocimetry (PIV) technique was used to investigate the swirling and precessing flows generated with four rotationally-symmetric inlet pipes at a fixed nozzle Reynolds number of ReD = 10,500 for two configurations of swirl angle (5° and 15°) and two alternative tilt angles (25° and 45°). The measurements reveal three distinctive rotational flow patterns within the external recirculation zone (ERZ) and the central recirculation zone (CRZ) for these configurations. It was found that the mean and root-mean-square flow characteristics of the swirl within the chamber depend strongly on the relative significance of the ERZ and CRZ, with the swirling velocity being higher in the CRZ than that in the ERZ. A precessing vortex core (PVC) was identified for all experimental conditions considered here, although its significance was less for the cases with a dominant CRZ.

Experimental Study of Unconfined and Confined Isothermal Swirling Jets

A 3C-2D PIV technique was applied to investigate the swirling flow generated by an axial plus tangential type swirl generator. This work is focused on the near-exit region of an isothermal swirling jet to characterize the effect of swirl on the flow field and to identify the large coherent structures both in unconfined and confined conditions for geometrical swirl number, Sg = 4.6. Effects of the Reynolds number on the flow structure were also studied. The experimental results show significant effects of the confinement on the mean velocity fields and its fluctuations. The size of the recirculation zone was significantly enlarged upon confinement compared to the free swirling jet. Increasing in the Reynolds number further enhanced the recirculation zone. The frequency characteristics have been measured with a capacitive microphone which indicates the presence of periodic oscillation related to the existence of precessing vortex core, PVC. Proper orthogonal decomposition of the jet velocity field was carried out, enabling the identification of coherent structures. The time coefficients of the first two most energetic POD modes were used to reconstruct the phase-averaged velocity field of the oscillatory motion in the swirling flow. The instantaneous minima of negative swirl strength values calculated from the instantaneous velocity field revealed the presence of two helical structures located in the inner and outer shear layers and this structure fade out at an axial location of approximately z/D = 1.5 for unconfined case and z/D = 1.2 for confined case. By phase averaging the instantaneous swirling strength maps, the 3D helical vortex structure was reconstructed.

Vortex dynamics and entrainment mechanisms in lobed jets

2007

Two isothermal turbulent air jets from lobed nozzles with inclined and respectively non inclined lobes and a circular reference jet with the same initial Reynolds number were experimentally studied. Quantitative image processing of time resolved visualizations as well as hot-wire measurements of the velocity spectra allowed an objective understanding of the vortex roll-up mechanisms. Unlike the circular jet, where the primary rings are continuous, the Kelvin-Helmholtz vortices in the lobed jet flows are discontinuous at the locations where the exit plane curvature turns to infinite. Primary structures detach at different frequencies whether they are shed in the lobe troughs or at the lobe sides. The ``cutting'' of the Kelvin-Helmholtz vortices enables the development of permanent secondary streamwise structures. Their momentum flux transport role is thus rendered more efficient and seems to be amplified by the double inclination of the injection boundary. The quantification of the entrained flow rates by means of LDA measurements perfectly agrees with these observations.

The naturally oscillating flow emerging from a fluidic precessing jet nozzle

Journal of Fluid Mechanics, 2008

Phase-averaged and directionally triggered digital particle image velocimetry measurements were taken in longitudinal and transverse planes in the near field of the flow emerging from a fluidic precessing jet nozzle. Measurements were performed at nozzle inlet Reynolds and Strouhal numbers of 59 000 and 0.0017, respectively. Results indicate that the jet emerging from the nozzle departs with an azimuthal component in a direction opposite to that of the jet precession. In addition, the structure of the 'flow convergence' region, reported in an earlier study, is better resolved here. At least three unique vortex-pair regions containing smaller vortical 'blobs' are identified for the first time. These include a vortex-pair region originating from the foci on the downstream face of the nozzle centrebody, a vortex-pair region shed from the edge of the centrebody and a vortex-pair region originating from the downstream surface of the nozzle exit lip.

Control of vortex breakdown in critical swirl regime using azimuthal forcing

Vortex Breakdown (VB) is a unique feature which may take place in swirling flows. It occurs when the ratio of axial to azimuthal velocity exceeds a certain threshold, while both quantities have to be of the same order of magnitude. VB is characterized by a sudden deceleration of the fluid on the jet axis and the formation of a stagnation point with a region of reversed flow further downstream. The control of swirling jets undergoing VB is of great interest for various industrial applications. For example, gas turbine combustion is adversely affected by thermoacoustic instabilities, that are stimulated by a fluctuating flame, located in the wake of VB. In recent years, particular attention was paid to the dominant role of the vortices generated by instabilities in the shear layers of swirling jets. Flow visualizations conducted at weakly and moderately swirling jets 1 and at highly swirling jets 2 indicate the occurrence of rotating, winding vortices in the outer shear layer. An experimental study on highly swirled jets 3 identifies dominant double-and triple-helices in the pre-breakdown stage as well as dominant single-and double-helices after VB. The single-helix is suggested to be a self-excited/globally unstable mode, possibly arising from a region of local absolute instability in the wake of VB. Forced experiments using vortex generators mounted on a rotating nozzle supported the absolute/convective nature of the dominating instabilities 3 . In the present study, a swirling air jet emanating into a large room is investigated in the critical swirl number range around the onset of VB. The swirling jet facility accommodates two blowers to regulate the rate of swirl independent of the Reynolds number, which was set to Re D = 20 000. The axial and azimuthal flows are merged in the swirl generator using inclined vanes. The integral swirl number S int = (2Ġ θ )/(DĠ x ), introduced by Chigier & Chervinsky 4 , is used to quantify the degree of swirl. Azimuthal waves are imposed on the shear layer separating from the nozzle lip. Eight loudspeakers, circumferentially mounted around the nozzle outlet (D = 51 mm), allows to force modes with azimuthal wavenumbers ranging from m = −4 to m = +4, negative when rotating with the bulk flow. Furthermore the excitation can be varied in amplitude and frequency. Stereo Particle Image Velocimetry (PIV) was conducted providing all three velocity components in a 2d-plane. A single probe hot-wire placed close to the PIV measurement plane was run simultaneously. Thereby, it is possible to sort the PIV snapshots, which were captured at ≈ 2.5 Hz, according to the phaseangle of the excited wave which was fluctuating at ≈ 45 Hz. This allows to decompose the instantaneous velocities u(t) into a mean u, a periodicũ and a fluctuating u ′ part. Measurements were conducted in the near-field close to the nozzle exit. With the laser sheet aligned with the jet axis the flow was measured in streamwise direction from x/D = 0.2 to x/D = 3 and with the laser sheet perpendicular to the jet axis in crosswise direction at x/D = 0.57. Four different swirl numbers were investigated. At the lowest, S int = 0.72, the mean axial velocity does not indicate the appearance of VB, nor does the investigation of each individual PIV snapshot. At the highest swirl number, S int = 1.22, VB occurs and a large recirculation area is present very close to the nozzle exit, giving the mean axial velocity profile a wake-like shape in the entire measurement domain. The axial velocity corresponding to the two intermediate swirl numbers, S int = 0.92 and S int = 1.12, change from jet-like to wake-like in downstream direction. At the axial location of this transition, u RMS reaches its maximum at the jet axis, unfolding a highly fluctuating location of the VB. There, histograms of the axial velocity (not shown) have two distinct peaks describing the transition between the two different flow states, where for S int = 0.92 the tendency is toward the jet-like profile while for S int = 1.12 it shifts toward the wake-like shape

The effect of swirl on jets and wakes: Linear instability of the Rankine vortex with axial flow

Physics of Fluids, 1998

The effect of swirl on jets and wakes is investigated by analyzing the inviscid spatiotemporal instability of the Rankine vortex with superimposed plug flow axial velocity profile. The linear dispersion relation is derived analytically as a function of two nondimensional control parameters: the swirl ratio S and the external axial flow parameter a ͑aϾϪ0.5 for jets, aϽϪ0.5 for wakes͒. For each azimuthal wave number m, there exists a single unstable Kelvin-Helmholtz mode and an infinite number of neutrally stable inertial waveguide modes. Swirl decreases the temporal growth rate of the axisymmetric Kelvin-Helmholtz mode (mϭ0), which nonetheless remains unstable for all axial wave numbers. For helical modes (m 0), small amounts of swirl lead to the widespread occurrence of direct resonances between the unstable Kelvin-Helmholtz mode and the inertial waveguide modes. Such interactions generate, in the low wave number range, neutrally stable wave number bands separated by bubbles of instability. As S increases above a critical value, all bubbles merge to give rise to unstable wave numbers throughout, but the growth rate envelope decreases continuously with increasing swirl. In the high wave number range, negative helical mode growth rates are enhanced for small swirl and decrease continuously for large swirl, while positive helical mode growth rates monotonically decrease with increasing swirl. For a given positive swirl, negative modes are more unstable than their positive counterparts, although their growth rates may not necessarily be larger than in the nonrotating case. The absolute/convective nature of the instability in swirling jets and wakes is determined in an aϪS control parameter plane by numerical implementation of the Briggs-Bers criterion. In the absence of swirl, jets (aϾϪ0.5) become absolutely unstable ͑AI͒ to the axisymmetric mode mϭ0 only for a sufficiently large external axial counterflow aϽϪ0.15. AI is found to be significantly enhanced by swirl: for SϾ1.61, AI occurs, even for coflowing jets (aϾ0). As S is gradually increased, the transitional mode to AI successively becomes mϭ0, Ϫ1, Ϫ2, Ϫ3, etc. In the absence of swirl, wakes (aϽϪ0.5) become absolutely unstable to the bending modes mϭϮ1 only for sufficiently large counterflow 1ϩaϾ0.091. For SϾ0.47, AI occurs even for coflowing wakes (aϽϪ1) and, as S increases, the transitional mode to AI successively becomes mϭϪ1, Ϫ2, Ϫ3, etc. This instability analysis is found to provide a preliminary estimate of the critical Rossby number for the onset of vortex breakdown: for zero external axial flow jets (aϭ0), absolute/convective transition first takes place at a Rossby number RoϵS Ϫ1 ϳ0.62, which very favorably compares with available experimental and numerical threshold values for vortex breakdown onset.

Experimental analysis of the precessing vortex core in a free swirling jet

Experiments in Fluids, 2007

An experimental analysis of the precessing vortex core (PVC) instability in a free swirling jet of air at ambient pressure and temperature is performed by means of laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Two parametric studies are considered, varying the swirl parameter and the Reynolds number. The range of parameters considered allowed to study conditions of strong precession as well as the inception and settlement of the instability. Mean velocity and standard deviation profiles, power spectral density functions and probability density functions for the axial and tangential velocity components are presented. Average as well as instantaneous PIV maps are considered in the characterization of the flowfield structure and detection of the instantaneous position of the vortex center. Joint analysis of velocity PDFs and power spectra shows that the PVC contribution to the global statistics of the velocity field can be properly separated from the contribution of the true flow turbulence, giving additional insight to the physics of the precession phenomenon. The results obtained in the explored range of conditions indicate that the true turbulence intensity is not dependent on the swirl parameter.