scholarly journals Elliptic jets. Part 2. Dynamics of coherent structures: pairing

1991 ◽  
Vol 233 ◽  
pp. 439-482 ◽  
Author(s):  
Hyder S. Husain ◽  
Fazle Hussain
Keyword(s):  
Flow Visualization ◽  
Coherent Structures ◽  
Coherent Structure ◽  
Nozzle Exit ◽  
Near Field ◽  
Major Axis ◽  
Equivalent Diameter ◽  
Longitudinal Vortices ◽  
Time Average ◽  
Exit Speed

The dynamics of coherent structure interactions, in particular the jet column mode of vortex pairing, in the near field of an elliptic jet have been investigated using hotwire measurements and flow visualization. A 2:1 aspect-ratio jet with an initially laminar boundary layer and a constant momentum thickness all around the nozzle exit perimeter is used for this study. While detailed hot-wire measurements were made in air at a Reynolds number ReDe (≡UeDe/ν) = 3.2 × 104, flow visualization was performed in water at a lower ReDe = 1.7 × 104; here Ue is the exit speed and De is the equivalent diameter of the nozzle exit cross-section. Excitation at the stable pairing mode induced successive pairings to occur periodically at the same location, allowing phase-locked measurements using a local trigger sensor. Coherent structures were educed at different phases of pairing in the planes of both the major and minor axes. These are compared with corresponding data in a circular jet, educed similarly.Pairing interactions are found to be quite different from those in a circular jet. Owing to non-planar and non-uniform self-induction of elliptical vortical structures and the consequent effect on mutual induction, pairing of elliptic vortices in the jet column does not occur uniformly around the entire perimeter, unlike in a circular jet. Merger occurs only in the initial major-axis plane through an entanglement process, while in the initial minor-axis plane, the trailing vortex rushes through the leading vortex without pairing and then breaks down violently. These motions produce considerably greater entrainment and mixing than in circular or plane jets. From distributions of dynamical properties over the extent of coherent structures, the production mechanism is explained in terms of the longitudinal vortices (or ribs) connecting the elliptic structures. Time-average measures and their modification by controlled excitation are also discussed in terms of coherent structure dynamics. A significant space in this paper is devoted to documenting phase-dependent and time-average flow measures; these new results should serve as target data for numerical simulations. Further details are given in Husain (1984).

The ‘preferred mode’ of the axisymmetric jet

1981 ◽  
Vol 110 ◽  
pp. 39-71 ◽  
Author(s):  
A. K. M. F. Hussain ◽  
K. B. M. Q. Zaman
Keyword(s):  
Reynolds Number ◽  
Reynolds Stress ◽  
Coherent Structure ◽  
Large Scale ◽  
Near Field ◽  
Saddle Points ◽  
Initial Boundary ◽  
Phase Average ◽  
Background Turbulence ◽  
Time Average

The ‘preferred mode’ of an incompressible axisymmetric free jet has been organized through controlled perturbation, and spatial distributions of time-average as well as phase-average flow properties in the near field are documented. The excitation produces noticeable changes in the time-average measures of the jet, although these changes are less dramatic than those for the excitation producing stable vortex pairing. For different stages in the evolution of the preferred-mode coherent structure, the phase-average vorticity, coherent Reynolds stress, and incoherent turbulence intensities and Reynolds stress have been educed through phase-locked hot-wire measurements, over the spatial extent of the structure and without invoking the Taylor hypothesis. For a particular stage of the evolution (i.e. when the structure is centred at x/D ≃ 3) the distributions of these quantities have been compared for both initially laminar and fully turbulent exit boundary layers, and for four jet Reynolds numbers. The relative merits of the coherent structure streamline and pseudo-stream-function patterns, as compared with phase-average velocity contours, for structure boundary identification have been discussed. The structure shape and size agree closely with those inferred from the average streamline pattern of the natural structure educed by Yule (1978).These data as well as τ-spectra show that even excitation at the preferred mode cannot sustain the initially organized large-scale coherent structure beyond eight diameters from the jet exit. The background turbulence is organized by the coherent motions in such a way that the maximum rate of decrease of the coherent vorticity occurs at the structure centres which are the saddle points of the background-turbulence Reynolds-stress distributions. The structure centres are also the locations of peak phase-average turbulence intensities. The evolving shape of the structure as it travels downstream helps explain the transverse variations of the wavelength and convection velocity across the mixing layer. The coherent structure characteristics are found to be independent of whether the initial boundary layer is laminar or turbulent, but depend somewhat on the jet Reynolds number. With increasing Reynolds number, the structure decreases in the streamwise length and increases in the radial width and becomes relatively more energetic, and more efficient in the production of coherent Reynolds stress.

Effect of Nozzle Geometry on the Near-Field Flow Characteristics of High-Pressure Gas Leak Jets

2018 ◽  
Author(s):  
Xiaopeng Li ◽  
Fakun Zhuang ◽  
Rui Zhou ◽  
Yian Wang ◽  
Libo Wang ◽  
...  
Keyword(s):  
High Pressure ◽  
Nozzle Exit ◽  
Flow Behavior ◽  
Near Field ◽  
Pressure Ratio ◽  
Three Dimensional ◽  
Major Axis ◽  
Minor Axis ◽  
Nozzle Geometry ◽  
Square Jets

Three-dimensional large eddy simulations of high-pressure jets at the same nozzle pressure ratio of 5.60 but issuing from different nozzles are conducted. Four different nozzle geometries, i.e., the circular, elliptic, square, and rectangular nozzles, are used to investigate the effect of the nozzle geometry on the near-field jet flow behavior. A high-resolution, hexahedral, and block-structured grid containing about 31.8 million computational cells is applied. The compressible flow solver, astroFoam, which is developed based on the OpenFOAM C++ library, is used to perform the simulations. The time-averaged near-field shock structures and the mean axial density are compared with the experiment data to validate the fidelity of the LES results, and the reasonable agreement is observed. The results indicate that the remarkable differences exist in the near-field flow structures of the jets. In particular, the circular and square jets correspond to a three-dimensional helical instability mode, while the elliptic and rectangular jets have a two-dimensional lateral instability in their minor axis planes. A subsonic flow zone exists after the Mach disk in the circular and square jets, but is lacking in the elliptic and rectangular jets. The intercepting shocks in the circular jet originate near the nozzle exit, and appear to be circular in cross-section. The intercepting shocks in the square jet originate at the four corners of the nozzle exit at first, and then are observed along the major axis plane some distance downstream of the nozzle exit. However, the formation of the intercepting shock is observed in the major axis planes but is lacking in the minor axis planes for the elliptic and rectangular jets. In addition, the real mass flow rates and discharge coefficients for different jets are computed based on the LES modeling, and their differences are explored.

Elliptic jets. Part 3. Dynamics of preferred mode coherent structure

1993 ◽  
Vol 248 ◽  
pp. 315-361 ◽  
Author(s):  
Hyder S. Husain ◽  
Fazle Hussain
Keyword(s):  
Coherent Structure ◽  
Near Field ◽  
Three Dimensional ◽  
Mixing Enhancement ◽  
Hot Wire ◽  
Vortical Structures ◽  
Mass Entrainment ◽  
Time Average ◽  
And Control ◽  
Made In

The dynamics of the preferred mode structure in the near field of an elliptic jet have been investigated using hot-wire measurements. A 2:1 aspect ratio jet with an initially turbulent boundary layer and a constant momentum thickness all around the nozzle exit perimeter was used for this study. Measurements were made in air at a Reynolds number ReDe (≡ UeDe/v) = 3.5 × 104. Controlled longitudinal excitation at the preferred mode frequency (StDe ≡ fDe/Ue = 0.4) induced periodic formation of structures, allowing phase-locked measurements with a local trigger hot wire. The dynamics of the organized structure are examined from educed fields of coherent vorticity and incoherent turbulence in the major and minor symmetry planes at five successive phases of evolution, and are also compared with corresponding data for a circular jet. Unlike in a circular jet, azimuthally fixed streamwise vortices (ribs) form without the aid of azimuthal forcing. The three-dimensional deformation of elliptic vortical structures and the rib formation mechanism have also been studied through direct numerical simulation. Differential self-induced motions due to non-uniform azimuthal curvature and the azimuthally fixed ribs produce greater mass entrainment in the elliptic jet than in a circular jet. The turbulence production mechanism, entrainment and mixing enhancement, and time-average measures and their modification by excitation are also discussed in terms of coherent structure dynamics and the rib-roll interaction. Various phase-dependent and time-average turbulence measures documented in this paper should serve as target data for validation of numerical simulations and turbulence modelling, and for design and control purposes in technological applications. Further details are given by Husain (1984).

The elliptic whistler jet

1999 ◽  
Vol 397 ◽  
pp. 23-44 ◽  
Author(s):  
HYDER S. HUSAIN ◽  
FAZLE HUSSAIN
Keyword(s):  
Flow Visualization ◽  
Near Field ◽  
Major Axis ◽  
Field Mass ◽  
Vortical Structures ◽  
Air Jet ◽  
Mass Entrainment ◽  
Circular Jets ◽  
Secondary Vortices ◽  
Axis Switching

Elliptic jets have decided advantages for technological applications over circular jets; this paper explores further advantages achieved by jet forcing due to self-excitation. Using hot-wire measurements and flow visualization, we have studied an elliptic whistler (i.e. self-excited) air jet of 2:1 aspect ratio which, in contrast to an elliptic jet issuing from a contoured nozzle, displays no axis switching, but significantly increased spread in the major-axis plane. Its near-field mass entrainment is considerably higher (by as much as 70%) than that of a non-whistling jet. Flow visualization reveals unexpected dynamics of the elliptic vortical structures in the whistler jet compared to that in the non-whistling jet. Vortices rolled up from the lip of the elliptic pipe impinge onto the collar, producing secondary vortices; interaction of these two opposite-signed vortices is shown to cause the different behaviour of the whistler jet.

Instability Modal Behavior of the Acoustically Excited Impinging Plane Jet With a Small Cylinder

2004 ◽  
Vol 20 (2) ◽  
pp. 145-157 ◽  
Author(s):  
Fei-Bin Hsiao ◽  
I-Che Hsu ◽  
Cheng-Chiang Hsu
Keyword(s):  
Flow Field ◽  
Coherent Structures ◽  
Nozzle Exit ◽  
Near Field ◽  
Jet Flow ◽  
Core Region ◽  
Far Field ◽  
Potential Core ◽  
Modal Behavior ◽  
Small Cylinder

AbstractThe Instability modal behavior of coherent structures in a jet-small cylinder impinging flow field is extensively studied by hot-wire anemometry measurements. The free jet is employed with a small cylinder of 3 mm in diameter located in the potential core region at the impinging length of L/H = 1.5 for the near field impingement and L/H = 4 for the far field impingement. The jet exit velocity is operated at 10 m/sec with the Reynolds number of 1.03 × 104 based on the nozzle exit width H = 15mm. The impinging jet is locally excited at the nozzle exit with varicose mode (m =0) and sinuous mode (m = 1) disturbances at the fundamental frequency of the natural jet flow. Data indicate that the jet flow is greatly altered and significantly enhanced by strengthening the coherent structures of the flow due to resonance according to the feedback mechanism. Although the original natural jet preferably exhibits the varicose mode, the strong sinuous mode is dominant in the flow field owing to the presence of the small cylinder in the potential core region. In the near field impingement, the wake region behind the cylinder preserves the pure sinuous mode to where the jet vortices merge and then mildly fades out. Whereas in the jet shear layer, the sinuous mode exists in the initial portion and gradually transforms to the varicose mode. In the far field impingement, the alternate mode dominates in each frequency stage in pure impinging case and the modal behavior follows the selected mode with the introducing acoustic waves in the acoustic excitation cases.

Rotating co-flow jets with horizontal lip and shape transition

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Dawn Pradeeb S. A. ◽  
Subramanian Thanigaiarasu ◽  
Nagarajakrishnan Premanand
Keyword(s):  
Nozzle Exit ◽  
Large Scale ◽  
Metal Cutting ◽  
Flame Length ◽  
Equivalent Diameter ◽  
Exit Plane ◽  
Interesting Phenomenon ◽  
Destructive Effect ◽  
Outer Flow ◽  
Content Type

Purpose Control over large-scale coherent structures and stream-wise vortices lead to enhanced entrainment/conservation of the jet which is desirable for most free jet applications such as design of combustion chamber in jet engines and flame length elongation of welding torch used for metal cutting. Design/methodology/approach A co-flow nozzle with lip thickness of 2 mm, between the primary (inner) and secondary (outer) flow, is selected. Three nozzle combinations are used, i.e. C–C (circle–circle), C–E (circle–ellipse) and C–S (circle–square) for acquiring comparative data. For these nozzle combinations, inner nozzle exit plane is kept as a circle, whereas the outer nozzle exit planes are varied to circle, ellipse and square. The exit plane area of outer nozzle for the nozzle combinations has equivalent diameter, De. The nozzles are fabricated in a way that the outer nozzle can be rotated along the longitudinal axis, keeping the inner nozzle intact. Findings The C–C nozzle combination is effective in low Mach number regime in decaying the jet, when the rotational component is introduced. Around 30% reduction in the jet core length is observed for the C–C nozzle combinations without any lip. The C–E nozzle shows sedative result in decaying or preserving the jet. The C–S nozzle combination shows interesting phenomenon, whereby the low subsonic case tends to conserve the jet by 15% and the higher subsonic case tends to decay the jet by 10%. Originality/value The developed nozzle systems show both conservative and destructive effect on the jet, which is desirable for the mentioned applications.

Trajectory and Characteristics of Buoyancy and Momentum Dominated Horizontal Jet Flames From Circular and Elliptic Burners

2005 ◽  
Vol 128 (4) ◽  
pp. 300-310 ◽  
Author(s):  
Tracy Smith ◽  
Chendhil Periasamy ◽  
Benjamin Baird ◽  
S. R. Gollahalli
Keyword(s):  
Nitric Oxide ◽  
Reynolds Number ◽  
Flame Temperature ◽  
Major Axis ◽  
Equivalent Diameter ◽  
Flame Stability ◽  
Temperature Profiles ◽  
Burner Exit ◽  
Wide Range ◽  
Nitric Oxide Emissions

Relative effects of buoyancy and momentum on the characteristics of horizontally oriented circular (Circ) and elliptic (E) burner flames in a quiescent environment over a wide range of jet exit velocities are presented. The major axis of the elliptic burner was oriented horizontally and vertically (referred to as Emaj and Emin flames, respectively). Propane was used as fuel and a small amount of hydrogen was piloted to attach flames to the burner. Global flame characteristics such as flame dimensions, centerline trajectory, emission indices (EI) and radiative fraction, and in-flame transverse concentration and temperature profiles were measured. At a jet exit Reynolds number (Rej) of 2000, based on the area-equivalent diameter of the burner, the flame characteristics were affected by the burner geometry and its orientation. Also, the vertical dimension of the burner exit dictated buoyancy effects. At Rej=12,500, the influence of burner geometry or its orientation was negligible. Elliptic burner flames exhibited lower liftoff and blowout velocities than circular burner flames. Furthermore, the flame stability and nitric oxide emissions were not much affected by the orientation of elliptic burner. Although the elliptic burners produced higher EINO at lower jet exit velocities, the variation in EINO among three burners (Circ, Emaj, and Emin) was insignificant at higher velocities. Some effects of buoyancy on EICO were observed at lower jet exit velocities and the EICO was the lowest for the burners with largest buoyancy flux. Elliptic burner flames produced greater peak flame temperature than the corresponding circular burner flames under most conditions.

scholarly journals Glaucomatous Macular Vasculature: A Quantitative Analysis

2020 ◽  
Vol 9 (3) ◽  
pp. 185-197
Author(s):  
Pedro Arede ◽  
Joanna Przezdziecka-Dolyk ◽  
Fabian Debowy ◽  
Jacek Olszewski ◽  
Carla Fernandes ◽  
...  
Keyword(s):  
Optical Coherence Tomography ◽  
Structural Changes ◽  
Open Angle Glaucoma ◽  
Major Axis ◽  
Foveal Avascular Zone ◽  
Control Group ◽  
Equivalent Diameter ◽  
Optical Coherence ◽  
Fiber Layer ◽  
Avascular Zone

Background: The aim of this study was to evaluate the characteristics of the macular vessel density (VD) and the foveal avascular zone (FAZ) in glaucoma quantitatively using the optical coherence tomography angiography (OCT-A). Methods: Twenty-five eyes of 13 patients with primary open angle glaucoma (POAG) and 12 eyes of 6 healthy participants were enrolled retrospectively. Functional visual field (VF) and structural Spectral-Domain optical coherence tomography (SD-OCT) Retinal Nerve Fiber Layer Thickness (RNFLT) were assessed in all participants. OCT-A was performed on a fovea centered, 15x10 degrees, macular region. OCT-A scans were processed with MATLAB software and automatically graded to define FAZ parameters. The parafoveal VD in the superficial and deep retinal vascular plexus (SVP and DVP) was analyzed by quadrant and circular segmented zones. Results: Foveal Avascular Zone -Major Axis Length (p=0.02), Area (p=0.04), Equivalent Diameter (p=0.04) and Perimeter (p=0.04) were significantly larger in glaucoma than the control group. Regarding SVP and DVP, the average macular total VD were lower in glaucoma patients compared to the control group (p<0.01; p<0.01). Additionally, the inner circular region (p=0.04; p<0.01 respectively for SVP and DVP) and all quadrants except for North had a lower VD in glaucoma group compared to the control group. Assessment of the total VD successfully predicted RNFLT (p<0.001) and was significantly associated with the probability of glaucoma (p=0.009). Conclusion: OCT-A parameters, namely the FAZ morphology and the macular VD, were associated with glaucomatous functional and structural changes. The macular VD showed a considerable diagnostic value. It may be a modern biomarker, representing microvascular network disruption of the macular perfusion in glaucoma.

Sign in / Sign up

Export Citation Format

Share Document