Noncircular Jets Research Articles

Effect of Jet Inclination Angle and Hole Exit Shape on Vortical Flow Structures in Low-Reynolds Number Jet in Cross-Flow

Numerical studies have been performed to visualize vortical flow structures emerged from jet cross-flow interactions. A single square jet issuing perpendicularly into a cross-flow was simulated first, followed by two additional scenarios, that is, inclined square jet at angles of 30° and 60° and round and elliptic jets at an angle of 90°, respectively. The simulation considers a jet to cross-flow velocity ratio of 2.5 and a Reynolds number of 225, based on the free-stream flow quantities and the jet exit width in case of square jet or minor axis length in case of elliptic jet. For the single square jet, the vortical flow structures simulated are in good qualitative agreement with the findings by other researchers. Further analysis reveals that the jet penetrates deeper into the cross-flow field for the normal jet, and the decrease of the jet inclination angle weakens the cross-flow entrainment in the near-wake region. For both noncircular and circular jet hole shapes, the flow field in the vicinity of the jet exit has been dominated by large-scale dynamic flow structures and it was found that the elliptic jet hole geometry has maximum “lifted-off” effect among three hole configurations studied. This finding is also in good qualitative agreement with existing experimental observations.

An Experimental Study on Comparison of Non-circular Co-flow Jet with Co-axial Jets and Computational Verification

An Experimental Study on Comparison of Non-circular Co-flow Jet with Co-axial Jets and Computational Verification

Effects of notch geometry and sharpness on turbulent jets issuing from indeterminate-origin notched nozzles

An experimental investigation using hot-wire anemometry to identify the effects of notch configuration and relative sharpness on notched indeterminate-origin turbulent jets at R e = 20,500 was carried out. The notches are characterized by their V- and A-shaped cuts, as well as their relative sharpness due to the design aspect ratio ( A R = 2 and 4) of the notch. Regardless of exact notch geometry, high aspect-ratio nozzles are shown to produce significant flow differences between measurements taken along peak and trough planes, in contrast to low aspect-ratio nozzles. Half-jet width results demonstrate that as the aspect-ratio increases, smooth peaks result in a constant increase within the measurement range while sharp peaks produce an initial decrease till some downstream location before increasing thereafter. In contrast, smooth and sharp trough produces significant nonlinear increases in the half-jet widths with the aspect-ratio increment. Cross-over points linked to axis-switching behaviour are observed for all nozzles except A R = 4 A-notched nozzle, when half-jet widths along peak and trough planes for each nozzle are compared. Increasing the aspect ratio also results in a 50% reduction in the distance away from the nozzle origin where the cross-over point occurs for V-notched nozzles. On the other hand, similar increase in the aspect ratio produces no cross-over point for A-notched nozzles. These results indicate that suitably configured notched nozzles are able to impart significant control over circular jet flow behaviour by inducing and manipulating axis-switching behaviour typically found in noncircular jets.

On Turbulent Jets Issuing from Notched-Rectangular and Circular Orifice Plates

This paper reports an experimental investigation of the near-field flow characteristics of two air/air turbulent jets issuing respectively from notched-rectangular and circular orifice plates with identical opening areas or equivalent diameters (De). Planar particle image velocimetry (PIV) was used to measure the velocity field at the same Reynolds number, based on De, of Re = 72,000. Consistent with previous work on other noncircular jets, the present study finds that the notched jet has a higher rate of mixing than does the circular counterpart. In particular, this jet in the very near field transfers its momentum to the surroundings at a greater rate, evidenced by a notably shorter unmixed core and faster turbulence intensity growth. The higher rates of overall decay and spread of the notched jet are maintained over the entire measurement region and likely beyond. In addition, the phenomenon of axis switching is also found to occur in this jet.

Manipulation of vortex rings for flow control

This paper reviews the dynamics of vortex rings and the control of flow by the manipulation of vortex rings. Vortex rings play key roles in many flows; hence, the understanding of the dynamics of vortex rings is crucial for scientists and engineers dealing with flow phenomena. We describe the structures and motions of vortex rings in circular and noncircular jets, which are typical examples of flows evolving into vortex rings. For circular jets the mechanism of evolving, merging and breakdown of vortex rings is described, and for noncircular jets the dynamics of three-dimensional deformation and interaction of noncircular vortex rings under the effect of self- and mutual induction is discussed. The application of vortex-ring manipulation to the control of various flows is reviewed with successful examples, based on the relationship between the vortex ring dynamics and the flow properties.

Characterization of jets issuing from circular nozzles with A-shaped notch lip-modifications

Jets exhausting from circular nozzles lip-modified with A-shaped notches of different relative sharpness were studied using digital particle image velocimetry. Results show that along the trough-to-trough planes, in addition to large-scale shear layer vortices, small-scale secondary vortex structures are formed close to the jet centrelines. Along the peak-to-peak planes, small-scale streamwise-aligned vortex structures are formed along the jet peripheral instead, while shear layer vortices move towards the jet centrelines. Flow stresses are also generally higher along peak-to-peak planes than along trough-to-trough planes. Half jet-width and momentum thickness profiles for the lip-modified nozzles are significantly wider than those for a non-lip-modified reference nozzle. Furthermore, half jet-width profiles taken along the peak-to-peak and trough-to-trough planes demonstrate trends and crossover locations, which are reminiscent of noncircular jet axis-switching behaviour. Lastly, increasing the nozzle-relative sharpness here accentuates the vortex dynamics but not necessarily leading to further favourable flow behaviour.

Circular and noncircular subsonic jets in cross flow

Experiments were conducted to investigate the impact of circular and noncircular injector nozzle geometries on the penetration, mixing, and structure of a jet injected perpendicular into subsonic cross flow. Circular, triangle, and elongated slot nozzles were studied including varying their orientations relative to the cross flow, to comprise a matrix of five different test cases. The cross-sectional areas of all geometries were equal. The interaction between the jets and the cross flow was strongly dependent on the different geometries and their orientations. Each of the injector geometries resulted in a distinctive region of backflow at the leeward side of the jet. The location and size of this region and the reverse flow magnitude were largely dependent on the aspect ratio and the jet cross section lateral dimension change in the axial direction. A combination of instantaneous and time-averaged particle image velocimetry images taken in the axial and lateral cross sections of the jet near the exit plane revealed several large scale turbulent structures downstream of the jets for two different blowing ratios r=3.2 and 8. Detailed flow field data are presented for the higher blowing ratio due to the better spatial resolution of the large scale structures in the near field. A counter-rotating vortex pair was identified in the near field of the jets resulting in a reverse flow region. These structures had an upright nature similar to the wake vortices with the exception that the former were attached to and constantly fed from jet fluid mixed with the cross flow. The latter was formed by the cross flow, fed from its boundary layer vorticity, and remained separate from the jet. It was attached to the surface of the flat plate at one end and to the trajectory of the bent jet at the other end. Both the shape and orientation of the nozzle geometries had a strong effect on the jet spread and penetration. Compared to the baseline circular nozzle, the slot major (slot nozzle’s major axis oriented along the cross stream) exhibited the highest penetration, whereas the slot minor penetrated the least. The triangular jets’ characteristics were significantly affected by their orientation relative to the cross flow. Several different scaling techniques were explored in order to compare the present jet trajectories with previously published data. Commonly used length scaling parameters of rd and r2d were shown to be inadequate to scale different experimental data. Newly proposed scaling that was based on the velocity profiles of the cross flow boundary layer and the jet, prior to their interaction, yielded better results.

Acoustic Characteristics of Non-Circular Slot Jets

Experimental investigations on the acoustic characteristics of non-circular jets are carried out. Non-circular geometries include triangular, rounded triangular, square, and elliptic configurations, with a circular jet serving as the baseline case. Acoustic characteristics in terms of the spectra, directivity of sound pressure, tonal properties, and broadband shock associated noise are obtained to unravel the basic differences in these jets. Experiments are carried out for various pressure ratios ranging from 2 to 6, corresponding to fully expanded jet Mach numbers ranging from 1.04 to 1.82. The reduced effects of screech and broadband shock associated noise in triangular and square jets render them quieter than circular jet by up to 10 dB. The broadband shock associated noise levels are higher along flat edge planes than vertex planes, at all pressure ratios. Tonal analysis of the acoustic radiation from elliptic jet reveals that the screech tones along the major axis plane are more intense than those along minor axis plane.

Mixing in Circular and Non-circular Jets in Crossflow

Coherent structures and mixing in the flow field of a jet in crossflow have been studied using computational (large eddy simulation) and experimental (particle image velocimetry and laser-induced fluorescence) techniques. The mean scalar fields and turbulence statistics as determined by both are compared for circular, elliptic, and square nozzles. For the latter configurations, effects of orientation are considered. The computations reveal that the distribution of a passive scalar in a cross-sectional plane can be single- or double-peaked, depending on the nozzle shape and orientation. A proper orthogonal decomposition of the transverse velocity indicates that coherent structures may be responsible for this phenomenon. Nozzles which have a single-peaked distribution have stronger modes in transverse direction. The global mixing performance is superior for these nozzle types. This is the case for the blunt square nozzle and for the elliptic nozzle with high aspect ratio. It is further demonstrated that the flow field contains large regions in which a passive scalar is transported up the mean gradient (counter-gradient transport) which implies failure of the gradient diffusion hypothesis.

B109 LESによる長方形噴流中の三次元渦構造の可視化

The vortex rings generated in non-circular jets are deformed and interacted three-dimensionally owing to self-induced velocity, and the vortex motion enhances mixing. To understand the mechanism of the mixing enhancement, it is crucial to make clear the three-dimensional vortical structure caused by the interaction of non-circular vortices. In the present study, the details of three-dimensional vortical structure of a rectangular jet of aspect ratio 4 is clarified using Large Eddy Simulation (LES), and the interaction mechanism of rectangular vortices is discussed, focusing the deformation of vortices. The simulation results reveal the details of the vortex interaction in the rectangular jet: the vortices deform owing to the non-uniform curvature effect, and the interaction of vortices generates hairpin vortices via vortex stretching.

Development of ICRF wall conditioning technique on divertor-type tokamaks ASDEX Upgrade and JET

Encouraging results of efficient wall conditioning with ICRF discharges achieved on the circular tokamaks (without divertors) TEXTOR, TORE SUPRA, HT-7 stimulated the next step in the development of an alternative wall conditioning technique that is relevant to reactor-scale superconducting fusion machines. Here, the first results on the ICRF discharge initiation and the wall conditioning on the large-size non-circular tokamaks ASDEX Upgrade and JET using the standard ICRF antennas are reported. Analysis of the mass-spectrum data on both machines indicates noticeable outgassing of deuterium, water and hydrocarbons from the walls despite operating with a non-optimized set of ICRF discharge parameters.

Effect of Delta Tabs on Axis Switching of a Coaxial Square Jet

The effects of delta tabs in the near field of a coaxial square jet have been studied experimentally using a laser Doppler anemometer. The tabs are added to the centers of the four sidewalls of the inner nozzle trailing edge, with the tip of each tab tilted into the inner jet, that is, θ = 45 deg, or into the outer jet, that is, θ = -45 deg. For each tab orientation, three velocity ratios have been considered: γ u = 0, 0.5, and 1.0 for θ = 45 deg and γ u = 1.0, 2.0, and oo for θ = -45 deg. The results illustrate that tabs increase the mixing between the inner jet and outer jet and may promote or suppress the phenomenon of axis switching, depending on the orientation of the tabs. Axis switching is defined as follows: As a noncircular (here square) jet spreads, its cross-section may evolve from the initial shape similar to that the jet exit, to have its axis rotated some angle at some downstream location. In the case of θ =45 deg, the cross-sectional shape of the inner mixing layers is bifurcated into a four-finger structure, which tends to suppress the onset of axis switching. When θ = -45 deg, it exhibits an eightfold mode, and a rapid axis-switching occurs in the near-field. The effect of tabs is briefly discussed using inviscid vortex dynamics.

Mixing and Entrainment Characteristics of Circular and Noncircular Confined Jets

The present work deals with the experimental investigation of entrainment characteristics of confined/semiconfined circular and noncircular jets. The jet fluid, after issuing out of a nozzle of circular or noncircular cross section, enters a circular mixing tube of larger area, and during this process it entrains some ambient fluid into the mixing tube. The flow is incompressible and isothermal at a jet Reynolds number of 7200. The experimental results obtained in the study are first validated with the approximate theoretical analysis of Pritchard et al. (1997) and also with the similarity solution proposed by Becker et al. (1963) for circular nozzles. It is observed that the similarity solution is applicable for circular as well as noncircular jets in the region close to the jet axis and away from the nozzle exit plane. The entrainment ratio increases to a maximum value as the jet location is shifted away from the tube inlet; for the configurations studied, enhancement up to 30% has been observed in the entrainment ratio with shift in jet location. For a smaller mixing tube diameter and jet located at the inlet of the mixing tube, the circular jet entrains more than noncircular jets. For a larger mixing tube or shifted jet locations, the noncircular jets entrain more of ambient fluid, in general. Among the different noncircular geometries considered, the jet having the cross section of an isosceles triangle causes maximum entrainment.

Direct numerical simulations of two-phase laminar jet flows with different cross-section injection geometries

Direct numerical simulations are performed of spatial, three-dimensional, laminar jets of different inlet geometric configurations for the purpose of quantifying the characteristics of the flows; both single-phase (SP) and two-phase (TP) free jets are considered. The TP jets consist of gas laden with liquid drops randomly injected at the inlet. Drop evaporation ensues both due to the gaseous flow being initially unvitiated by the vapor species corresponding to the liquid drops, and to drop heating as the initial drop temperature is lower than that of the carrier gas. The conservation equations for the TP flow include complete couplings of mass, momentum, and energy based on thermodynamically self-consistent specification of the vapor enthalpy, internal energy, and latent heat of vaporization. Inlet geometries investigated are circular, elliptic, rectangular, square, and triangular. The results focus both on the different spreading achieved according to the inlet geometry, as well as on the considerable change in the flow field due to the presence of the drops. The most important consequence of the drop interaction with the flow is the production of streamwise vorticity that alters entrainment and species mixing according to the inlet geometry. Similar to their SP equivalent, TP jets are shown to reach steady-state entrainment; examination of the flows at this time station shows that the potential cores of TP jets are shorter by an order of magnitude than their SP counterpart. Moreover, whereas the TP circular jet exhibits a symmetric entrainment pattern well past the streamwise location of the potential core, noncircular jets display at the same location strong departures from symmetry. Furthermore, the SP-jet phenomenon of axis switching is no longer present in TP jets. The distributions of drop-number density, liquid mass, and evaporated species are compared for different inlet cross sections and recommendations are made regarding the optimal choice for different applications.

Direct Numerical Simulation of Transitional Noncircular Buoyant Reactive Jets

The near field dynamics of transitional buoyant reactive jets established on noncircular geometries, including a rectangular nozzle with an aspect ratio of 2:1 and a square nozzle with the same cross-sectional area, are investigated by three-dimensional spatial direct numerical simulations. Without applying external perturbations at the inflow boundary, large vortical structures develop naturally in the flow field due to buoyancy effects. Simulation results and analysis describe the details and clarify mechanisms of vortex dynamics of the noncircular buoyant reactive jets. The interaction between density gradients and gravity initiates the flow vorticity. Among the major vorticity transport terms, the gravitational term mainly promotes flow vorticity in the cross-streamwise direction. For the baroclinic torque, it can either create or destroy flow vorticity depending on the local flow structure. The vortex stretching term has different effects on the streamwise and cross-streamwise vorticity. Streamwise vorticity is mainly created by vortex stretching, while this term can either create or destroy cross-streamwise vorticity. Under the coupling effects of buoyancy and noncircular nozzle geometry, three-dimensional vortex interactions lead to the transitional behavior of the reactive jets. Simulations also show that the rectangular jet is more vortical than the square jet. The rectangular jet has a stronger tendency of transition to turbulence at the downstream due to the aspect ratio effect. Mean flow property calculations show that the rectangular buoyant reactive jet has a higher entrainment rate than its square counterpart.

Asymmetric turbulent jet flows

Some of the novel features of non-circular asymmetric turbulent jet flows are introduced. The computational methods employed to verify those features are outlined and details about the differences in power levels are presented. Typical applications of asymmetric nozzles to mixing and cleaning are discussed. Significant advantages over standard circular nozzles are shown in some cases.

FLOW CONTROL WITH NONCIRCULAR JETS

▪ Abstract Noncircular jets have been the topic of extensive research in the last fifteen years. These jets were identified as an efficient technique of passive flow control that allows significant improvements of performance in various practical systems at a relatively low cost because noncircular jets rely solely on changes in the geometry of the nozzle. The applications of noncircular jets discussed in this review include improved large- and small-scale mixing in low- and high-speed flows, and enhanced combustor performance, by improving combustion efficiency, reducing combustion instabilities and undesired emissions. Additional applications include noise suppression, heat transfer, and thrust vector control (TVC). The flow patterns associated with noncircular jets involve mechanisms of vortex evolution and interaction, flow instabilities, and fine-scale turbulence augmentation. Stability theory identified the effects of initial momentum thickness distribution, aspect ratio, and radius of curvature on the initial flow evolution. Experiments revealed complex vortex evolution and interaction related to self-induction and interaction between azimuthal and axial vortices, which lead to axis switching in the mean flow field. Numerical simulations described the details and clarified mechanisms of vorticity dynamics and effects of heat release and reaction on noncircular jet behavior. The research on noncircular jets has also led to technology transfer. A topic that started as an academic curiosity—an interesting flow phenomenon—subsequently has had various industrial applications. The investigations reviewed include experimental, theoretical, numerical, and technological aspects of the subject.

Jet flames from noncircular burners

Gas jets from noncircular exits entrain more air from surroundings than jets from circular exits of equivalent area. Because the mixing rate of fuel and air governs the combustion and pollutant emission of diffusion flames and partially premixed flames, noncircular geometries offer a passive control of the flame characteristics. In this paper, the literature on nonburning, noncircular jets is reviewed and recent studies on noncircular jet flames are discussed with focus on the work conducted in the author’s laboratory.

Simulation of Chaotic Particle Motion in Particle-Laden Jetflow and Application to Abrasive Waterjet Machining

A novel method is presented for modeling the abrasive waterjet machining process. Particle motion on the cross section of particle-laden jetflow is simulated in order to quantify the erosion contributions of millions of particles having different kinematic behaviors. The simulation is performed using fractal point sets with chaotic behavior for the cases of circular and noncircular (elliptical, triangular) jets. The jetflow constructed can be endowed with any desired velocity profile and/or particle flowrate. In association with a classical constitutive equation for estimating the penetration ability of a particle, the drilling and cutting operations are simulated and verified by experiment for titanium, glass, and other materials. Roughness and waviness of the cutting surfaces are also simulated and there is good consistency between theory and experiment. Triangular and elliptical jetflows are utilized to explore potential applications of noncircular abrasive waterjets.

Studies on Twin Non-Parallel Unventilated Axisymmetric Jets

Twin non-parallel jet configurations occur in many practical devices like burners, combustion chambers, and also in the area of fluidics etc. Despite the importance of such configurations, studies on the interaction of twin non-parallel jets are very limited. The present work is an attempt to investigate the mixing of two axisymmetric jets obliquely oriented towards each other. Experimental studies were made on the interaction of twin intersecting axisymmetric jets issuing from two unventilated convergent nozzles. The nozzles of exit diameter (D) 10 mm were set on a common end wall with their axes intersecting each other at half angles (α3) of 0°, 5°, 10° and 15°. The centre-to-centre spacing (S) of the nozzles, non-dimensionalized, as S/D, was 3.1. The jet exit Mach number (Me) studied was 0.2. The results indicate that the near field characteristics are strongly influenced by α3. However, the potential core of the individual jets and the far field characteristics of the twin jet flow field, that is beyond downstream distances of 40 nozzle diameters, are not significantly influenced by α3. The cross-section of the jet just downstream of the combining point is approximately elliptic. The axis switching phenomenon normally associated with non-circular jets is observed in the jet flow field. The spread of the combined jet is more in the transverse direction than in the spanwise direction. Entrainment of the ambient fluid is found to be more in the case of twin parallel jets (α3 = 0°). The entrainment decreases with increasing α3. The self-preserving nature of the combined jet is almost independent of the initial geometric conditions. The combined jet is axisymmetric with regard to the normalized velocity and length scales.