Elliptic Jet Research Articles

Control of Supersonic Elliptic Jet with Ventilated Tabs

Abstract Control of Mach 1.5 elliptic jet with ventilated triangular tabs is studied experimentally, in the presence of different levels of pressure gradient at the nozzle exit. Three different sets of ventilated tabs with circular, triangular and trapezoidal ventilations were studied. Two tabs were placed, at the ends of major and minor axes, at the exit of the elliptic nozzle of aspect ratio 3.37. The mixing enhancement caused by these tabs was studied in the presence of adverse and favorable pressure gradients, corresponding to nozzle pressure ratio (NPR) from 3 to 8. For Mach 1.5 jet NPR 3 corresponds to 18 % adverse pressure gradient and NPR 8 corresponds to 118 % favorable pressure gradient. The results of ventilated tabs are compared with unventilated truncated triangular tabs of identical geometry. The difference between the mixing promoting efficiency of the unventilated and ventilated tabs is only marginal (around 5–6 %). All tabs cause jet bifurcation and weaken the waves in the jet core. The tab with trapezoidal ventilation, at NPR 3, promotes mixing to an extent of reducing the core to about 92 %. At higher NPRs the mixing caused by unventilated tab is slightly better than the ventilated tabs.

Flow field and acoustic characteristics of elliptical throat CD nozzle

ABSTRACTThe mixing characteristic of circular jet with elliptical throat is studied at different levels of expansion of the nozzle. Two kinds of configurations are studied in this paper, elliptical throat and circular throat jet. At all pressure ratios, the jet with non-circular throat experiences superior mixing than circular throat. The core length of elliptical throat jet is shorter than circular jet. Also, at all pressure ratios, the profile of both configurations looks identical in oscillation; however, the efficiency of the mixing promotion is high in non-circular throat. The promotion of mixing is most efficient at nozzle pressure ratio (NPR) level of 8. As the NPR increases, the core length of the jet reduces nearly 5–10% at each range of NPR. The centreline Pitot pressure profile of non-circular and circular throat shows that the decay of the jet from elliptical throat nozzle is faster than the jet from circular jet. At different levels of expansion, the shock structure shows notable changes with elliptical shape. The amplitude of screech frequency of the jet noise decreases with an increase in NPR. Introduction of elliptical throat to circular throat modifies the shock-cell structures, which significantly change the screech amplitude due to weakening of waves.

Statistical Properties of Round, Square, and Elliptic Jets at Low and Moderate Reynolds Numbers

An experimental study was conducted to investigate the effect of nozzle geometries on the statistical properties of free orifice jets at low and moderate Reynolds numbers. The studied cross sections were round, square, and ellipses with aspect ratios of 2 and 3. For each jet, detailed velocity measurements were made using a particle image velocimetry (PIV) system at Reynolds numbers of 2500 and 17,000. The results showed that at both Reynolds numbers, the elliptic jets had relatively higher velocity decay and jet spreading; however, the nozzle geometry effects were more pronounced at Re = 17,000 than at Re = 2500. Analysis of the swirling strength revealed that the rotational motions induced by vortices within the minor planes of the elliptic jets were stronger than observed in the major planes, square and round jets which were consistent with the relatively higher spreading observed in the minor planes. It was observed that the streamwise locations of the switchover points were independent of Reynolds number but are a strong function of aspect ratio. Based on the present results and those documented in the literature, a linear correlation was proposed for the location of axis-switching in orifice jets. Due to the axis-switching phenomena, a sign change was observed in the distribution of the Reynolds shear stress in the major planes of the elliptic jets. This results in the existence of regions with negative eddy viscosity in the near field regions, an observation that has an important implication for the predictive capabilities of standard eddy viscosity models.

Nozzle Aspect Ratio Effect on Supersonic Elliptic Jet Mixing

Nozzle aspect ratio effect on the mixing of Mach 2 elliptic free jet, issuing from convergent–divergent elliptic nozzles of aspect ratios 2, 3, and 4, in the presence of adverse and marginally favorable pressure gradients at the nozzle exit has been studied experimentally. The results show that AR4 jet enjoys better mixing than AR2 and AR3 jets at all nozzle pressure ratios. The AR2 and AR3 jets displayed axis switching, whereas there is no axis switching for AR4 jet. The shadowgraph shows that the waves in AR4 jet are weaker than those in AR2 and AR3 jets.

Acoustic and mixing characteristic of CD nozzle with inverted triangular tabs

ABSTRACTIn this paper, we study the pressure energy decay, acoustic and mixing characteristics of non-circular jet with inverted triangular tabs at different ranges of aspect ratio. The centre line pitot pressure profile of the inverted triangular tab and ordinary tab shows that the decay of the jet from elliptical jet with inverted tab is faster than the jet from circular jet. When the aspect ratio of nozzle increased, the core length of the jet changes significantly due to the pressure loss. At different aspect ratios, the shock structure shows a prominent change with respect to the mixing promotion. The screech frequency of the jet noise gives appreciable change as the nozzle pressure is increased, and nearly 20–30% of decrease is witnessed in the magnitude of screech frequency. Introduction of inverted triangular tab jet modifies the shock cell formation which extensively changes the screech amplitude due to axis switching.

Flow characteristic of highly underexpanded jets from various nozzle geometries

Flow characteristics of highly underexpanded jets at the same nozzle pressure ratio of 5.60 but issuing from four different nozzles, i.e., the circular, elliptic, square, and rectangular nozzles, are studied using large eddy simulations. The results show that the square jet penetrates fastest, although the turbulence transition is similar for different jets. The penetration rates of different jets show the similar linear dependency on the square root of time, but the penetration constant Γ for the noncircular jets deviates more than 5% from the theoretical value of 3.0. The circular and square jets both correspond to a three-dimensional helical instability mode, while the elliptic and rectangular jets have a two-dimensional flapping instability in their minor axis planes. All the jets undergo a Mach reflection forming the Mach disk, but the Mach disk in the elliptic and rectangular jets is not easily visible. The intercepting shocks in the square jet originate at the four corners of the nozzle exit at first, while the formation of the intercepting shocks is only observed in the major axis planes for the elliptic and rectangular jets. In addition, great differences are observed on the mixing characteristics between different jets. In particular, the elliptic jet penetrates slowest, has the shortest length of jet potential core, and takes the largest mixing area.

Instability evolution of the viscous elliptic liquid jet in the Rayleigh regime.

For jet flow emanating from noncircular orifices, an unbalanced surface tension force leads to capillary instability, which is independent of influence from the ambient air in the Rayleigh regime. In the present article, the dynamic behavior of incompressible elliptical jets in the Rayleigh regime is investigated. Theoretically, with the consideration of the fluid viscosity, the solution of the Cosserat equation consists of a particular solution and a complementary solution. For the complementary solution the wave number of disturbance modes has two complex conjugate roots, which are responsible for the jet breakup. To match the nonzero particular solution, a spatial wave needs to be introduced, which is independent of external perturbations. Physically, such a spatial wave is interpreted as the axis-switching phenomenon. The predicted features of the axis-switching wavelength and the damping effect from the fluid viscosity have been successfully verified by experimental results. Moreover, the dispersion relations from the present theory suggest that the growth rate of spatial instability is influenced by orifice eccentricity, the Weber number, and the Ohnesorge number.

PIV Measurements in the Near and Intermediate Field Regions of Jets Issuing from Eight Different Nozzle Geometries

An experimental study was conducted to investigate the effect of nozzle geometry on the mixing characteristics and turbulent transport phenomena in turbulent jets. The nozzle geometry examined were round, square, cross, eight-corner star, six-lobe daisy, equilateral triangle as well as ellipse and rectangle each with aspect ratio of 2. The jets were produced from sharp linear contoured nozzles which may be considered intermediate to the more widely studied smooth contraction and orifice nozzles. A high resolution particle image velocimetry was used to conduct detailed velocity measurements in the near and intermediate regions. It was observed that the lengths of the potential cores and the growth rates of turbulence intensities on the jet centerline are comparable with those of the orifice jets. The results indicate that the decay and spreading rates are lower than reported for orifice jets but higher than results for smooth contoured jets. The jets issuing from the elliptic and rectangular nozzles have the best mixing performance while the least effective mixing was observed in the star jet. The distributions of the Reynolds stresses and turbulent diffusion clearly showed that turbulent transport phenomena are quite sensitive to nozzle geometry. Due to the specific shape of triangular and daisy jets, the profiles of mean velocity and turbulent quantities are close to each other in their minor and major planes while in the elliptic and rectangular jets are considerably different. They also exhibit more isotropic behavior compared to the elliptic and rectangular jets. In spite of significant effects of nozzle geometry on mean velocity and turbulent quantities, the integral length scales are independent of changes in nozzle geometry.

Control of Elliptic Supersonic Jet of Aspect Ratio 3

AbstractControl of a Mach 2 elliptic jet from a convergent-divergent elliptic nozzle of Aspect Ratio 3, with rectangular and triangular tabs of identical blockage, was studied experimentally at dif...

Spray Characteristics of Elliptical Power-Law Fluid-Impinging Jets

The spray characteristics of a liquid sheet contribute much to the investigation of atomization efficiency. Considering the jet contracting effect of elliptical jets, an improved model of elliptical power-law fluid jets is proposed herein to derive the spray characteristics. Some experiments have been conducted to verify its feasibility, and the results show a good agreement with theoretical predictions. The effect of the aspect ratio on sheet shape and thickness has been studied to interpret the phenomenon that liquid sheets formed by the impinging elliptical jets are more likely to disintegrate. The relationships between rheological parameters (K and n) and the spray features are also discussed.

Flow and acoustic characteristics of non-axisymmetric jets at subsonic conditions

Flow and acoustic behavior of two asymmetric, rectangular (AR = 4) and elliptic (AR = 2.5), jets are studied and compared to an equivalent area round jet. The jets are operated at a Mach number of 0.9 and temperature ratio of 1. Time-averaged flow field measurements are carried out using planar and stereoscopic particle image velocimetry. In addition, far-field microphone measurements are performed to compare jet acoustics. Mean flow field results demonstrate that for the given Mach number and aspect ratios, rectangular and elliptic jet properties are somewhat modified compared to the round jet. The elliptic jet exhibits properties that are intermediate between two geometric extremes. Moderately enhanced mixing in asymmetric jets as a result of weak streamwise vortices is evidenced by overall shorter potential core, faster centerline velocity decay, and higher shear layer growth rates. Centerline turbulence levels and transverse shear stress distribution also show enhanced fluctuations for non-circular jets. Compared to their major axis planes, relatively higher turbulence levels are measured in the minor axis planes for both rectangular and elliptic jets. Far-field acoustic measurements reveal the asymmetric nature of the sound field. Compared to the round jet, major axis orientation for asymmetric jets is observed to provide moderate acoustic benefit in the downstream direction. However, enhanced fluctuations in the minor axis plane result in a marginal noise augmentation at moderate to high frequencies in this plane for downstream polar angles.

Elliptic nozzle aspect ratio effect on controlled jet propagation

The present study deals with the control of a Mach 2 elliptic jet from a convergent–divergent elliptic nozzle of aspect ratio 4 using tabs at the nozzle exit. The experiments were carried out for rectangular and triangular tabs of the same blockage, placed along the major and minor axes of the nozzle exit, at different levels of nozzle expansion. The triangular tabs along the minor axis promoted superior mixing compared to the other controlled jets and caused substantial core length reduction at all the nozzle pressure ratios studied. The rectangular tabs along the minor axis caused core length reduction at all pressure ratios, but the values were minimal compared to that of triangular tabs along the minor axis. For all the test conditions, the mixing promotion caused by tabs along the major axis was inferior to that of tabs along the minor axis. The waves present in the core of controlled jets were visualized using a shadowgraph. Comparison of the present results with the results of a controlled Mach 2 elliptic jet of aspect ratio 2 (Aravindh Kumar and Sathakrishnan 2016 J. Propulsion Power 32 121–33, Aravindh Kumar and Rathakrishnan 2016 J. Aerospace Eng. at press (doi:10.1177/0954410016652921)) show that for all levels of expansion, the mixing effectiveness of triangular tabs along the minor axis of an aspect ratio 4 nozzle is better than rectangular or triangular tabs along the minor axis of an aspect ratio 2 nozzle.

Broad band shock associated noise predictions in axisymmetric and asymmetric jets using an improved turbulence scale model

Supersonic jets that are subject to off-design operating conditions are marked by three distinct regions in their far-field spectra: mixing noise, screech and Broadband Shock Associated Noise (BBSAN). BBSAN is conspicuous by the prominent multiple peaks. The Morris and Miller BBSAN model that is based on an acoustic analogy, offering a straightforward implementation for RANS, forms the foundation of the present work. The analogy model robustly captures the peak frequency noise, that occurs near Strouhal number of about 1, based on the nozzle exit diameter but leads to major sound under prediction for higher frequencies. In the jet mixing noise literature, it has been shown that an inclusion of frequency dependence into the characteristic length and temporal scales of the effective noise sources improves the far-field noise predictions. In the present paper, several modifications of the original Morris and Miller model are considered that incorporate the frequency dependent scales as recommended in the jet mixing noise literature. In addition to these, a new mixed scale model is proposed that incorporates a correlation scale that depends both on the mean-flow velocity gradient and the standard mixing noise-type scaling based on the dissipation of turbulent kinetic energy. In comparison with the original Morris and Miller model, the mixed scale model shows considerable improvements in the noise predictions for the benchmark axisymmetric convergent-divergent and convergent jets. Further to this validation, the new model has been applied for improved predictions for elliptic jets of various eccentricity. It has been shown that, for the same thrust conditions, the elliptical nozzles lead to noise reduction at the source in comparison with the baseline axisymmetric jets.

Study of decay characteristics of rectangular and elliptical supersonic jets

Experiments were carried out to study the decay characteristics of non-circular supersonic jets issued from rectangular and elliptical nozzles, and results were compared with circular case. Numerical simulations and Schlieren image study were carried out to validate the experimental results obtained from total pressure data. The supersonic core lengths of the jets were found to be different for different exit shaped geometries and area ratio for those nozzles was same. To avoid the losses in divergent section, the shape of cross-section of throat was maintained same that of as the exit. The exit shape geometry played the important role to enhance the mixing of the jet with ambient air, without requiring secondary method to increase the mixing characteristics. The mixing with ambient air in case of non-circular jets was more intense when compared to that of circular jets, which resulted in reduction of supersonic core length. The behavior of supersonic core length had identical signature for both under-expanded and over-expanded operation. The supersonic core length was characterized by exit shape factor. In literature, the supersonic jet characterization and the related experimental correlation are available for optimum expansion conditions whereas for other expansion (under and over) conditions the experimental correlations are barely available. While investigating experimentally, a new empirical relation was obtained which was the improved form of earlier correlations for supersonic core length. The current results obtained from three different methods (total pressure data, schlieren image and numerical simulation) had shown the reasonable agreement with the experimentally obtained relation.

Corrugated Limiting Tab for Supersonic Jet Mixing

The mixing efficiency of limiting tabs of three geometries with triangular corrugations, in promoting the mixing of a Mach 2 elliptic jet has been studied. Limiting tabs of rectangular, triangular and circular-arc geometries with 5% blockage are placed along major and minor axis at the nozzle exit are tested for nozzle pressure ratio from 4 to 8 in steps of one. All the tabs are found to be efficient mixing promoter at all tested NPRs, when placed along major axis at the nozzle exit. However at NPR 4 and 5 circular-arc tab along minor axis is found to retard the near field jet mixing. But the corrugated rectangular tab along minor axis is found to be the best mixing promoter among tested geometries. It causes a maximum reduction of 89% in core length at NPR 6. The corresponding core length reduction for corrugated triangular and circular-arc tab is 78% and 84% respectively. The maximum reduction in core length obtained for corrugated triangular and circular arc tab is found when placed along major axis at the exit of elliptical nozzle. The shadowgraph pictures of the jet reveal that the waves prevailing in the elliptic jet controlled with corrugated limiting tabs along major axis are significantly weaker than those in uncontrolled and controlled (with tabs along minor axis) elliptic jet.

Vortex dynamics and wall shear stress behaviour associated with an elliptic jet impinging upon a flat plate

Vortical structures and dynamics of a Re h = 2100 elliptic jet impinging upon a flat plate were studied at H/d h = 1, 2 and 4 jet-to-plate separation distances. Flow investigations were conducted along both its major and minor planes using laser-induced fluorescence and digital particle image velocimetry techniques. Results show that the impingement process along the major plane largely consists of primary jet ring-vortex and wall-separated secondary vortex formations, where they subsequently separate from the flat plate at smaller H/d h = 1 and 2 separation distances. Key vortex formation locations occur closer to the impingement point as the separation distance increases. Interestingly, braid vortices and rib structures begin to take part in the impingement process at H/d h = 4 and wave instabilities dominate the flow field. In contrast, significantly more coherent primary and secondary vortices with physically larger vortex core sizes and higher vortex strengths are observed along the minor plane, with no signs of braid vortices and rib structures. Lastly, influences of these different flow dynamics on the major and minor plane instantaneous and mean skin friction coefficient levels are investigated to shed light on the effects of separation distance on the wall shear stress distributions.

Elliptic jet control with triangular tab

Elliptic jet mixing influenced by triangular tabs is demonstrated in this work. Mixing modification of a Mach 2 jet from a convergent-divergent elliptic nozzle of aspect ratio 2, in the presence of two triangular tabs along the major and minor axis at the nozzle exit, at different levels of nozzle expansion has been studied. The results show that the mixing caused by tabs along the minor axis is impressive compared to the uncontrolled jet at all the pressure ratios. But for tabs along the major axis, mixing enhancement is significant only for nozzle pressure ratios above 5. Tabs along the minor axis cause better mixing than tabs along the major axis. The iso-pitot pressure contours reveal that the tabs along the minor axis enhance the mixing by bifurcating the jet. Shadowgraphs show that the tabs render the waves in the jet weaker. The present study demonstrates the superior mixing promotion caused by triangular tab than rectangular tab, studied by Aravindh Kumar and Rathakrishnan (2015).

Aspect ratio effect on elliptical sonic jet mixing

ABSTRACTThe effects of aspect ratio on elliptical sonic jet decay at different levels of under-expansion has been studied experimentally. Elliptical sonic jets from orifices of same area with aspect ratios (AR) 2, 4 and 6 at nozzle pressure ratios 2 to 5 in steps of 1 have also been studied. A circular jet from an orifice with an area equal to that of elliptical orifice was also studied for comparison. Jet centreline pressure decay, spread and waves present in the jet core were analysed. The results show that the mixing of the elliptical jet is superior to the circular jet, at all the nozzle pressure ratios of the present study. Also, the aspect ratio of the elliptical orifice has a strong influence on the jet mixing. Elliptical jets of aspect ratio 4 and 6 experience a significantly higher mixing than the aspect ratio 2 jet, till the under-expansion level corresponding to Mach disk formation. For higher under-expansion levels, the mixing of AR 4 and 6 jets become inferior to that of the AR 2 jet.

Characteristics of a supersonic elliptic jet

ABSTRACTComparative mixing of a Mach 2 elliptical free jet from a convergent-divergent elliptic nozzle with an aspect ratio of 2:1 in the presence of adverse and marginally favourable pressure gradients has been studied experimentally. It is found that the mixing of the elliptical jet is higher than that for the equivalent circular jet at all the levels of expansion. The decay of elliptic jet is significantly higher than the equivalent circular jet in all three zones of the jet field – the core, characteristic decay and fully developed regions. The reason for the faster decay of the elliptic jet is found to be the continuous variation in the size of the mixing-promoting vortices shed from the nozzle exit owing to its azimuthal asymmetry. The evolution of the jet and its axis-switching phenomenon has been studied using iso-pitot pressure contours taken at different axial locations in the plane normal to the jet axis. As expected, the elliptic jet spreads faster along the minor axis plane than the major axis plane, leading to axis-switching at all the levels of expansion studied. The axis-switching of the elliptic jet shifts upstream with increase in nozzle pressure ratio (NPR) from 4 to 5; from 5 to 7, it shifts downstream. But at marginally under-expanded condition of NPR 8, the axis-switching is found to shift slightly upstream. The occurrence of axis-switching in the elliptic jet indicates enhanced near-field mixing, compared to the equivalent circular jet. The shadowgraph pictures of the jet reveal that the waves prevailing in the elliptic jet are significantly weaker than those in the circular jet.

Tab Geometry Effect on Supersonic Elliptic Jet Control

AbstractThe efficiency of tabs of two geometries in promoting the mixing of a Mach 2 elliptic jet has been studied. Limiting tab of triangular and circular geometry (crosswire) of 5 % blockage placed along major and minor axis at the nozzle exit, are tested for nozzle pressure ratio from 4 to 8, in steps of one. Both tabs are efficient mixing promoters, at all the tested NPRs, when placed along the minor axis. But along major axis the crosswire retards the mixing, at all the NPRs. The triangular tab along the major axis is also found to retard the mixing at NPRs 4 and 5, but for nozzle pressure ratios above 5 it causes mixing enhancement even when placed along the major axis. The triangular tab is found to be a better mixing promoter than the crosswire. The maximum core length reduction of 88 % is caused by triangular tab along the minor axis is at NPR4. The corresponding core length reduction for the crosswire is only 72 %. Shadowgraph pictures of controlled jets show that both tabs weaken the waves in jet core. The geometry and orientation of the tab and the expansion level influence the mixing caused by the tab.