Minor Axis Plane Research Articles

Implicit large-eddy simulation of an over-expanded screeching rectangular jet

Implicit large-eddy simulation of an over-expanded screeching rectangular jet is performed with a seventh-order finite difference scheme. Good agreement is found between the predicted flow- and acoustic fields with the experimental observations. Fourier decomposition, phase-averaging analysis and Spectral Proper Orthogonal Decomposition (SPOD) are used to investigate the origin of the screech, the shock leakage during the shear-layer flapping, and the distinguishing fluctuating characteristics in the minor- and major-axis plane of the rectangular jet. It finds that the screech is radiated from the end of the forth shock cell, where the interaction of the shock waves with the shear layer causes periodic leakages of shock-wave tips in the minor-axis plane, resulting in the generation of intense acoustic waves in the surrounding air. An obvious flapping mode at the same frequency of the screech is captured in the minor-axis plane and dominates the dynamic motions of the rectangular jet. The SPOD modes of pressure and velocity fluctuations at the screech frequency help to reveal the relationship between the screech generation and the coherent structures.

Effects of Aspect Ratio on Vortex Dynamics in Starting Rectangular Jets

The vortex dynamics of starting rectangular jets with aspect ratios of 1, 4, 8, and 10 is investigated numerically at Reynolds number of 1000. The 45°-axis-switching for the square vortex ring and 90°-axis-switching for the rectangular vortex ring are found to be controlled by the secondary flow induced by the ring deformation. Vortex stretching appears near both the and centerline planes, which is weakened by the increasing aspect ratio. Vortex ring pinch-off on both centerline planes can be identified by the magnitude of the vorticity flux delivered into the vortex ring. The dimensionless time for pinch-off (the formation numbers) for the square jet is at about 2.0 on both diagonal plane and midplane. For the rectangular jets, the formation number is reduced from 2.3 to 1.5 on the major axis plane by increasing aspect ratio, while it approaches 1.7 on the minor axis plane, which is independent of the aspect ratio. Our results suggest that the vortex deformation accelerates the pinch-off process on both the major and minor axis planes. In particular, the appearance of secondary vortices on the major plane and the arc-shaped vortices on the minor plane signals the onset of pinch-off.

Flow dynamics and noise generation mechanisms in supersonic underexpanded rectangular and planar jets

In this study, large eddy simulations of two rectangular jets with different aspect ratios and one planar jet with spanwise periodic boundaries are carried out in an attempt to investigate and clarify some key issues regarding the flow dynamics and noise generation mechanisms for the screech tone. The substantial effects of the nozzle configuration on shock structures and noise are first revealed in detail. It is found that when the lateral confinement is weakened, the spacing of the shock cells increases and the jet oscillates more intensively in the minor-axis plane, increasing the noise level and altering the screeching frequency. By analyzing the pressure fluctuations of the shear layer in the wavenumber-frequency space, different kinds of waves in these supersonic jets are examined. Importantly, it is revealed that the guided jet wave should play a dominant role in closing the resonance loops rather than the acoustic wave. Moreover, the energy-containing structures with pertinent frequencies are extracted by employing the reduced-order variational mode decomposition, and some underlying flow dynamics are presented. Especially, a novel mechanism has been identified: the low-frequency stretching motions of shock cells have a significant modulation effect on the screeching amplitude in the planar jet. Furthermore, with the aid of the multi-process acoustic theory, the characteristics of physical noise sources are diagnosed, particularly the source mechanisms related to the screech tone. The general structures and distribution of the kinematic noise source Sβ and entropy noise source Se are presented. Sβ mainly exhibits a vortex-like structure near the nozzle, while Se exhibits a lamellar bilayer structure. The spatiotemporal correlations between the physical sources and far-field noise show that the dominant mechanisms for the screech tone rely on the nozzle configuration and the screech tone tends to be produced by multiple sources in all cases.

Numerical investigation of the cryogenic underexpanded hydrogen jets exiting rectangular shaped openings

Cryo-compressed hydrogen shows increasing potentials in hydrogen utilization. The development and revision of safety codes and standards for cryo-compressed hydrogen infrastructure requires studies of accidental leaks for various scenarios. However, the actual leaks are more likely to result in jets through slots rather than round nozzles in most previously studies. In this work, a numerical model of cryogenic hydrogen underexpanded jets was employed to evaluate the effect of jet temperature and aspect ratio of rectangular nozzles on the shock structure and dispersion characteristics. Correlations were proposed to evaluate the effect of jet temperature and aspect ratio of rectangular nozzles on the shock structure and dispersion characteristics. The numerical results revealed that cryogenic hydrogen release caused axial extension of the hazard zone. The aspect ratio of rectangular nozzle shows significant effect on the shock structure and hydrogen dispersion. The shock intensity decreases with aspect ratio and Mach disk disappear when the ratio larger than eight. The effect of nozzle shape on hydrogen diffusion was pronounced at high pressure ratio between nozzle exits and environment. Moreover, the normalized radial volume fraction with various jet temperature collapsed into a Gaussian distribution with low aspect ratio (AR) smaller than 8, and the centerline. The decay rates of centerline normalized volume fraction increased with the aspect ratio. The normalized volume fraction on centerline decay rates increased with the aspect ratio. However, the normalized volume fraction on the minor axis plane did not fit a Gaussian distribution when the aspect ratio was larger than eight due to axis switching, which also produced a wider hazard zone.

Anisotropic Turbulent Kinetic Energy Budgets in Compressible Rectangular Jets

Turbulence is governed by various mechanisms, such as production, dissipation, diffusion, dilatation and convection, which lead to its evolution and decay. In high-speed flows, turbulence becomes complicated due to compressibility effects. Therefore, the goal of the current work is to characterize these mechanisms in rectangular supersonic jets by directly evaluating their contributions in turbulent kinetic energy (TKE) budget equation. The budgets are obtained using high-fidelity Large Eddy Simulations that employ WALE subgrid-scale model. Jet nearfield data are validated with PIV experimental measurements, available from the literature, which include mean flow and second-order statistics. To ensure spatial resolution and temporal convergence of higher-order statistics, qualitative performance metrics are presented. The results indicate that TKE production is the major source term, while pressure-dilatation term acts as a sink throughout the development of the jet. The diffusion term has the highest contribution from triple-velocity correlations, followed by pressure diffusion and molecular diffusion. Subgrid-scale diffusion and dissipation are also evaluated and their contributions are minimal. Each term is presented on both minor and major axis plane and reveals asymmetry in the statistics. A detailed explanation of budget contributions is provided, leading to the mechanisms responsible for the anisotropy of TKE.

Effects of Expansion Ratio and Nozzle Asymmetry on Flowfield of Diamond Jets

A joint experimental and computational study is performed to quantify the effects of various expansion ratios on the flowfields and acoustic properties of a jet, issuing from a diamond-shaped nozzle. Three expansion ratios are studied corresponding to perfectly expanded, overexpanded, and underexpanded regimes, which progressively increase the far-field acoustic impact. Nozzle asymmetry leads to distinct mean flow properties across the two principal (major/minor) axes of the jets, with the potential core and shock-expansion cells exhibiting a faster collapse on the major axis plane. Asymmetry is also observed in the computed near-field pressure spectra, which exhibits higher amplitudes in the minor axis plane, consistent with the higher far-field sound levels experimentally measured on this plane for all three jets. Near-field spectra also identify three specific bands, corresponding to broadband shock-associated noise (BBSAN), mixing noise, and that associated with nonradiating subsonic convecting eddies. Further analysis of noise mechanisms through the turbulent kinetic energy (TKE) budget shows enhanced levels of TKE on the minor axis plane, resulting from correlations between fluctuations in streamwise and cross-stream velocities, and gradients across the shear layers. Fourier analysis of near-field coherent structures in pressure identified axisymmetric features in the most energetic modes, followed by the first helical mode, which was enhanced in the presence of shock-expansion cells in the imperfectly expanded jets. The most energetic modes in the overexpanded shock-expansion cells have a strong harmonic nature to it, corresponding to the peak frequencies of its near-field BBSAN component.

Experimental investigation on flow characteristics and unsteady heat transfer of noncircular impinging synthetic jets

This study investigates the flow characteristics and unsteady heat transfer of noncircular synthetic jets impinging on a heated plate. Thermographic phosphor thermometry and time-resolved particle image velocimetry are used to acquire quantitative wall temperature and flow field information, respectively. At a constant orifice-to-wall distance (H/De = 7.5), five orifice configurations (circular, square, elliptic with aspect ratio (AR) of 3, and rectangular with ARs of 3 and 5) are examined at a wall temperature of 80 °C. The orifice configuration has a significant effect on the heat transfer and vortex behavior of the impinging synthetic jets. The square orifice displays better stagnation cooling performance with 42% increase in the maximum stagnation cooling coefficient relative to the circular case due to the larger impinging velocity and deeper penetration into the wall shear layer. Axis-switching of the vortex rings is detected for both the elliptic and rectangular orifices before they impinge onto the wall. The axis-switching enhances the near-wall mixing and turbulent kinetic energy in the elliptic and rectangular orifices, and thus heat transfer compared with the circular case. Additionally, stronger vortices and deeper penetration into the wall shear layer are observed in the major-axis plane than in the minor-axis plane, leading to enhanced heat transfer in the major-axis direction. In particular, the rectangular orifice with an AR of 5 induces strong secondary structures in the trailing jet following the primary vortex ring, and these are responsible for the double-peak distribution of wall temperature in the major-axis direction. This is the first time a double-peak temperature distribution has been reported for noncircular impinging synthetic jets. The rectangular orifice with an AR of 5 also yields a larger cross-stream velocity in the minor-axis direction, indicating a stronger wall jet. Thus, among the cases considered in this study, the rectangular orifice with AR = 5 achieves the optimal cooling performance and the maximum time-averaged cooling area reaches more than 14 times the jet orifice area.

Investigation of mixing characteristics of coflow rectangular nozzle of aspect ratio 2 and 3

PurposeThe purpose of this study is to understand experimentally the mixing characteristics of a two-stream exhaust system with a supersonic Mach 1.5 primary jet that exits the rectangular C-D nozzle surrounded by a sonic secondary jet from a convergent rectangular nozzle by varying the aspect ratio (AR = 2 and 3) similar to those that can be available for future high-speed commercial aircraft.Design/methodology/approachThis paper focuses on the experimental results of effects of AR at various expansion levels of jets issued/delivered from a central rectangular convergent-divergent nozzle of AR 2 and 3 surrounded by a coflow from a convergent rectangular sonic nozzle. The lip thickness of the primary nozzle is 2.2 mm. various nozzle pressure ratios (NPRs) ranging from 2, 3, 3.69 and 4 were chosen for pressure measurements.FindingsFor all the NPRs, AR 3 had a shorter core than AR 2. Also, AR 3 was found to decay faster in the transition and fully developed zones. The lateral plots show that the AR has an influence on the jet spread.Originality/valueThe structure of waves existing in the potential core of the rectangular coflow jet along with the major and minor axis planes was visualized by the shadowgraph technique.

Characterization of a Novel Elliptical Air-port Inverse Jet Flame

ABSTRACT This paper reports the experimental and numerical investigation of an inverse jet flame (IJF) burner with an elliptical central port and circumferentially arranged circular fuel ports (CAFP). In order to enhance the air-fuel entrainment, a passive technique using an elliptical central jet is implemented. The flame appearance of elliptical central air-port IJF denoted a difference in the jet spread on major and minor axis planes. The azimuthal structure of elliptical central port IJF exhibited properties similar to elliptical normal jet flames. The experiments also revealed that, unlike in circular central port IJF burner, elliptical IJF generates a wide range of turbulent scales beneficial for combustion applications as visualized through schlieren images. The numerical model based on a steady laminar flamelet model and the standard k-ε model having a modified model constant predicted the thermal characteristics of elliptical central port CAFP IJF. The experiments reported a significant reduction in flame height for IJF with an elliptical central port due to differences in spreading and entrainment rates. Thermal and emission characteristics are significantly different compared with circular IJF for a similar momentum flux ratio. Interestingly, the elliptical central port IJF demonstrates a lower NO emission index at higher and lower momentum flux ratios. Apart from the temperature, flame height, and NO emission index variation, the mixing and entrainment characteristics on major and minor axis planes are vividly illustrated and quantified as compared to the conventional design has been brought out explicitly.

Effect of aspect ratio on Reynolds shear stress distribution of jet particles from rectangular convergent transitions

The flow field and Reynolds shear stress characteristics of the jet particles from circular and different circular-rectangular orifices into still air surroundings are experimentally investigated with hot wire apparatus. The distribution of dimensionless Reynolds shear stress at different radial and axial positions in the major and minor axis planes is measured. The Reynolds number is 2.2 × 105 in all jets. It is found that circular-rectangular jet induces stronger mixing than the equivalent circular jet does, and the increase in aspect ratio can make even stronger mixing. The result shows that in the radial direction, the dimensionless Reynolds shear stress firstly increases and then decreases after it reaches the peak value near the interface of jet and ambient. The interface moves outward along the radial side when jet goes downstream. The distribution of dimensionless Reynolds Stress is similar both in major and minor axis planes in different jets and the magnitude increases as the aspect ratio increases. On the central lines of all jets, dimensionless Reynolds stress near transition exit increases, and then decreases after it reaches the peak value. The maximum value of AR4 is three times larger than the maximum value of AR1, and the former is even 330% larger than the latter on the narrow side. The increase in aspect ratio results in an increase in dimensionless Reynolds stress at corresponding positions.

Optimal Design of Elliptical Beam Cassegrain Antenna

An optimal design method of elliptical beam Cassegrain antenna based on differential evolution with global and local neighborhoods algorithm is proposed to achieve high efficiency, low sidelobe, and low profile at Ku/Ka dual band. A rapid shaping method of elliptical beam antenna is given by applying the algorithm of steepest descent. Constraining of the first sidelobe level, antenna efficiency is optimized with only 17 variables, where aperture field distribution functions in the major and minor axis planes are parameterized by Non-uniform rational B-spline curves, and the transition function is expressed with a sinusoidal function. A prototype of optimized antenna with an aperture of 630mm $\times1150$ mm is fabricated and measured. The measured results show that the total efficiency is greater than 54.5%, and the first sidelobe levels are lower than −14.2dB across the Ku/Ka dual-band, which agree well with simulated ones. It is verified that the optimal design method is very effective.

Aspect Ratio Effect on Mach 1.5 Rectangular Jet Mixing

An experimental investigation on the mixing characteristics of Mach 1.5 rectangular jet emanating from aspect ratios (ARs) 2 and 3 convergent-divergent nozzles is carried out at nozzle pressure ratios (NPRs) 3, 3.69, 4 and 5, respectively. At overexpanded (NPR3), correctly expanded (NPR 3.69) and underexpanded (NPRs 4, 5) levels of Mach 1.5 jet, the Pitot pressure measured along the centerline of AR2 and AR3 rectangular jets showed that the influence of aspect ratio on the jet mixing is not as strong as expected. The effect of aspect ratio is noticeable only in the core of both the jets. The core length of AR2 jet is shorter than AR3 jet at all the NPRs, which indicates faster mixing of AR2 jet with ambient fluid as compared to AR3 jet. But after core, the decay of AR2 and AR3 jets in the characteristic decay region remained almost same at all the NPRs except at correctly expanded NPR of 3.69, the AR3 jet decays slightly faster than AR2 jet. For all the cases, the jets have become fully developed together at far downstream location. The pressure profiles taken along major axis and minor axis of Mach 1.5 rectangular jets confirm the faster spreading of AR2 and AR3 jets along minor axis than along major axis. Both the jets are found to switch axes between 8D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> to 10D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> . The shadowgraph visualization reveals the complex structure of waves present in the major axis and minor axis planes of AR2 and AR3 rectangular jets.

The mechanism of vortex bifurcation vis-à-vis axial switching in rectangular synthetic jets

The present study investigates the vortex dynamics of the rectangular shaped synthetic jet and reports the occurrence of vortex ring bifurcation along with other reported modes such as axial switching and the vortex suction. The novel finding of vortex ring bifurcation of rectangular synthetic jets has been observed without any other mode of excitation except the periodic axial actuation. The experiments on synthetic jets have been conducted at different actuation frequencies and both qualitative and quantitative characterization of the flow structures has been carried out using Laser Induced Fluorescence (LIF) and Laser Doppler Velocimetry, respectively. LIF flow visualization provides insights into the size of the vortex and the vortex evolution with respect to time, enabling us to propose the flow physics behind the axial switching and the vortex bifurcation processes for rectangular synthetic jets. The proposed flow physics is then quantitatively evidenced by the time-averaged velocity measurements. Vortex splitting or bifurcation is found to occur in the minor axis plane of orifice and the divergence angle depends on the actuation frequency and average velocity of fluid expelled through the orifice in the forward stroke of diaphragm. In the case of occurrence of axial switching, a maximum of three axial switching events are observed before vortex breakup. Finally, by systematically carrying out experiments across a wide range of operational parameters, a narrow region corresponding to the vortex bifurcation has been identified on a Reynolds Number–Strouhal Number map, along with other modes such as axial switching regime and the vortex suction regime. Based on our measurements, a mechanism of vortex bifurcation vis-à-vis axial switching has also been suggested.

A Numerical Study of the Bifurcation of Rectangular Synthetic Jets

The present paper explores the mechanism of vortex formation and its bifurcation process for a rectangular orifice synthetic jet. The orifice aspect ratio is equal to two (7 x 14 mm2) with hydraulic diameter 9.33 mm and 3 mm thickness. A 3-D numerical simulation using Large-eddy -simulation (LES) model on Fluent software is performed to investigate the vortex formation and its bifurcation process. The diaphragm oscillates at amplitude of 1.6 mm and 2.6mm compromising a volumetric ejection of 1500mm3 and 2500mm3 respectively through orifice at 1 Hz of actuation frequency. The geometry of the cavity and the diaphragm displacement profile as well as the volumetric ejection is kept identical in both the experimental study and the numerical simulations. It has been observed that vortex ring bifurcates in both the cases and the simulation results are in good agreement with the experimental observations. In both the bifurcation cases, the vortex ring undergoes axial switching followed by bifurcation in the minor axis plane. Further the bifurcated rings moves at away from each other in the same plane at different divergence angle depending on the vortex ejection velocity during forward stroke of the diaphragm and its actuation frequency.

On the Bifurcation of Rectangular Orifice Synthetic Jets

The present study investigates the influence of actuation frequency and diaphragm displacement of the rectangular shaped synthetic jet and reports the occurrence of vortex ring bifurcation for a rectangular orifice, along with other reported modes such as axial switching and vortex suction. The novel finding of vortex ring bifurcation of rectangular synthetic jets has been observed without any other mode of excitation except the periodic axial actuation. The experiments on synthetic jets have been conducted at different actuation frequencies and both qualitative and quantitative characterization of the flow structures has been carried out using Laser Induced Fluorescence and Laser Doppler Velocimetry, respectively. LIF flow visualization provides insights into the size of the vortex and the vortex evolution with respect to time, enabling us to propose the flow physics behind the axial switching and the vortex bifurcation processes for rectangular synthetic jets. The proposed flow physics is then quantitatively evidenced by the time-averaged velocity measurements. Vortex splitting or bifurcation is found to occur in the minor axis plane of orifice and their divergence angle depends on the actuation frequency and average velocity of fluid expelled through the orifice in the forward stroke of diaphragm. In the case of occurrence of axial switching, a maximum of three axial switching events are observed before vortex breakup. Finally, by systematically carrying out experiments across a wide range of operational parameters, a narrow region corresponding to the vortex bifurcation has been identified on an Re-St map, along with other modes such as axial switching regime and the vortex suction regime.

Antisymmetric Oscillation Modes in Rectangular Screeching Jets

In this paper, the origin and the properties of the oscillation modes in screeching non-ideally expanded rectangular jets are investigated using compressible implicit LES of rectangular supersonic jets. At the exit of a converging diverging rectangular nozzle of aspect ratio 2 and of design Mach number 1.5, the jets are under- and over-expanded. Seven simulations with four different temperature ratios ranging from 1 to 3 and two different nozzle pressure ratios are performed. The geometry of the nozzle and the exit conditions are chosen such that to match the experimental study conducted at the University of Cincinnati. First, the over-expanded jets are studied. It is shown that the total number of shock cells decreases with increased temperature ratio. However, the temperature does not influence the size of the first shock cell and the linear decrease of the shock cell size in the downstream direction. The spreading of the jet is observed to be higher along the minor axis plane than along the major axis plane. The intensity of the screech noise increases with the temperature ratio in the present study although the opposite is observed in the experiments. Moreover, for jet temperature ratios of 2.5 and 3, the strong flapping motion of the jet along the minor axis plane due to the screech feedback mechanism yields to an antisymmetric organization of the Mach wave radiation. Thereafter, the near- and far-field acoustic are studied. In the near-field, screech tones are captured, whose frequencies are consistent with both experimental data and theoretical models. In the far-field, four acoustic components typical of non-ideally expanded supersonic jets are observed, namely the screech noise, the broadband shock-associated noise, the mixing noise and the Mach wave noise. Their directivities and frequencies are in agreement with experimental results and models. The mechanism of the screech noise generation is studied by using a Fourier decomposition of the pressure field. For the four over-expanded jets, a flapping motion along the diagonal or along the minor axis plane of the jet is noted. Finally, the hypothesis that the acoustic waves completing the feedback loop in these jets are linked to the upstream-propagating acoustic wave modes of the equivalent ideally expanded jets is tested. Using a jet vortex sheet model to describe the dispersion relations of these modes, it is found that this hypothesis allows us to explain the antisymmetric jet oscillation observed at the screech frequencies. Based on frequency-wavenumber decomposition of the pressure fluctuations in the jets, it is shown that at the screech frequencies, acoustic waves propagating in the upstream direction at the ambient speed of sound exist also in the jet flow, additionally to the acoustic waves propagating outside of the jet. These acoustic waves belong to the neutral acoustic wave modes of the equivalent ideally expanded jet. These results support the idea that a vortex sheet model of the corresponding 2-D planar ideally expanded jet is capable of predicting the wave modes of a non-ideally expanded rectangular supersonic jet. They also suggest that these waves are involved in the feedback part of the screech mechanism; explaining why, for the simulated screeching rectangular jets, the associated oscillation mode is antisymmetric.

Modeling of underexpanded hydrogen jets through square and rectangular slot nozzles

The development and revision of safety codes and standards for hydrogen infrastructure requires a solid scientific basis, including studies of unignited releases from high pressure systems for various scenarios. Most hydrogen releases are modeled as axisymmetric jets, but real leaks are more likely to be non-axisymmetric jets issuing from high aspect ratio cracks or slots. In the present study, underexpanded hydrogen jets from square and rectangular nozzles with aspect ratios of 1–16 were numerically modeled for stagnation pressures up to 20 MPa. The near and far flow fields were modeled separately using two sequential computational domains to accurately and efficiently capture the flow characteristics. The numerical models were first validated with experimental data from a previous experimental study and literature data. The mass fraction and velocity distributions show that the centerline decay rates increase as the nozzle aspect ratio increases, but this increase is dependent on the pressure. This means that the canonical decay law of round turbulent jets and plumes no longer applies to the slot nozzle jets for high pressures. The radial profiles collapse onto a Gaussian curve in the major axis plane, but neither collapse, nor are they Gaussian in the minor axis plane with peaks away from the jet centerline. Different shock patterns were identified along the major and minor axes and the axis switching phenomenon seen in the literature was also reproduced. The axis switching resulted in significantly wider flattened concentration distributions compared with the axisymmetric jet which may require consideration during safety analyses for non-circular nozzles. A scaling factor taking both the nozzle shape and pressure effects into account was then developed to better scale the centerline decay rates for jets from both the square and rectangular nozzles. The present study demonstrates that the nozzle shape effects on the jet spreading should not be overlooked and proper scaling factors are required to collapse the data and calculate decay rates.

The Effect of wall on subsonic non-circular jets

The present experimental study aims at characterizing the mixing characteristics of subsonic elliptic and rectangular orifice jets (Ve = 51 m/s) in the presence of wall of different surface roughness, namely smooth, fine and coarse, placed parallel to the jet axis at the edge of the orifice exit. The wall of length 120 cm of different surface roughness was oriented either parallel to the minor axis plane or major axis plane if the orifice. The wall had a strong effect on the mixing characteristics of elliptic and rectangular jets. The centerline velocity measurement showed that the potential core of elliptic and rectangular orifice jets was around 3De and 3.5De. For both jets, all the wall configurations retarded the mixing of jet with the ambient fluid only in the characteristic decay zone but not in the core and fully developed zones. The level of mixing retardation was found to be maximum when wall was placed parallel to the major axis plane of the jet. The effect of wall on jet mixing was intense for rectangular jet compared to elliptic jet.

Impinging jet noise reduction using non-circular jets

Experimental studies in the acoustic far field and flow visualization are carried out on circular and non-circular impinging jets at a nozzle to plate distance ratio of 5, for the nozzle pressure ratios of 1.8, 4, and 6. Spectra and directivity of the overall sound pressure level of circular jets are compared with those of non-circular jets for the same nozzle pressure ratios, at various emission angles. Non-circular jets mitigate the transonic impinging tones at around the critical pressure ratio, compared to the circular jet. Square jet is quieter at low nozzle pressure ratios by 5–8 dB compared to the circular jet. At higher under expansion conditions, the triangular and square jets almost completely eliminate the supersonic impinging tones. Elliptical jet is noisy at a nozzle pressure ratio of 6, even compared to the circular jet. Elliptical minor axis plane is quieter at lower nozzle pressure ratios compared to the major axis plane and the trend is reversed for the higher nozzle pressure ratios. Schlieren flow visualization studies reveal that the shock cell structures are weakened and symmetry of the shock cells are lost for the non-circular jets compared to the circular jet.

Numerical investigation on the three-dimensional flow characteristics of unsteady subsonic elliptic jet

Based on large eddy simulation (LES), combined with high order tuned centered-difference (TCD) scheme, the initial flow characteristics of the unsteady subsonic elliptic jet was investigated numerically. The numerical results illustrate the formation, variation and development of the subsonic elliptic jets’ initial flow structures. Moreover, the self-induced deformation of the noncircular vortex loops and the distribution of the streamwise vorticity field have been completely clear using the three-dimensional (3D) view. The vorticity field of the results qualitatively shows that the self-induced Biot-Savart deformation of primary vortex loop (PVL) leads to the streamwise vortices generation in the flow, and the streamwise vortex pairs enlarge along the primary vortex core and augment the deformation of PVL. These two mechanisms govern the phenomenon of axis switching. Subsequently, the coherent structures present in the major and minor axis planes of the elliptic vortex loop and jet shear layer are identified with proper orthogonal decomposition (POD) method. Moreover, the mechanisms of vortex dynamics and the entrainment features of various noncircular jets have also been described in detail.