Primary Jet Research Articles

Role of jet core in splashing patterns of diesel jet impingement on a solid wall

Under certain conditions diesel jet wall impingement results in triangular (polygonal) splashing shapes. This process has been found to be almost independent of injection pressure, injection frequency or of the injection duration. The triangular splashing shapes are observed at a given transition time after diesel jet wall contact. This time depends on the distance from the wall and on the injection pressure (jet velocity). This triangular splashing process is dependent on the distance from the wall, and does not correspond to a typical hydraulic jump process. The wall jet created after jet impingement along the solid wall is not responsible for triangular splashing. The process investigated depends on the degree of the diesel jet development and is characterized by impingement of the jet core on the wall only. A hypothesis of a non-homogeneous jet core has been formulated and verified to explain triangular splashing regions. For very small distances between nozzle and wall, the resulting splashing may be observed as a star-like three-jet structure. These jets, rebounding in three pre-dominating directions, will spread the preliminary circular splashing (just after wall contact of the primary jet), rendering the splashing geometry more and more triangular in shape until the transition time occurs.

The Kaye effect**This article is a written version of the oral contribution of the German team to the 21st IYPT competition, which was awarded first prize by an international jury. The article has been edited by European Journal of Physics.

The International Young Physicists' Tournament (IYPT) is a worldwide, annual competition for secondary school students. This is our solution to problem number 10, The Kaye effect, as presented in the final round of the 21st IYPT in Trogir, Croatia. The Kaye effect occurs when a thin stream of shampoo or a different adequate non-Newtonian liquid is poured onto a surface. Suddenly, a jet leaves the heap that is formed by the shampoo and begins to ‘dance’ around the primary jet like a lasso. The phenomenon ends when the ‘dancing’ jet hits the primary jet and subsequently collapses. We started our investigations based on available literature (Kaye 1963 Nature 197 1001, Versluis et al 2006 J. Stat. Mech., Collyer and Fischer 1976 Nature 261 682). We made experiments with a similar experimental set-up in which we could determine the velocities of both shampoo streams as well as the angle of the ‘dancing’ stream. From there on, we developed a theoretical model for the energy loss of the jet in the heap. We discovered that the air layer between the jet and the heap is a necessity for the Kaye effect to occur. At this point, our observations differ from the aforementioned literature. This also accounts for the shampoo beam acting as a light guide. Further experiments concerning the viscoelasticity of the shampoo revealed that the elastic property of the shampoo is necessary for the effect to occur.

Cavitation within a droplet

The rapid dynamics of vapor bubbles, the so-called cavitation bubbles, in confined geometries may lead to surprisingly rich dynamics. The example presented here in Fig. 1 a shows a cavitation bubble expanding and collapsing inside a liquid droplet, which creates two high-speed liquid jets shooting upwards. The primary jet is formed during the initial bubble expansion and collapse, and the secondary cylindrical jet forms when the bubble rebounds, i.e., after the collapse. The top picture captures both jets taken 400 s after the cavitation bubble has been created. The central jet is surrounded by the circular one. Both jet tips become unstable and pinch off droplets. The cavitation bubble inside the droplet creates a jet flow downwards and transforms into a torus, which disintegrates; its remains are still visible. The bottom row shows the stages leading to the double jet. Figure 1 b depicts the bubble 40 s after creation at the apex of the droplet. Bubble collapse occurs around 120 s Fig. 1 c , leading to flow separation, with an upward jet and a jet inside the droplet flowing through the bubble’s center, deforming the bubble into a torus. During re-expansion of the bubble a second upward pointing, now cylindrical, jet emanates; see Fig. 1 d . Technical details: a Nd:YAG laser =532 nm with 6 ns pulse duration is focused with a 40 microscope objective from below through a microscope slide into a sessile water droplet. The laser pulse energy 3 mJ explosively vaporizes the water and creates a bubble of approximately 1 mm maximum radius. The stroboscopic pictures are taken with a commercial digital camera Nikon D200 equipped with a macrolens. The droplet is illuminated from behind with a high power light emitting diode Seoul Semiconductor, P7 with a strobe duration 2 s . The light is mildly diffused to obtain a clear view into the droplet interior. For each picture, a new droplet is placed with a precision syringe onto a partially hydrophobic microscope slide.

Hydrodynamics of pulsed jetting in juvenile and adult brief squid Lolliguncula brevis: evidence of multiple jet `modes' and their implications for propulsive efficiency

The dynamics of pulsed jetting in squids throughout ontogeny is not well understood, especially with regard to the development of vortex rings, which are common features of mechanically generated jet pulses (also known as starting jets). Studies of mechanically generated starting jets have revealed a limiting principle for vortex ring formation characterized in terms of a ;formation number' (F), which delineates the transition between the formation of isolated vortex rings and vortex rings that have; pinched off' from the generating jet. Near F, there exists an optimum in pulse-averaged thrust with (potentially) low energetic cost, raising the question: do squids produce vortex rings and if so, do they fall near F, where propulsive benefits presumably occur? To better understand vortex ring dynamics and propulsive jet efficiency throughout ontogeny, brief squid Lolliguncula brevis ranging from 3.3 to 9.1 cm dorsal mantle length (DML) and swimming at speeds of 2.43-22.2 cms(-1) (0.54-3.50 DMLs(-1)) were studied using digital particle image velocimetry (DPIV). A range of jet structures were observed but most structures could be classified as variations of two principal jet modes: (1) jet mode I, where the ejected fluid rolled up into an isolated vortex ring; and (2) jet mode II, where the ejected fluid developed into a leading vortex ring that separated or ;pinched off' from a long trailing jet. The ratio of jet length [based on the vorticity extent (L(omega))] to jet diameter [based on peak vorticity locations (D(omega))] was <3.0 for jet mode I and >3.0 for jet mode II, placing the transition between modes in rough agreement with F determined in mechanical jet studies. Jet mode II produced greater time-averaged thrust and lift forces and was the jet mode most heavily used whereas jet mode I had higher propulsive efficiency, lower slip, shorter jet periods and a higher frequency of fin activity associated with it. No relationship between L(omega)/D(omega) and speed was detected and there was no apparent speed preference for the jet modes within the speed range considered in this study; however, propulsive efficiency did increase with speed partly because of a reduction in slip and jet angle with speed. Trends in higher slip, lower propulsive efficiency and higher relative lift production were observed for squid <5.0 cm DML compared with squid >/=5.0 cm DML. While these trends were observed when jet mode I and II were equally represented among the size classes, there was also greater relative dependence on jet mode I than jet mode II for squid <5.0 cm DML when all of the available jet sequences were examined. Collectively, these results indicate that approximately 5.0 cm DML is an important ontogenetic transition for the hydrodynamics of pulsed jetting in squids. The significance of our findings is that from early juvenile through to adult life stages, L. brevis is capable of producing a diversity of vortex ring-based jet structures, ranging from efficient short pulses to high-force longer duration pulses. Given that some of these structures had L(omega)/D(omega)s near F, and F represented the delineation between the two primary jet modes observed, fluid dynamics probably played an integral role in the evolution of squid locomotive systems. When this flexibility in jet dynamics is coupled with the highly versatile fins, which are capable of producing multiple hydrodynamic modes as well, it is clear that squid have a locomotive repertoire far more complex than originally thought.

Optimization of Second Throat Ejectors for High-Altitude Test Facility

DOI: 10.2514/1.39219 In the present work, a second throat ejector using nitrogen as the primary fluid is considered for the creation of a low vacuum in a high-altitude testing facility for large-area-ratio rocket motors. Detailed numerical investigations have been carried out to evaluate the performance of the ejector for various operational conditions and geometric parametersduring thenonpumpingandpumpingmodesof operation.Inthenonpumpingmode,the lowestvacuum chamberpressureisattainedwhentheprimaryjetjustexpandsuptothemixerthroatandtheresultingsingleshock cellsealsthethroatagainstanybackflow.Thestudyillustrateshoweachgeometricandoperationalparameterofthe ejector can be optimized to meet the test requirements in a high-altitude testing facility byensuring that the primary jet completely expands without a strong impact on the duct wall. When the rocket motor is fully started, due to the self-pumping action, the required vacuum is almost maintained by the exhaust flow itself and the external nitrogen ejector plays only a supplementary role. Numerical predictions for both nonpumping and pumping modes of operation have been validated with experimental data obtained from a scaled-down model of a high-altitude testing facility.

Study of Sound Generated by Large-Scale Structures in Low Speed Coaxial Jets

Sound generation by low speed circular and elliptic coaxial jets is investigated computationally. The secondary to primary jet velocity ratio is limited up to 0.4 and the elliptic jet profile has a 2:1 aspect ratio. Large Eddy Simulations are coupled with the Lighthill's acoustic analogy. Flow incompressibility is assumed limiting the study to the compact quadrupole structures generated during the jet mixing process. As expected, adding a secondary stream is found to extend the mixing region, while changing the jet geometry from circular to elliptic leads to vice versa. Good qualitative agreement in the flow behaviour is achieved with known results. Lighthill's longitudinal streamwise quadrupole is found to dominate the sound generation leading to a mildly high streamwise emission. Increasing the velocity ratio of the secondary stream to the primary stream increases the dominance of the streamwise sound emission. Good qualitative agreement with a previously suggested sound directivity pattern is achieved. Adding a secondary stream to the jet is found to have a mixed effect on the emitted acoustic power. A plausible explanation based on a large scale instability wave model is presented. Dominance of the longitudinal streamwise quadrupole Qxx was observed in all jets and the lateral quadrupoles Qxy and Qxz were of about the same magnitude and only secondary to Qxx. This resulted in high sound emission in the streamwise direction particularly as U1 /U0 was increased to 0.4. Good qualitative agreement was found with a previously suggested expression for the sound directivity pattern [18]. Adding a secondary stream did not cause a significant reduction in the overall acoustic power output as changing the jet geometry from circular to elliptic which was also limited to about 2 dB reduction. Further evidence for the existence of a large scale wave packet structure in the mixing region was produced. The wave packet source model was used to point to the opposing effects on the acoustic power output when adding a secondary stream, which could result in its mild and mixed effect.

Broadband shock-cell noise from dual stream jets

Broadband shock-cell noise from dual stream jets is investigated. The present work is confined to the cases of supersonic secondary jet and subsonic primary jet. Previous studies on single stream supersonic jets reveal that broadband shock-cell noise from these jets is generated by the interaction of the large turbulence structures and the shock cells in the jet plume as the former propagate downstream through the latter. A prominent characteristic of the radiated noise is that it is most intense in the upstream direction and drops off with an increase in inlet angle. An important result of the present investigation is the discovery that there are two sets of broadband shock-cell noise. One set is the classical shock-cell noise, similar to that of single stream jets. The second set radiates sound primarily in the downstream direction. Its intensity is low at 90°, but the intensity increases with inlet angle until a critical angle is reached beyond which it starts to decrease rapidly. It is believed that the first sound field is generated by the interaction of the large turbulence structures and the shock cells in the outer shear layer of the dual stream jet. The second sound field is generated in a similar manner involving the large turbulence structures and the shock cells in the inner shear layer of the jet. The inner shear layer separates the secondary and the primary jet. A simplified mathematical model capable of elucidating the generation, transmission and radiation of broadband shock-cell noise from dual stream jets is developed. The model provides formulas relating the direction of radiation and the frequencies of broadband shock-cell noise at the peaks of the noise spectra for both sets of sound fields. Good agreement is found between the predictions from these formulas and experimental measurements, thereby providing support for the validity of the model.

Experimental study of the effect of some geometric variables and number of nozzles on the performance of a subsonic air—air ejector

An ejector is a pumping device that uses a high-speed primary fluid jet to entrain a secondary stream. The pumping action is due to the exchange of momentum between the primary and secondary streams. The air ejectors are widely used in mixing systems and many engineering applications. The purpose of the present work is to study experimentally the effects of using different numbers of nozzles with various geometric variables on the performance of the air ejector. The test rig, which is designed particularly for this study, is constructed so that the overall performance of the air ejector can be measured under various operating conditions. To the best knowledge of the authors, there is no published work concerning the study of all the geometric variables with different numbers of nozzles, showing their effect on the ejector performance. The experimental investigations consist of the performance measurements for 52 ejectors. They are classified into three main groups The following conclusions were drawn

Numerical Investigation on the Effects of Inlet Air Temperature on Spray Combustion in a Wall Jet Can Combustor Using the k − ϵ Turbulence Model

A three-dimensional numerical study was performed to assess the effects of inlet temperature and equivalence ratio on the spray combustion and subsequent NOx emission in a wall jet can combustor (WJCC) installed with twin-fluid air-assisted fuel atomizers. The RNG k − ϵ turbulence model, eddy breakup (EBU) combustion model, and the Zeldovich model of NOx formation were utilized in the numerical study. The WJCC was implemented with a swirling air jet at the fuel nozzle exit and two other air jets, known as primary and dilute jets, at downstream locations. The inlet air temperature and overall equivalence ratio were varied from 373 to 1000 K and from 0.3 to 0.6, respectively. Our computational study showed that the inlet air of high temperature induced flow acceleration and sufficient jet penetration, which were desirable for achieving uniform temperature distribution at the combustor outlet but unfavorably yielded increased NOx emission. While the inlet air temperature had no prominent influence on the evaporation rate of the fuel drops in the upstream primary zone, its influence appeared to be prominent further downstream.

Mixing Enhancement in Subsonic Jet Flow Using the Air-Tab Technique

M IXING enhancement in jet flow has long been the topic of extensive research due to its important applications in aircraft propulsion and combustion. It also has military importance when mixing enhancement is used to suppress the infrared red signature for the fighter jet exhausts. Many passive or active approaches or techniques to enhance jet mixing have been developed and explored, and some of these techniques had also been reviewed and summarized by Gutmark and Grinstein [1]. In passive control strategies, the tabs or vortex generators in particular are simple but effective techniques in flow control. Bradbury and Khadem [2] were the pioneers to study the effect of solid tabs on jet flow. They found that the nozzle boundary-layer thickness, turbulence level, and convergent ratio did not have very strong influence on the jet development. On the other hand, to insert small rectangular tabs into the jet flow on the nozzle perimeter would induce some profound effects on the jet development. Since then, many researchers have been working on this technique. Notable examples include Samimy et al. [3], Reeder and Samimy [4], Zaman et al. [5], Yu and Koh [6], Paoli et al. [7] and many others. Zaman [8] conducted a systematic test on a series of nozzles with various shapes or orifices and found that the spreading of most asymmetric jets was not much different from that of a round jet, but the biggest increase in jet spreading was observed with the tabs. However, the penalty of introducing the tabs is also found to be significant. The thrust lost due to the tabs varies from 4.1 to 23.7%when theflowblockage (proportional to the facing area of tabs) increases from 1.1 to 14.1%. Tominimize the thrust lost caused by tabs but yet maintain its effectiveness in mixing enhancement is important to practical applications. Researchers have been searching for new techniques with better effects on mixing but minimum penalty on thrust. This Note presents results using the socalled air-tab technique. The air-tab technique is achieved by injecting a small amount of air (less than 1% of the volume flow rate of the primary jet) into the plume of the primary jet at choked speed and at certain attacking angles (45 and 90 deg with respect to the primary jet direction). The air-tab technique shows minimum or no impact on thrust, but it would be able to provide significant effect as the solid tab on the mixing enhancement. A similar idea had been used in air and fuel mixing: for example, the experiments conducted byMilanovic and Zaman [9,10]. The description of the experimental setup is presented subsequently. It will be followed by the results and discussion. The Note ends with brief concluding remarks.

Computation of Shock Cell Structure of Dual-Stream Jets for Noise Prediction

Broadband shock cell noise is an important component of aircraft interior noise during cruise. At cruise, the secondary jet of most modern day jet engines is supersonic. As a result, a shock cell structure develops in the jet plume. The interaction of large turbulence structures of the jet flow and the periodic components of the shock cells results in the emission of highly directional broadband shock cell noise. The primary objective of this investigation is to develop a computational method to calculate the shock cell structures ofdual-stream jets issued from separate flow nozzles. Computation of the Fourier modes of such shock cell structures is also considered. Based on the dominant wave numbers computed by the method developed, the frequencies at the peaks of broadband shock cell noise spectra at various radiation directions are calculated. Good agreements are found with experimental measurements over a wide range of primary and secondary jet Mach numbers. The good agreements provide a validation of the accuracy of the computation method.

Experimental study of physical mechanisms in the control of supersonic impinging jets using microjets

Supersonic impinging jet(s) inherently produce a highly unsteady flow field. The occurrence of such flows leads to many adverse effects for short take-off and vertical landing (STOVL) aircraft such as: a significant increase in the noise level, very high unsteady loads on nearby structures and an appreciable loss in lift during hover. In prior studies, we have demonstrated that arrays of microjets, appropriately placed near the nozzle exit, effectively disrupt the feedback loop inherent in impinging jet flows. In these studies, the effectiveness of the control was found to be strongly dependent on a number of geometric and flow parameters, such as the impingement plane distance, microjet orientation and jet operating conditions. In this paper, the effects of some of these parameters that appear to determine control efficiency are examined and some of the fundamental mechanisms behind this control approach are explored. Through comprehensive two- and three-component velocity (and vorticity) field measurements it has been clearly demonstrated that the activation of microjets leads to a local thickening of the jet shear layer, near the nozzle exit, making it more stable and less receptive to disturbances. Furthermore, microjets generate strong streamwise vorticity in the form of well-organized, counter-rotating vortex pairs. This increase in streamwise vorticity is concomitant with a reduction in the azimuthal vorticity of the primary jet. Based on these results and a simplified analysis of vorticity transport, it is suggested that the generation of these streamwise vortices is mainly a result of the redirection of the azimuthal vorticity by vorticity tilting and stretching mechanisms. The emergence of these longitudinal structures weakens the large-scale axisymmetric structures in the jet shear layer while introducing substantial three-dimensionality into the flow. Together, these factors lead to the attenuation of the feedback loop and a significant reduction of flow unsteadiness.

The near pressure field of co-axial subsonic jets

Results are presented from pressure measurements performed in the irrotational near field of unbounded co-axial jets. Measurements were made for a variety of velocity and temperature ratios, and configurations both with and without serrations on the secondary nozzle lip. The principal objective of the study is to better understand the near pressure field of the jet, what it can tell us regarding the underlying turbulence structure, and in particular how it can be related to the source mechanisms of the flow.A preliminary analysis of the axial, temporal and azimuthal structure of the pressure field shows it to be highly organized, with axial spatial modes (obtained by proper orthogonal decomposition) which resemble Fourier modes. The effects of serrations on the pressure fluctuations comprise a global reduction in level, a change in the axial energy distribution, and a modification of the evolution of the characteristic time scales.A further analysis in frequency–wavenumber space is then performed, and a filtering operation is used to separate the convective and propagative footprints of the pressure field. This operation reveals two distinct signatures in the propagating component of the field: a low-frequency component which radiates at small angles to the flow axis and is characterized by extensive axial coherence, and a less-coherent high-frequency component which primarily radiates in sideline directions. The serrations are found to reduce the energy of the axially coherent propagating component, but its structure remains fundamentally unchanged; the high-frequency component is found to be enhanced. A further effect of the serrations involves a relative increase of the mean-square pressure level of the acoustic component – integrated over the measurement domain – with respect to the hydrodynamic component. The effect of increasing the velocity and temperature of the primary jet involves a relative increase in the acoustic component of the near field, while the hydrodynamic component remains relatively unchanged: this shows that the additional acoustic energy is generated by the mixing region which is produced by the interaction of the inner and the outer shear layers, whereas the hydrodynamic component of the near field is primarily driven by the outer shear layer.

The low-mass YSO CB230-A: investigating the protostar and its jet with NIR spectroscopy and Spitzer observations

Context. To investigate the earliest phases of star formation and study how newly-born stars interact with the surrounding medium, we performed a line and continuum survey at NIR and mm-wavelengths of a sample of relatively isolated Bok globules. Aims. We present a follow-up observational program of a star-forming site in the globule CB230. From narrow-band continuum observations of this site, we had discovered a bright [FeII] jet, which originates in the low-mass YSO CB230-A. We aim to investigate the physical properties of the region from where the jet is launched. Methods. Our analysis was carried out using low-resolution NIR spectra acquired with the camera NICS at the TNG telescope, with JH and HK grisms and a 1 arcsec-wide slit. These observational data were complemented with infrared photometric data from the Spitzer space telescope archive. Results. The relevant physical properties of CB230-A were constrained by SED fitting of fluxes from the NIR to the mm. The YSO spectrum exhibits a significant number of atomic and molecular emission lines and absorption features. The characteristics of this spectrum suggest that we are observing a region in the close vicinity of CB230-A, i.e. its photosphere and/or an active accretion disk. The spectra of the knots in the jet contain a large number of emission lines, including a rich set of [FeII] lines. Emission due to H2 and [FeII] are not spatially correlated, confirming that [FeII] and H2 are excited by different mechanisms, in agreement with the models where [FeII] traces dissociative J-shocks and molecular hydrogen traces slower C-shocks. By using intensity ratios involving density-sensitive [FeII] lines, we estimated the electron densities along the jet to be 6 × 10 3 –1 × 10 4 cm −3 . This indicates either high density post-shock regions of ionised gas or regions with a high degree of ionisation. Conclusions. By combining the present data with previously obtained maps at NIR- and mm-wavelengths, the emerging scenario is that CB230-A is a Class 0/I YSO driving an atomic jet that is observed to be almost monopolar probably due to its inclination to the plane of the sky and the resulting higher extinction of its red side. This primary jet appears to be sufficiently energetic to open the cavity visible in the NIR images and drive the large-scale molecular outflow observed at mm-wavelengths. CB230-A was revealed to be a good location to test the innermost structure of accreting low-mass protostars.

Experimental study of overexpanded co-flowing jets

AbstractAn experimental investigation was carried out to find the effect of an annular co-flow jet on the primary supersonic jet from Mach 2 nozzle at different levels of overexpansion. In this study, a convergent-divergent circular nozzle of exit Mach number 2, surrounded by an annular convergent circular nozzle with an annular gap of 4·4mm was used. Nozzle pressure ratios (NPRs) 3, 4, 5, 6, 7 are investigated for overexpanded states of the primary jet and NPR 8 is investigated for almost correctly expanded state. The centreline pressure distributions were taken at all NPRs for both with and without co-flow case, to investigate the supersonic core extent and mixing activity in the jet field. In the radial direction pitot pressure at different axial locations at all NPRs for both the cases are measured to find the jet development and shadowgraph visualisation of jet structure was done to visualise the shock structure in near-field. It is found that the co-flow acts as mixing inhibitor at all levels of overexpansion for Mach 2 nozzle.

Active flow control of jet mixing using steady and pulsed fluid tabs

Flow control techniques for increasing the rate of jet mixing in axisymmetric nozzle flows have been investigated. A combination of water tunnel and high-speed airflow facilities is used to assess the near-field jet behaviour. Solid tabs, steady fluid tabs (i.e. discrete radially discharged control jets located close to the core jet exit), and pulsed fluid tabs are compared. The effect of fluid tab velocity amplitude, pulse rate, and pulse phase are studied using open-loop control. The measurements indicate that fluid tabs generate a similar streamwise vortex formation process (and hence display increased mixing) as previously observed in solid-tabbed nozzle flows. In incompressible testing the mixing effectiveness with a pair of pulsed fluid tabs 180° out-of-phase was as good as a twin solid tab nozzle for a control jet flowrate of only 0.5 per cent of the primary (core) jet flow. In preliminary high-speed testing similar benefits of fluid tabs over solid tabs were observed. Further study of pulsed fluid tabs is recommended; they have the attractive performance benefit that they can be easily switched off when not needed and offer increased flexibility as the basis of an optimized active control jet mixing device.

Axisymmetric Coanda-assisted vectoring

An experimental demonstration of a jet vectoring technique used in our novel spray method called Coanda-assisted Spray Manipulation (CSM) is presented. CSM makes use of the Coanda effect on axisymmetric geometries through the interaction of two jets: a primary jet and a control jet. The primary jet has larger volume flow rate but generally a smaller momentum flux than the control jet. The primary jet flows through the center of a rounded collar. The control jet is parallel to the primary and is adjacent to the convex collar. The Reynolds number range for the primary jet at the exit plane was between 20,000 and 80,000. The flow was in the incompressible Mach number range (Mach < 0.3). The control jet attaches to the convex wall and vectors according to known Coanda effect principles, entraining and vectoring the primary jet, resulting in controllable r − θ directional spraying. Several annular control slots and collar radii were tested over a range of momentum flux ratios to determine the effects of these variables on the vectored jet angle and spreading. Two and Three-component Particle Image Velocimetry systems were used to determine the vectoring angle and the profile of the combined jet in each experiment. The experiments show that the control slot and expansion radius, along with the momentum ratios of the two jets predominantly affected the vectoring angle and profile of the combined jets.

Linear stability of a compressible coaxial jet with continuous velocity and temperature profiles

This paper describes an investigation of the stability of a jet with velocity and temperature profiles characteristic of the exit region for a turbofan engine. Because the bypass stream mixes with both the exhaust and the ambient air, these profiles contain thin layers in which the velocity and temperature may vary rapidly. As a consequence, multiple instability modes are possible. In accordance with Rayleigh’s theorem for axisymmetric incompressible shear flows, it turns out that there are three possible modes, only two of which are unstable. We consider the effect on spatial growth rates of varying the diameter and velocity ratios, compressibility, and azimuthal wavenumber. Radiating modes, that are possible when the primary jet is heated, are also studied.

Similarity between the Primary and Secondary Air-Assisted Liquid Jet Breakup Mechanisms

We report an ultrafast synchrotron x-ray phase-contrast imaging study of the primary breakup mechanism of a coaxial air-assisted water jet. There exist great similarities between the primary (jet) and the secondary (drop) breakup, and in the primary breakup on different length scales. A transition from a ligament- to a membrane-mediated breakup is identified around an effective Weber number We' approximately 13. This observation reveals the critical role an effective Weber number plays in determining the atomization process and strongly supports the cascade breakup model.

Influence of a secondary, parallel, low Reynolds number, round jet on a turbulent axisymmetric jet

The influence of a secondary, parallel, low Reynolds number, round jet on the development of a turbulent axisymmetric jet with Re 5500 is studied experimentally by means of 2 D laser Doppler anemometry. Two weakly turbulent jets of Reynolds numbers, 1200 and 1800, are considered. In both test cases the secondary jet is fully absorbed by the primary one. The mean and turbulent flow structure of the standalone primary jet is compared to previous studies referring to the jet’s development before and inside the self-similarity region. The flow field of interacting jets is then analyzed and discussed. Characteristic profiles are presented at locations within the merging and the mixing zones. In the early stages of development, the patterns of both jets can be identified. Within the merging region, besides the absorption of the secondary jet, the measurements indicate a spatial suppression of primary jet’s characteristics. Further downstream, the profiles resemble to those of a standalone jet. Higher values of the mean and turbulent terms are observed while the profiles extend over a wider region, in accordance with the secondary jet’s Reynolds number. Results of the present study demonstrate that skewness and flatness factors can be used as indicators of small scale mixing.