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This study is aimed at understanding the effects of changing the upstream nozzle geometry on the development of rectangular free jets. An existing converging rectangular nozzle with an exit aspect ratio of 4 and a circular inlet (AR4 nozzle) has been used as the basic configuration for this work. The study is primarily based on the results of numerical simulations wherein the internal geometry variation is accomplished by changing the inlet aspect ratio (AR,) and the length of the converging section, expressed as a ratio with respect to the length of the nozzle (called 'converging section ratio*, CSR); all the other parameters are kept constant. The results from LDA experiments done on the AR4 nozzle are presented and used as validation data for the CPD simulations. Analyses of the numerical results help in understanding the variation of the jet spreading for different combinations of AR, and CSR. Two parameters are identified for describing the jet development: the cross-over point (XC), defined as the location downstream of the exit where the jet half-velocity-widths (B) along the major and minor axes are equal, and the difference in the half-velocity-widths at 30 nozzle equivalent diameters (Dm) from the exit (AB30), to ascertain the occurrence of axis-switching. For a given AR, XC varies linearly with CSR; the variation of XC is non-linear with AR, for a constant CSR. The A1330 variation is non-linear with both AR, and CSR; the other variable being kept constant. The data obtained from the simulations are further used to propose two parametric models which can be used to predict the occurrence of axis-switching, within the scope of this work. The parametric models are validated and future work is proposed.

Dans ce travail de recherche, on présente des développements spécifiques de diagnostiques optiques et leur application à l’étude aéroacoustique des jets rapides à haut nombre de Reynolds. Les résultats expérimentaux présentés ici résultent de visualisation par strioscopie et, de manière prépondérante dans ce manuscrit, de mesure de masse volumique par diffusion Rayleigh. Ces méthodes de caractérisation d’écoulement, appliquées aux jets subsoniques ou supersoniques, ont été associées à des mesures de bruit en champ lointain. La mesure par diffusion Rayleigh, qui repose sur la lumière diffusée par les molécules constituantes du gaz, et n’est donc pas intrusive. Des difficultés apparaissent néanmoins pour exploiter les résultats lorsque le milieu diffusant contient des poussières. Bien que l’air des écoulements obtenu en soufflerie soit filtré, la quantité résiduelle de poussières a rendu nécessaire le développement d’une méthode de nettoyage du signal en post-traitement. Le niveau des signaux obtenus par diffusion Rayleigh est très faible, et dominé par du bruit appelé shot noise. Un gain significatif sur le niveau de ce bruit a été obtenu en optimisant la chaîne d’acquisition après analyse des systèmes existants. De plus une méthode de traitement du signal dérivée d’une méthode existante a permis de calculer des spectres de masse volumique malgré le shot noise avec un seul capteur, là où il en fallait deux auparavant. Les profils de p obtenus par cette technique ont montré qu’il existe une loi de similarité permettant de superposer les profils mesurés à différentes positions axiales. Cette loi est identique pour les jets issus de trois tuyères aux géométries différentes, et à des nombres de Mach de 0.7 et 0.9. Une loi de similarité est également observée pour p’rms si les profils sont mesurés suffisamment loin de la tuyère. L’étude des spectres dans la couche de mélange a mis en évidence un maximum faiblement marqué autour d’une fréquence centrale comprise entre St = 0:2 et St = 2 dans les régions mesurées, plus marqué que dans les spectres de vitesse, et dont le comportement diffère selon l’état initialement laminaire ou turbulent du jet. L’évolution de la forme des spectres en fonction de la différence de masse volumique entre le jet et le milieu ambiant, ainsi qu’en fonction du nombre de Mach, a également été étudiée. Une loi permettant de superposer les spectres a été définie empiriquement sur la plage de variation des différents paramètres. Des mesures simultanées entre l’acoustique en champ lointain et la masse volumique dans l’écoulement ont été réalisées pour un jet à Mj = 0:9 et un jet à Mj = 1:32. Ces résultats ont permis l’estimation de cohérences spectrales et de moyennes conditionnelles. Les résultats obtenus mettent en évidence la présence de structures liées au rayonnement acoustique dans une région située proche de l’axe du jet en aval du cône potentiel. Pour finir, une étude a été réalisée sur le screech dans les jets supersoniques sous-détendus. Elle a permis d’identifier la position de la source de la rétroaction acoustique pour les modes A1, A2, et B, ainsi que la structure du cycle de la boucle qui détermine les changements de fréquences observés aux sauts de modes. / In this research study, developments of optical diagnosis techniques are presented, and are put into practice in an aeroacoustics study of high speed and high Reynolds jets. The results described here are obtained from Schlieren visualization, and in the majority from density measurements through Rayleigh scattering. These measurement methods are applied for subsonic and supersonic jets, and associated with far field acoustics measurements. Rayleigh scattering measurements rely on laser light scattered by the molecules constituting the flow, thus they are non-intrusive. However, some difficulties in interpreting the results arise when the flow contains dust particles. This problem is partly solved by using air filters in the wind tunnel, but some residual dust particles remain, thus a software based signal cleaning method has been developed and is applied to the signals during post-processing. Besides, the signals obtained from Rayleigh scattering are very weak, and dominated by a shot noise. The noise level has been significantly reduced following the optimization of the chain of acquisition in comparison with actual apparatus. Moreover, a method to compute spectra despite the shot noise level has been adapted from a classic method to be usable with a single sensor instead of two. The analysis of radial profiles of density measured with the Rayleigh scattering apparatus shows that a similarity law allows to superimpose radial profiles measured in jets exhausted from three different nozzles at Mach 0.9 and Mach 0.7. Another similarity law is observed for radial profiles of 0 rms when they are measured far enough form the nozzle. The analysis of density spectra in the mixing layer points out the presence of a local maximum of central frequency found between St = 0:2 and St = 2 depending on the probed location, which is not as clearly observed in velocity spectra. The spatial evolution of this maximum is found different whether the flow is initially laminar or turbulent at the nozzle exit. The dependence of spectra to the Mach number, and to the difference of density between the jet and the surrounding is also studied. A scaling law is empirically determined for the scope of the study. Simultaneous measurements of density and farfield acoustic acquired in a Mach 0.9, and a supersonic ideally expanded Mach 1.32 jets are processed to obtain coherence, and conditional averaging. The results emphasis the role of large structures downstream the end of the potential core in acoustic radiations. Finally, the screech in underexpended jets is studied. This work results in the identification of the acoustic feedback source location for A1, A2 and B modes, and in the characterization of the changes in the loop structure associated with mode switching.

Thesis (M.Sc.Eng.) PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / Synthetic jet is a quasi-steady jet of fluid generated by oscillating pressure drop across an orifice, produced by a piston-like actuator. A unique advantage of the synthetic jet is that it is able to transfer linear momentum without requiring an external fluid source, and has therefore attracted much research within the past decade. Principal applications include aerodynamic flow boundary-layer separation control, heat transfer enhancement, mixing enhancement, and flow-generated sound minimization. In this thesis, the method of deriving the volume flux equation for a duct is first reviewed, combined with this method, a simplified synthetic jet model is presented, based on the principles of aerodynamic sound, the pressure fluctuation in the acoustic far field is predicted. This model is then been used to predict the minimum synthetic jet cavity resonance frequency, acoustic power, acoustic efficiency, root-mean-square jet speed, acoustic spectrum and their dependence on the following independent parameters: the duct length and radius, the aperture radius, the piston vibration frequency, and the maximum piston velocity. / 2031-01-01

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No description available.

It is well known that high-speed liquid jetting is one of the most powerful techniques available to cut or penetrate material. Recently, it has been conjectured that high-speed liquid jets may be beneficial in improving combustion in such applications as SCRAM jets and direct injection diesel engines. Although there are practical limitations on maximum jet velocity, a fundamental study of the characteristics of high-speed liquid fuel jets and their auto-ignition feasibility is necessary. Important benefits could be increased combustion efficiency and enhanced emission control from improved atomisation. The generation of high-speed liquid jets (water and diesel fuel) in the supersonic to hypersonic ranges by use of a vertical single stage powder gun is described. The effect of the projectile velocity and projectile mass on the jet velocity is found experimentally. Jet exit velocities from a range of different nozzle inner profiles and nozzle hardness are thoroughly examined. The characteristics and behaviour of the high-speed liquid jet and its leading bow shock wave have been studied with the aid of a shadowgraph technique. This provides a clearer picture of each stage of the generation of hypersonic liquid jets. It makes possible the study of hypersonic diesel fuel jet characteristics and their potential for auto-ignition. The fundamental processes by which a supersonic liquid jet is generated by projectile impact have been investigated. The momentum transfer from the projectile to the liquid and the shock wave reflection within the nozzle cavity are the key items of interest. A new one-dimensional analysis has been used in order to simplify this complex and difficult problem. The impact pressure obtained from the projectile was firstly derived. Then, an investigation of the intermittent pressure increase in a closed end cavity and a simple stepped, cross-sectional nozzle were carried out. The nozzle pressure and final jet velocity were estimated and compared to a previous method and to experimental results. Some interesting characteristics found in the experiments relate well to those anticipated by the analysis. The characteristics of a hypersonic diesel fuel jet and its leading edge shock wave were assessed for their potential for auto-ignition using fuel with cetane numbers from 50-100. The investigations were performed at normal ambient air and at elevated air (110 ???C) temperature. So far, there is no sign of auto-ignition that may occur because of the temperature rise of the induced shock.

<p>Introduction</p><p><p>Very light jet is a hot subject growing stronger and stronger. The new type of air craft is an air plane that weighs less than 10000 pounds and uses a jet engine.</p><p>Problem</p><p>The student was proposed to designing a conceptual very light jet that could be used for inspiration and accepted the challenge.</p><p>Method</p><p>In this thesis the reader can follow the project progress in detail, the proposed methods and the results. The student divided the project into four activities analysis, creation, development and documentation.</p><p>Result</p><p>The project ended with a concept very light jet with simple specifications. Illustrations for inspirational usage and a simulation testing for verification of the proposed concept specifications.</p></p>

Droplet formation in inkjet cartridges has been studied for several decades to improve print qualities. Currents and rigid nozzle plates are used inside the cartridge to fix the size of droplets exiting the chamber. This research is focused on studying the effect of nozzle flexibility on droplet formation. A flexible nozzle can be modeled as an flexible annular plate with a clamped outside edge. In this thesis, plate vibration equations are studied for an annular shape. An analytical solution is formed for the small deflection plate equation. To solve the large deflection plate vibration equation, Galerkin's method is used. Droplet formation is predicted by a one dimensional fluid dynamic model. An integrated plate vibration and droplet formation model is created with these basic building blocks. Results from simulation indicate flexible plates yield shorter droplet breakoff time and longer breakoff distance while generating a slightly droplet compared to a rigid nozzle. / Graduation date: 2004