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1
Oren, Liran. "Fluid dynamics of pulsating jets and voice". University of Cincinnati / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1353155395.
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2
Lai, Chung-kei Chris, e 黎頌基. "Mixing of inclined dense jets". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2009. http://hub.hku.hk/bib/B4423661X.
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3
Smith, Barton Lee. "Synthetic jets and their interaction with adjacent jets". Diss., Georgia Institute of Technology, 1999. http://hdl.handle.net/1853/18889.
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4
Or, Chun-ming, e 柯雋銘. "Flow development in the initial region of a submerged round jet in a moving environment". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2009. http://hub.hku.hk/bib/B42664512.
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5
Shen, Jihua. "Formation and characteristics of sprays from annular viscous liquid jet breakup". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp02/NQ32723.pdf.
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6
Davis, Staci Ann. "The manipulation of large- and small-scale flow structures in single and coaxial jets using synthetic jet actuators". Diss., Georgia Institute of Technology, 2000. http://hdl.handle.net/1853/17313.
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7
Cutler, Philip Robert Edward. "On the structure and mixing of a jet in crossflow : Ph.D. thesis". Title page, abstract and table of contents only, 2002. http://web4.library.adelaide.edu.au/theses/09PH/09phc9895.pdf.
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8
Hunter, Hanif. "Formation and break up of microscale liquid jets". Thesis, Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/28194.
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9
Soo, Jin Hou. "Direct and large-eddy simulations of three-dimensional jets using the lattice Boltzmann method". Thesis, Georgia Institute of Technology, 2002. http://hdl.handle.net/1853/12013.
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10
Li, Larry. "Forcing of globally unstable jets and flames". Thesis, University of Cambridge, 2012. https://www.repository.cam.ac.uk/handle/1810/242373.
Texto completo da fonteResumo:
In the analysis of thermoacoustic systems, a flame is usually characterised by the way its heat release responds to acoustic forcing. This response depends on the hydrodynamic stability of the flame. Some flames, such as a premixed bunsen flame, are hydrodynamically globally stable. They respond only at the forcing frequency. Other flames, such as a jet diffusion flame, are hydrodynamically globally unstable. They oscillate at their own natural frequencies and are often assumed to be insensitive to low-amplitude forcing at other frequencies. If a hydrodynamically globally unstable flame really is insensitive to forcing at other frequencies, then it should be possible to weaken thermoacoustic oscillations by detuning the frequency of the natural hydrodynamic mode from that of the natural acoustic modes. This would be very beneficial for industrial combustors. In this thesis, that assumption of insensitivity to forcing is tested experimentally. This is done by acoustically forcing two different self-excited flows: a non-reacting jet and a reacting jet. Both jets have regions of absolute instability at their base and this causes them to exhibit varicose oscillations at discrete natural frequencies. The forcing is applied around these frequencies, at varying amplitudes, and the response examined over a range of frequencies (not just at the forcing frequency). The overall system is then modelled as a forced van der Pol oscillator. The results show that, contrary to some expectations, a hydrodynamically self-excited jet oscillating at one frequency is sensitive to forcing at other frequencies. When forced at low amplitudes, the jet responds at both frequencies as well as at several nearby frequencies, and there is beating, indicating quasi-periodicity. When forced at high amplitudes, however, it locks into the forcing. The critical forcing amplitude required for lock-in increases with the deviation of the forcing frequency from the natural frequency. This increase is linear, indicating a Hopf bifurcation to a global mode. The lock-in curve has a characteristic ∨ shape, but with two subtle asymmetries about the natural frequency. The first asymmetry concerns the forcing amplitude required for lock-in. In the non-reacting jet, higher amplitudes are required when the forcing frequency is above the natural frequency. In the reacting jet, lower amplitudes are required when the forcing frequency is above the natural frequency. The second asymmetry concerns the broadband response at lock-in. In the non-reacting jet, this response is always weaker than the unforced response, regardless of whether the forcing frequency is above or below the natural frequency. In the reacting jet, that response is weaker than the unforced response when the forcing frequency is above the natural frequency, but is stronger than it when the forcing frequency is below the natural frequency. In the reacting jet, weakening the global instability – by adding coflow or by diluting the fuel mixture – causes the flame to lock in at lower forcing amplitudes. This finding, however, cannot be detected in the flame describing function. That is because the flame describing function captures the response at only the forcing frequency and ignores all other frequencies, most notably those arising from the natural mode and from its interactions with the forcing. Nevertheless, the flame describing function does show a rise in gain below the natural frequency and a drop above it, consistent with the broadband response. Many of these features can be predicted by the forced van der Pol oscillator. They include (i) the coexistence of the natural and forcing frequencies before lock-in; (ii) the presence of multiple spectral peaks around these competing frequencies, indicating quasi-periodicity; (iii)the occurrence of lock-in above a critical forcing amplitude; (iv) the ∨-shaped lock-in curve; and (v) the reduced broadband response at lock-in. There are, however, some features that cannot be predicted. They include (i) the asymmetry of the forcing amplitude required for lock-in, found in both jets; (ii) the asymmetry of the response at lock-in, found in the reacting jet; and (iii) the interactions between the fundamental and harmonics of both the natural and forcing frequencies, found in both jets.
11
劉國強 e Kwok-keung Lau. "Interactions of coherent structures in annular jets". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1991. http://hub.hku.hk/bib/B31232632.
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12
Ieong, Chao. "Exploring the complexity in an oil jet falling into an oil bath /". View abstract or full-text, 2007. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202007%20IEONG.
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13
Tamburello, David A. "Parametric analysis of the synthetic air jet using numerical simulations". Thesis, Georgia Institute of Technology, 2003. http://hdl.handle.net/1853/17075.
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14
Ali, Md Shahjahan. "Mixing of a non-buoyant multiple jet group in crossflow". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2003. http://hub.hku.hk/bib/B29485587.
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15
林傑明 e Kit-ming Lam. "Time and frequency domain analyses of large-scale flow structures of abasic annular jet". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1985. http://hub.hku.hk/bib/B31230507.
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16
張華倫 e Valiant Cheung. "Mixing of a round buoyant jet in a current". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1991. http://hub.hku.hk/bib/B3123253X.
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17
李偉德 e Wai-tak Lee. "Mixing of a vertical round buoyant jet in lateral confinement". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1993. http://hub.hku.hk/bib/B31233697.
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18
周健強 e Kin-keung Chow. "Acceleration of coherent structures in free shear layer". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1999. http://hub.hku.hk/bib/B31240069.
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19
Lai, Chun-hin Adrian, e 黎駿軒. "Mixing of a rosette buoyant jet group". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2009. http://hub.hku.hk/bib/B44140514.
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20
Rees, Simon John. "Hydrodynamic instability of confined jets & wakes & implications for gas turbine fuel injectors". Thesis, University of Cambridge, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609152.
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21
Sahajwalla, Veena. "The spout of air jets upwardly injected into a water bath". Thesis, University of British Columbia, 1988. http://hdl.handle.net/2429/28515.
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The spout formed at the free surface of a gas-stirred liquid has received little attention even though it has both theoretical and practical significance. In steelmaking ladles, for example, the spout is the site of strong metal-slag-air mixing which affects: the kinetics of reactions at the slag-metal interface, the absorption of oxygen by the bath and the temperature drop of the bath. Notwithstanding its importance, the spout is usually neglected in flow models of gas-stirred baths because it has not been characterized quantitatively; assumption of a flat top surface, however, reduces the accuracy of the velocity and kinetic energy predictions, particularly close to the spout region.
Thus in this study, the spout of upwardly injected gas jets in water was characterized
experimentally in terms of gas fraction, bubble frequency and axial velocity distributions. The measurements were made with a two-element electroresistivity probe coupled to a microcomputer. Special hardware and software were developed to analyze
the signals generated by contact of the bubbles with the sensor, in real time, for the turbulent flow conditions prevailing in the jet plume and spout. Correlations of the gas fraction with axial and radial position for different gas flow rates have been established from the measurements. The dimensions of the spout were obtained from time-exposure photographs; when compared with the gas fraction measurements, the spout boundary always corresponded to values ranging from 0.82 to 0.86. The radial profiles of bubble frequency at different levels in the spout have a bell shape; the bubble frequency decreases with increasing height. The velocity of the bubbles in the spout drops linearly with increasing axial position. Measurements of bath velocity near the walls of the vessel were also conducted with a laser doppler velocimeter for comparison to model predictions.
The gas fraction data obtained for the spout then were incorporated into a mathematical
model of turbulent recirculatory flow with which predictions of velocity, kinetic energy and effective viscosity in the bath were made. Predictions of the model were compared with the experimental measurements as well as with predictions assuming a flat bath surface (no spout); and the importance of incorporating the spout thus was demonstrated. The variation of the total kinetic energy in the spout with gas flow rate was determined. The energy increased with flow rate, as expected, but at a critical value, the rate of increase abruptly rose. Based on photographs taken of the gas/liquid dispersion, the increased spout kinetic energy appears to be related to the location of bubble break-up and possibly to gas channeling. At lower flow rates below the critical
value, the bubble break-up occurs relatively close to the nozzle, whereas at higher flow rates bubble disintegration is nearer to the surface. At the lower flow rates the gas/liquid interaction was maximum which promoted the gas/liquid momentum transfer.
Moreover, at the higher flow rates the gas dispersion was observed intermittently to be a continuous chain of large envelopes which could permit a fraction of the gas to channel through the bath for a considerable distance. The channeling phenomenon could lead to an inefficient gas/liquid energy transfer resulting in a reduced efficiency of bath mixing and enhanced energy release at the surface. These results can explain the observations of previous investigators who found that beyond a critical gas injection rate, the rate of decrease of mixing time with flow rate decreased. The metallurgical consequences of the spout and its influence on the flow field, especially in the near-surface region, have been highlighted, thus unveiling the practical bearing of the spout on the gas injection process.Applied Science, Faculty of
Materials Engineering, Department of
Graduate
22
Coe, David James. "Fabrication technology approaches to micromachined synthetic jets". Diss., Georgia Institute of Technology, 2002. http://hdl.handle.net/1853/15485.
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23
Gilbertson, Mark. "Mixing in multiphase jet flow : experimental comparison with a computational model". Thesis, University of Oxford, 1993. http://ora.ox.ac.uk/objects/uuid:98fae523-d738-4392-8b63-ab9cfbeaf37b.
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A series of experiments has been conducted for comparison with the results of a computer code called CHYMES. It is intended to calculate the coarse mixing of molten metal with water by solving the equations of the Separated Flow Model. These are derived by volume averaging and the terms that relate them to the particular case of participate flow are discussed. An experimental apparatus that is compatible with CHYMES and coarse mixing has been constructed which projects a jet of ball bearings into a thin tank of water. Experiments over a wide range of conditions were conducted at room temperature. Owing to practical difficulties only one, poorly controlled experiment with hot ball bearings was performed. Under nearly all sets of conditions an arrow-shaped plume was obtained. The speed of penetration of the plume varied little with changes in experimental conditions. The width of the plume was most strongly influenced by the widths of the tank and the jet. The individual paths of some particles were followed; it appeared that their motion was mostly dependent on their position in the plume. A model of the plume is proposed, based upon its front being impermeable to water in the vertical direction. Much of the detail of the experimental plumes was not present in the computational results and they responded differently to changes in conditions. It is proposed that this is a result of the different forms of the two sets of plumes. To rectify this an experimental plume was volume averaged. A method to determine a suitable averaging volume size is described. The process results in a plume similar to the computational ones. The length scales required for volume averaging to be successful are discussed and the possibility that this method is inappropriate for describing coarse mixing is admitted.
24
Loretz, Yves Daniel. "Flow control on a NACA 4418 airfoil using streamwise synthetic jet actuators". Thesis, Georgia Institute of Technology, 2001. http://hdl.handle.net/1853/16377.
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Honohan, Andrew M. "The interaction of synthetic jets with cross flow and the modification of aerodynamic surfaces". Diss., Georgia Institute of Technology, 2003. http://hdl.handle.net/1853/20836.
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26
Lee, Wing-yan, e 李永仁. "Global behavior of a round buoyant jet in a counterflow". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2006. http://hub.hku.hk/bib/B35312531.
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黃傳輝 e Chuen-fai Wong. "Advected line thermals and puffs". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1991. http://hub.hku.hk/bib/B31210582.
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28
陳孝章 e Hau-cheung Chan. "Investigation of a round jet into a counterflow". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1998. http://hub.hku.hk/bib/B3123818X.
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29
Sanderson, V. E. "Turbulence modelling of turbulent buoyant jets and compartment fires". Thesis, Cranfield University, 2001. http://hdl.handle.net/1826/137.
Texto completo da fonteResumo:
Turbulent buoyant jets are a major feature in fire hazards. The solution of the
Reynolds Averaged Navier-Stokes (RANS) equations through computational fluid
dynamic (CFD) techniques allow such flows to be simulated. The use of Reynolds
averaging requires an empirical model to close the set of equations, this is known as
the turbulence model. This thesis undertakes to investigate linear and nonlinear
approaches to turbulence modelling and to apply the knowledge gained to the
simulation of compartment fires. The principle contribution of this work is the reanalysis
of the standard k- ε turbulence model and the implementation and
application of more sophisticated models as applied to thermal plumes.
Validation in this work, of the standard k- ε model against the most recent
experimental data, counters the established view that the model is inadequate for the
simulation of buoyant flows. Examination of previous experimental data suggests
that the measurements were not taken in the self-similar region resulting in
misleading comparisons with published numerical solutions. This is a significant
conclusion that impacts of the general approach taken to modelling turbulence in
this field.
A number of methods for modelling the Reynolds stresses and the turbulent scalar
fluxes have been considered and, in some cases for the first time, are applied to nonisothermal
flows. The relative influence of each model has been assessed enabling
its performance to be gauged. The results from this have made a valuable
contribution to the knowledge in the field and have enabled the acquired experience
to be applied to the simulation of compartment fires.
The overall conclusion drawn from this thesis is that for the simulation of
compartment fires, the most appropriate approach with current computational
resources, is still the buoyancy corrected standard k- ε model. However, the
turbulence scalar flux should be modelled by the generalised gradient diffusion
hypothesis (GGDH) rather than the eddy-diffusivity assumption.
30
Coelho, Sergio Luis Villares. "The dynamics of the near field of entraining jets in cross-flows". Thesis, University of Cambridge, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.292919.
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31
Daniel, Kyle Andreas. "Space-time Description of Supersonic Jets with Thermal Non-uniformity". Diss., Virginia Tech, 2019. http://hdl.handle.net/10919/95942.
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The supersonic jet plumes that exhaust from the engines of tactical aircraft produce intense noise signatures that expose the Navy personnel working on the deck of aircraft carriers to dangerously high levels of noise that often results in hearing damage. Reducing the noise radiated by these supersonic plumes is of interest to the Department of Defense and is the primary motivation of this research. Fundamentally, jet noise reduction is achieved by manipulating the nozzle boundary condition to produce changes in the turbulence development and decrease the acoustic efficiency of coherent structures. The research presented here focuses on a novel jet noise reduction technique involving a centered thermal non-uniformity that alters the base flow by introducing a temperature-driven centerline velocity deficit into a perfectly expanded Mach 1.5 jet. The results indicate $2 pm 0.5$ dB reductions in peak narrowband spectral sound pressure levels upstream of peak directivity directions for the non-uniform jet compared to a thermally uniform baseline, even for static thrust matched conditions. This reduction is hypothesized to be related to perturbations induced by the thermal non-uniformity that convect inside the irrotational core and reduce the correlation length scales of turbulence at locations far downstream. This hypothesis was evaluated by studying the coherent turbulence via its convective hydrodynamic footprint in the near-field. An indirect investigation of the near-field using a far-field-informed model of the wavenumber-frequency spectra indicate a reduction in the energy contained in the tail of the wavenumber spectra amplitude, suggesting a reduction in the size of large scale structures. A direct evaluation of the spatio-temporal behavior of the near-field was performed using temporally resolved schlieren images. Space-time correlations of the frequency-filtered near-field identified high frequency acoustic waves radiated by compactly coherent turbulent structures and low frequency Mach waves produced by large scale instabilities. In the thermally non-uniform case these features and their sources were found to be decorrelated at downstream regions. These results provide strong evidence that a centered thermal non-uniformity reduces the radiated noise compared to a uniform baseline by shortening the correlation length scales of coherent structures in regions far from the nozzle exhaust.Doctor of Philosophy
A more complete understanding of the intense noise sources present in supersonic jet plumes is of value to both government and industry, and is a necessary step towards optimizing noise reduction techniques. Tactical aircraft that operate on the deck of aircraft carriers expose Navy personnel to dangerously high levels of noise that often results in permanent hearing damage. Supersonic jet noise reduction is also of relevance to the recent efforts to revitalize supersonic air transport over land. For supersonic air transport to become a reality, the noise produced by these future aircraft during takeoff and landing must meet the increasingly stringent community noise requirements. Fundamental jet noise research is needed to guide the design of future engine architectures for these aircraft to ensure their commercial success. The research presented herein examines a novel noise reduction technique that involves a centered thermal non-uniformity consisting of a heated jet plume with a spot of locally cooler, slower moving air concentrated along the centerline of a Mach 1.5 jet. This temperature driven velocity deficit is shown to reduce the radiated noise by up to 2.5 dB at peak frequencies and at angles just outside of the peak directivity direction. The cause of the noise reduction is hypothesized be related to a reduction in the size of the coherent structures that radiate a majority of the noise produced by turbulent jets. This hypothesis is evaluated by examining the 'footprint' of the coherent structures in the ambient field directly outside of the jet shear layer in an area called the near-field. An indirect investigation of the near-field using a far-field informed analytic model suggests a reduction in the size of large scale structures. A direct evaluation of the space time structure of the near-field was performed using temporally resolved schlieren images. Statistical processing of the density gradient provided by the schlieren images revealed acoustically intense structures known as Mach waves and high frequency acoustic waves. These features and their sources, large scale instabilities and compactly coherent turbulence, were found to be decorrelated by the introduction of the thermal non-uniformity. These results provide strong evidence that the centered thermal non-uniformity produces a noise benefit by reducing the size of the turbulent structures.
32
Elwell, Lance Christopher. "Dynamics of stationary and obliquely oscillating free plane jets". Thesis, Georgia Institute of Technology, 2000. http://hdl.handle.net/1853/17371.
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Barker, James. "The assimilation of heated axisymmetric jets into external streams". Thesis, Keele University, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.341302.
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Raad, Layla M. "Simultaneous measurements of velocity and concentration for a turbulent jet". Thesis, Georgia Institute of Technology, 1999. http://hdl.handle.net/1853/20713.
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Krikellas, Dimitrios. "Improvement of the performance of a turbo-ramjet engine for UAV and missile applications". Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2003. http://library.nps.navy.mil/uhtbin/hyperion-image/03Dec%5FKrikellas.pdf.
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Thesis (M.S. in Applied Physics and M.S. in Aeronautical Engineering)--Naval Postgraduate School, December 2003.Thesis advisor(s): Garth V. Hobson, Kai E. Woehler. Includes bibliographical references (p. 133). Also available online.
36
Huang, Shiling. "Linear stability analysis of non-reacting and reacting elliptical jets". Diss., This resource online, 1994. http://scholar.lib.vt.edu/theses/available/etd-06062008-164706/.
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Bond, Jean-François. "The influence of turbulence on dust and gas explosions in closed vessels /". Thesis, McGill University, 1985. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=65522.
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Shipton, Jemma. "Balance, gravity waves and jets in turbulent shallow water flows". Thesis, University of St Andrews, 2009. http://hdl.handle.net/10023/708.
Texto completo da fonteResumo:
This thesis contains a thorough investigation of the properties of freely decaying turbulence in a rotating shallow water layer on a sphere. A large number of simulations, covering an extensive range of Froude and Rossby numbers, have been carried out using a novel numerical algorithm that exploits the underly- ing properties of the flow. In general these flows develop coherent structures; vortices interact, merge and migrate polewards or equatorwards depending or their sign, leaving behind regions of homogenized potential vorticity separated by sharp zonal jets. In the first half of the thesis we investigate new ways of looking at these structures. In the second half of the thesis we examine the properties of the potential vorticity (PV) induced, balanced component and the residual, unbalanced component of the flows. Cyclone-anticyclone asymmetry has long been observed in atmospheric and oceanic data, laboratory experiments and numerical simulations. This asymmetry is usually seen to favour anticyclonic vorticity with the asymmetry becoming more pronounced at higher Froude numbers (e.g. Polvani et al. [1994a]). We find a similar result but note that the cyclones, although fewer, are significantly more intense and coherent. We present several ways of quantifying this across the parameter space. Potential vorticity homogenization is an important geophysical mechanism responsible for sharpening jets through the expulsion of PV gradients to the edge of flow structures or domains. Sharp gradients of PV are obvious in contour plots of this field as areas where the contours are bunched together. This suggests that we can estimate the number of zonal jets by performing a cluster analysis on the mean latitude of PV contours (this diagnostic is also examined by Dritschel and McIntyre [2007]). This provides an estimate rather than an exact count of the number of jets because the jets meander signficantly. We investigate the accuracy of the estimates provided by different clustering techniques. We find that the properties of the jets defy such simple classification and instead demand a more local examination. We achieve this by examining the palinstrophy field. This field, calculated by taking the gradient of the PV, highlights the regions where PV contours come closer together, exactly what we would expect in regions of strong jets. Plots of the palinstrophy field reveal the complex structure of these features. The potential vorticity field is even more central to the flow evolution than the strong link with jets suggests. From a knowledge of the spatial distribution of PV, it is possible to diagnose the balanced components of all other fields. These components will not contain inertia-gravity waves but will contain the dominant, large scale features of the flow. This inversion, or decomposition into balanced (vortical) and unbalanced (wave) components, is not unique and can be defined to varying orders of accuracy. We examine the results of four dfferent definitions of this decomposition, two based on truncations of the full equations and two based on an iterative procedure applied to the full equations. We find the iterative procedure to be more accurate in that it attributes more of the flow to the PV controlled, balanced motion. However, the truncated equations perform surprisingly well and do not appear to suffer in accuracy at the equator, despite the fact that the scaling on which they are based has been thought to break down there. We round off this study by considering the impact of the unbalanced motion on the flow. This is accomplished by splitting the integration time of the model into intervals τ < t < τ+dτ and comparing, at the end of each interval, the balanced components of the flow obtained by a) integrating the model from t = 0 and b) integrating the full equations, initialised at t = τ with the balanced components from a) at t = τ. We find that any impact of the unbalanced component of the flow is less than the numerical noise of the model.
39
Mutter, Troy Blake. "Numerical simulations of elliptical jets : a study of jet entrainment /". Thesis, This resource online, 1994. http://scholar.lib.vt.edu/theses/available/etd-07102009-040630/.
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Schneider, Gerald Manfred. "Structures and turbulence characteristics in a precessing jet flow /". Title page, contents and summary only, 1996. http://web4.library.adelaide.edu.au/theses/09PH/09phs358.pdf.
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Funk, Denise Reneé Martin. "Application of laser techniques to experimental studies of jets and plumes". Thesis, Georgia Institute of Technology, 1993. http://hdl.handle.net/1853/21487.
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Jarrah, Yousef Mohd. "Nonlinear interactions in mixing layers and compressible heated round jets". Diss., The University of Arizona, 1989. http://hdl.handle.net/10150/184652.
Texto completo da fonteResumo:
The nonlinear interactions between a fundamental instability mode and both its harmonics and the changing mean flow are studied using the weakly nonlinear stability theory of Stuart and Watson, and numerical solutions of coupled nonlinear partial differential equations. The first part of this work focuses on incompressible cold (or isothermal; constant temperature throughout) mixing layers, and for these, the first and second Landau constants are calculated as functions of wavenumber and Reynolds number. It is found that the dominant contribution to the Landau constants arises from the mean flow changes and not from the higher harmonics. In order to establish the range of validity of the weakly nonlinear theory, the weakly nonlinear and numerical solutions are compared and the limitation of each is discussed. At small amplitudes and at low-to-moderate Reynolds numbers, the two results compare well in describing the saturation of the fundamental, the distortion of the mean flow, and the initial stages of vorticity roll-up. At larger amplitudes, the interaction between the fundamental, second harmonic, and the mean flow is strongly nonlinear and the numerical solution predicts flow oscillations, whereas the weakly nonlinear theory yields saturation. Beyond the region of exponential growth, the instability waves evolve into a periodic array of vortices. In the second part of this work, the weakly nonlinear theory is extended to heated (or nonisothermal mean temperature distribution) subsonic round jets where quadratic and cubic nonlinear interactions are present, and the Landau constants also depend on jet temperature ratio, Mach number and azimuthal mode number. Under exponential growth and nonlinear saturation, it is found that heating and compressibility suppress the growth of instability waves, that the first azimuthal mode is the dominant instability mode, and that the weakly nonlinear solution describes the early stages of the roll-up of an axisymmetric shear layer. The receptivity of a typical jet flow to pulse type input disturbances is also studied by solving the initial value problem and then examining the behavior of the long-time solution. The excitation produces a wave packet which consists of a few oscillations and is convected downstream by the mean flow. The magnitude of the disturbance in the jet depends on the location of the excitation and there is an optimum position at which little energy input will produce large perturbations. It is found that in order to generate the largest perturbations at any point in the jet, the disturbance should be deposited into the flow at a point where the phase velocity of the most amplified wave equals the fluid velocity (of the base flow).
43
Kastner, Jeffrey F. "Far-field radiated noise mechanisms in high reynolds number and high-speed jets". The Ohio State University, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=osu1181753004.
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Perrault-Joncas, Dominique C. "Linear stability of coaxial jets with application to aeroacoustics". Thesis, McGill University, 2008. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=112343.
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Motivated by a practical interest in noise generated by turbofan engine, this thesis studies the stability of parallel coaxial jets with velocity and temperature profiles characteristic of the exhaust region of the 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 follows that there are three possible modes, only two of which are unstable. To complement the study of parallel flow stability, this thesis also includes the derivation of the amplitude evolution equation for slowly varying axisymmetric incompressible flows.
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Moore, Matthew Richard. "New mathematical models for splash dynamics". Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:c94ff7f2-296a-4f13-b04b-e9696eda9047.
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In this thesis, we derive, extend and generalise various aspects of impact theory and splash dynamics. Our methods throughout will involve isolating small parameters in our models, which we can utilise using the language of matched asymptotics. In Chapter 1 we briefly motivate the field of impact theory and outline the structure of the thesis. In Chapter 2, we give a detailed review of classical small-deadrise water entry, Wagner theory, in both two and three dimensions, highlighting the key results that we will use in our extensions of the theory. We study oblique water entry in Chapter 3, in which we use a novel transformation to relate an oblique impact with its normal-impact counterpart. This allows us to derive a wide range of solutions to both two- and three-dimensional oblique impacts, as well as discuss the limitations and breakdown of Wagner theory. We return to vertical water-entry in Chapter 4, but introduce the air layer trapped between the impacting body and the liquid it is entering. We extend the classical theory to include this air layer and in the limit in which the density ratio between the air and liquid is sufficiently small, we derive the first-order correction to the Wagner solution due to the presence of the surrounding air. The model is presented in both two dimensions and axisymmetric geometries. In Chapter 5 we move away from Wagner theory and systematically derive a series of splash jet models in order to find possible mechanisms for phenomena seen in droplet impact and droplet spreading experiments. Our canonical model is a thin jet of liquid shot over a substrate with a thin air layer trapped between the jet and the substrate. We consider a variety of parameter regimes and investigate the stability of the jet in each regime. We then use this model as part of a growing-jet problem, in which we attempt to include effects due to the jet tip. In the final chapter we summarise the main results of the thesis and outline directions for future work.
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Boland, Emma Joan Douglas. "Jets, mixing, and topography in the Southern Ocean". Thesis, University of Cambridge, 2013. https://www.repository.cam.ac.uk/handle/1810/245073.
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The Southern Ocean holds a unique place in our planet. It is home to the world’s longest and strongest ocean current, the Antarctic Circumpolar Current (or ACC), which is formed of jets (alternating velocity structures), thought to be significant surface transport barriers. The dynamical processes (particularly mixing processes) in the Southern Ocean are crucial to driving the global overturning circulation, which is in turn responsible for the global transport of heat, CO2, and nutrients. Despite the evident importance of the Southern Ocean to current and future climate, the important dynamical processes that occur there are poorly understood. This thesis attempts to contribute towards the understanding of some of the open questions in Southern Ocean dynamics. In particular, we investigate the effect that topography might have on the jets that form the ACC, with regards to their formation and in particular, their transport properties. Through a quasi-geostrophic model we investigate the properties of jets that form over a zonal slope in bottom topography, and find that the jets become tilted, aligning perpendicular to the large-scale barotropic potential vorticity gradient. As the jets tilt more, they become significantly more energetic, corresponding with an increase in across-jet transport. We compare various theories regarding the formation of such jets, involving linear analysis of the system. It is found that the analytical form of the Rossby wave frequencies correctly predicts the anisotropy of the energy spectra of simulations, and so the jet direction. Additionally, there is a need to characterise accurately the isopycnal mixing occurring throughout the Southern Ocean. We utilise satellite measurements to estimate isopycnal diffusivities in the Southern Ocean in two different studies. Using an effective diffusivity diagnostic to extend a previous study, we find reduced surface horizontal mixing at the latitudes of the ACC core. By comparing a tracer advection simulation with measurements from an experiment in the Southern Ocean, we find that simulations with a vertically averaged horizontal diffusivity of 20m2s−1 best match observations in the Pacific sector of the ACC.
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Chakravorty, Saugata. "A Numerical Study On Absolute Instability Of Low Density Jets". Thesis, Indian Institute of Science, 2000. https://etd.iisc.ac.in/handle/2005/227.
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A spectacular instability has been observed in low density round jets when the density ratio of jet fluid to ambient fluid falls below a threshold of approximately 0.6. This phenomenon has been observed in non-buoyant jets of helium in air, heated air jets and heated buoyant jets. The oscillation of the flow near the nozzle is extremely regular and periodic and consists of ring vortices. Even the smaller scale structures that appear downstream exhibit similar regularity. A theory for predicting the onset of this oscillation is based on finding regions of absolute instability from linear stability analysis of parallel flow. However, experiments suggest that the theory is at least incomplete and fortuitous as the oscillation is not a linear process. The present work is to observe and understand the process of regeneration of these oscillations by conducting numerical simulations. Here, two-dimensional, plane jets were simulated because they undergo a qualitatively similar process.
A spatial and temporal picture of a heated jet has been obtained numerically. A perturbation expansion was used to obtain a system of conservation laws for compressible flows which is valid for low Mach numbers. The low Mach number approximation removes the high frequency acoustic waves from the flow field. This enables a larger time step to be taken without making the calculation unstable. To ensure that all the scales of motion are properly resolved, calculations were done at a low Reynolds number. The governing equations were discretized in space using second-order finite difference formulas on a staggered grid. Velocity fields were advanced using a second-order Adams-Bashforth explicit scheme and then corrected by solving for pressure such that continuity is satisfied at every time step. The Poisson problem for pressure requires the time derivative of the density which was approximated by a third-order backward difference formula. Gauss-Siedel iteration was used to find the pressure.
Several numerical tests were conducted prior to simulations of variable density jets to check the stability and accuracy of the code. Two dimensional driven cavity flow calculations were done as a first test. Then a calculation of a forced, spatially developing, incompressible, plane mixing layer was done to check the time accuracy of the code. After obtaining satisfactory performance of the code for the different test cases, two-dimensional, variable density jets were simulated.
Since the plane jet extends ad infinitum in the streamwise direction, a sufficiently large domain was used to capture all the relevant physics in the downstream regions of the jet. An advective boundary condition was imposed at the exit plane. Rigid, slipwall conditions were employed to prescribe lateral boundary conditions.
A 2-D, incompressible plane jet was simulated first. The jet profile was approximated by two hyperbolic tangent shear layers. The most unstable mode of the inviscid shear layer for this profile, along with its first and second harmonics, was imposed on the velocity profile at the inlet plane. The amplitude of oscillation of the harmonics was chosen so as to provide sufficient energy in the perturbation to accelerate the growth of the layer. No explicit phase lag was introduced in the perturbation. The flow was allowed to develop long enough to wash out the effect of the initial condition. The results obtained for this case indicate that experimentally realized phenomena such as vortex pairing were captured in this simulation. Furthermore, to check the convective nature of instability of the incompressible jet, the forcing at the inlet plane was turned off. The disturbances were gradually convected downstream, out of the computational domain.
Next, two-dimensional heated, non-buoyant jets were studied numerically. The effects of the ratio of jet density to ambient density S, the velocity ratio R, and jet width W, on the near field behavior of an initial laminar jet and the regeneration mechanism of the self-sustaining vortices were explored. The theory based on domain of absolute/convective instability identifies these three parameters. No initial perturbation was necessary to start roll-up of the shear layer. For certain choices, e.g., S= 0.75, R = 20, W =10.5, self-sustaining oscillations appeared spontaneously, and these cycles repeated for very long simulation intervals. Waviness on the jet shear layers grow and roll-up into vortices as in constant density shear layers. But unlike the incompressible plane jet, these vortices grow much larger and mixes more with the surrounding fluid. As these vortices evolve, packets of fluid break away as trailing legs similar to side jet expulsions observed in round jets and plumes. The growing vortices disturb the upstream shear layer. Consistently with linear theory, which predicts absolute instability for these parameters, these disturbances are able to grow and roll up. If these disturbances travelled faster than the downstream vortices, it would not be possible for the cycle to repeat. With sufficient shear between the co-flowing streams (R not too small), the entire regeneration process was found to begin from roughly the same streamwise location. Furthermore, it is the symmetric, varicose mode which occurs. At a slightly larger density ratio (S = 0.8, R = 10), self-sustaining oscillations appeared, but each new cycle began slightly farther downstream. It seems likely that these values are close to the boundary in parameter space between self-sustained oscillatory and convectively unstable behaviors. Jet width also influences the selection of these two behaviors. When jet width was reduced, W = 6, even for S = 0.75,R = 20, each new cycle began to shift downstream. For larger jet width (W = 12.3), self-sustaining oscillations occur but the response is now as an asymmetric sinuous mode after a short initial varicose mode.
The detailed processes that have now been revealed in plane jets should serve as guidelines for the study of such processes in the technologically more important round jets.
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Chakravorty, Saugata. "A Numerical Study On Absolute Instability Of Low Density Jets". Thesis, Indian Institute of Science, 2000. http://hdl.handle.net/2005/227.
Texto completo da fonteResumo:
A spectacular instability has been observed in low density round jets when the density ratio of jet fluid to ambient fluid falls below a threshold of approximately 0.6. This phenomenon has been observed in non-buoyant jets of helium in air, heated air jets and heated buoyant jets. The oscillation of the flow near the nozzle is extremely regular and periodic and consists of ring vortices. Even the smaller scale structures that appear downstream exhibit similar regularity. A theory for predicting the onset of this oscillation is based on finding regions of absolute instability from linear stability analysis of parallel flow. However, experiments suggest that the theory is at least incomplete and fortuitous as the oscillation is not a linear process. The present work is to observe and understand the process of regeneration of these oscillations by conducting numerical simulations. Here, two-dimensional, plane jets were simulated because they undergo a qualitatively similar process.
A spatial and temporal picture of a heated jet has been obtained numerically. A perturbation expansion was used to obtain a system of conservation laws for compressible flows which is valid for low Mach numbers. The low Mach number approximation removes the high frequency acoustic waves from the flow field. This enables a larger time step to be taken without making the calculation unstable. To ensure that all the scales of motion are properly resolved, calculations were done at a low Reynolds number. The governing equations were discretized in space using second-order finite difference formulas on a staggered grid. Velocity fields were advanced using a second-order Adams-Bashforth explicit scheme and then corrected by solving for pressure such that continuity is satisfied at every time step. The Poisson problem for pressure requires the time derivative of the density which was approximated by a third-order backward difference formula. Gauss-Siedel iteration was used to find the pressure.
Several numerical tests were conducted prior to simulations of variable density jets to check the stability and accuracy of the code. Two dimensional driven cavity flow calculations were done as a first test. Then a calculation of a forced, spatially developing, incompressible, plane mixing layer was done to check the time accuracy of the code. After obtaining satisfactory performance of the code for the different test cases, two-dimensional, variable density jets were simulated.
Since the plane jet extends ad infinitum in the streamwise direction, a sufficiently large domain was used to capture all the relevant physics in the downstream regions of the jet. An advective boundary condition was imposed at the exit plane. Rigid, slipwall conditions were employed to prescribe lateral boundary conditions.
A 2-D, incompressible plane jet was simulated first. The jet profile was approximated by two hyperbolic tangent shear layers. The most unstable mode of the inviscid shear layer for this profile, along with its first and second harmonics, was imposed on the velocity profile at the inlet plane. The amplitude of oscillation of the harmonics was chosen so as to provide sufficient energy in the perturbation to accelerate the growth of the layer. No explicit phase lag was introduced in the perturbation. The flow was allowed to develop long enough to wash out the effect of the initial condition. The results obtained for this case indicate that experimentally realized phenomena such as vortex pairing were captured in this simulation. Furthermore, to check the convective nature of instability of the incompressible jet, the forcing at the inlet plane was turned off. The disturbances were gradually convected downstream, out of the computational domain.
Next, two-dimensional heated, non-buoyant jets were studied numerically. The effects of the ratio of jet density to ambient density S, the velocity ratio R, and jet width W, on the near field behavior of an initial laminar jet and the regeneration mechanism of the self-sustaining vortices were explored. The theory based on domain of absolute/convective instability identifies these three parameters. No initial perturbation was necessary to start roll-up of the shear layer. For certain choices, e.g., S= 0.75, R = 20, W =10.5, self-sustaining oscillations appeared spontaneously, and these cycles repeated for very long simulation intervals. Waviness on the jet shear layers grow and roll-up into vortices as in constant density shear layers. But unlike the incompressible plane jet, these vortices grow much larger and mixes more with the surrounding fluid. As these vortices evolve, packets of fluid break away as trailing legs similar to side jet expulsions observed in round jets and plumes. The growing vortices disturb the upstream shear layer. Consistently with linear theory, which predicts absolute instability for these parameters, these disturbances are able to grow and roll up. If these disturbances travelled faster than the downstream vortices, it would not be possible for the cycle to repeat. With sufficient shear between the co-flowing streams (R not too small), the entire regeneration process was found to begin from roughly the same streamwise location. Furthermore, it is the symmetric, varicose mode which occurs. At a slightly larger density ratio (S = 0.8, R = 10), self-sustaining oscillations appeared, but each new cycle began slightly farther downstream. It seems likely that these values are close to the boundary in parameter space between self-sustained oscillatory and convectively unstable behaviors. Jet width also influences the selection of these two behaviors. When jet width was reduced, W = 6, even for S = 0.75,R = 20, each new cycle began to shift downstream. For larger jet width (W = 12.3), self-sustaining oscillations occur but the response is now as an asymmetric sinuous mode after a short initial varicose mode.
The detailed processes that have now been revealed in plane jets should serve as guidelines for the study of such processes in the technologically more important round jets.
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Hsu, Cheng-Hsing. "Laser Doppler anemometry measurements of a confined turbulent water jet with a uniform background flow". Diss., Virginia Polytechnic Institute and State University, 1989. http://hdl.handle.net/10919/54194.
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An axisymmetric, turbulent water jet with several very slow, coflowing external streams was measured with a frequency shifted laser Doppler anemometer. The objective was to approximate a jet submerged in an ambient fluid of infinite domain by using a confined jet in a uniform coflow. The coflow prevents flow reversal outside the jet, but if the coflow velocity is not small compared to the jet velocity, the jet will no longer be self-preserving. Thus, the objective is reached in the limit as the coflow approaches zero, but in the absence of reverse flow. In the present study, a jet with several slow coflows was examined to investigate this behavior, the data was extrapolated to the limit (Us/Uj) to obtain the free jet results and reduce uncertainty in earlier data.
The Reynolds number based on the jet diameter and exit velocity was 32100. Conservation of momentum of the jet was demonstrated up to the measurement limit of x/d = 100. Its distribution suggests that the near fleld axial pressure variation has significant effects on the momentum flux. The results also indicate that momentum flux measurements require accurate data to the edge of the jet. The similarity of mean and rms velocity profiles suggest the existence of a region of self-preservation.
The entrainment rate, centerline velocity decay rate and spreading rate of the jet were determined and compared to previous measurements with and without a coflowing stream. The variation of these jet parameters with respect to the veIocity ratios was obtained. The limiting values of the jet parameters were determined by extrapolation to zero velocity ratio.
This study indicates that a slow coflowing stream is an ideal way to eliminate the recirculating zone present outside jets without coflows. By reducing the coflow to a negligible velocity with constant Craya-Curtet number, researchers can greatly reduce the wide experimental variation in jet entrainment and spreading rates found in different facilities. The results also indicated that a confined jet with a very slow coflow without recirculation can asymptotically approach the conditions of a free jet. An estimate of the variation of the duct size versus the velocity ratio is obtained. It suggests that it is not possible to reduce the velocity ratio to an arbitrarily small value without backflow because the duct would become impractically large.Ph. D.
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Ahmad, Imtiaz 1962. "Simulation of turbulent flow and heat transfer under an impinging round jet discharging into a crossflow". Thesis, McGill University, 1987. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=66202.
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