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1
Boga, Gabriele. "Numerical Experiments on Turbulent Entrainment." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2020. http://amslaurea.unibo.it/20559/.
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The aim of this thesis work is the study of the turbulent entrainment phenomenon in jets through numerical experiments. More specifically, an attempt to study the effect of engulfment and nibbling mechanisms separately was made. The flow chosen for the numerical experiments is the temporal planar jet. The idea behind these experiments is to study the spreading and the mixing of a passive scalar under the effect of two modified velocity fields. The first is a large-scale velocity field obtained through a filtering operation, while the second is a small-scale velocity field obtained subtracting the large-scale velocity field from the total one and then adding the mean velocity.
Initially, the post-processing of a spatially developing planar jet, performed by Doctor Andrea Fregni and Professor Andrea Cimarelli, has been carried out in order to analyse the main features of spatially evolving jets compared with the temporal ones. A co-flow and a passive scalar are present in the simulation. The Reynolds number is set to Re = 3000 and the Schmidt number is Sc = 1. After this first step, a benchmark DNS of a temporal planar jet with Re = 3000 and Sc = 1 has been performed in order to evaluate the main differences with respect to the spatially evolving jet. Once the settings were validated, the numerical experiments with large and small scale velocity fields have been performed. The filter used in all the experiments is the box filter. The results of two different filter lengths are presented, the first is Δ = 1.5λcl and the second is Δ = 3λcl. Since λcl is function of time, the two filter lengths are themselves varying in time. The results of the experiments were then compared with those of the unfiltered solution. The passive scalar spread approximatively the same amount under the effect of the large-scale velocity fields and under the effect of the unfiltered velocity. On the other hand, the small-scale fluctuations have been proved to be important in the mixing process.
2
Mickett, John B. "Turbulent entrainment fluxes within the eastern Pacific warm pool /." Thesis, Connect to this title online; UW restricted, 2007. http://hdl.handle.net/1773/11005.
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Witz, Matthew J. "Mechanics of particle entrainment in turbulent open-channel flows." Thesis, University of Aberdeen, 2015. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=225690.
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An advanced understanding of particle entrainment is required to optimise the design and maintenance of numerous open channel hydraulic systems and structures placed in these systems; including river channels and canals. This study is on particle entrainment (defined as the movement of a particle from a stationary position to being mobile in the flow). Three aspects of particle entrainment were identified as the focus of this work: First, the waiting time for an exposed particle to entrain under constant flow conditions. Second, the flow features responsible for the entrainment of an individual exposed particle. Third, the motion of an entrained particle immediately after entrainment. Waiting time was found to be highly sensitive to protrusion, with a small increase in protrusion resulting in a significant decrease in waiting time. Contrary to previous suggestions the waiting time to entrainment was found to be poorly described by an exponential distribution; instead Weibull or gamma distributions provide an improved fit in both qualitative and quantitative terms. Ensemble averaged flow fields at the point of entrainment were computed to determine the features responsible for entrainment. The data from the transverse vertical PIV plane indicated the presence of two counter-rotating vortices, with the boundary between the vortices located directly over the entrainment particle. The streamwise vertical PIV measurements showed the presence of a structure extending for a considerable distance in the streamwise direction, the length of which appeared to be independent of submergence. Further, the inclination of the downstream end of the structure appeared to increase with submergence. From the point of entrainment particle dffusion in all three coordinate directions displays an exponent significantly greater than that of ballistic diffusion. From the point of entrainment particle diffusion in all three coordinate directions displays an exponent significantly greater than that of ballistic diffusion. The results highlight the clear difference in the local scale between the diffusion of an already mobile particle with one starting from a position of rest.
4
Cocconi, Giacomo. "Numerical investigation of turbulent/non-turbulent interface." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2013. http://amslaurea.unibo.it/5237/.
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The subject of this work is the diffusion of turbulence in a non-turbulent flow. Such phenomenon can be found in almost every practical case of turbulent flow: all types of shear flows (wakes, jet, boundary layers) present some boundary between turbulence and the non-turbulent surround; all transients from a laminar flow to turbulence must account for turbulent diffusion; mixing of flows often involve the injection of a turbulent solution in a non-turbulent fluid. The mechanism of what Phillips defined as “the erosion by turbulence of the underlying non-turbulent flow”, is called entrainment. It is usually considered to operate on two scales with different mechanics. The small scale nibbling, which is the entrainment of fluid by viscous diffusion of turbulence, and the large scale engulfment, which entraps large volume of flow to be “digested” subsequently by viscous diffusion. The exact role of each of them in the overall entrainment rate is still not well understood, as it is the interplay between these two mechanics of diffusion. It is anyway accepted that the entrainment rate scales with large properties of the flow, while is not understood how the large scale inertial behavior can affect an intrinsically viscous phenomenon as diffusion of vorticity. In the present work we will address then the problem of turbulent diffusion through pseudo-spectral DNS simulations of the interface between a volume of decaying turbulence and quiescent flow. Such simulations will give us first hand measures of velocity, vorticity and strains fields at the interface; moreover the framework of unforced decaying turbulence will permit to study both spatial and temporal evolution of such fields. The analysis will evidence that for this kind of flows the overall production of enstrophy , i.e. the square of vorticity omega^2 , is dominated near the interface by the local inertial transport of “fresh vorticity” coming from the turbulent flow. Viscous diffusion instead plays a major role in enstrophy production in the outbound of the interface, where the nibbling process is dominant. The data from our simulation seems to confirm the theory of an inertially stirred viscous phenomenon proposed by others authors before and provides new data about the inertial diffusion of turbulence across the interface.
5
Drew, Brady Patterson. "Entrainment Characteristics of Turbulent Round Gas Jets Submerged in Water." Thesis, Virginia Tech, 2011. http://hdl.handle.net/10919/76852.
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The entrainment process in two-phase buoyant jets differs significantly from their singlephase counterparts, and is not well understood. Entrainment models developed for singlephase flow are often used in two-phase jetting simulations, albeit with limited success. In this work, Particle Image Velocimetry (PIV) and shadowgraph flow visualization experiments have been conducted on submerged round gas jets of varying speeds and nozzle diameters with the goal of improving our understanding of the entrainment process in a two-phase (gas-liquid) jet. The total entrainment estimated using the PIV measurements is higher than the respective values suggested by a common empirical model developed for singlephase buoyant jets. A two-phase theoretical entrainment model used for comparison shows an overestimation of entrainment, but predicts the increase in the rate of entrainment with axial distance from the jet nozzle seen in the PIV results.
This thesis also presents advances in PIV processing methodology that were developed concurrently with the entrainment research. The novel Spectral Phase Correlation (SPC) allows for particle displacement to be determined directly from phase information in the Fourier domain. Some of the potential benefits of the SPC explored here include (1) avoidance of errors introduced by spatial peak-finding routines; (2) use of a modal analysis that can be used to provide information such as correlation quality; and (3) introduction of a means of incorporating information from multiple image windows. At low image noise levels, the method performs as well as an advanced CC-based method. However, difficulties unwrapping the aliased phase information cause the SPC's performance to degrade at high noise levels.Master of Science
6
Holzner, Markus. "Experimental and numerical study on the small scale features of turbulent entrainment." kostenfrei, 2007. http://e-collection.ethbib.ethz.ch/view/eth:29729.
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Robertson, Zachary Burton Smith. "Deposition of Newtonian Particles Entrained in a Turbulent Axisymmetric Free Jet." Thesis, Virginia Tech, 2012. http://hdl.handle.net/10919/31884.
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In the past 10 years there has been a significant amount of research into two-phase particle transport. The terrorist events of September 11, 2001 sparked a series of studies analyzing particle entrainment and deposition in turbulent airflows. One area of research needing further attention has been the study of particles entrained in axisymmetric air jets. An experimental rig was designed and built to study entrainment properties and deposition of Newtonian particles, after injection into a turbulent axisymmetric free air jet.
Newtonian spherical particles, ranging from 1mm to 6mm in diameter, were injected into a turbulent airstream and blown through a nozzle into a large, open space. As the particles fell
out of the jet stream, their linear distances, from nozzle to initial-ground-contact, were recorded and analyzed.
The experiments conducted indicated particle size and density to be significant factors when considering Newtonian particle entrainment. Additionally, particle deposition distribution revealed a consistent positive skewness, as opposed to an expected Gaussian form.
The data presented in this paper provide a starting point for understanding entrainment of Newtonian spherical particles in jets. The simple experimental rig geometry and results also provide an opportunity for computational fluid dynamics models to be validated, answering a call from the 2006 Annual Review of Fluid Mechanics.Master of Science
8
Ma, Jianmin. "Numerical analysis of flow structures and bed entrainment in turbulent open-channel flow." Thesis, Queen Mary, University of London, 2009. http://qmro.qmul.ac.uk/xmlui/handle/123456789/524.
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The results from a Direct Numerical Simulation (DNS) and a Large Eddy Simulation (LES) are employed to study the large-scale coherent structures and bed entrainment in the turbulent open-channel flow. The gravel bed is represented by a hexagonal arrangement of uniform spheres. The large-scale coherent structures are composed of a group of quasistreamwise vortices and asymmetric hairpin vortices. The meandering structures are shown to be longer than the length of the computational box, more than 20 times the effective flow depth in this study, and the width tends to be one order of magnitude smaller than the length. The signature of the large-scale motion is elongated local maximum of streamwise velocity. It is also found that these structures contribute substantially to both of the Reynolds Stress (RS) and the Turbulent Kinetic Energy (TKE). The entrainment of bed gravels is investigated by the three-dimensional analysis of the relationship between near-wall coherent structures and the force moments exerted on the particles. It is found that the spanwise drag moment (MD2) is of the same order of magnitude compared with the streamwise drag moment (MD1). The majority of MD2 originates from pressure whilst the viscous force plays as an important role as pressure for MD1. The contributions of the forces at different heights of the particle to MD1 and MD2 are explored. The quasi-streamwise vortices are strongly associated with MD2 and the ejections are shown to be more favorable for bed entrainment than the sweeps in this bed condition.
9
Sebesta, Christopher James. "Modeling the Effect of Particle Diameter and Density on Dispersion in an Axisymmetric Turbulent Jet." Thesis, Virginia Tech, 2012. http://hdl.handle.net/10919/31987.
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Creating effective models predicting particle entrainment behavior within axisymmetric turbulent jets is of significant interest to many areas of study. Research into multiphase flows within turbulent structures has primarily focused on specific geometries for a target application, with little interest in generalized cases. In this research, the entrainment characteristics of various particle sizes and densities were simulated by determining the distribution of particles across a surface after the particles had fallen out of entrainment within the jet core. The model was based on an experimental set-up created by Lieutenant Zachary Robertson, which consists of a particle injection system designed to load particles into a fully developed pipe [1]. This pipe flow then exits into an otherwise quiescent environment (created within a wind tunnel), creating an axisymmetric turbulent round jet. The particles injected were designed to test the effect of both particle size and density on the entrainment characteristics.
The data generated by the model indicated that, for all particle types tested, the distribution across the bottom surface of the wind tunnel followed a standard Gaussian distribution. Experimentation yielded similar results, with the exception that some of the experimental trials showed distributions with significantly non-zero skewness. The model produced results with the highest correlation to experimentation for cases with the smallest Stokes number (small size/density), indicating that the trajectory of particles with the highest level of interaction with the flow were the easiest to predict. This was contrasted by the high Stokes number particles which appear to follow standard rectilinear motion.Master of Science
10
Papanicolaou, Athanasios N. "The Role of Turbulence on the Initiation of Sediment Motion." Diss., Virginia Tech, 1997. http://hdl.handle.net/10919/30591.
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The present study examines the role of turbulence on the
incipient motion of sediment. For this purpose,
well-controlled experiments are performed at the
laboratory in a tilting flume. In these tests glass beads of
the same size and density are used as the testing material
to isolate the role of turbulence. State of the art
equipment are used during the course of this study.
Specifically, a 3-D Laser Doppler Velocimetry system is
employed to measure the instantaneous velocity
components at different points near the vicinity of a ball
while the ball motion is monitored with a video camera.
An image analysis program is developed here to analyze
the motion of the particles within a test area. To examine
the importance of the different stress components in the
entrainment of sediment, five tests of different packing
configuration are performed. Specifically three different
roughness regimes are examined namely, the isolated, the
wake interference, and the skimming flow. The results
reveal that the instantaneous normal stress in the
streamwise direction is the most dominant component of
the instantaneous stress tensor. The backbone of this
study is the development of a methodology to link the
effects of turbulence with the commencement of
sediment motion. It is considered that the metastable
bursting cycle (i.e. sweeps, ejections, inward and
outward interactions) is responsible for the sediment
entrainment. And that the sediment entrainment, if any,
occurs within a bursting period. The main concept
behind the determination of the critical conditions is that
the probability of the entrainment of sediment (effect) is
equal to the probability of occurrence of these highly
energetic turbulent events that have magnitude greater
than the critical (cause). The probability of sediment
entrainment is computed by means of the image analysis
tool. The balance of moments is obtained here to
determine the minimum moment that is required for the
commencement of sediment motion. The balance of
moments yields the deduction of a new variable that is
used to describe the probability of occurrence of the
different turbulent events. This variable is the summation
of the instantaneous normal stresses in the streamwise
and vertical direction. It is shown here that a
two-parameter gamma density function describes quite
well the statistical behavior of this variable. The results
that are obtained from the existing model suggest that the
present methodology can adequately describe the
commencement of sediment motion. It is shown here that
the traditionally used shear stress term uw may not be
the appropriate measure for the determination of the
critical conditions.Ph. D.
11
Loganathan, Ramanan Mayoorathen. "Solutions for the flows induced by lazy, forced and pure turbulent plumes." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/273741.
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In this thesis an analytical modelling approach is employed to predict and gain insight into the flows induced by turbulent plumes and jets above slender horizontal slots, in otherwise quiescent uniform environments. To supplement the solutions, the effect on the environment of a plume driven by an off-source supply of buoyancy was also considered. The solutions derived provide an advancement on existing idealised models for the jet and plume induced flows, and moreover, complement a number of key advances that have been made in our understanding of plume flows in recent years. The theory of functions of a complex variable, which has not previously been applied in such an application, has been utilised as a fundamental tool throughout the work. This has enabled the entrainment behaviour and geometry of the plumes to be accounted for when developing the induced flow solutions. A novel conformal mapping has been devised specifically to account for the curved perimeter of the contracting lazy plume. This modelling approach is robust in that future developments to aspects of the modelling, for instance, the formulation of a new entrainment closure, can be straightforwardly accounted for using the method. The induced flow solutions exhibit a range of flow patterns which are dependent on the source Richardson number of the plume flow. A pure plume induces a uniform horizontal flow. Forced and lazy plumes correspond to a relative deficit and excess in source buoyancy flux compared to the pure plume, respectively. Generally, forced plumes induce downwardly inclined flows, in contrast to lazy plumes, which induce upwardly inclined flows. Consistent with these solutions, the notionally lazy plume driven by a vertical uniform off-source supply of buoyancy induces an upwardly inclined flow. In addition to an improved understanding of induced flows, our solutions have provided us with insight into the plume flow. Notably, the solution corresponding to the forced plume has led us to fundamentally question existing models describing the plume and, in particular, closures that have been employed to model entrainment. We find that the existing well accepted closures exhibit some form of non-physical flow behaviour.
12
Valyrakis, Manousos. "Initiation of Particle Movement in Turbulent Open Channel Flow." Diss., Virginia Tech, 2011. http://hdl.handle.net/10919/27289.
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The objective of this thesis is to investigate the flow conditions that lead to coarse grain entrainment at near incipient motion conditions. Herein, a new conceptual approach is proposed, which in addition to the magnitude of hydrodynamic force or flow power, takes into account the duration of the flow event. Two criteria for inception of grain entrainment, namely the critical impulse and critical energy concepts, are proposed and compared. These frameworks adopt a force or energy perspective, considering the momentum or energy transfer from each flow event to the particle respectively, to describe the phenomenon.
A series of conducted mobile particle experiments, are analyzed to examine the validity of the proposed approaches. First a set of bench-top experiments incorporates an electromagnet which applies pulses of known magnitude and duration to a steel spherical particle in a controlled fashion, so as to identify the critical level for entrainment. The utility of the above criteria is also demonstrated for the case of entrainment by the action of turbulent flow, via analysis of a series of flume experiments, where both the history of hydrodynamic forces exerted on the particle as well as its response are recorded simultaneously.
Statistical modeling of the distribution of impulses, as well as conditional excess impulses, is performed using distributions from Extreme Value Theory to effectively model the episodic nature of the occurrence of these events. For the examined uniform and low mobility flow conditions, a power law relationship is proposed for describing the magnitude and frequency of occurrence of the impulse events. The Weibull and exponential distributions provide a good fit for the time between particle entrainments. In addition to these statistical tools, a number of Adaptive Neuro-Fuzzy Inference Systems employing different input representations are used to learn the nonlinear dynamics of the system and perform statistical prediction. The performance of these models is assessed in terms of their broad validity, efficiency and forecast accuracy.
Even though the impulse and energy criteria are deeply interrelated, the latter is shown to be advantageous with regard to its performance, applicability and extension ability. The effect of single or multiple highly energetic events carried by certain coherent flow structures (mainly strong sweep events) with regard to the particle response is also investigated.Ph. D.
13
Stella, Francesco. "Caractérisation d’un décollement turbulent sur une rampe : entraînement et lois d’échelle." Thesis, Orléans, 2017. http://www.theses.fr/2017ORLE2043/document.
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Les décollements turbulents massifs sont des phénomènes communs qui peuvent causer des pertes et de nuisances aérodynamiques importantes dans les écoulements industriels, par exemple à l’arrière d’une aile d’avion. Ce travail contribue à leur compréhension par l’analyse phénoménologique d’un décollement turbulent, représentatif d’un grand nombre d’écoulements réels. Le premier objectif est d’identifier les lois d’échelle des décollements turbulents, notamment en rapport avec les caractéristiques de l’écoulement à l’amont de la rampe. Un deuxième objectif est l’analyse, à grande et à petite échelle, des mécanismes de transport de fluide qui pilotent le fonctionnement des décollements. A cet effet, une approche originale est proposée, basée sur une description expérimentale et analytique de la couche cisaillée décollée et des interfaces turbulentes qui la délimitent. Nos résultats suggèrent que les lois d’échelle du décollement varient de façon complexe selon l’interaction de la couche limite à l’amont, de la couche cisaillée et de l’écoulement potentiel extérieur. La taille du décollement est liée à l’intensité de l’entraînement turbulent de masse dans la couche cisaillée, qui à son tour dépend de la turbulence dans la couche limite, bien à l’amont du point de décollement. Cette dépendance pourrait s’appliquer à toute la gamme d’échelles turbulentes responsables du transport de masse. Ces observations montrent clairement le rôle de la couche cisaillée dans le fonctionnement des décollements massifs et suggèrent la faisabilité de stratégies de contrôle nouvelles, de type retro-action ou prédictif, basée sur l’entrainement turbulentMassive turbulent separations are common phenomena that can cause sizeable aerodynamical losses and detrimental effects in industrial flows, for example on airplane wings. This work contributes to their understanding with a phenomenological analysis of a canonical turbulent separation, representative of a large number of real flows. The first objective is to identify the scaling laws of turbulent separations, in particular with respect to their dependencies on the characteristics of the flow upstream of the ramp. A second objective is the analysis, both at large and small scale, of the transfert mechanisms that drive the functioning of separated flows. To this end, a new approach is proposed, centered on the experimental and analytical description of the separated shear layer and of the turbulent interfaces that bound it. Our results suggest that the scaling laws of the separated flow vary in a complex way, in function of the interaction of the incoming boundary layer, the separated shear layer and the free-stream. The size of the separation is related to the intensity of turbulent mass entrainment within the shear layer, which in turn depends on the turbulence in the incoming boundary layer, well upstream of the separation point. This dependency might apply over the entire range of turbulent length scales that are responsible for mass transfer. These observations clearly show the role of the shear layer in the functioning of massive separation. They also suggest the feasibility of new control strategies, both of feedback and feed-forward type, based on turbulent entrainment
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内山, 知実, Tomomi UCHIYAMA, 正章 成瀬, and Masaaki NARUSE. "自由落下粒子群が形成する粒子噴流の三次元渦法解析." 日本機械学会, 2005. http://hdl.handle.net/2237/9206.
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内山, 知実, Tomomi UCHIYAMA, 佳伸 北野, and Yoshinobu KITANO. "自由落下粒子群が形成する粒子噴流の数値解析." 日本機械学会, 2003. http://hdl.handle.net/2237/9212.
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Johnson, Stephen. "A quasi-dimensional model for performance and emissions predictions in a dual fuel engine." Thesis, Loughborough University, 2012. https://dspace.lboro.ac.uk/2134/10975.
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A new quasi-dimensional, multi-zone model has been developed to describe the combustion processes occurring inside a dual fuel engine. A dual fuel engine is a compression ignition engine in which a homogeneous lean premixed charge of gaseous fuel and air is ignited by a pilot fuel spray. The atomisation and preparation of the pilot leads to the formation of multiple ignition centres from which turbulent flame fronts develop. The energy release in a dual fuel engine is therefore a combination of that from the combustion of the pilot fuel spray and lean premixed charge. Hence, the dual fuel combustion process is complex, combining elements of both conventional spark and compression ignition engines. The dual fuel engine is beneficial as it can achieve significant reductions in emissions of carbon dioxide (CO2), as well as reducing emissions of oxides of nitrogen (NOx) and particulate matter (PM).
17
Alberti, Luca. "Statistical Breakdown of Numerically Simulated Shear-Free Flows." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2020.
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A statistical analysis of several DNSs of incompressible turbulent shear-free flows (with Reλ ranging from ≈ 86 to ≈ 245) was performed, aimed to study the small scale processes that promote turbulent entrainment at the TNTI. After a certain transient, the shearless flows reached a statistically steady state. From that time instant, the main features of the various flow configurations were statistically investigated by assessing the classical and conditional mean profiles of total kinetic energy (K) and anisotropy ratio (urms/ξ), the classical and conditional mean profiles of the terms of the total kinetic energy budget (advection C, pressure strain Π, viscous diffusion Dv, dissipation ε and power injection Pi), the classical PDFs of the velocity components (u, v and w), the classical and conditional mean profiles of the invariants of the velocity gradient (R and Q), rate-of-strain (RS and QS) and rate-of-rotation (QW) tensors and the classical and conditional J PDFs of the same invariants. Conditional statistical quantities were computed in relation to the distance from the TNTI. To avoid the accidental mixing of turbulent and non-turbulent points in the conditional statistics, all the nodes belonging to irrotational bubbles and turbulent islands were neglected.
All the conditional mean profiles are characterized by very sharp gradients at the TNTI, meanwhile in the classical mean profiles the variations are distributed on a much wider portion of the domain, indicating how the conditional averages are able to successfully discern turbulent and irrotational regions, avoiding the smoothening of the mean profiles via large scale intermittency. The similarity between the conditional statistics obtained here for shearless flows, and those extracted from temporal jets in previous works, is very interesting, since it suggests some sort of universality in the TNTI dynamics, as well asin the turbulent entrainment processes.
18
Bahner, Mark A. "A reduced-turbulence, reduced-entrainment electrostatic precipitator." Thesis, This resource online, 1990. http://scholar.lib.vt.edu/theses/available/etd-03122009-040702/.
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Beard, Robert L. "Oceanic mixed layer entrainment zone dynamics." Thesis, Monterey, California : Naval Postgraduate School, 1990. http://handle.dtic.mil/100.2/ADA240896.
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Thesis (M.S. in Meteorology and Physical Oceanography)--Naval Postgraduate School, September 1990.Thesis Advisor(s): Garwood, R. W. Second Reader: Chu, P. C. "September 1990." Description based on title screen viewed on December 16, 2009. DTIC Descriptor(s): Marine meteorology, thermoclines, thickness, production, predictions, models, energy, theses, turbulence, temperature gradients, vertical orientation, heating, kinetic energy, budgets, oceans, weather stations, entrainment, north pacific ocean, shear properties, transients. DTIC Identifier(s): Mixed layer (marine) air sea interaction, entrainment shear production, mixed layer. Author(s) subject terms: air-sea interaction, entrainment shear production, mixed layer. Includes bibliographical references (p. 46-47). Also available in print.
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Cotel, Aline J. "Entrainment and detrainment of a jet impinging on a stratified interface /." Thesis, Connect to this title online; UW restricted, 1995. http://hdl.handle.net/1773/9964.
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Mehaddi, Rabah. "Contribution à l'étude des fontaines turbulentes." Thesis, Aix-Marseille, 2014. http://www.theses.fr/2014AIXM4356/document.
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Une fontaine peut se créer quand la flottabilité d'un rejet vertical s'oppose à sa quantité de mouvement. Ce type d'écoulement connaît beaucoup d'applications que ce soit dans la nature (panaches issus des éruptions volcaniques) ainsi que dans l'industrie du bâtiment (chauffage et refroidissement) ou dans le domaine des risques (rejets accidentel de gaz lourd). Dans cette thèse, nous nous focalisons sur l'étude des fontaines turbulentes miscibles. Dans le premier chapitre nous reformulons le modèle théorique de Morton et al. (1956) pour traiter le cas des fontaines en milieu linéairement stratifié. La résolution de ce modèle permet d'obtenir des relations analytiques pour la hauteur de la fontaine et sa hauteur d'étalement. Ce modèle est, par la suite, étendu au cas des panaches et des jets turbulents en milieu linéairement stratifié. Dans le second chapitre, nous proposons un modèle théorique permettant d'étudier une fontaine turbulente miscible en régime établi. Pour calibrer ce modèle, des simulations numériques aux grandes échelles (LES) sont utilisées pour obtenir une estimation des valeurs des constantes associées aux phénomènes d'échanges turbulents entre les parties ascendante et descendante de la fontaine. L'objectif du dernier chapitre est d'apporter, à partir d'expérimentations en laboratoire, des informations quantitatives sur l'influence de forts écarts de masses volumiques dans les écoulements de type fontaine. Les expériences sont réalisées pour des fontaines gazeuses (mélange air/hélium) en régime établiA fountain can occur when the buoyancy of a vertically released fluid opposes its momentum. Such flows have many applications in nature (plumes issuing from volcanic eruption), building industry (cooling or heating) or in the area of risk management (accidental release of heavy dangerous gas). In this thesis, we focus on the study of miscible turbulent fountains. In the first chapter, we revisit the theoretical model of Morton et al. (1956) to handle the case of fountains in linearly stratified fluid. The resolution of this model allows us to obtain analytical relations for the fountain height as well as the spreading height of its horizontal layer. This model is subsequently extended to the case of turbulent jets and plumes in linearly stratified fluid. In the second chapter, we propose a theoretical model for the study of a turbulent miscible fountain in a steady state. To calibrate this model, large eddy simulations (LES) are used to obtain an estimate of the values of the constants associated with the additional terms appearing in the equations. The objective of the final chapter is to provide, from laboratory experiments, quantitative information on the influence of strong density differences on the behaviour of a turbulent fountain. These experiments shows that all the classical relations valid for the Boussinesq case can be extended to the non-Boussinesq case by using an appropriate definition of the Froude number
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Wilson, Wesley M. "The development of a droplet formation and entrainment model for simulations of immiscible liquid-liquid flows." Morgantown, W. Va. : [West Virginia University Libraries], 1999. http://etd.wvu.edu/templates/showETD.cfm?recnum=1161.
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Thesis (M.S.)--West Virginia University, 1999.Title from document title page. Document formatted into pages; contains xvi, 219 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 166-169).
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Majidi, Seyyed Hojjat. "Modeling of air entrainment and oxide inclusion formation during pouring of metal castings." Diss., University of Iowa, 2018. https://ir.uiowa.edu/etd/6611.
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Oxide inclusions are among the most commonly reported defects in ferrous and non-ferrous castings. They affect the surface quality, machinability, and mechanical performance of a cast part. Air entrainment during mold filling is the main source of the oxygen that is consumed in inclusion formation. A quantitative understanding of the formation mechanisms or the prediction of final amounts and locations of oxide inclusions in metal castings is not available. Ductile iron experiments are conducted to study the formation of oxide inclusions during pouring. Oxide inclusions are measured by serial sectioning of the solidified castings. The effect of different gating systems, section thicknesses, and surface orientations on the inclusion formation and final distribution is studied. In addition, a computational model is developed for predicting the formation, motion and final location of oxide inclusions during pouring of metal castings, with the focus on the important mechanism of generation of oxide inclusions due to air entrainment during mold filling. The developed model calculates the local air entrainment rate as a function of the turbulent kinetic energy and the magnitude of the normal velocity gradient of the liquid metal at the liquid-air interface. The turbulent kinetic energy is estimated from the sum of the squares of the fluctuating velocity components relative to a spatially averaged mean velocity. The air entrainment model is implemented in a casting simulation software and validated by comparing its predictions to experimental air entrainment measurements for a circular water jet plunging into a quiescent pool. The liquid velocity, diameter and the turbulence intensity dependence is determined by a single entrainment coefficient. Oxide inclusions are then generated at the liquid-air interface, transported with the melt flow under the combined influences of drag and buoyancy, and captured by the solidifying casting surface. The developed model provides a powerful technique for predicting the oxide inclusion formation and final location.
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Balakrishnan, Mahalingam III. "The Role of Turbulence on the Entrainment of a Single Sphere and the Effects of Roughness on Fluid-Solid Interaction." Diss., Virginia Tech, 1997. http://hdl.handle.net/10919/30732.
Full textAbstract:
Incipient motion criterion in sediment transport is very important, as it defines the flow condition that initiates sediment motion, and is also frequently employed in models to predict the sediment transport at higher flow conditions as well. In turbulent flows, even a reasonably accurate definition of incipient motion condition becomes very difficult due to the random nature of the turbulent process, which is responsible for sediment motion under incipient conditions. This work investigates two aspects, both of which apply to incipient sediment transport conditions. The first one deals with the role of turbulence in initiating sediment motion. The second part deals with the nature of sediment-fluid interaction for more general and complex flows where the number of sediment particles that form the rough surface is varied.
The first part of this work that investigates the role of turbulence in initiating sediment motion, uses a video camera to simultaneously monitor and record the sediment (glass ball) motion and corresponding fluid velocity events measured by a three-component laser Doppler Velocimeter (LDV). The results of the single ball experiment revealed that the number of LDV flow measurements increase dramatically (more than four folds) just prior to the ball motion. The fluid mean velocity and its root-mean-square (rms) values also are significantly higher than the values that correspond to the flow conditions that yield no ball motion.
The second part of the work, investigation of the fluid-sediment interaction, includes five tests with varying number of sediment particles. In order to understand the nature and extent of fluid-solid interaction, velocity profile measurements using the 3-D laser system were carried out at three locations for each of these five cases. Plots of mean velocities, rms quantities located the universal layer at about 1.5 ball diameters above the porous bed. However, at higher sediment particle concentrations, this distance reduced and the beginning of the universal layer approached the top of the porous bed.Ph. D.
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Pergaud, Julien. "Étude et paramétrisation de la turbulence et des nuages de couche limite." Toulouse 3, 2008. http://thesesups.ups-tlse.fr/2340/.
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L'objectif principal de cette thèse était de développer une nouvelle paramétrisation améliorant le mélange et la représentation des nuages dans les couches limites convectives. Nous avons développé ainsi une paramétrisation basée sur le concept EDMF pour Eddy-Diffusivity/Mass Flux dans lequel le schéma en flux de masse représente un thermique idéalisé qui peut échanger de l'air avec son environnement et dont les parcelles d'air peuvent condensées. Dans la partie sèche de l'updraft, l'entraînement et le détrainement sont définis comme proportionnels à la flottabilité et inversement proportionnels à la vitesse verticale dans l'updraft. Ce formalisme permet à ces échanges de s'adapter en fonction du type de couche limite ou encore de l'intensité de la convection. Dans le nuage, c'est le formalisme en tri par flottabilité qui a été choisi. Il est en effet bien adapté pour les échanges entre le nuage et son environnement. Les tests dans un modèle unicolonne ont permis de mettre en valeur l'apport d'un tel schéma dans la représentation du mélange mais aussi des nuages de couche limite. Il permet notamment une bonne représentation de la zone à contre-gradient et corrige un défaut bien connu des paramétrisations en coefficient d'échange turbulent. Des résultats 3D pré-opérationnels sont présentés et montrent la nécessité d'un tel schéma pour des modèles à 2. 5km de résolution. Il a été ainsi implémenté dans le nouveau modèle opérationnel de Météo-France, AROMEThe main objective of this thesis was to develop a new parameterization enabling to improve the mixing and representation of clouds in convective boundary layers. We thus developed a parameterization based on the concept EDMF for Eddy- Diffusivity/Mass Flux and in which the mass _ux scheme represents idealized thermal which can exchange air with its environment and whose air parcels can condensate. In the dry part of the updraft, entrainment and detrainment are defined as proportional to the buoyancy and inversely proportional to the vertical velocity in the updraft. This formalism allows the exchanges to adapt depending on the type of boundary layer or the intensity of convection. In the cloud, the buoyancy sort formalism was chosen. Indeed, it is well suited to represent exchanges between the cloud and its environment. Tests in a single column model helped highlight the contribution of such a scheme in the representation of the turbulence but also boundary layer clouds. It allows a good representation of the counter-gradient area and corrects a well-knwon defect of eddy-diffusivity parameterizations. 3D results are presented and demonstrate the need for such a scheme for models with 2. 5km resolution. It has been well implemented in the new operational mesoscale model at Météo-France named AROME
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Fonty, Thomas. "Modélisation de l’entraînement d’air dans l'eau avec la méthode SPH." Thesis, Paris Est, 2019. http://www.theses.fr/2019PESC1013.
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Les écoulements au sein d'ouvrages hydrauliques – déversement au-dessus d'un barrage, déferlement d'une vague sur une digue, etc. – sont le siège de forts mélanges d'eau et d'air qui se traduisent visuellement par la formation d'eaux blanches à la dynamique complexe. Représenter fidèlement le phénomène d'entraînement/capture des bulles d'air dans l'eau revêt donc un aspect stratégique important pour le dimensionnement de ces ouvrages. La modélisation tant physique que numérique de tels cas s'avère délicate à cause du fort rapport de densité entre les phases et de la nature multi-échelle de ces écoulements impliquant des effets de turbulence et de tension de surface. La méthode numérique SPH (Smoothed Particle Hydrodynamics), approche totalement lagrangienne qui représente l'écoulement comme un ensemble de particules en mouvement sans recours à un maillage, est particulièrement adaptée à la simulation de tels écoulements fortement déformés. Néanmoins, les limites actuelles de puissance de calcul empêchent encore de simuler finement des cas d'application industriels à large emprise en hydraulique. On se propose donc dans cette thèse de modéliser ces écoulements de manière macroscopique via un modèle de mélange qui consiste à voir chaque particule SPH comme un volume de mélange d'eau et d'air en mouvement. On détaille d'abord la dérivation des équations continues de ce modèle de mélange, puis on présente un état de l'art des simulations multiphasiques SPH. A partir du modèle continu et des outils actuels de discrétisation, un modèle de mélange diphasique SPH est ensuite mis en place en vue de son implémentation sur GPU (Graphics Processing Unit). Un accent tout particulier est mis sur les éléments originaux de discrétisation développés, notamment la dérivation d'un schéma aux bonnes propriétés numériques pour le suivi de l'évolution des volumes par phase et l'écriture d'un formalisme de frontières ouvertes pour un mélange. La turbulence, centrale dans le phénomène d'entraînement d’air, est modélisée via un modèle k-ϵ incluant un terme de flottabilité. Ce modèle de mélange est validé sur des cas académiques bidimensionnels de complexité croissante tels que la séparation d'un mélange eau-huile, un écoulement de Poiseuille diphasique, l'instabilité de Rayleigh–Taylor et un lâché de sédiments, illustrant sa polyvalence. La phénoménologie de l'entraînement d'air est ensuite décrite, et le modèle appliqué à des structures communément rencontrées en hydraulique, comme des jets plongeants et des coursiers en marches d'escalier, en introduisant une fermeture spécifique de la vitesse relative entre les phases. Enfin, on présente des premiers cas d'application industriels aux géométries et dynamiques complexesFlows over hydraulic works – a nappe falling over a spillway, a wave breaking on a dike, etc. – undergo strong mixtures of air and water that lead to the appearance of white waters with complex dynamics. Faithfully capture the phenomenon of air bubbles entrainment/entrapment in the flowing water is therefore pivotal for the design of those works. Both experimental and numerical modeling prove to be complex due to high density ratio between phases and the multiscale nature of those flows involving turbulence and surface tension effects. The SPH (Smoothed Particle Hydrodynamics) method, a fully Lagrangian approach that models the flow as a set of moving particles without any mesh, is particularly well-suited to simulate such highly-distorted flows. Nevertheless, the current computational limits still prevent one from finely simulating industrial application cases with large domains in hydraulics. In this work, we aim at simulating macroscopically those flows with a mixture model in which each SPH particle stands for a moving volume of air and water. The derivation of the continuous equations of this mixture model is first detailed, then a state of the art of multiphase simulations in SPH is presented. Equipped with this continuous model and the existing discretization tools, a two-phase SPH mixture model is then derived and implemented on GPU (Graphics Processing Unit). A focus is made on original elements developed in the discretization, especially the derivation of a scheme with good numerical properties to follow the phase volume variations and the writing of an open boundary framework for mixtures. Turbulence, prominent for the air entrainment phenomenon, is modeled with a k-ϵ model including a buoyancy term. This model is validated against bidimensional academic test cases of increasing complexity, namely an oil-water separation, a two-phase Poiseuille flow, the Rayleigh-Taylor instability and a sand dumping case, proving its versatility. The air entrainment phenomenology is then described and the model is applied to common structures in hydraulics such as plunging jets and stepped spillways by introducing a specific closure for the relative velocity between phases. Finally, first industrial application cases with complex geometries and dynamics are presented
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Grasreiner, Sebastian. "Combustion modeling for virtual SI engine calibration with the help of 0D/3D methods." Doctoral thesis, Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2012. http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-90518.
Full textAbstract:
Spark ignited engines are still important for conventional as well as for hybrid power trains and are thus objective to optimization. Today a lot of functionalities arise from software solutions, which have to be calibrated. Modern engine technologies provide an extensive variability considering their valve train, fuel injection and load control. Thus, calibration efforts are really high and shall be reduced by introduction of virtual methods. In this work a physical 0D combustion model is set up, which can cope with a new generation of spark ignition engines. Therefore, at first cylinder thermodynamics are modeled and validated in the whole engine map with the help of a real-time capable approach. Afterwards an up to date turbulence model is introduced, which is based on a quasi-dimensional k-epsilon-approach and can cope with turbulence production from large scale shearing. A simplified model for ignition delay is implemented which emphasizes the transfer from laminar to turbulent flame propagation after ignition. The modeling is completed with the calculation of overall heat release rates in a 0D entrainment approach with the help of turbulent flame velocities. After validation of all sub-models, the 0D combustion prediction is used in combination with a 1D gas exchange analysis to virtually calibrate the modern engine torque structure and the ECU function for exhaust gas temperature with extensive simulationsModerne Ottomotoren spielen heute sowohl in konventionellen als auch hybriden Fahrzeugantrieben eine große Rolle. Aktuelle Konzepte sind hochvariabel bezüglich Ventilsteuerung, Kraftstoffeinspritzung und Laststeuerung und ihre Optimierungspotentiale erwachsen zumeist aus neuen Softwarefunktionen. Deren Applikation ist zeit- und kostenintensiv und soll durch virtuelle Methoden unterstützt werden. In der vorliegenden Arbeit wird ein physikalisches 0D Verbrennungsmodell für Ottomotoren aufgebaut und bis zur praktischen Anwendung geführt. Dafür wurde zuerst die Thermodynamik echtzeitfähig modelliert und im gesamten Motorenkennfeld abgeglichen. Der Aufbau eines neuen Turbulenzmodells auf Basis der quasidimensionalen k-epsilon-Gleichung ermöglicht anschließend, die veränderlichen Einflüsse globaler Ladungsbewegung auf die Turbulenz abzubilden. Für den Brennverzug wurde ein vereinfachtes Modell abgeleitet, welches den Übergang von laminarer zu turbulenter Flammenausbreitung nach der Zündung in den Vordergrund stellt. Der restliche Brennverlauf wird durch die physikalische Ermittlung der turbulenten Brenngeschwindigkeit in einem 0D Entrainment-Ansatz dargestellt. Nach Validierung aller Teilmodelle erfolgt die virtuelle Bedatung der Momentenstruktur und der Abgastemperaturfunktion für das Motorsteuergerät
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Guibu, Pereira Gabriel. "Modélisation numérique d'écoulements turbulents avec entraînement d'air au sein d'ouvrages hydrauliques." Thesis, Strasbourg, 2019. http://www.theses.fr/2019STRAD023.
Full textAbstract:
La dispersion de l'air dans l'eau est un phénomène observé fréquemment pour une large variété , d'écoulements dans des structures hydrauliques, qu'elles soient naturelles ou conçues par l'homme. Or, la présence de bulles d'air immergées dans l'eau modifie de façon significative les caractéristiques de l'écoulement. Ces modifications peuvent être bénéfiques ou nuisibles selon la situation. En particulier, dans le contexte des milieux confinés des réseaux d'eaux urbains, l'évaluation du transport d'air est un facteur important pour le dimensionnement des ouvrages, tels que les puits de chute. Le travail présenté dans cette thèse s'intéresse à l'identification et au développement des techniques de modélisation numérique tridimensionnelle, permettant de mieux évaluer les interactions eau-air et le transport diphasique dans le contexte des ouvrages hydrauliques. Une approche de modélisation diphasique hybride est implémentée, dans le but de représenter les écoulements à la fois en régime ségrégué et dispersé. Différentes méthodologies ont ainsi été appliquées pour simuler des écoulements aérés, et leurs résultats ont été confrontés à des mesures expérimentalesThe mixing and dispersion of air into water is a frequently observed feature of flows occurring in nature and in man-made hydraulic structures. The addition of immersed air bubbles in the water modifies substantially the flow's characteristics, which may have a desirable or harmful effect depending on the context. Furthermore, the assessment of the air currents is of value in the study of confined spaces. For instance, this is important in the design of air vents in structures such as dropshafts in sewage systems. This thesis focuses on identifying and developing three-dimensional computational models for a better description of air-water interactions and two-phase flows in the context of hydraulic structures. A hybrid two-phase modelling approach is implemented in this thesis, thus allowing the simultaneous representation of segregated and dispersed flow regimes. The results from different methods employed for the aerated flow simulations are finally confronted with experimental data
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Lobosco, Raquel Jahara. "Investigação teórico-numérica da aeração em estruturas de vertedouros em degraus com uso de software livre." Universidade de São Paulo, 2013. http://www.teses.usp.br/teses/disponiveis/18/18138/tde-07102014-152612/.
Full textAbstract:
Em inúmeras aplicações práticas da engenharia a camada interfacial entre dois fluidos é de extrema importância para a correta caracterização do escoamento. Especificamente na Engenharia Ambiental e Hidráulica na interface entre o ar e a água existem propriedades capazes de relacionar a aeração e oxigenação. Focado na dinâmica dos processos turbulentos, o estudo proposto se aplica a problemas de sub-pressão em vertedores e visa analisar a troca de gases da interface ar-água em situações de fases dispersas e avaliar a qualidade da água a jusante dos mesmos. O objetivo maior é descrever a relação entre o comportamento da superfície da água, o desenvolvimento da camada limite e os regimes de escoamento de acordo com a vazão volumétrica. Para representar a distribuição da fração de vazios da fase dispersa, foram usados os princípios da formulação dos escoamentos em vertedores escalonados e da formulação para enlaçamento de ar. As argumentações e justificativas das formulações propostas para os perfis de concentrações foram baseadas na distribuição da fração de vazios prevista numericamente. A abordagem numérica do problema fluidodinâmico diferencia da maioria dos estudos concentrados na literatura, por estar baseada na quebra da superfície livre.In many pratical aplications of engineering the interfacial layer between two fluids has a strong relevance in the process of right measurements of the flow. Espeficically in the enviromental and hydraulics engineering the interface between air and water is the property related with oxigen and areation of the flow. With focus on the dynamics of the turbulent processes, the proposed study is applied to sub-pressure problems in spillways. It investigates the gases exchange in the air- water interface in the dispersed phase and evaluate the water quality downstream. The major goal is to describe a relation between the boundary layer development and the flow regimes with discharge. To represent the distribution of void fractions of the dispersed phase, the physical formulation concepts of stepped spillways and entrapped air were used. The arguments and reasons to justify the proposed concentration profile formulation are based on the numerical provided void fraction distribution. The results as well as the physical model are in good agrement with described literature data. The Numerical approach of the fluid dynamics problem differs from major of described literature studies because it is based on the surface breakup.
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Parisi, Valerio. "Large Eddy Simulation of a Stagnation Point Reverse Flow Combustor." Thesis, Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/13995.
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In this study, numerical simulations of a low emission lab-scale non-premixed combustor are conducted and analyzed. The objectives are to provide new insight into the physical phenomena in the SPRF (Stagnation Point Reverse Flow) combustor built in the Georgia Tech Combustion Lab, and to compare three Large Eddy Simulation (LES) combustion models (Eddy Break-Up [EBU], Steady Flamelet [SF] and Linear Eddy Model [LEM]) for non-premixed combustion. The nominal operating condition of the SPRF combustor achieves very low NOx and CO emissions by combining turbulent mixing of exhaust gases with preheated reactants and chemical kinetics. The SPRF numerical simulation focuses on capturing the complex interaction between turbulent mixing and heat release. LES simulations have been carried out for a non-reactive case in order to analyze the turbulent mixing inside the combustor. The LES results have been compared to PIV experimental data and the code has been validated. The dominating features of the operational mode of the SPRF combustor (dilution of hot products into reactants, pre-heating and pre-mixing) have been analyzed, and results from the EBU-LES, SF-LES and LEM-LES simulations have been compared. Analysis shows that the LEM-LES simulation achieves the best agreement with the observed flame structure and is the only model that captures the stabilization processes observed in the experiments. EBU-LES and SF-LES do not predict the correct flow pattern because of the inaccurate modeling of sub-grid scale mixing and turbulence-combustion interaction. Limitations of these two models for this type of combustor are discussed.
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Khan, Muhammad. "RANS and LES of multi-hole sprays for the mixture formation in piston engines." Phd thesis, Ecole Centrale de Lyon, 2014. http://tel.archives-ouvertes.fr/tel-01002111.
Full textAbstract:
Cette thèse porte sur la simulation des jets de gouttes générés par des pulvérisateurs essence haute pression, pulvérisateurs qui sont un point clef des systèmes de combustion automobile de la présente et future génération devant diminuer les émissions de CO2 et de polluants. Dans un premier temps les jets de gouttes (" sprays ") sont simulés par simulation moyennée. Les résultats de simulation d'un jet donnant des résultats en moyenne satisfaisant, l'interaction de jets en injecteurs multi-trous est alors simulée. Les résultats sont cohérents par rapport aux mesures d'entraînement d'air. La simulation permettant d'avoir accès au champ complet 3D, le mécanisme d'interaction jet à jet et de développement instationnaire du spray est décrit en détail. La formation d'un mouvement descendant au centre du spray et celle d'un point d'arrêt central sont trouvés. Finalement, Ces résultats sont étendus au cas surchauffé, cas où la pression dans la chambre est inférieure à la pression de vapeur saturante. Un modèle simple semi-empirique est proposé pour tenir compte de la modification des conditions proches de la buse d'injection. Le modèle prédit correctement les tendances des variations de paramètres et capture la forme générale du spray qui se referme sur lui-même. La seconde grande partie est consacrée au développement d'un modèle de spray par l'approche des grandes échelles (SGE), limité ici aux cas non évaporant. Il comprend la modélisation de sous-maille de la dispersion turbulente, des collisions-coalescence et des termes d'échange de quantité de mouvement de sous-maille. L'effet du choix du modèle de sous-maille pour la viscosité turbulente de sous-maille est montré, le choix retenu étant le modèle de Smagorinski dynamique. Afin d'améliorer la représentativité cruciale des conditions d'injections, un couplage faible est réalisé à partir de résultats de simulations existantes de l'écoulement interne aux buses. Les fonctions densité de probabilité simple et jointes extraits des résultats de simulations sont validés par rapport aux mesures PDA en situation pseudo-stationnaire et la pénétration liquide et la forme du spray est comparée aux visualisations par ombroscopie. Enfin, différentes zones caractéristiques sont identifiées et des longueurs sont notées pour les cas d'injection à 100 et 200bar.
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Dahm, Werner Johann Anton. "Experiments on Entrainment, Mixing and Chemical Reactions in Turbulent Jets at Large Schmidt Number." Thesis, 1985. https://thesis.library.caltech.edu/4815/1/Dahm_wja_1985.pdf.
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Entrainment, mixing and chemical reactions are investigated in the far field of steady, axisymmetric, momentum-driven, turbulent jets issuing into an unconfined, quiescent medium in the large Schmidt number (liquid-phase) regime. Visualization experiments using both passive and chemically sensitive planar laser induced fluorescence (LIF) techniques show the importance of large scale transport in the jet far field, and suggest that entrainment, mixing and chemical reactions in the far field are dominated by a large scale organization of the flow. Successive instantaneous profiles of the jet fluid concentration along the axial and radial directions in the jet far field are measured by combining these LIF techniques with direct, high-resolution, linear photodiode array imaging and high-speed digital data acquisition. These imaging measurements have revealed an axial similarity concentration variable for which probability density functions (PDFs) in the jet far field are self-similar along rays. A chemical reaction method is presented which allows the self-similar form of these PDFs to be measured with full resolution at all scales of transport and mixing. Furthermore, these imaging measurements have shown that instantaneous radial profiles of the jet fluid concentration do not resemble the mean concentration profile. Specifically, unmixed ambient fluid is found deep within the jet and the composition of molecularly mixed fluid within large regions in the jet is approximately uniform. The results from these experiments are interpreted in the context of a simple conceptual model for large scale organization of entrainment, mixing and chemical reactions in the far field of turbulent jets.
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Lin, Tzi-Liang, and 林自樑. "Numerical Study of Turbulent Jet Impinging on a Moving Surface. Including the Effect of Entrainment." Thesis, 1993. http://ndltd.ncl.edu.tw/handle/94889569180639039344.
Full textAbstract:
碩士國立成功大學
機械工程研究所
81
The paper presents the influence of crossflow, surface motion and entrainment boundary on the flow and heat transfer characteristics of a turbulent slot jet impinging on an isothermal surface. The parameters studied include the jet Reynolds number (Rej ), dimensionless nozzle-to-surface spacing (H/W), the ratio of crossflow mass flow rate to the jet mass flow rate (M) and the ratio of impingement surface velcity to the mean jet velocity (Us/Vj ). The governing equations are solved by a control- volume based finite-difference method with power-law scheme and the well known ｋ - ε model and its associate wall function to describe the turbulence structure. The velocity and pressure terms of momentum equations are solved by SIMPLE (Semi-Implicit Methods for Pressure-Linked Equation). It is found that the flow between the walls closer to a channel flow when both sides of the slot jet is confined. In the case of confined surface, as the nozzle-to- surface spacing (H/W) increases, discrepancy of the shear stress and local Nusselt number between the prediction and experiment of Gardon & Akfirat [1] is about 20 ％. As M increases, the maximum value of the shear stress decreases and its position shift downstream a little with considering the effect of crossflow. The increase of shear stress due to the increase of surface velocity is presented when both the crossflow and surface motion effect are imposed. The local Nusselt number increases with increasing surface motion velocity in the upstream, and decreases in the downstream. In the case with entrainment boundary and surface motion, the ambient fluid enter the jet flow field, it is not like a channel flow. It is found that near the jet the influence of the surface motion is more significant than the upstream and downstream region.
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Ningtyas, Alviani Hesthi Permata, and Alviani Hesthi Permata Ningtyas. "A new experimental design to measure the penetrative entrainment flow rate due to a transition or turbulent fountain at a density interface." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/fxahzy.
Full textAbstract:
碩士國立臺灣科技大學
機械工程系
104
Penetrative entrainment by a turbulent fountain has many practical applications in nature. The examples include the cooling system in the commercial building, recirculating a water reservoir and the process of improving the water quality. This research studied the penetrative entrainment flow rate quantitatively. A new experimental design was developed to estimate the penetrative entrainment volume flow rate by a turbulent fountain. Simulation experiments used the salt bath technique. Two sources, a salt solution source and a fresh water source, were placed in a plexiglass tank. Two sources were located at different heights, the fresh water source at the top and the salt solution source at 2 cm above the floor of the tank. There were 9 experimental runs conducted in this research, and the flow from the fresh water source hit the floor boundary in 3 runs of them. Two data acquisition methods included the measurements of solution density and the recording of flow images. Experiments started from the 2 layers initially with the fresh water as the upper layer and the salt solution as the lower layer. After the experiment began, the density in the upper dilute layer increased with the time until the steady state and the density in the lower dense layer remained the same in the transient state except for the cases in which the fountain flow hit the floor boundary. When the fresh water flow hit the floor boundary, the density of the lower dense layer was less than that of the salt solution and the interface was unclear. For the experimental cases in which the fountain flow does not reach the floor, this new experimental set-up gives three independent equations to estimate the penetrative entrainment flow rate by a fountain flow. Using the numerical turbulent fountain flow model gives the Richardson number at the density interface between 0.029 and 1.816 and the entrainment rate in the range between 0.13 and 2.21, when the initial fountain momentum is 4/3 Q2f/A , and the Richardson number at the density interface between 0.044 and 14.389 and the entrainment rate in the range between 0.12 and 2.08, when the initial fountain momentum is Q2 /A .
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Lo, Chi-Hao, and 羅濟灝. "An experimental arrangement on measuring the penetrative entrainment flow rate due to a turbulent fountain at a density interface in the steady state." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/9xdyj6.
Full textAbstract:
碩士國立臺灣科技大學
機械工程系
105
This research uses an experimental arrangement to measure the penetrative entrainment flow rate due to a turbulent fountain at a density interface in the steady state. The salt-bath technique is employed to conduct the experiments in an acrylic model. Experiments start with two layers of fluids initially, i.e. fresh water as the top and salt water as the bottom. Two sources, a fresh water source and a salt water source, are located at different heights in the acrylic model. A fresh water source is placed at the top of the acrylic model, and there is a stainless steel mesh placed at the outlet of the fresh water nozzle to make the outflow become turbulent. A salt water source is placed at 2 cm above the bottom of the acrylic model by using an acrylic pipe to supply salt water to the bottom layer. Dye attenuation technique is used to analyze the light intensity data derived from the recorded images of experiments. According to the density of salt water and the volume flow rate of fresh water, there are eight experiments in this research. The experimental results show that the interface height is highly related to the density of salt water and the volume flow rate of fresh water in the steady state. For the experiments having the same density of salt water, the distance between the density interface and the fresh water source increases with the increasing volume flow rate of fresh water. For the experiments having the same flow rate of fresh water, when the density of salt water increases, the distance between the density interface and the fresh water sources decreases. The theoretical model gives three equations to estimate the penetrative entrainment flow rate in the steady state. The theoretical turbulent fountain model gives the turbulent fountain radius, the vertical velocity and the volume flow rate at the density interface to estimate the penetrative entrainment rate, the Richardson number, and the densimetric Froude number. The research results show the Richardson number at the density interface between 0.030 and 4.261, and the penetrative entrainment rate in the range of 0.13 to 2.32. Keywords: Density interface, turbulent fountain, penetrative entrainment, steady-state experimental arrangement.
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Peterson, Jill Elizabeth. "Entrainment and turbulence characteristics of low-velocity isothermal and buoyant jets." Thesis, 1991. http://hdl.handle.net/1911/16474.
Full textAbstract:
The current study examined the transition region of axisymmetric isothermal and buoyant jets of low Reynolds number, directed vertically upwards into a stagnant, unstratified ambient. These flows were examined experimentally and numerically. The region in which measurements were obtained allows examination of two types of transition occurring in the jet: from nozzle exit dominated to fully developed, and from momentum to buoyancy dominated flow. Velocity data were acquired using a two channel Laser-Doppler Anemometer for isothermal Reynolds numbers of 850 to 7405, and buoyant Froude numbers of 12 to 6425 and Reynolds numbers from 525 to 6500. Curve fit approximations of the data were developed by assuming polynomial similarity profiles for the measured quantities. Correlation equations were developed which allow prediction of the downstream velocity flow field and turbulent flow field as a function of the Reynolds number, Froude number and density ratio at the nozzle exit. Profile width and entrainment increased at low Reynolds number. Axial and radial velocity fluctuations were found to increase at low Reynolds number. The buoyant cases studied were found to have lower velocity fluctuations and significantly lower Reynolds stresses than isothermal cases of similar Reynolds number.
Once comprehensive correlation equations were developed predicting mean and turbulent flow quantities, the basis was formed for development of a new turbulence model, the analytic turbulence model. This method involved substitution of correlation equation approximations to the mean flow quantities into the boundary layer forms of the governing equations. The turbulent terms were then solved for explicitly. It was shown that this method predicts the fully developed boundary layer turbulent flow field, and can be used as a criterion for a flow attaining boundary layer form. Comparison of turbulent values predicted analytically with those measured empirically revealed that the transition flow examined experimentally had not fully developed to boundary layer form.
Testing of the turbulent correlation equations was performed by using them as a turbulence model in a finite difference numerical solution. While the correlation equations were representative of the turbulent flow field, they did not produce correct results when used in a marching, boundary layer simulation. As verified by the analytic turbulence model, this occurred because the transition flow had not yet reached fully developed boundary layer form.
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Seol, Dong Guan. "Entrainment and mixing properties of multiphase plumes: Experimental studies on turbulence and scalar structure of a bubble plume." 2008. http://hdl.handle.net/1969.1/ETD-TAMU-2794.
Full textAbstract:
This dissertation presents a series of laboratory experiments to study flow and mixing
properties of multiphase plumes. The particle image velocimetry (PIV) and laserinduced
fluorescence (LIF) techniques are developed to measure two-dimensional velocity
and concentration fields of multiphase plumes. The developed measurement
techniques are applied to bubble plumes in different ambient conditions.
The problems and errors in the two-phase PIV application to a bubble plume case
are addressed through a comparative study between the optical separation method
using fluorescent particles and a new phase separation method using vector postprocessing.
The study shows that the new algorithm predicts well the instantaneous
and time-averaged velocity profiles and has errors comparable to those for image
masking techniques.
The phase separation method developed in the previous section is applied to
study the mean flow characteristics of a bubble plume in quiescent and unstratified
condition. The entrainment coefficients representing the mixing properties of a bubble
plume are calculated to lie between 0.08 near the plume source and 0.05 in the upper
region, and to depend on the non-dimensional quantity us/(B/z)1/3, where us is the
bubble slip velocity, B is the initial buoyancy flux, and z is the height from the diffuser.
Further, the LIF technique is investigated to measure the scalar concentration
field around a bubble plume in quiescent, unstratified condition. This new application
to bubble plumes accounts for light scattering by bubbles using an attenuation coef-
ficient that is proportional to the local void fraction. Measured scalar concentration
fields show similar trend in concentration fluctuation to turbulent plume cases.
Finally, the velocity and concentration field measurements using the developed
two-phase PIV and LIF methods are applied for a bubble plume in a density-stratified
ambient. The turbulent flow characteristics induced by a bubble plume in a stratified
ambient water are studied. The plume fluctuation frequency is measured as about 0.1
Hz and compares well to plume wandering frequency measured in unstratified plume
cases.