Дисертації з теми "Turbulent jet flows"
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1
Mastorakos, Epaminondas. "Turbulent combustion in opposed jet flows." Thesis, Imperial College London, 1994. http://hdl.handle.net/10044/1/11820.
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2
Müller, Sebastian. "Numerical investigations of compressible turbulent swirling jet flows." kostenfrei, 2007. http://e-collection.ethbib.ethz.ch/view/eth:30052.
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3
Pouransari, Zeinab. "Numerical studies of turbulent flames in wall-jet flows." Doctoral thesis, KTH, Turbulens, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-160609.
Повний текст джерелаАнотація:
The present thesis deals with the fundamental aspects of turbulent mixing and non-premixed combustion in the wall-jet flow, which has a close resemblance to many industrial applications. Direct numerical simulations (DNS) of turbulent wall-jets with isothermal and exothermic reactions are performed. In the computational domain, fuel and oxidizer enter separately in a nonpremixed manner and the flow is compressible, fully turbulent and subsonic. The triple “turbulence-chemistry-wall” interactions in the wall-jet flow have been addressed first by focusing on turbulent flow effects on the isothermal reaction, and then, by concentrating on heat-release effects on both turbulence and flame characteristics in the exothermic reaction. In the former, the mixing characteristics of the flow, the key statistics for combustion and the near-wall effects in the absence of thermal effects are isolated and studied. In the latter, the main target was to identify the heat-release effects on the different mixing scales of turbulence. Key statistics such as the scalar dissipation rates, time scale ratios, two-point correlations, one and two-dimensional premultiplied spectra are used to illustrate the heat release induced modifications. Finer small mixing scales were observed in the isothermal simulations and larger vortical structures formed after adding significant amounts of heat-release. A deeper insight into the heat release effects on three-dimensional mixing and reaction characteristics of the turbulent wall-jet flow has been gained by digging in different scales of DNS datasets. In particular, attention has been paid to the anisotropy levels and intermittency of the flow by investigating the probability density functions, higher order moments of velocities and reacting scalars and anisotropy invariant maps for different reacting cases. To evaluate and isolate the Damkohler number effects on the reaction zone structure from those of the heat release a comparison between two DNS cases with different Damkohler numbers but a comparable temperature rise is performed. Furthermore, the wall effects on the flame and flow characteristics, for instance, the wall heat transfer; the near-wall combustion effects on the skin-friction, the isothermal wall cooling effects on the average burning rates and the possibility of formation of the premixed mode within the non-premixed flame are addressed. The DNS datasets are also used for a priori analysis, focused on the heat release effects on the subgrid-scale (SGS) statistics. The findings regarding the turbulence small-scale characteristics, gained through the statistical analysis of the flow have many phenomenological parallels with those concerning the SGS statistics. Finally, a DNS of turbulent reacting wall-jet at a substantially higher Reynolds number is performed in order to extend the applicability range for the conclusions of the present study and figuring out the possible differences.QC 20150225
4
Kakhi, M. "The transported probability density function approach for predicting turbulent combusting flows." Thesis, Imperial College London, 1994. http://hdl.handle.net/10044/1/8729.
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5
Rana, Zeeshan Ahmed. "Implicit LES of turbulent compressible high-speed flows with transverse jet injection." Thesis, Cranfield University, 2011. http://dspace.lib.cranfield.ac.uk/handle/1826/7218.
Повний текст джерелаАнотація:
Implicit Large Eddy Simulation (ILES) has rapidly emerged as a powerful technique
which is utilised to explore the unsteady compressible turbulent flows.
Apart from o ering accuracy in numerical simulations, ILES is also computationally
e cient compared to Direct Numerical Simulations or conventional Large Eddy Simulations.
This report focuses on the validation of the existing high-resolution methods
within the framework of ILES and explores its applications to the high-speed compressible
turbulent flows such as a typical flow field inside a scramjet engine. The
methodology applied in the current work employs a fifth-order MUSCL scheme with
a modified variable extrapolation and a three-stage second-order Runge-Kutta scheme
for temporal advancement.
In order to simulate a realistic and accurate supersonic turbulent boundary layer (STBL)
a synthetic turbulent inflow data generation method based upon digital filters has been
implemented. This technique has been validated and compared against various other
turbulent inflow data generation methods in order to find the most accurate, reliable
and computationally e cient technique. The high-speed complex multi-species flow
of a transverse sonic jet injection into a supersonic crossflow (JISC), which is typical
fuel injection strategy inside a scramjet engine, has been investigated for time-averaged
and instantaneous flow. It has been demonstrated that the incoming STBL plays a vital
role in establishing the correct flow dynamics in JISC study as it enhances the KH
instabilities in the flow field.
Thermally perfect gas formulation has been implemented according to the NACA-
1135 report to study the e ects of high temperatures on the ratio of specific heats (
).
Using this, the full geometry of the HyShot-II scramjet engine is investigated to obtain
the inflow conditions for the HyShot-II combustion chamber. Although the design of
HyShot-II allowed to disgorge the shock and boundary layer which could otherwise
enter the combustion chamber, but, it has been demonstrated that the flow field inside
the combustion chamber still consists of a weak shock-train. Finally, the hydrogen
injection is analysed inside the HyShot-II combustion chamber, with the shock-train
travelling inside and the incoming STBL using digital filters based technique, to explore
various time-averaged and instantaneous flow structures and parameters with a
view to enhance the understanding of the complex flow field inside the combustion
chamber. It is demonstrated from the detailed investigations of a complex high-speed
flow that ILES methodology has the potential to develop the understandings of the
high-speed compressible turbulent flows using comparatively less computational resources.
6
Örlü, Ramis. "Experimental studies in jet flows and zero pressure-gradient turbulent boundary layers /." Stockholm : Skolan för teknikvetenskap, Kungliga Tekniska högskolan, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-10448.
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7
Smith, Benjamin Scott. "Wall Jet Boundary Layer Flows Over Smooth and Rough Surfaces." Diss., Virginia Tech, 2008. http://hdl.handle.net/10919/27597.
Повний текст джерелаАнотація:
The aerodynamic flow and fluctuating surface pressure of a plane, turbulent, two-dimensional wall jet flow into still air over smooth and rough surfaces has been investigated in a recently constructed wall jet wind tunnel testing facility. The facility has been shown to produce a wall jet flow with Reynolds numbers based on the momentum thickness, Re&delta = &deltaUm/&nu, of between 395 and 1100 and nozzle exit Reynolds numbers, Rej = Umb/&nu, of between 16000 and 45000. The wall jet flow properties (&delta, &delta*, &theta, y1/2, Um, u*, etc.) were measured and characterized over a wide range of initial flow conditions and measurement locations relative to the wall jet source. These flow properties were measured for flow over a smooth flow surface and for flow over roughness patches of finite extent. The patches used in the current study varied in length from 305 mm to 914 mm (between 24 and 72 times the nozzle height, b) and were placed so that the leading edge of the patch was fixed at 1257 mm (x/b = 99) downstream of the wall jet source. These roughness patches were of a random sand grain roughness type and the roughness grain size was varied throughout this experiment. The tests covered roughness Reynolds numbers (k+) ranging from less than 2 to over 158 (covering the entire range of rough wall flow regimes from hydrodynamically smooth to fully rough). For the wall jet flows over 305 mm long patches of roughness, the displacement and momentum thicknesses were found to vary noticeably with the roughness grain size, but the maximum velocity, mixing layer length scale, y1/2, and the boundary layer thickness were not seen to vary in a consistent, determinable way. Velocity spectra taken at a range of initial flow conditions and at several distinct heights above the flow surface showed a limited scaling dependency on the skin friction velocity near the flow surface.
The spectral density of the surface pressure of the wall jet flow, which is not believed to have been previously investigated for smooth or rough surfaces, showed distinct differences with that seen in a conventional boundary layer flow, especially at low frequencies. This difference is believed to be due to the presence of a mixing layer in the wall jet flow. Both the spectral shape and level were heavily affected by the variation in roughness grain size. This effect was most notable in overlap region of the spectrum. Attempts to scale the wall jet surface pressure spectra using outer and inner variables were successful for the smooth wall flows. The scaling of the rough wall jet flow surface pressure proved to be much more difficult, and conventional scaling techniques used for ordinary turbulent boundary layer surface pressure spectra were not able to account for the changes in roughness present during the current study. An empirical scaling scheme was proposed, but was only marginally effective at scaling the rough wall surface pressure.Ph. D.
8
Küng, Marco. "Large eddy simulation of turbulent channel and jet flows using the approximate deconvolution model." Zürich : ETH, 2007. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=17466.
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9
Xu, Hongyi. "Large eddy simulation of turbulent flows in square and annular ducts and confined square coaxial jet." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape17/PQDD_0006/NQ27860.pdf.
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10
Örlü, Ramis. "Experimental studies in jet flows and zero pressure-gradient turbulent boundary layers". Doctoral thesis, KTH, Mekanik, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-10448.
Повний текст джерелаАнотація:
This thesis deals with the description and development of two classical turbulent shear flows, namely free jet and flat plate turbulent boundary layer flows. In both cases new experimental data has been obtained and in the latter case comparisons are also made with data obtained from data bases, both of experimental and numerical origin. The jet flow studies comprise three parts, made in three different experimental facilities, each dealing with a specific aspect of jet flows. The first part is devoted to the effect of swirl on the mixing characteristics of a passive scalar in the near-field region of a moderately swirling jet. Instantaneous streamwise and azimuthal velocity components as well as the temperature were simultaneously accessed by means of combined X-wire and cold-wire anemometry. The results indicate a modification of the turbulence structures to that effect that the swirling jet spreads, mixes and evolves faster compared to its non-swirling counterpart. The high correlation between streamwise velocity and temperature fluctuations as well as the streamwise passive scalar flux are even more enhanced due to the addition of swirl, which in turn shortens the distance and hence time needed to mix the jet with the ambient air. The second jet flow part was set out to test the hypothesis put forward by Talamelli & Gavarini (Flow, Turbul. & Combust. 76), who proposed that the wake behind a separation wall between two streams of a coaxial jet creates the condition for an absolute instability. The experiments confirm the hypothesis and show that the instability, by means of the induced vortex shedding, provides a continuous forcing mechanism for the control of the flow field. The potential of this passive mechanism as an easy, effective and practical way to control the near-field of interacting shear layers as well as its effect towards increased turbulence activity has been shown. The third part of the jet flow studies deals with the hypothesis that so called oblique transition may play a role in the breakdown to turbulence for an axisymmetric jet.For wall bounded flows oblique transition gives rise to steady streamwise streaks that break down to turbulence, as for instance documented by Elofsson & Alfredsson (J. Fluid Mech. 358). The scenario of oblique transition has so far not been considered for jet flows and the aim was to study the effect of two oblique modes on the transition scenario as well as on the flow dynamics. For certain frequencies the turbulence intensity was surprisingly found to be reduced, however it was not possible to detect the presence of streamwise streaks. This aspect must be furher investigated in the future in order to understand the connection between the turbulence reduction and the azimuthal forcing. The boundary layer part of the thesis is also threefold, and uses both new data as well as data from various data bases to investigate the effect of certain limitations of hot-wire measurements near the wall on the mean velocity but also on the fluctuating streamwise velocity component. In the first part a new set of experimental data from a zero pressure-gradient turbulent boundary layer, supplemented by direct and independent skin friction measurements, are presented. The Reynolds number range of the data is between 2300 and 18700 when based on the free stream velocity and the momentum loss thickness. Data both for the mean and fluctuating streamwise velocity component are presented. The data are validated against the composite profile by Chauhan et al. (Fluid Dyn. Res. 41) and are found to fulfil recently established equilibrium criteria. The problem of accurately locating the wall position of a hot-wire probe and the errors this can result in is thoroughly discussed in part 2 of the boundary layer study. It is shown that the expanded law of the wall to forth and fifth order with calibration constants determined from recent high Reynolds number DNS can be used to fix the wall position to an accuracy of 0.1 and 0.25 l_ * (l_* is the viscous length scale) when accurately determined measurements reaching y+=5 and 10, respectively, are available. In the absence of data below the above given limits, commonly employed analytical functions and their log law constants, have been found to affect the the determination of wall position to a high degree. It has been shown, that near-wall measurements below y+=10 or preferable 5 are essential in order to ensure a correctly measured or deduced absolute wall position. A number of peculiarities in concurrent wall-bounded turbulent flow studies, was found to be associated with a erroneously deduced wall position. The effect of poor spatial resolution using hot-wire anemometry on the measurements of the streamwise velocity is dealt with in the last part. The viscous scaled hot-wire length, L+, has been found to exert a strong impact on the probability density distribution (pdf) of the streamwise velocity, and hence its higher order moments, over the entire buffer region and also the lower region of the log region. For varying Reynolds numbers spatial resolution effects act against the trend imposed by the Reynolds number. A systematic reduction of the mean velocity with increasing L+ over the entire classical buffer region and beyond has been found. A reduction of around 0.3 uƬ, where uƬ is the friction velocity, has been deduced for L+=60 compared to L+=15. Neglecting this effect can lead to a seemingly Reynolds number dependent buffer or log region. This should be taken into consideration, for instance, in the debate, regarding the prevailing influence of viscosity above the buffer region at high Reynolds numbers. We also conclude that the debate concerning the universality of the pdf within the overlap region has been artificially complicated due to the ignorance of spatial resolution effects beyond the classical buffer region on the velocity fluctuations.QC 20100820
11
Semeraro, Onofrio. "Active Control and Modal Structures in Transitional Shear Flows." Doctoral thesis, KTH, Stabilitet, Transition, Kontroll, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-117916.
Повний текст джерелаАнотація:
Flow control of transitional shear flows is investigated by means of numerical simulations. The attenuation of three-dimensional wavepackets of Tollmien-Schlichting (TS) and streaks in the boundary layer is obtained using active control in combination with localised sensors and actuators distributed near the rigid wall. Due to the dimensions of the discretized Navier-Stokes operator, reduced-order models are identified, preserving the dynamics between the inputs and the outputs of the system. Balanced realizations of the system are computed using balanced truncation and system identification. We demonstrate that the energy growth of the perturbations is substantially and efficiently mitigated, using relatively few sensors and actuators. The robustness of the controller is analysed by varying the number of actuators and sensors, the Reynolds number, the pressure gradient and by investigating the nonlinear, transitional case. We show that delay of the transition from laminar to turbulent flow can be achieved despite the fully linear approach. This configuration can be reproduced in experiments, due to the localisation of sensing and actuation devices. The closed-loop system has been investigated for the corresponding twodimensional case by using full-dimensional optimal controllers computed by solving an iterative optimisation based on the Lagrangian approach. This strategy allows to compare the results achieved using open-loop model reduction with model-free controllers. Finally, a parametric analysis of the actuators/ sensors placement is carried-out to deepen the understanding of the inherent dynamics of the closed-loop. The distinction among two different classes of controllers – feedforward and feedback controllers - is highlighted. A second shear flow, a confined turbulent jet, is investigated using particle image velocimetry (PIV) measurements. Proper orthogonal decomposition (POD) modes and Koopman modes via dynamic mode decomposition (DMD) are computed and analysed for understanding the main features of the flow. The frequencies related to the dominating mechanisms are identified; the most energetic structures show temporal periodicity.QC 20130207
12
Dia, Aliou. "Simulation de jets d'air lobés pour l'optimisation des Unités Terminales de Diffusion d'Air." Phd thesis, Université de La Rochelle, 2012. http://tel.archives-ouvertes.fr/tel-00818414.
Повний текст джерелаАнотація:
La thèse traite de la simulation numérique de jets d'air lobés pour l'optimisation des Unités Terminales de Diffusion de l'Air pour le bâtiment. Il est présenté tout d'abord une analyse bibliographique exhaustive sur les jets d'air et sur les moyens passifs de leur contrôle. Vient ensuite un exposé des principes de la modélisation des écoulements turbulents. Sur la base des précédents éléments, nous abordons la phase de simulation et d'analyse de jets d'air lobés en trois parties. La première partie traite de la simulation d'un jet d'air libre à très faible nombre de Reynolds (Reynolds 800) en régime instationnaire. Dans cette étude, nous avons présenté plusieurs cas tests de simulation d'un jet d'orifice en forme de croix que nous avons confrontés à des résultats expérimentaux. Du point de vue quantitatif, la simulation du quart du jet a permis d'obtenir des caractéristiques globales assez satisfaisantes. Pour détecter d'une façon significative l'instationnarité du jet, il a été nécessaire de simuler la totalité du jet, mais ceci a été fait au détriment du nombre de mailles dans le domaine de calcul, ce qui conduit par conséquent à des résultats quantitativement inappropriés. La deuxième partie simule le même jet d'air mais en régime turbulent (Reynolds 3000). Sept modèles de turbulence ont été confrontés à des résultats expérimentaux. Nous montrons la pertinence du modèle RSM (Reynolds Stress Model) pour la prédiction de l'écoulement. La dernière partie est dédiée à la simulation de jets d'air turbulents en interaction. Cette partie se décline en trois études successives et complémentaires. La première a pour objet de rechercher le modèle de turbulence le plus pertinent capable de reproduire les phénomènes et les grandeurs utiles à l'application visée. La conclusion est qu'aucun des modèles de turbulence évalués n'est capable de prédire l'ensemble des caractéristiques dynamiques de l'écoulement de jets lobés en interaction. Cependant, parmi ces modèles, SST apparait nettement supérieur dans la prédiction de l'interaction des jets, de l'expansion dynamique globale et de l'entrainement de l'air ambiant lorsque l'écoulement est résolu à travers le diffuseur lobé. Sur la base des grandeurs dynamiques pertinentes identifiées et dont la prédiction par le modèle SST est jugée satisfaisante, nous entreprenons dans la suite une analyse de l'influence de la géométrie du lobe sur la capacité d'induction de l'écoulement. Un lobe de forme arrondie est alors trouvé plus avantageux qu'un lobe à angles droits. Enfin, la dernière étude s'intéresse à l'effet de la disposition et de l'espacement des orifices lobés sur les grandeurs globales de l'écoulement. Une solution géométrique permettant l'augmentation de la transparence de l'Unité Terminale de Diffusion de l'Air lobée est alors proposée. Elle devra être testée expérimentalement en conditions réelles, dans la cellule test échelle 1 thermiquement gardée nouvellement construite au LEPTIAB.
13
Sainte-Rose, Bruno. "Simulations numériques d'écoulements réactifs massivement décollés par une approche hybride RANS/LES." Phd thesis, Ecole Centrale Paris, 2010. http://tel.archives-ouvertes.fr/tel-00635538.
Повний текст джерелаАнотація:
Les premières simulations numériques d'écoulements réactifs sur des configurationscomplexes ont été réalisées à l'aide d'approches RANS (Reynolds Averaged Navier Stokes). Ces dernières, bien adaptées aux écoulements de type couches limites attachées et relativement peu coûteuses en temps de calcul, ne donnent accès qu'à des résultats stationnaires qui s'éloignent parfois de la réalité. Pour réaliser des simulations instationnaires d'écoulements, les méthodes de type LES (Large Eddy Simulation) -- plus précises mais plus coûteuses -- sont de plus en plus utilisées. Cependant, ces méthodes sont mal adaptées à la simulation de la dynamique pariétale, car elles nécessitent un effort de maillage souvent prohibitif près de la paroi. Cette thèse est consacrée au développement dans le code CEDRE (code de simulation d'écoulements réactifs complexes de l'Onera) d'une méthode hybride RANS/LES, appelée Delayed Detached Eddy Simulation (DDES), et à son application à des écoulements réactifs massivement décollés. Après une étape de validation sur des couches limites attachées, la DDES a été appliquée à la simulation des écoulements inerte et réactif dans une chambre de combustion en forme de marche descendante (A3C) et comparée aux résultats des approches RANS et LES classiques, ainsi qu'aux résultats expérimentaux. Cette méthode a ensuite permis de réaliser l'étude de la dynamique de l'écoulement réactif décollé dans la tuyère ATAC montée sur le banc cryotechnique MASCOTTE de l'Onera.
14
Levin, Ori. "Numerical studies of transtion in wall-bounded flows." Doctoral thesis, KTH, Mechanics, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-546.
Повний текст джерелаАнотація:
Disturbances introduced in wall-bounded flows can grow and lead to transition from laminar to turbulent flow. In order to reduce losses or enhance mixing in energy systems, a fundamental understanding of the flow stability and transition mechanism is important. In the present thesis, the stability, transition mechanism and early turbulent evolution of wall-bounded flows are studied. The stability is investigated by means of linear stability equations and the transition mechanism and turbulence are studied using direct numerical simulations. Three base flows are considered, the Falkner-Skan boundary layer, boundary layers subjected to wall suction and the Blasius wall jet. The stability with respect to the exponential growth of waves and the algebraic growth of optimal streaks is studied for the Falkner-Skan boundary layer. For the algebraic growth, the optimal initial location, where the optimal disturbance is introduced in the boundary layer, is found to move downstream with decreased pressure gradient. A unified transition prediction method incorporating the influences of pressure gradient and free-stream turbulence is suggested. The algebraic growth of streaks in boundary layers subjected to wall suction is calculated. It is found that the spatial analysis gives larger optimal growth than temporal theory. Furthermore, it is found that the optimal growth is larger if the suction begins a distance downstream of the leading edge. Thresholds for transition of periodic and localized disturbances as well as the spreading of turbulent spots in the asymptotic suction boundary layer are investigated for Reynolds number Re=500, 800 and 1200 based on the displacement thickness and the free-stream velocity. It is found that the threshold amplitude scales like Re^-1.05 for transition initiated by streamwise vortices and random noise, like Re^-1.3 for oblique transition and like Re^-1.5 for the localized disturbance. The turbulent spot is found to take a bullet-shaped form that becomes more distinct and increases its spreading rate for higher Reynolds number. The Blasius wall jet is matched to the measured flow in an experimental wall-jet facility. Both the linear and nonlinear regime of introduced waves and streaks are investigated and compared to measurements. It is demonstrated that the streaks play an important role in the breakdown process where they suppress pairing and enhance breakdown to turbulence. Furthermore, statistics from the early turbulent regime are analyzed and reveal a reasonable self-similar behavior, which is most pronounced with inner scaling in the near-wall region.
15
Bagheri, Shervin. "Analysis and control of transitional shear flows using global modes." Doctoral thesis, Stockholm : Department of Mechanics, Royal Institute of Technology, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-11894.
Повний текст джерела
16
Lanitis, Nicolas. "The turbulent structure of the jet in cross-flow." Thesis, University of Cambridge, 2014. https://www.repository.cam.ac.uk/handle/1810/246593.
Повний текст джерелаАнотація:
In this thesis the structure of the jet in cross flow in the far field was investigated experimentally using time-resolved, multi-scale and statistically independent Stereoscopic Particle Image Velocimetry measurements to reveal the mean and instantaneous three-dimensional (3D) structures. All of the measurements were performed in the Counter-rotating Vortex Pair (CVP) plane for a high velocity ratio and jet Reynolds number. Statistical measurements at various downstream locations and velocity ratios are presented. Probability density functions of the streamwise vorticity field showed that each CVP core is instantaneously made of a number of small vortex tubes rather than a single vortex core. The characteristic ‘kidney’ shape was illustrated in the rms velocity profiles and the Reynolds stress profiles exhibited a high level of organisation which showed an evolving shape with downstream distance and persisted well into the far field. Two point spatial correlations pointed to a common structure for all conditions whose mean shape generates the ‘kidney’ shape, as well as evidence of wake structures. Time-resolved measurements were carried out in a moving and stationary frame of reference, converted to 3D measurements via the use of Taylor’s hypothesis. The origin of the ‘kidney’ shape and large degree of spatial order in the far field was found to be a result of an organised ‘train’ of consecutive hairpin, roller and wake structures. Together, these structures provide a physical explanation that reconciles the statistical and instantaneous structure of the CVP.
17
McManus, Thomas Andrew. "An Experimental Investigation of the Relationship between Flow Turbulence and Temperature Fields in Turbulent Non-premixed Jet Flames." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1558370669358194.
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18
Catlett, Matthew Ryan. "Flow Induced Noise from Turbulent Flow over Steps and Gaps." Thesis, Virginia Tech, 2010. http://hdl.handle.net/10919/32926.
Повний текст джерелаАнотація:
The existence of small surface discontinuities on a flow surface generate significant pressure fluctuations which can manifest as radiated far field sound and affect the fluctuating near wall pressure field exerted on the flow surface. A significant amount
of research has been performed on various step and gap flows; however few have dealt with step heights that are small relative to the incoming boundary layer. Fewer still have been concerned with measuring the effect on the fluctuating wall pressure field or the radiated far field sound from these small surface discontinuities. This study presents the work aimed at scaling the radiated sound from small forward and backward steps, detailing the surface pressure field as a result of these steps, and detailing the far field sound radiated from gap configurations of similar dimension. These measurements were
performed in the Virginia Tech Anechoic Wall Jet facility for step heights that ranged from approximately 10% to 100% of the incoming boundary layer height. The results show the influence of step height and boundary layer velocity on the far field sound from forward and backward steps. Very little directivity is seen for either source and the larger step heights considered in this study are shown to not be acoustically compact. A new
mixed scaling normalization is proposed for the far field spectra from both types of step, which is shown to reliably collapse the data. Backward steps are shown to be much weaker producers of far field sound than a similarly sized forward step. The implications of this behavior are discussed with respect to the far field sound measured from various gap flows. The fluctuating wall pressure field was measured upstream and downstream of both step configurations. The data shows a slow recovery of the wall pressure field with lasting disturbances up to 100 step heights downstream of the step feature.Master of Science
19
Gan, Subhadeep. "Active Separation Control of High-Re Turbulent Separated Flow over a Wall-Mounted Hump using RANS, DES, and LES Turbulence Modeling Approaches." University of Cincinnati / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1275924069.
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Crook, Andrew. "The control of turbulent flows using synthetic jets." Thesis, University of Manchester, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488311.
Повний текст джерелаАнотація:
Flow control in the modern meaning of the term, is a technology that enables gains in performance, greater than that achievable using conventional design tools. To maximise the potential of this technology, forms of actuation have to be developed that require low power, do not modify the structure of the vehicle considerably, are reliable and above all efficient and effective in controlling the flowfield. A promising form of actuation is known as the synthetic or massless jet, so called because for an axi-symmetric orifice, a jet is formed from a train of vortex rings, with zero net mass flux. The synthetic jet actuator often consists of an orifice plate from which the vortex rings are formed, mounted to a cavity with a diaphragm at the other end. The periodic oscillation of the diaphragm causes the roll-up of a vortex ring at the orifice exit, that for sufficient levels of forcing, convects away under its self-induced velocity, before the next suction stroke commences. The present research is focused upon the fundamental understanding of the structure of an axi-symmetric synthetic jet embedded in a turbulent boundary layer, with the aim of understanding how non-dimensional parameters of practical importance affect the structure and the dynamics of the synthetic jet formation. A series of basic experiments that gradually added the salient features of an embedded synthetic jet were undertaken, and demonstrated that parameters such as the jet velocity ratio and Strouhal number have a large effect upon the dynamics and structure of a synthetic jet subjected to quiescent conditions, a cross-flow and shear in the form of a turbulent boundary layer. A model of a synthetic jet embedded in a boundary layer has been proposed based upon the results of these experiments, and hypothesises that the ejected vortex rings form two periodic counter-rotating streamwise vortical structures, provided that the vortex rings do not penetrate beyond the boundary layer. The effectiveness of synthetic jets in delaying the separation of a turbulent boundary layer from a circular cylinder by up to 5% in azimuth, was demonstrated. Surface flow visualisation provided further evidence of the presence of two periodic counter-rotating streamwise structures, and that three-dimensionality of the boundary layer near to separation, has a large effect on the structure of the synthetic jet and its interaction with the separation.
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Hayes-McCoy, Declan. "Direct computations of a synthetic jet actuator." Thesis, Brunel University, 2012. http://bura.brunel.ac.uk/handle/2438/6454.
Повний текст джерелаАнотація:
Synthetic jet actuators have previously been defined as having potential use in both internal and external aerodynamic applications. The formation of a jet flow perpendicular to the surface of an aerofoil or in a duct of diffuser has a range of potential flow control benefits. These benefits can include both laminar to turbulent transition control, which is associated with a drag reduction in aerodynamic applications. The formation and development of zero-net-mass-flux synthetic jets are investigated using highly accurate numerical methods associated with the methodology of Direct Numerical Simulation (DNS). Jet formation is characterised by an oscillating streamwise jet centreline velocity, showing net momentum flux away from the jet orifice. This momentum flux away from the orifice takes the form of a series of vortex structures, often referred to as a vortex train. Numerical simulations of the synthetic jet actuator consist of a modified oscillating velocity profile applied to a wall boundary. The Reynolds numbers used vary from 85 ≤ Re ≤ 300. A complete numerical study of both axisymmetric and fully three-dimensional jet flow is performed. A parametric axisymmetric simulation is carried out in order to study the formation criterion and evolution of zero-net-mass-flux synthetic jets under variations in actuator input parameters. From the results of these simulations the conditions necessary for the formation of the synthetic jet along with the input parameters that provide an optimal jet output are deduced. Jet optimisation is defined by the mass flow, vortex strength and longevity of the vortex train as it travels downstream. Further investigations are carried out on a fully three-dimensional DNS version of the optimised axisymmetric case. Comparisons between the jet evolution and flow-field structures present in both the axisymmetric and three-dimensional configurations are made. This thesis examines the vortex structures, the jet centreline velocities along with time dependent and time averaged results in order to deduce and visualise the effects of the input parameters on the jet formation and performance. The results attained on altering the oscillation frequency of the jet actuator indicated that synthetic jets with zero mean velocity at the inflow behave significantly differently from jets with non-zero mean velocity at the inflow. A study into the evolution and formation of the train of vortex structures associated with the formation of a synthetic jet is performed. This study is accompanied with a series of time averaged results showing time dependent flow-field trends. The time history of the jet centreline velocity, showing the net momentum flux of the fluid away from the orifice of a fully developed synthetic jet, is analysed for both axisymmetric and three-dimensional cases. Differences in the fluid dynamics between the idealised axisymmetric configuration and the three-dimensional case have been identified, where three-dimensional effects are found to be important in the region near the jet nozzle exit. The effect of a disturbance introduced into the three-dimensional simulation in order to break its inherent symmetr around the jet centreline is examined by altering the input frequency of the disturbance. It was found that the effect of this relatively minor disturbance had a major effect on the jet flow field in the region adjacent to the orifice. The effect of which was deemed to be caused by discontinuities in the surface of the jet orifice due to manufacturing tolerances. Although the effects of these disturbances on the jet flow-field are large, they seem to have been neglected from numerical simulations to date. The effect of a synthetic jet on an imposed cross-streamwise velocity profile was examined. It was found that the synthetic jet flow-field resulted in a deformation of the velocity profile in the region downstream of the synthetic jet. It is suggested that this region of deformed flow could interact with coherent structures in a transitional boundary layer in order to delay flow transition to turbulence. The effect of varying the Strouhal number of a synthetic jet in a cross-flow is also analysed. It is clear from the results presented that, in the presence of a cross-flow velocity the Strouhal number effect on the synthetic jet flow field evolution, while dominant in a quiescent fluid is surpassed by the effect of the cross-flow.
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Tabrizi, Seyed Pariviz Alavi. "Jet impingement onto a circular cylinder." Thesis, University of Liverpool, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.263841.
Повний текст джерела
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Lögdberg, Ola. "Turbulent Boundary Layer Separation and Control." Doctoral thesis, KTH, Linné Flow Center, FLOW, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-9821.
Повний текст джерелаАнотація:
Boundary layer separation is an unwanted phenomenon in most technical applications, as for instance on airplane wings, ground vehicles and in internal flow systems. If separation occurs, it causes loss of lift, higher drag and energy losses. It is thus essential to develop methods to eliminate or delay separation.In the present experimental work streamwise vortices are introduced in turbulent boundary layers to transport higher momentum fluid towards the wall. This enables the boundary layer to stay attached at larger pressure gradients. First the adverse pressure gradient (APG) separation bubbles that are to be eliminated are studied. It is shown that, independent of pressure gradient, the mean velocity defect profiles are self-similar when the scaling proposed by Zagarola and Smits is applied to the data. Then vortex pairs and arrays of vortices of different initial strength are studied in zero pressure gradient (ZPG). Vane-type vortex generators (VGs) are used to generate counter-rotating vortex pairs, and it is shown that the vortex core trajectories scale with the VG height h and the spanwise spacing of the blades. Also the streamwise evolution of the turbulent quantities scale with h. As the vortices are convected downstream they seem to move towards a equidistant state, where the distance from the vortex centres to the wall is half the spanwise distance between two vortices. Yawing the VGs up to 20° do not change the generated circulation of a VG pair. After the ZPG measurements, the VGs where applied in the APG mentioned above. It is shown that that the circulation needed to eliminate separation is nearly independent of the pressure gradient and that the streamwise position of the VG array relative to the separated region is not critical to the control effect. In a similar APG jet vortex generators (VGJs) are shown to as effective as the passive VGs. The ratio VR of jet velocity and test section inlet velocity is varied and a control effectiveness optimum is found for VR=5. At 40° yaw the VGJs have only lost approximately 20% of the control effect. For pulsed VGJs the pulsing frequency, the duty cycle and VR were varied. It was shown that to achieve maximum control effect the injected mass flow rate should be as large as possible, within an optimal range of jet VRs. For a given injected mass flow rate, the important parameter was shown to be the injection time t1. A non-dimensional injection time is defined as t1+ = t1Ujet/d, where d is the jet orifice diameter. Here, the optimal t1+ was 100-200.QC 20100825
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Pasumarti, Venkata-Ramya. "Large eddy simulation of heated pulsed jets in high speed turbulent crossflow." Thesis, Georgia Institute of Technology, 2010. http://hdl.handle.net/1853/37291.
Повний текст джерелаАнотація:
The jet-in-crossflow problem has been extensively studied, mainly because of its applications in film cooling and injector designs. It has been established that in low-speed flows, pulsing the jet significantly enhances mixing and jet penetration.
This work investigates the effects of pulsing on mixing and jet trajectory in high speed (compressible) flow, using Large Eddy Simulation. Jets with different density ratios, velocity ratios and momentum ratios are pulsed from an injector into a crossflow.
Density ratios used are 0.55 (CH4/air), 1.0 (air/air) and 1.5 (CO2/air). Results are compared with the low speed cases studied in the past and then analyzed for high speed scaling. The simulations show that the lower density jet develops faster than a
higher density jet. This results in more jet spread for the lower density jet. Scaling for jet spread and the decay of centerline jet concentration for these cases are established, and variable density scaling law is developed and used to predict jet penetration in the far field.
In most non-premixed combustor systems, the fuel and air being mixed are at different initial temperatures and densities. To account for these effects, heated jets at temperatures equal to 540K and 3000K have been run. It has been observed that, in addition to the lower density of heated jets, the higher kinematic viscosity effects the jet penetration. This effect has been included and validated in the scaling law for the heated jet trajectory.
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Landel, Julien Rémy Dominique Gérard. "Dynamics of quasi-two-dimensional turbulent jets." Thesis, University of Cambridge, 2012. https://www.repository.cam.ac.uk/handle/1810/244569.
Повний текст джерелаАнотація:
The study of quasi-two-dimensional turbulent jets is relevant to chemical reactors, the coking process in oil refinement, as well as rivers flowing into lakes or oceans. In the event of a spillage of pollutants into a river, it is critical to understand how these agents disperse with the flow in order to assess damage to the environment. For such flows, characteristic streamwise and cross-stream dimensions can be much larger than the fluid-layer thickness, and so the flow develops in a confined environment. When the distance away from the discharge location is larger than ten times the fluid-layer thickness, the flow is referred to as a quasi-two-dimensional jet. From experimental observations using dyed jets and particle image velocimetry, we find that the structure of a quasi-two-dimensional jet consists of a high-speed meandering core with large counter-rotating eddies developing on alternate sides of the core. The core and eddy structure is self-similar with distance from the discharge location. The Gaussianity of the cross-stream distribution of the time-averaged velocity is due, in part, to the sinuous instability of the core. To understand the transport and dispersion properties of quasi-two-dimensional jets we use a time-dependent advection--diffusion equation, with a mixing length hypothesis accounting for the turbulent eddy diffusivity. The model is supported by experimental releases of dye in jets or numerical releases of virtual passive tracers in experimentally-measured jet velocity fields. We consider the statistical properties of this flow by releasing and then tracking large clusters of virtual particles in the jet velocity field. The probability distributions of two-point properties (such as the distance between two particles) reveal large streamwise dispersion. Owing to this streamwise dispersive effect, a significant amount of tracers can be transported faster than the speed predicted by a simple advection model. Using potential theory, we determine the flow induced by a quasi-two-dimensional jet confined in a rectangular domain. The streamlines of the induced flow predicted by the theory agree with experimental measurements away from the jet boundary. Finally, we investigate the case of a quasi-two-dimensional particle-laden jet. Depending on the bulk concentration of dense particles, we identify different flow regimes. At low concentrations, the jet features the same core and eddy structure observed without the particles, and thus quasi-two-dimensional jet theory can apply to some extent. At larger concentrations, we observe an oscillating instability of the particle-laden jet.
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YAMAMOTO, Kazuhiro, Satoshi INOUE, Hiroshi YAMASHITA, Daisuke SHIMOKURI, and Satoru ISHIZUKA. "Flow Field of Turbulent Premixed Combustion in a Cyclone-Jet Combustor." The Japan Society of Mechanical Engineers, 2007. http://hdl.handle.net/2237/9384.
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Johnstone, Henry Webb 1956. "CONFINED JET-INDUCED MIXING AT A DENSITY INTERFACE (TURBULENT, SHEAR FLOW)." Thesis, The University of Arizona, 1987. http://hdl.handle.net/10150/292003.
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Semlitsch, Bernhard. "Large Eddy Simulation of Turbulent Compressible Jets." Doctoral thesis, KTH, Mekanik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-156230.
Повний текст джерелаАнотація:
Acoustic noise pollution is an environmental aggressor in everyday life. Aero- dynamically generated noise annoys and was linked with health issues. It may be caused by high-speed turbulent free flows (e.g. aircraft jet exhausts), by airflow interacting with solid surfaces (e.g. fan noise, wind turbine noise), or it may arise within a confined flow environment (e.g. air ventilation systems, refrigeration systems). Hence, reducing the acoustic noise levels would result in a better life quality, where a systematic approach to decrease the acoustic noise radiation is required to guarantee optimal results. Computational predic- tion methods able to provide all the required flow quantities with the desired temporal and spatial resolutions are perfectly suited in such application areas, when supplementing restricted experimental investigations. This thesis focuses on the use of numerical methodologies in compressible flow applications to understand aerodynamically noise generation mechanisms and to assess technologies used to suppress it. Robust and fast steady-state Reynolds Averaged Navier-Stokes (RANS) based formulations are employed for the optimal design process, while the high fidelity Large Eddy Simulation (LES) approach is utilized to reveal the detailed flow physics and to investigate the acoustic noise production mechanisms. The employment of fast methods on a wide range of cases represents a brute-force strategy used to scrutinize the optimization parameter space and to provide general behavioral trends. This in combination with accurate simulations performed for particular condi- tions of interest becomes a very powerful approach. Advance post-processing techniques (i.e. Proper Orthogonal Decomposition and Dynamic Mode Decomposition) have been employed to analyze the intricate, highly turbulent flows. The impact of using fluidic injection inside a convergent-divergent nozzle for acoustic noise suppression is analyzed, first using steady-state RANS simulations. More than 250 cases are investigated for the optimal injection location and angle, amount of injected flow and operating conditions. Based on a-priori established criteria, a few optimal candidate solutions are detected from which one geometrical configuration is selected for being thoroughly investigated by using detailed LES calculations. This allows analyzing the unsteady shock pattern movement and the flow structures resulting with fluidic injec- tion. When investigating external fluidic injection configurations, some lead to a high amplitude shock associated noise, so-called screech tones. Such unsteady phenomena can be captured and explained only by using unsteady simulations. Another complex flow scenario demonstrated using LES is that of a high ve- locity jet ejected into a confined convergent-divergent ejector (i.e. a jet pump). The standing wave pattern developed in the confined channel and captured by LES, significantly alters the acoustic noise production. Steady-state methods failed to predict such events. The unsteady highly resolved simulations proved to be essential for analyzing flow and acoustics phenomena in complex problems. This becomes a very powerful approach when is used together with steady-state, low time-consuming formulations and when complemented with experimental measurements.QC 20141202
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Shevade, Shantanu S. "Simulation of Turbulent Air Jet Impingement for Commercial Cooking Applications." Scholar Commons, 2018. https://scholarcommons.usf.edu/etd/7362.
Повний текст джерелаАнотація:
The research work in this dissertation focuses on turbulent air jet heat transfer for commercial cooking applications.
As a part of this study, convective heat transfer coefficient and its interdependency with various key parameters is analyzed for single nozzle turbulent jet impingement. Air is used as the working fluid impinging on the flat surface. A thorough investigation of velocity and temperature distributions is performed by varying nozzle velocity and height over diameter ratio (H/D). Nusselt number and Turbulent Energy are presented for the impingement surface. It was found that for H/D ratios ranging between 6 and 8, nozzle velocities over 20 m/s provide a large percentage increase in heat transfer.
Single nozzle jet impingement is followed by study of turbulent multi-jet impingement. Along with parameters mentioned above, spacing over diameter ratio (S/D) is varied. Convective heat transfer coefficient, average impingement surface temperature and heat transfer rate are calculated over the impingement surface. It was found that higher S/D ratios result in higher local heat transfer coefficient values near stagnation point. However, increased spacing between the neighboring jets results in reduced coverage of the impingement surface lowering the average heat transfer. Lower H/D ratios result in higher heat transfer coefficient peaks. The peaks for all three nozzles are more uniform for H/D ratios between 6 and 8. For a fixed nozzle velocity, heat transfer coefficient values are directly proportional to nozzle diameter. For a fixed H/D and S/D ratio, heat transfer rate and average impingement surface temperature increases as the nozzle velocity increases until it reaches a limiting value. Further increase in nozzle velocity causes drop in heat transfer rate due to ingress of large amounts of cold ambient air in the control volume.
The final part of this dissertation focuses on case study of conveyor oven. Lessons learned from analysis of single and multi-jet impingement are implemented in the case study. A systematic approach is used to arrive to an optimal configuration of the oven. As compared to starting configuration, for optimized configuration the improvement in average heat transfer coefficient was 22.7%, improvement in average surface heat flux was 24.7% and improvement in leakage air mass flow rate was 59.1%.
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Debugne, Antoine Louis René. "A study of round, line-like and meandering turbulent fountains." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/284881.
Повний текст джерелаАнотація:
The dynamics of different classes of turbulent and miscible fountains are stud- ied: from classic axisymmetric fountains issuing from round sources, to confined fountains propagating in a quasi-two-dimensional environment, to line fountains which form when release conditions are approximately two-dimensional at the source. Each class is characterised by distinct dynamical behaviour, which this the- sis analyses both through theoretical arguments and experimental measurements. A model for the entrainment of ambient fluid into a fluctuating fountain top is developed and implemented into a first complete description for round fountains. The solutions of the resulting 'three-region-model' lie in improved agreement with available data and, uniquely, do not diverge near the top of the fountain. Next, con- fined fountains (unexplored to date) are classified into four flow regimes and their behaviour collapsed according to a single governing parameter that captures the severity of confinement. Finally, new experiments on line foutains shed light on the quasi-steady structure of these flows, revealing (and motivating) a strong con- nection between their motion in the vertical and lateral planes. Round, confined and line fountains are then contrasted in the conclusions, where we reflect on what is required to progress towards a unified theory of turbulent fountains.
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Cresswell, R. W. "An experimental study of a turbulent jet in which buoyancy acts against initial momentum." Thesis, Open University, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.233227.
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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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Shipton, Jemma. "Balance, gravity waves and jets in turbulent shallow water flows /." St Andrews, 2008. http://hdl.handle.net/10023/708.
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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.
Повний текст джерелаАнотація:
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.
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Cutbill, Sue. "A study of the turbulent flow of a high speed Coanda jet." Thesis, Durham University, 1998. http://etheses.dur.ac.uk/4743/.
Повний текст джерелаАнотація:
This thesis presents an experimental investigation into a compressible turbulent wall jet issuing from a slot, and flowing over a surface with streamwise curvature, followed by a plane wall recovery region. The purpose of this data was to provide suitable test cases to aid in the design and validation of turbulence models used for curved flow situations in computational fluid dynamics. The presence of streamwise curvature provides an extra rate of strain to the flow which effects both the mean flow field and the turbulence structure. The effects of curvature are dependent on the ratio of the slot width to the radius of curvature. The effects are increased with the magnitude of this ratio. Hot film anemometry was used to measure the mean flow and Reynolds stresses under six different flow regimes. Four experiments were performed using a constant slot to radius ratio, and the supply pressure ratio was varied. Two further experiments were performed at a constant pressure ratio, but at differing slot to radius ratios. In all cases, the extra rate of strain was found to increase the jet growth and velocity decay rates beyond those of a plane wall jet. 'History effects' were apparent where there was a change from flow with streamwise wall curvature to that without. The turbulence structure was found to require a finite distance along the recovery section before it reverts to a plane wall turbulence structure. The presence of the extra rate of strain, and the history effects at sudden changes in surface curvature, make strong demands on any turbulence model. Various turbulence models have been tested, and although no one turbulence model has proved robust enough for all flow situations, a number of models have been identified to provide superior performance under certain conditions. The use of a higher order numerical scheme has also been found to reduce the effects of numerical diffusion, leading to improvements in the predictions of the flow shock cell structure and the breakaway performance of the jet.
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Landers, Brian D. "Mixing Characteristics of Turbulent Twin Impinging Axisymmetric Jets at Various Impingement Angles." University of Cincinnati / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1467987856.
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Al-Hussyni, Saad Kohel Ali. "Numerical study of turbulent plane jets in still and flowing environments employing two-equation k-ε model". Thesis, University of Edinburgh, 1987. http://hdl.handle.net/1842/11065.
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Bray, Tim P. "A Parametric Study of Vane and Air-jet Vortex Generators." Thesis, Cranfield University, 1998. http://hdl.handle.net/1826/3569.
Повний текст джерелаАнотація:
An experimental parametric sturdy of vane and air-jet vortex generators in a turbulent boundary
layer has been carried out. Experiments were carried out in two facilities, one with a free-stream
velocity of 20 m/s and a boundary layer thickness (6) of 41.5 mm, and one in a high speed facility
at free-stream Mach numbers of between 0.45 and 0.75 and a boundary layer thickness of 20 mm.
Cross-stream data were measured at a number of downstream locations using a miniature five-hole
pressure probe, such that local cross-stream velocity vectors could be derived. Streamwise
vorticity was calculated using the velocity vector data.
In the low speed study, vortex generator parameters were as follows:
" Vane vortex generators: thin rectangular vanes with a vane aspect ratio of unity (2h/c = 1),
free-stream velocity 20 m/s, incidence (cc = 10', 15', 18', 20'), height-to-boundary- layer-
thickness-ratio (h/8 0.554,0.916,1.27,1.639), and strearnwise distance from the vortex
generator (x/6 = 3.855,12.048,19.277,26.506).
" Air-jet vortex generators: circular jet nozzles, free-stream velocity = 20 m/s, jet nozzle pitch
and skew angles (cc, P= 30', 45', 60'), hole diameter-to-boundary-layer-thickness-ratio (D/5 =
0.098,0.193,0.289), jet-to-free-stream-velocity ratio (VR = 0.7,1.0,1.3,1.6,2.0), and
strearnwise distance from the vortex generator (x/8 = 3.855,12.048,19.277,26.506).
In the high-speed study, the vortex generator parameters were as follows:
Vane vortex generators: thin rectangular vanes with an aspect ratio of unity, incidence ((X
1505 20'), he i ght-to- boundary- I ayer-th i ckne s s-rati o (h/8 = 0.75), strearnwise distance from the
vortex generator (x/6 = 8.755 16.25,23.75), and free-stream Mach numbers of 0.45,0.6 and
0.75.
Air-jet vortex generators: jet pitch ((x = 30', 45'), jet skew angle (P = 30', 45', 60'), hole
diameter-to-boundary-layer-thickness-ratio (D/8 = 0.15,0.3), j et-to- free- strearn-ve loc ity ratio
(VR = 1.6), and strearnwise distance from the vortex generator (x/6 = 8.75,16.25,23.75,
31.25), and free-stream Mach numbers of 0.50,0.6 and 0.75.
Streamwise vorticity data from the experiment was used to generate prediction techniques that
would allow the vorticity profiles, downstream of vane or air-jet vortex generators, to be predicted.
Both techniques are based on the approximation of the experimental cross-stream vorticity data to
Gaussian distributions of vorticity through the vortex centre. The techniques, which are
empirically derived, are simple equations that give the peak vorticity and vortex radius based on
the vortex generator parameters. Use of these descriptors allows the assembly of the Gaussian
vorticity equation.
Both techniques are compared with the experimental data set and were seen to produce peak
vorticity results to within 12% and 20% (for the vanes and air-jets respectively), 15% for the
radius of the vortex, and 15% and 20% in vortex circulation (for the vanes and air-jets
respectively). The two simple prediction techniques allow good prediction of the vortex structure
at extremely low computational effort.
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Riley, Benjamin Matthew. "Magnetohydrodynamic lattice Boltzmann simulations of turbulence and rectangular jet flow." [College Station, Tex. : Texas A&M University, 2007. http://hdl.handle.net/1969.1/ETD-TAMU-1222.
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Gampert, Markus Matthias [Verfasser]. "Decomposition of the scalar field in a turbulent jet flow / Markus Matthias Gampert." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2013. http://d-nb.info/1037326733/34.
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Gampert, Markus [Verfasser]. "Decomposition of the scalar field in a turbulent jet flow / Markus Matthias Gampert." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2013. http://nbn-resolving.de/urn:nbn:de:hbz:82-opus-45049.
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Ball, Stephen. "Near wall flow characteristics in jet impingement heat transfer." Thesis, Nottingham Trent University, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.388866.
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Xia, Liping. "An experimental and numerical investigation of a turbulent round jet issuing into an unsteady crossflow /." Hong Kong : University of Hong Kong, 1998. http://sunzi.lib.hku.hk/hkuto/record.jsp?B19605377.
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Ward, Duncan Estcourt. "The two-phase plane turbulent mixing layer /." Title page, contents and abstract only, 1986. http://web4.library.adelaide.edu.au/theses/09PH/09phw257.pdf.
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Bocquet, Sébastien. "Modélisation de paroi et injection de turbulence pariétale pour la Simulation des Grandes Echelles des écoulements aérothermiques." Phd thesis, Toulouse, INPT, 2013. http://oatao.univ-toulouse.fr/10307/1/bocquet.pdf.
Повний текст джерелаАнотація:
Lors du développement d’un nouvel avion, l’estimation des échanges d’énergie entre l’air ambiant et les parois est une donnée cruciale pour la conception aérothermique. Cette conception repose de plus en plus sur des simulations numériques mais certains phénomènes d’aérothermique externe, comme le jet débouchant du système de dégivrage des nacelles moteur, montrent les limites des modèles RANS classiques. La simulation des grandes échelles (LES) se révèle bien adaptée à ce type de phénomène mais se heurte à un coût de calcul extrêmement élevé pour ces écoulements pariétaux à très grand nombre de Reynolds. Pour lever cette limitation, cette thèse propose l’étude de deux briques fondamentales : la LES avec loi de paroi (WMLES) conjuguée à l’injection d’une couche limite turbulente à l’entrée du domaine. Pour une meilleure compréhension et une utilisation fiable de l’approche loi de paroi, on se concentre tout d’abord sur les sources d’erreur qui lui sont associées. Après les avoir identifiées, on propose une correction de l’erreur de sous-maille ainsi qu’une loi de paroi adaptée aux écoulements compressibles. Grâce à ces deux éléments, on obtient une estimation correcte du flux de chaleur pariétal sur des simulations WMLES de canal plan supersonique sur parois froides. Puis, pour préparer la transition vers des applications plus industrielles, on introduit un schéma numérique plus dissipatif ce qui nous permet d’étudier l’influence de la méthode numérique sur l’approche loi de paroi. Dans une seconde partie dédiée à l’injection de couche limite pour la WMLES, on sélectionne une méthode basée sur l’injection de perturbations combinée à un terme de contrôle volumique. On montre que des simulations WMLES utilisant cette méthode d’injection permettent d’établir une couche limite turbulente réaliste à une courte distance en aval du plan d’entrée, à la fois sur une plaque plane mais également sur un écoulement de jet débouchant à la géométrie plus complexe, représentative d’un cas avion.
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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.
Повний текст джерелаАнотація:
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
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Grissom, Dustin Leonard. "A Study of Sound Generated by a Turbulent Wall Jet Flow Over Rough Surfaces." Diss., Virginia Tech, 2007. http://hdl.handle.net/10919/28336.
Повний текст джерелаАнотація:
The far field acoustics generated by turbulent flow over rough surfaces has been experimentally investigated in an acoustically treated wall jet facility. The facility allows direct measurement of the far field sound from small patches of surface roughness, without contamination from edge or other aerodynamic noise sources. The facility is capable of generating turbulent boundary layer flows with momentum thickness Reynolds numbers between 450 and 1160. The variation of surface conditions tested cover the range from hydrodynamically smooth surfaces through most of the transitional range, with h+ variations from 3 to 85. Single microphone narrow band acoustic spectra, measured in the far field, show sound levels as much as 15 dB above the background from 0.186 m2 roughness patches. The measurements revealed the spectral shape and level variations with flow velocity, boundary layer thickness, and roughness size; providing the first data set large enough to assess the affects of many aerodynamic properties on the acoustic spectra. Increases in the size of grit type roughness produced significant increases in acoustic levels. Patches of hydrodynamically smooth roughness generated measurable acoustic levels, confirming that acoustic scattering is at least one of the physical mechanisms responsible for roughness noise. The shapes of the measured spectra show a strong dependence on the form of the surface roughness. The acoustic spectra generated by periodic two-dimensional surfaces have a much narrower louder peak than that generated by three-dimensional grit type roughness. Measurements also show the orientation of the two-dimensional surface significantly affects the acoustic levels and directivity.
The variation of sound levels with flow velocity and roughness size suggests the acoustic field is significantly affected by changes in the near wall flow due to the presence of the roughness. Current models of noise generated by rough surfaces predict the general trends seen in measurements for flows over grit and two-dimensional roughness in the range of 20
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山本, 和弘, Kazuhiro YAMAMOTO, 泰樹 西澤 та Yasuki NISHIZAWA. "強乱流予混合火炎の流れ場と構造". 日本機械学会, 2002. http://hdl.handle.net/2237/9347.
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夏麗萍 and Liping Xia. "An experimental and numerical investigation of a turbulent round jet issuing into an unsteady crossflow." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1998. http://hub.hku.hk/bib/B31237897.
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Tasar, Gursu. "Experimental Investigation Of Near And Far Field Flow Characteristics Of Circular And Non-circular Turbulent Jets." Master's thesis, METU, 2008. http://etd.lib.metu.edu.tr/upload/12610256/index.pdf.
Повний текст джерелаАнотація:
The atomization problem of high speed viscous jets has many applications in industrial processes and machines. In all these applications, it is required that the droplets have high surface area/volume ratio meaning that the droplets should be as small as possible. This can be achieved with high rates of turbulence and mixing of the flow. In order to constitute a foresight of geometry eects on droplet size, experimental investigation and the determination of flow characteristics in near and far fields of a low-speed air jet have been performed. In order to fulfill this task, three components of instantaneous velocity are measured, using a triple sensor Constant Temperature Anemometer (CTA) system. Through these measurements, mean velocity, Reynolds stress, velocity decay, spreading rate, turbulent kinetic energy, vorticity, and mass entrainment rate values are obtained. Stress-Strain relationship is also observed. Measurements are obtained for a baseline circular nozzle (round jet) as well as for an equilateral triangular and a square nozzle. On the basis of these measurements, the equilateral triangular jet is found to be the best option in order to get highest turbulence and mixing level with smallest core length.