Wall transpiration, in the form of wall-normal suction or blowing through a permeable wall, is a relatively simple and effective technique to control the behaviour of a boundary layer. For its potential applications for laminar-turbulent transition and separation delay (suction) or for turbulent drag reduction and thermal protection (blowing), wall transpiration has over the past decades been the topic of a significant amount of studies. However, as far as the turbulent regime is concerned, fundamental understanding of the phenomena occurring in the boundary layer in presence of wall transpiration is limited and considerable disagreements persist even on the description of basic quantities, such as the mean streamwise velocity, for the rather simplified case of flat-plate boundary-layer flows without pressure gradients. In order to provide new experimental data on suction and blowing boundary layers, an experimental apparatus was designed and brought into operation. The perforated region spans the whole 1.2 m of the test-section width and with its streamwise extent of 6.5 m is significantly longer than previous studies, allowing for a better investigation of the spatial development of the boundary layer. The quality of the experimental setup and measurement procedures was verified with extensive testing, including benchmarking against previous results on a canonical zero-pressure-gradient turbulent boundary layer (ZPG TBL) and on a laminar asymptotic suction boundary layer. The present experimental results on ZPG turbulent suction boundary layers show that it is possible to experimentally realize a turbulent asymptotic suction boundary layer (TASBL) where the boundary layer mean-velocity profile becomes independent of the streamwise location, so that the suction rate constitutes the only control parameter. TASBLs show a mean-velocity profile with a large logarithmic region and without the existence of a clear wake region. If outer scaling is adopted, using the free-stream velocity and the boundary layer thickness (δ99) as characteristic velocity and length scale respectively, the logarithmic region is described by a slope Ao=0.064 and an intercept Bo=0.994, independently from the suction rate (Γ). Relaminarization of an initially turbulent boundary layer is observed for Γ&gt;3.70×10−3. Wall suction is responsible for a strong damping of the velocity fluctuations, with a decrease of the near-wall peak of the velocity-variance profile ranging from 50% to 65% when compared to a canonical ZPG TBL at comparable Reτ. This decrease in the turbulent activity appears to be explained by an increased stability of the near-wall streaks. Measurements on ZPG blowing boundary layers were conducted for blowing rates ranging between 0.1% and 0.37% of the free-stream velocity and cover the range of momentum thickness Reynolds number 10000&lt;Reθ&lt;36000. Wall-normal blowing strongly modifies the shape of the boundary-layer mean-velocity profile. As the blowing rate is increased, the clear logarithmic region characterizing the canonical ZPG TBLs gradually disappears. A good overlap among the mean velocity-defect profiles of the canonical ZPG TBLs and of the blowing boundary layers for all the Re number and blowing rates considered is obtained when normalization with the Zagarola-Smits velocity scale is adopted. Wall blowing enhances the intensity of the velocity fluctuations, especially in the outer region. At sufficiently high blowing rates and Reynolds number, the outer peak in the streamwise-velocity fluctuations surpasses in magnitude the near-wall peak, which eventually disappears. / Genom att använda sig av genomströmmande ytor, med sugning eller blåsning, kan man relativt enkelt och effektivt påverka ett gränsskikts tillstånd. Genom sin potential att påverka olika strömningsfysikaliska fenomen så som att senarelägga både avlösning och omslaget från laminär till turbulent strömning (genom sugning) eller som att exempelvis minska luftmotståndet i turbulenta gränsskikt och ge kyleffekt (genom blåsning), så har ett otaligt antal studier genomförts på området de senaste decennierna. Trots detta så är den grundläggande förståelsen bristfällig för de strömningsfenomen som inträffar i turbulenta gränsskikt över genomströmmande ytor. Det råder stora meningsskiljaktigheter om de mest elementära strömningskvantiteterna, såsom medelhastigheten, när sugning och blåsning tillämpas även i det mest förenklade gränsskiktsfallet nämligen det som utvecklar sig över en plan platta utan tryckgradient. För att ta fram nya experimentella data på gränsskikt med sugning och blåsning genom ytan så har vi designat en ny experimentell uppställning samt tagit den i bruk.Den genomströmmande ytan spänner över hela bredden av vindtunnelns mätsträcka (1.2 m) och är 6.5 m lång i strömningsriktningen och är därmed betydligt längre än vad som använts i tidigare studier. Detta gör det möjligt att bättre utforska gränsskiktet som utvecklas över ytan i strömningsriktningen. Kvaliteten på den experimentella uppställningen och valda mätprocedurerna har verifierats genom omfattande tester, som även inkluderar benchmarking mot tidigare resultat på turbulenta gränsskikt utan tryckgradient eller blåsning/sugning och på laminära asymptotiska sugningsgränsskikt. De experimentella resultaten på turbulenta gränsskikt med sugning bekräftar för första gången att det är möjligt att experimentellt sätta upp ett turbulent asymptotiskt sugningsgränsskikt där gränsskiktets medelhastighetsprofil blir oberoende av strömningsriktningen och där sugningshastigheten utgör den enda kontrollparametern. Det turbulenta asymptotiska sugningsgränsskiktet visar sig ha en medelhastighetsprofil normalt mot ytan med en lång logaritmisk region och utan förekomsten av en yttre vakregion. Om man använder yttre skalning av medelhastigheten, med friströmshastigheten och gränsskiktstjockleken som karaktäristisk hastighet respektive längdskala, så kan det logaritmiska området beskrivas med en lutning på Ao=0.064 och ett korsande värde med y-axeln på Bo=0.994, som är oberoende av sugningshastigheten. Om sugningshasigheten normaliserad med friströmshastigheten överskrider värdet 3.70x10^-3 så återgår det ursprungligen turbulenta gränsskiktet till att vara laminärt. Sugningen genom väggen dämpar hastighetsfluktuationerna i gränsskiktet med upp till 50-60% vid direkt jämförelse av det inre toppvärdet i ett turbulent gränsskikt utan sugning och vid jämförbart Reynolds tal. Denna minskning av turbulent aktivitet verkar härstamma från en ökad stabilitet av hastighetsstråken närmast ytan. Mätningar på turbulenta gränsskikt med blåsning har genomförts för blåsningshastigheter mellan 0.1 och 0.37% av friströmshastigheten och täcker Reynoldstalområdet (10-36)x10^3, med Reynolds tal baserat på rörelsemängds-tjockleken. Vid blåsning genom ytan får man en stark modifiering av formen på hastighetesfördelningen genom gränsskiktet. När blåsningshastigheten ökar så kommer till slut den logaritmiska regionen av medelhastigheten, karaktäristisk för turbulent gränsskikt utan blåsning, att gradvis försvinna. God överens-stämmelse av medelhastighetsprofiler mellan turbulenta gränsskikt med och utan blåsning erhålls för alla Reynoldstal och blåsningshastigheter när profilerna normaliseras med Zagarola-Smits hastighetsskala. Blåsning vid väggen ökar intensiteten av hastighetsfluktuationerna, speciellt i den yttre regionen av gränsskiktet. Vid riktigt höga blåsningshastigheter och Reynoldstal så kommer den yttre toppen av hastighetsfluktuationer i gränsskiktet att överskrida den inre toppen, som i sig gradvis försvinner. / <p>QC 20171101</p>

L'objectif de la thèse est d'étudier les mécanismes d'allumage d'un noyau de flamme en écoulements laminaires et turbulents. Dans un premier temps, une étude préliminaire est consacrée à l'analyse du dispositif d'allumage par étincelle induite par laser et à l'étude de l'initiation du noyau de flamme en écoulement laminaire prémélangé. Dans un second temps, l'étude de l'allumage est réalisée en écoulement turbulent prémélangé, afin de mettre en évidence l'effet des fluctuations turbulentes de vitesse sur l'initiation de noyau de flamme. Enfin, dans le cas d'un écoulement turbulent nonprémélangé, l'influence du champ local et instantané de fraction de mélange sur l'allumage et le développement du noyau de flamme est analysée. / The aim of the Ph-D thesis is to study ignition mechanisms of a flame kernel in laminar and turbulent flows. First, a preliminary study is devoted to the analysis of the laser-induced spark ignition system and to the study of the flame kernel initiation in premixed laminar flow. Then, the study of the ignition is performed in turbulent premixed flow, to highlight the influence of velocity turbulent fluctuations on the flame kernel initiation. Finally, in turbulent non-premixed flows, the effect of the local and instantaneous mixture fraction on the flame kernel initiation and development is analyzed.

La problématique associée à ce travail de thèse concerne la détermination des causes de fatigue thermique dans des conduites hydrauliques. Ce travail porte plus particulièrement sur l’étude d’une zone de mélange orthogonale dans laquelle débouchent deux écoulements à différentes températures et présentant des régimes dynamiques turbulents. Ce type de configuration est utilisé afin de favoriser la baisse de température de l’écoulement (ou l’augmentation de température selon les cas). Pour des valeurs élevées du nombre de Reynolds et des écarts de températures des écoulements en confluence, la charge thermique engendrée par les fluctuations pariétales de température peut entraîner la formation de fissures dans les canalisations.Pour faire face à ce problème, les méthodes de dimensionnement des conduites hydrauliques mettent en évidence la nécessité de connaître la charge thermique imposée sur la paroi sensible. Le besoin attendu en termes de caractérisation dynamique et thermique de ces écoulements est à l’origine de ce projet de recherche qui s’attache ainsi à déterminer pour une configuration industrielle simplifiée, les phénomènes dynamiques et thermiques instationnaires et tridimensionnels à proximité de la paroi. Pour cela, l’étude s’organise en deux phases. Une approche expérimentale, faisant appel à la vélocimétrie par images de particules et la thermographie infrarouge, est mise en oeuvre dans un écoulement turbulent à la jonction entre une conduite de section rectangulaire et un piquage cylindrique. L’étude expérimentale permet de mesurer l’effet de différents paramètres de l’écoulement (régime dynamique, rapport des vitesses de l’écoulement, écart de température). Une des configurations expérimentales est de plus modélisée par simulations des grandes échelles dans laquelle le transfert thermique conjugué entre le fluide et la paroi d’intérêt est pris en compte. Les champs statistiques de vitesse et température issus du calcul numérique sont comparés à la base de données expérimentale. L’analyse des mécanismes thermodynamiques près de la paroi et dans la paroi, est de plus réalisée au moyen des grandeurs statistiques de l’écoulement, des corrélations de vitesse-température et de l’enregistrement temporel de sondes numériques placées dans l’écoulement et dans la paroi. / This research work originates from thermal stress issues in hydraulic pipes. It specifically focuses on determination of thermal load created by mixing flows in orthogonal pipe junctions. Such mixing zones are commonly used to cool or heat the main flow. For high Reynolds number values and high difference of temperature of the two flows, thermal load created by strong temperature fluctuations at the wall can initiate and propagate cracks through the pipe. Standards in pipe design pointed out the need in knowing these thermal loads applied at the downstream junction wall. Thus, this study aims to characterize three-dimensional and unsteady dynamical and thermal phenomena in a simplified industrial configuration. First, an experimental investigation based on particle image velocimetry and infrared thermography has been carried out on a cylindrical jet pipe flowing through a main rectangular channel flow. Different parameters have been studied such as Reynolds number influence, velocity ratio between secondary and main flow and temperature difference. One of the experimental configurations has also been investigated using numerical simulations. LargeEddy Simulations with conjugate heat transfer model has been developed and compared to the experimental database. Dynamics and Thermal mechanisms have been analysed near the wall and within the wall with obtained statistical flow fields, velocity-temperature moments and numerical probes extracted from fluid and solid parts.

External gear machines (EGMs) are used in a variety of industries ranging from fluid power machinery to fluid handling systems and fuel injection applications. Energy efficiency requirements and new trends in hydraulic technology necessitate the development of novel EGMs optimized for efficiency and reliability in all of these applications. A crucial piece in the novel EGM development process is a numerical model that can simulate the operation of EGM and predict its volumetric and hydro-mechanical performance.<div><br></div><div>The EGM simulation models developed in the past have focused mostly on the challenges related to the modeling of the theoretical behavior and elementary fluid dynamics, and determining appropriate modeling schemes. Key aspects related to the flow and geometry are either considered in a simplified manner or not considered at all. In particular, the current simulation models assume the fluid to be Newtonian and the leakage flows to be laminar. However, EGMs working in fluid handling applications operate with non-Newtonian fluids. Further, in fuel injection applications, due to low fluid viscosity and high operating speed, the internal leakage flows may not remain laminar.</div><div><br></div><div>With respect to the geometric aspects, the gears in EGMs are prone to manufacturing errors that are not accounted by any simulation model. In addition, there is no method available in the literature for accurately modeling the leakage flows through curve-constricted geometries in EGMs. Further, the goal of current simulation tools is related to the prediction of the volumetric performance of EGMs. However, an equally important characteristic, hydro-mechanical performance, is often ignored. Finally, the energy flow during EGM operation can result in the variation of the fluid temperature. Thus, the isothermal assumption of current simulation tools is another major limitation.</div><div><br></div><div>The work presented in this dissertation is focused on developing numerical methodologies for the modeling of EGMs that addresses all the aforementioned limitations of the current models. In this work, techniques for evaluating non-Newtonian internal flows in EGMs is developed to permit an accurate modelling of EGMs working with non-Newtonian fluids. For fuel injection EGMs, flow regime at the tooth tips of the gears is investigated and it is shown that the flow becomes turbulent for such EGMs. A methodology for modeling this turbulent flow is proposed and its impact on the performance of EGMs is described. To include gear manufacturing errors in the simulation model, numerical techniques are developed for modeling the effects of two common gear manufacturing errors: conicity and concentricity. These two errors are shown to have an opposite impact on the volumetric efficiency of the EGM. For the evaluation of flows through curve-constricted leakage paths in EGMs, a novel flow model is developed in this work that is applicable for a wide range of geometry and flow conditions. Modeling of the hydro-mechanical efficiency of EGMs is accomplished by developing methodologies for the evaluation of torque losses at key interfaces. Finally, to account for the thermal effects in EGMs, a thermal model is developed to predict the temperature distribution in the EGM and its impact on the EGM performance.</div><div><br></div><div><div>To validate the numerical methodologies developed in this work, several experiments are conducted on commercial gear pumps as well as on a custom apparatus designed and manufactured in the course of this research work. The results from the experiments are found to match those obtained from the simulations which indicates the validity of the methodologies developed in this work. </div><div><br></div><div>These numerical methodologies are based on the lumped parameter approach to allow the coupling with mechanical models for gear micromotion and permit fast computations so that the model can be used in optimization algorithms to develop energy efficient and reliable EGMs.</div><div><br></div><div>The methodologies described in the dissertation are useful for accurate analysis of a variety of EGMs working with different types of fluids and at wide range of operating conditions. This capability will be valuable for pump designers in developing novel better performing EGM designs optimized for various applications.</div><div><br></div></div>

Le transport de particules par des écoulements turbulents est un phénomène présent dans de nombreux écoulements naturels et industriels, tels que la dispersion de polluants dans l'atmosphère ou du phytoplancton et plastiques dans et à la surface des océans. Les modèles prédictifs classiques ne peuvent prévoir avec précision la formation de larges fluctuations de concentrations. La première partie de cette thèse concerne une étude de la dispersion turbulente de traceurs émis à partir d'une source ponctuelle et continue. Les fluctuations spatiales de masse sont déterminées en fonction de la distance à la source et à l'échelle d'observation. La combinaison de plusieurs phénomènes physiques à l'origine du mélange limite la validité d'une caractérisation de géométrie fractale. Une approche alternative est proposée, permettant d'interpréter les fluctuations massiques en terme des différents régimes de séparation de pair dans des écoulements turbulents. La seconde partie concerne des particules ayant une inertie finie, dont la dispersion dans l'espace des vitesses requiert de développer des techniques de modélisation adaptées. Une méthode numérique originale est proposée pour exprimer la distribution des particules dans l'espace position-vitesse. Cette méthode est ensuite utilisée pour décrire la modulation de la turbulence bi- dimensionnelle par des particules inertielles. A grand nombres de Stokes, l'effet montré est analogue à celui d'une friction effective à grande échelle. Aux petits Stokes, le spectre de l'énergie cinétique du fluide et les transferts non-linéaires sont modifiées d'une manière non triviale. / The transport of particles by turbulent flows is ubiquitous in nature and industry. It occurs in planet formation, plankton dynamics and combustion in engines. For the dispersion of atmospheric pollutants, traditional predictive models based on eddy diffusivity cannot accurately reproduce high concentration fluctuations, which are of primal importance for ecological and health issues. The first part of this thesis relates to the dispersion by turbulence of tracers continuously emitted from a point source. Mass fluctuations are characterized as a function of the distance from the source and of the observation scale. The combination of various physical mixing processes limits the use of fractal geometric tools. An alternative approach is proposed, allowing to interpret mass fluctuations in terms of the various regimes of pair separation in turbulent flows. The second part concerns particles with a finite and possibly large inertia, whose dispersion in velocity requires developing efficient modelling techniques. A novel numerical method is proposed to express inertial particles distribution in the position-velocity phase space. Its convergence is validated by comparison to Lagrangian measurements. This method is then used to describe the modulation of two-dimensional turbulence by large-Stokes-number heavy particles. At high inertia, the effect is found to be analogous to an effective large-scale friction. At small Stokes numbers, kinetic energy spectrum and nonlinear transfers are shown to be modified in a non-trivial way which relates to the development of instabilities at vortices boundaries.

No description available.

<p dir="ltr">The increasing availability of spatio-temporal information in the form of detailed time-resolved images sampled at very high framing rates has resulted in a search for mathematical techniques capable of extracting and relaying the pertinent underlying physics governing complex flows. Analysis relying on the usage of a solitary spectral, correlation, or modal decomposition techniques to identify dynamically significant information from large datasets may give an incomplete description of these phenomena. Moreover, fully resolved spatio-temporal measurements of these complex flow fields are needed for a complete and accurate description across a wide spectrum of length and time scales. The primary goals of this dissertation are address these challenges in two key aspects: (1) to improve and demonstrate the novel application of complementary data analysis and modal decomposition techniques to quantify the dynamics of flow systems that exhibit intricate patterns and behaviors in both space and time, and (2) to make advancements in achieving and characterizing high-resolution and high-speed quantitative measurements of turbulent mixing fields.</p><p dir="ltr">In the first goal, two canonical flow fields are considered, including an acoustically excited co-axial jet and a bluff-body stabilized flame. The local susceptibility of a nonreacting, cryogenic, coaxial-jet, rocket injector to transverse acoustics is characterized by applying dynamical systems theory in conjunction with complementary wavelet-based spectral decomposition to high-speed backlit images of flow field. The local coupling of the jet with external acoustics is studied as a function of the relative momentum flux ratio between the outer and inner jets, giving a quantitative description of the dynamical response of each jet to external acoustics as a function of the downstream distance from the nozzle.</p><p dir="ltr">Bluff bodies are a common feature in the design of propulsion systems owing to their ability to act as flame holders. The reacting wake behind the bluff body consists of a recirculation bubble laden with hot-products and wrapped between separated shear layers. The wake region of a bluff body is systematically investigated utilizing a technique known as robust dynamic mode decomposition (DMD) to discern the onset of the thermoacoustic instability mode, which is highly detrimental to aerospace propulsion systems. The approach enables quantification of the spatial distribution and behavior of coherent structures observed from different flows as a function of the equivalence ratio.</p><p dir="ltr">As modal decomposition techniques employ a certain degree of averaging in time, a novel space-and-time local filtering technique utilizing the well-defined characteristics of wavelets is introduced with a goal of temporally resolving the spatial evolution of irregular flow instabilities associated with specific frequencies. This provides insight into the existence of transient sub-modal characteristics representing intermittencies within seemingly stable modes. The flow fields obtained from the same two canonical flows are interrogated to demonstrate the utility of the technique. It has been shown that temporally resolved flow features obtained from wavelet filtering satisfactorily track the same modal featured derived from DMD, but reveal sub-modal spatial distortions or local non-stationarity of specific modal frequencies on a frame-by-frame basis.</p><p dir="ltr">Finally, to improve the ability to study the dynamical behavior of complex flows across the full range of spatio-temporal scales present, advancements are reported in the spatial and temporal quantitative measurement of the scalar quantities in turbulent mixing fields utilizing 100 kHz planar laser-induced fluorescence (PLIF) and Rayleigh scattering imaging of acetone. The imaging system provided a resolution of 55 µm with a field-of-view mapping of 18.5 µm/pixel on the camera sensor, which is three times better spatial resolution than the previous reported work to-date for similar flow fields that were investigated at 1/10^th the current measurement rate. The power spectra of instantaneous mixture fraction fluctuations adhere to Kolmogorov's well-established -5/3 law, showing that the technique captures a significant range of dissipation scales. This observation underscores the ability to study mixing dynamics throughout the turbulent by fully resolving scalar fluctuations up to 30 kHz. This enhanced spatio-temporal resolution allows for a more detailed investigation of the dynamical behavior of turbulent flows with complex modal interactions down to the smallest diffusion limited mixing scales.</p>

<p>This work is a five-article-based collection of published and to-be-published research articles that explore a novel combination of inlet conditions, wind turbine design, and non-flat topography by performing scaled-down experiments in a wind tunnel.</p>

No description available.

The interest towrds machine learning applied to turbulence has experienced a fast-paced growth in the last years. Thanks to deep-learning algorithms, flow-control stratigies have been designed, as well as tools to model and reproduce the most relevant turbulent features. In particular, the success of recurrent neural networks (RNNs) has been demonstrated in many recent studies and applications. The main objective of this project is to assess the capability of these networks to reproduce the temporal evolution of a minimal turbulent channel flow. We first obtain a data-driven model based on a modal decomposition in the Fourier domain (FFT-POD) on the time series sampled from the flow. This particular case of turbulent flow allows us to accurately simulate the most relevant coherent structures close to the wall. Long-short-term-memory (LSTM) networks and a Koopman-based framework (KNF) are trained to predict the temporal dynamics of the minimal channel flow modes. Tests with different configurations highlight the limits of the KNF method compared to the LSTM, given the complexity of the data-driven model. Long-term prediction for LSTM show excellent agreement from the statistical point of view, with errors below 2% for the best models. Furthermore, the analysis of the chaotic behaviour thorugh the use of the Lyapunov exponent and of the dynamic behaviour through Pointcaré maps emphasizes the ability of LSTM to reproduce the nature of turbulence. Alternative reduced-order models (ROMS), based on the identification of different turbulent structures, are explored and they continue to show a good potential in predicting the temporal dynamics of the minimal channel.