Turbulente Austauschprozesse in der atmosphärischen Grenzschicht spielen eine Schlüsselrolle beim vertikalen Impuls-, Energie- und Stofftransport in der Erdatmosphäre. In meso- und globalskaligen Atmosphärenmodellen sind turbulente Austauschprozesse jedoch subskalig und müssen unter Verwendung geeigneter Schliessungsansätze parametrisiert werden. Hierbei spielt die Spezifikation der charakteristischen Turbulenzlängenskala in Abhängigkeit vom Stabilitätszustand der Atmosphäre eine entscheidende Rolle. Gegenwärtig verwendete Ansätze, die auf der Verwendung der turbulenten Mischungslänge für neutrale Schichtung sowie dimensionsloser Stabilitätsfunktionen basieren, zeigen vor allem Defizite im oberen Bereich der konvektiven Grenzschicht sowie in der Entrainmentzone, wo starke vertikale Gradienten auftreten. In der vorliegenden Arbeit wurden hochaufgelöste dreidimensionale Grobstruktursimulationen der trockenen und feuchten Grenzschicht für ein weites Spektrum von Labilitätsbedingungen durchgeführt. Erste und zweite Momente atmosphärischer Strömungsvariablen wurden aus den simulierten hydro- und thermodynamischen Feldern berechnet und diskutiert. Die Spektraleigenschaften turbulenter Fluktuationen der Strömungsvariablen, das raumzeitliche Verhalten kohärenter Strukturen sowie charakteristische Turbulenzlängenskalen wurden abgeleitet. Eine Verifizierung der charakteristischen Turbulenzlängenskalen erfolgte durch Vergleich mit Ergebnissen früherer numerischer Simulationen, mit Turbulenzmessungen in der atmosphärischen Grenzschicht sowie mit Laborexperimenten. Mit Hilfe der nichtlinearen Datenmodellierung wurden leicht verwendbare Approximationen der charakteristischen Turbulenzlängenskalen abgeleitet und deren statistische Signifikanz diskutiert. Unter Verwendung dieser Approximationen wurde ein existierendes Parametrisierungsmodell revidiert und mit Hilfe von Grobstruktursimulationen verifiziert. Desweiteren wurde der Einfluß der turbulenten Mischungslänge auf die Prognose mesokaliger Felder untersucht. Hierzu wurde mit dem Lokal-Modell des Deutschen Wetterdienstes eine entsprechende Sensitivitätsstudie durchgeführt. Anhand von Satellitendaten und Analysedaten aus der 4D-Datenassimilation wurden die Simulationsergebnisse verifiziert.

Turbochargers play an important role in increasing the energetic efficiency andreducing emissions of modern power-train systems based on downsized recipro-cating internal combustion engines (ICE). The centrifugal compressor in tur-bochargers is limited at off-design operating conditions by the inception of flowinstabilities causing rotating stall and surge. They occur at reduced enginespeeds (low mass flow rates), i.e. typical operating conditions for a betterengine fuel economy, harming ICEs efficiency. Moreover, unwanted unsteadypressure loads within the compressor are induced; thereby lowering the com-pressors operating life-time. Amplified noise and vibration are also generated,resulting in a notable discomfort. The thesis aims for a physics-based understanding of flow instabilities andthe surge inception phenomena using numerical methods. Such knowledge maypermit developing viable surge control technologies that will allow turbocharg-ers to operate safer and more silent over a broader operating range. Therefore,broadband turbulent enabled compressible Large Eddy Simulation (LES) cal-culations have been performed and several flow-driven instabilities have beencaptured under unstable conditions. LES produces large amounts of 3D datawhich has been post-processed using Fourier spectra and Dynamic Mode De-composition (DMD). These techniques are able to quantify modes in the flowfield by extracting large coherent flow structures and characterize their relativecontribution to the total fluctuation energy at associated. Among the mainfindings, a dominant mode was found which describes the filling and emptyingprocess during surge. A narrowband feature at half of the rotating order wasidentified to correspond to co-rotating vortices upstream of the impeller faceas well as elevated velocity magnitude regions propagating tangentially in thediffuser and the volute. Dominant mode shapes were also found at the rotatingorder frequency and its harmonics, which manifest as a spinning mode shapelocalized at the diffuser inlet. From the compressible LES flow solution one can extract the acoustic infor-mation and the noise affiliated with the compressor. This enable through datacorrelation quantifying the flow-acoustics coupling phenomena in the compres-sor. Detailed comparison of flow (pressure, velocity) and aeroacoustics (soundpressure levels) predictions in terms of time-averaged, fluctuating quantities,and spectra is carried out against experimental measurements. / <p>QC 20160406</p>

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A numerical study has been performed to investigate the mechanics of the turbulent wake of a circular cylinder that is controlled by a plasma actuator. The numerical investigation implements a straightforward moving wall boundary condition to model the actuator's effects on the flow. Validations of the moving wall for this simulation are set forth with the understanding that the moving wall can model the plasma actuator bulk flow effects at a distance downstream and not in a region near or on the plasma actuator. The moving wall boundary condition is then applied to a circular cylinder at a Reynolds number of 8,000. At this unsteady transitional flow regime, a large eddy simulation solver is utilized to resolve flow features. The moving walls are placed at the top and bottom ninety degree points of the cylinder and alternately activated at a frequency to produce lock-in behavior. Investigation into the flowfield mechanics reveals that a harmonic frequency of the forced frequency occurs from the creation of sub-vortices from the instantaneous starting and stopping of the moving-wall actuators. With the forcing frequency close to the natural shedding frequency it is found that the aerodynamic drag increases due to the moving wall creating an average low pressure region on to the downstream side of the cylinder. It is also found that drag can be reduced when the forcing frequency is closer to half the natural shedding frequency. This happens because of a decrease in the average pressure on the downstream side of the cylinder.

Understanding the physics of mixing for two fluids is a complicated problem and has always been an interesting phenomenon to study. Surface discharge is the oldest, least expensive and simplest way of discharging industrial or domestic wastewater into rivers and estuaries. Because of the lower degree of dilution in surface discharges, critical conditions are more likely to occur. Having a better understanding of the mixing phenomenon in these cases will help to predict the environmental effects more accurately. In this study, surface discharges of jets into waterbodies with or without crossflows were investigated numerically and experimentally. Three-dimensional (3-D) Computational Fluid Dynamics (CFD) models were developed for studying the surface discharge of jets into water bodies using different turbulence models. Reynolds stress turbulence models and spatially filtered Large Eddy Simulation (LES) were used in the numerical models. The effects of inclusion of free surface water in the CFD models on the performance of the numerical model results were investigated. Numerical model results were compared with the experimental data in the literature as well as the experimental works performed in this study. Experimental works for buoyant and non-buoyant surface discharge of jets into crossflow and stagnant water were conducted in this study. A new setup was designed and built in the Civil Engineering Hydraulics Laboratory at the University of Ottawa to perform the desired experiments. Stereoscopic Particle Image Velocimetry (Stereo-PIV) was used to measure the instantaneous spatial and temporal 3-D velocity distribution on several planes of measurement downstream of the jet with the frequency of 40 Hz. Averaged 3-D velocity distribution was extracted on different planes of measurement to show the transformation of the velocity vectors from a “jet-like” to a “plume-like” flow regime. Averaged 3-D velocity distribution and streamlines illustrated the flow transformation of the surface jets. Experimental results detected the formation and evolution of vortices in the surface jet’s flow structure over the measurement zone. Additional turbulent flow characteristics such as the turbulent kinetic energy (k), turbulent kinetic energy dissipation rate (ϵ), and turbulent eddy viscosity (υt) were calculated using the measured time history of the 3-D velocity field.

[ES] En los últimos años, el interés por el desarrollo de motores de aviación limpios y eficientes se ha incrementado debido al impacto perjudicial sobre la salud y el medio ambiente ocasionado por los sistemas de combustión convencionales. En este contexto, la comunidad científica ha ido centrando cada vez más sus esfuerzos en el estudio de la combustión turbulenta y la generación de emisiones contaminantes como las partículas de hollín. Con los recientes avances en lo que respecta a potencia de cálculo, las simulaciones de alta fidelidad emergen como una valiosa alternativa para reproducir y analizar estos fenómenos. En concreto, las simulaciones basadas en el modelado de la turbulencia LES son consideradas como una de las herramientas numéricas más prometedoras a la hora de profundizar en la comprensión sobre los complejos procesos dinámicos que caracterizan el flujo reactivo turbulento y predecir emisiones de hollín en aplicaciones aeronáuticas. En el presente trabajo, se estudia y analiza la combustión turbulenta y producción de hollín en aplicaciones de turbina de gas mediante LES de alta fidelidad. El modelado de la combustión se aborda a través de un método flexible de química tabulada basado en el concepto flamelet, el cual es capaz de representar fenómenos químicos complejos con un coste computacional asequible. Además, se emplea una aproximación Euleriana-Lagrangiana para la descripción de la fase gaseosa y las gotas, de forma que se represente correctamente el flujo reactivo multifásico. Para la predicción de hollín en simulaciones computacionalmente eficientes, se emplea un novedoso enfoque de modelado basada en el método seccional y acoplada al modelo de combustión de química tabulada. Esta estrategia de modelado numérica es utilizada en este trabajo para analizar el proceso de combustión y evaluar sus capacidades para predecir hollín y las características de la llama en quemadores de turbina de gas representativos. En primer lugar, se estudia la combustión de flujo bifásico en una llama atmosférica sin torbellinador con inyección líquida de combustible. Este quemador presenta una estructura doble del frente reactivo y las simulaciones numéricas son capaces de capturar adecuadamente los fenómenos de extinción local que tienen lugar en la zona interna de la llama debido a la interacción de las gotas y la turbulencia con el frente reactivo. Posteriormente, se investiga la combustión y producción de hollín en un quemador presurizado con torbellinador que incluye aire secundario de dilución en el interior de la cámara de combustión. La validación del flujo reactivo y hollín se lleva a cabo tanto en la configuración del quemador con aire secundario como sin el mismo, mostrando unas excelentes capacidades predictivas en ambos casos. La presente estrategia de modelado reproduce de forma precisa el complejo patrón de flujo, la estructura de la llama y la dinámica de generación de hollín, además de que es capaz de proporcionar diferentes distribuciones de tamaño de partícula dependiendo de las variaciones en los procesos de formación y oxidación del hollín. En resumen, los diferentes casos prácticos estudiados permiten consolidar y validar la metodología computacional seguida en la presente tesis. La estrategia de modelado basada en química tabulada propuesta demuestra ser lo suficientemente válida y adecuada para reproducir los complejos fenómenos de la combustión y la formación de hollín, en vista de la consistencia del análisis, las precisas predicciones y la concordancia satisfactoria con las medidas experimentales. / [CA] En els últims anys, l'interés pel desenvolupament de motors d'aviació nets i eficients s'ha incrementat a causa de l'impacte perjudicial sobre la salut i el medi ambient ocasionat pels sistemes de combustió convencionals. En aquest context, la comunitat científica ha anat centrant cada vegada més els seus esforços en l'estudi de la combustió turbulenta i la generació d'emissions contaminants com les partícules de sutge. Amb els recents avanços pel que fa a potència de càlcul, les simulacions d'alta fidelitat emergeixen com una valuosa alternativa per a reproduir i analitzar aquests fenòmens. En concret, les simulacions basades en el modelatge de la turbulència LES són considerades com una de les eines numèriques més prometedores a l'hora d'aprofundir en la comprensió sobre els complexos processos dinàmics que caracteritzen el flux reactiu turbulent i predir emissions de sutge en aplicacions aeronàutiques. En el present treball, s'estudia i analitza la combustió turbulenta i la producció de sutge en aplicacions de turbina de gas mitjançant LES d'alta fidelitat. El modelatge de la combustió s'aborda a través d'un mètode flexible de química tabulada basat en el concepte flamelet, el qual és capaç de representar fenòmens químics complexos amb un cost computacional assequible. A més, s'empra una aproximació Euleriana-Lagrangiana per a la descripció de la fase gasosa i les gotes, de manera que es represente correctament el flux reactiu multifàsic. Per a la predicció de sutge en simulacions computacionalment eficients, s'empra un nou plantejament de modelatge basat en el mètode seccional i acoblat al model de combustió de química tabulada. Aquesta estratègia de modelatge numèrica és utilitzada en aquest treball per a analitzar el procés de combustió en cremadors de turbina de gas representatius, i avaluar les seues capacitats per a predir sutge i les característiques de la flama. En primer lloc, s'estudia la combustió de flux bifàsic en una flama atmosfèrica sense remolinador amb injecció líquida de combustible. Aquest cremador presenta una estructura doble del front reactiu i les simulacions numèriques són capaces de capturar adequadament els fenòmens d'extinció local que tenen lloc en la zona interna de la flama a causa de la interacció de les gotes i la turbulència amb el front reactiu. Posteriorment, s'investiga la combustió i producció de sutge en un cremador pressuritzat amb remolinador que inclou aire secundari de dilució a l'interior de la cambra de combustió. La validació del flux reactiu i sutge es duu a terme tant en la configuració del cremador amb aire secundari com sense aquest, mostrant unes estupendes capacitats predictives en tots dos casos. La present estratègia de modelatge reprodueix de manera precisa el complex patró de flux, l'estructura de la flama i la dinàmica de generació de sutge, a més de que és capaç de proporcionar diferents distribucions de grandària de partícula depenent de les variacions en els processos de formació i oxidació del sutge. En resum, els diferents casos pràctics estudiats permeten consolidar i validar la metodologia computacional seguida en la present tesi. L'estratègia de modelatge basada en química tabulada proposada demostra ser prou vàlida i adequada per a reproduir els complexos fenòmens de la combustió i la formació de sutge, en vista de la consistència de l'anàlisi, les precises prediccions i la concordança satisfactòria amb les mesures experimentals. / [EN] In recent years, interest in the development of efficient and clean aviation powerplants has increased due to the detrimental impact on health and the environment caused by conventional combustion systems. In this context, the research community has increasingly focused its efforts on the study of turbulent combustion and the generation of pollutant emissions such as soot particulates. With recent advances in computational power, high-fidelity simulations emerge as a valuable alternative to reproduce and analyze these phenomena. Specifically, Large Eddy Simulations (LES) are considered as one of the most promising numerical tools to provide further insight into the complex dynamic processes that characterize reactive turbulent flows and predict soot emissions in aeronautical applications. In the present work, turbulent combustion and soot production is studied and analyzed in gas turbine engine applications by means of high-fidelity LES. Combustion modelling is addressed by a flexible tabulated chemistry method based on the flamelet concept, which is able to represent complex chemical phenomena with an affordable computational cost. In addition, an Eulerian- Lagrangian description is employed for the gas phase and droplets in order to correctly represent the multiphase flow in spray flames. A recently developed approach based on the sectional method and coupled to the tabulated chemistry framework is considered for soot prediction in computationally efficient simulations. This numerical modelling framework is used in this work to analyze the combustion process and evaluate its capabilities to predict soot and flame characteristics in representative gas turbine burners. First, an atmospheric non-swirled spray flame is studied in terms of two-phase flow combustion. This burner shows a double reaction front structure and local extinction occurs in the inner layer due to both droplet-flame and turbulence-flame interactions, which is properly characterized by LES. Subsequently, combustion and soot production is investigated in a pressurized swirled model combustor which includes secondary dilution jets inside the combustion chamber. The assessment of the reacting flow field and soot is addressed for burner configurations with and without secondary air, showing excellent predictive capabilities in both cases. The present modelling approach accurately reproduce the complex swirled flow field, flame structure and soot dynamics and is able to provide different particle size distributions depending on the variations of the soot formation and oxidation processes. In summary, the different practical cases studied allow to consolidate and validate the computational methodology followed in the present thesis. The proposed tabulated modelling strategy is sufficiently valid and suitable for reproducing complex combustion and soot formation phenomena, in view of the consistency of the analysis, the accurate predictions and the satisfactory agreement with the experimental measurements. / El desarrollo de la presente tesis ha sido posible gracias a una ayuda para la Formación de Profesorado Universitario (FPU 18/03065) perteneciente al Subprograma Estatal de Formación del Ministerio de Ciencia, Innovación y Universidades de España. Además, el trabajo desarrollado está enmarcado en el proyecto ESTiMatE (Emissions SooT ModEl), que ha sido financiado por el consorcio Clean Sky 2 bajo el programa de investigación e innovación Horizonte 2020 de la Unión Europea (acuerdo No. 821418). Las actividades de simulación numérica han sido posibles gracias a la Red Española de Supercomputación y al Centro de Supercomputación de Barcelona por los recursos computacionales proporcionados en MareNostrum, además del grupo PRACE por conceder el acceso a HAWK (GCS, HLRS, Alemania) a través del proyecto SootAero. / Olmeda Ramiro, I. (2023). Assessment of Detailed Combustion and Soot Models for High-Fidelity Aero-Engine Simulations [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/202284

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No description available.

In gas stirred steelmaking ladles, entrainment of slag into metal and vice versa takes place. The slag entrainment has been shown to abruptly increase the mass transfer rates of refining reactions through high temperature and water modeling studies of the past. However such an effect has not been correlated with the degree of entrainment, since the latter has not been quantified in terms of operating parameters like gas injection rate and fluid properties. Much of the past works are limited to finding the critical conditions for onset of entrainment. The difficulty lies in measuring the degree of entrainment in industrial ladles or even in a water model. Mathematical modeling is also challenging due to the complexity of the multiphase phenomena. So in this thesis, a modular mathematical modeling approach is presented wherein the phenomena of slag entrainment into metal is resolved into four aspects, models developed for each and finally integrated to study its role. The individual models are (1) multiphase large eddy simulations to simulate slag entrainment in a narrow domain that receives its boundary conditions from (2) single phase RANS simulation of a full ladle, (3) a Lagrangian particle tracking method to compute the residence times of slag droplets in metal phase and (4) a kinetic model that integrates the above three models to compute mass transfer rate as a function of degree of entrainment. Mass transfer rate predictions comparable to a literature correlation were obtained. This supports the modeling approach and also the assessment of role of various system parameters on entrainment characteristics. In essence, the present work shows a systematic approach to model and study the complex multiphase phenomena. / Thesis / Doctor of Philosophy (PhD) / The entrainment of liquid slag into liquid steel in gas stirred-steelmaking ladles is known to increase the rate of refining drastically. However, there is lack of correlation between degree of entrainment and ladle operating conditions, which this thesis addresses through mathematical modeling.

The work presented in this dissertation is the first numerical investigation conducted to study leading edge film cooling with Large Eddy Simulation (LES). A cylindrical leading edge with a flat after-body represents the leading edge, where coolant is injected with a 30Ë compound angle. Three blowing ratios of 0.4, 0.8, and 1.2 are studied. Free-stream Reynolds number is 100,000 and coolant-to-mainstream density ratio is unity. At blowing ratio of 0.4, the effect of coolant inlet condition is investigated. Results show that the fully-turbulent coolant jet increases mixing with the mainstream in the outer shear layer but does not influence the flow dynamics in the turbulent boundary layer at the surface. As a result, the turbulent jet decreases adiabatic effectiveness but does not have a substantial effect on the heat transfer coefficient. At B.R.=0.4, three types of coherent structures are identified which consist of a primary entrainment vortex at the leeward aft-side of the coolant hole, vortex tubes at the windward side of the coolant hole, and hairpin vortices typical of turbulent boundary layers produced by the turbulent interaction of the coolant and mainstream downstream of injection. At B.R. = 0.8 and 1.2, coherent vortex tubes are no longer discernable, whereas the primary vortex structure gains in strength. In all cases, the bulk of the mixing occurs by entrainment which takes place at the leeward aft-side of the coolant jet. This region is characterized by a low pressure core and the primary entrainment vortex. Turbulent shear interaction between coolant jet and mainstream increases substantially with blowing ratio and contributes to the dilution of the coolant jet. As a result of the increased mixing in the shear layer and primary structure, adiabatic effectiveness decreases and heat transfer coefficient increases with increase in blowing ratio. The dissertation also investigates the deposition and erosion of Syngas ash particles in the film cooled leading edge region. Three ash particle sizes of 1, 5, and 10 microns are investigated at all blowing ratios using Lagrangian dynamics. The 1 micron particles with momentum Stokes number St = 0.03 (based on approach velocity and cylinder diameter), show negligible deposition/erosion. The 10 micron particles, on the other hand with a high momentum Stokes number, St = 3, directly impinge and deposit on the surface, with blowing ratio having a minimal effect. The 5 micron particles with St=0.8, show the largest receptivity to coolant flow and blowing ratio. On a mass basis, 90% of deposited mass is from 10 micron particles, with 5 micron particles contributing the other 10%. Overall there is a slight decrease in deposited mass with increase in blowing ratio. About 0.03% of the total incoming particle energy can potentially be transferred as erosive energy to the surface and coolant hole, with contribution coming from only 5 micron particles at B.R.=0.4 and 0.8, and both 5 and 10 micron particles at B.R.=1.2. / Ph. D.