Les lanceurs ont un impact sur la composition de l'atmosphere, et en particulier sur l'ozone stratospherique. Parmi tous les types de propulsion, les moteurs à propergol solide ont fait l'objet d'une attention particulière car leurs émissions sont responsables d'un appauvrissement significatif d'ozone dans le panache des lanceurs lors des premières heures suivant le lancement. Ce phénomène est principalement dû à la conversion de l'acide chlorhydrique, un composé chimique présent en grandes quantités dans les émissions de ce type de moteur, en chlore actif qui réagit par la suite avec l'ozone dans un cycle catalytique similaire à celui responsable du "trou de la couche d'ozone", cette diminution périodique de l'ozone en Antarctique. Cette conversion se produit dans le panache supersonique, où les hautes températures favorisent une seconde combustion entre certaines espèces chimiques du panache et l'air ambiant. L'objectif de cette étude est d'évaluer la concentration de chlore actif dans le panache d'un moteur à propergol solide en utilisant la technique des Simulations aux Grandes Echelles (SGE). Le gaz est injecté à travers la tuyère d'un moteur et une méthode de couplage entre deux instances du solveur de mécanique des fluides est utilisée pour étendre autant que possible le domaine de calcul derrière la tuyère (jusqu'à l'équivalent de 400 diamètres de sortie de la tuyère). Cette méthodologie est validée par une première SGE sans chimie, en analysant les caractéristiques de l'écoulement supersonique avec co-écoulement obtenu par ce calcul. Ensuite, le chimie mettant en jeu la conversion des espèces chlorées a été étudiée au moyen d'un modèle "hors-ligne" permettant de résoudre une chimie complexe le long de lignes de courant extraites d'un écoulement moyenné dans le temps résultant du calcul précédent (non réactif). Enfin, une SGE multi-espèces est réalisée, incluant un schéma chimique auparavant réduit afin de limiter le coût de calcul. Cette simulation représente une des toutes premières SGE d'un jet supersonique réactif, incluant la tuyère, effectuée sur un domaine de calcul aussi long. En capturant avec précision le mélange du panache avec l'air ambiant ainsi que les interactions entre turbulence et combustion, la technique des simulations aux grandes échelles offre une évaluation des concentrations des espèces chimiques dans le jet d'une precision inédite. Ces résultats peuvent être utilisés pour initialiser des calculs atmosphériques sur de plus larges domaines, afin de modéliser les réactions entre chlore actif et ozone et de quantifier l'appauvrissement en ozone dans le panache. / Rockets have an impact on the chemical composition of the atmosphere, and particularly on stratospheric ozone. Among all types of propulsion, Solid-Rocket Motors (SRMs) have given rise to concerns since their emissions are responsible for a severe decrease in ozone concentration in the rocket plume during the first hours after a launch. The main source of ozone depletion is due to the conversion of hydrogen chloride, a chemical compound emitted in large quantities by ammonium perchlorate based propellants, into active chlorine compounds, which then react with ozone in a destructive catalytic cycle, similar to those responsible for the Antartic "Ozone hole". This conversion occurs in the hot, supersonic exhaust plume, as part of a strong second combustion between chemical species of the plume and air. The objective of this study is to evaluate the active chlorine concentration in the far-field plume of a solid-rocket motor using large-eddy simulations (LES). The gas is injected through the entire nozzle of the SRM and a local time-stepping method based on coupling multi-instances of the fluid solver is used to extend the computational domain up to 400 nozzle exit diameters downstream of the nozzle exit. The methodology is validated for a non-reactive case by analyzing the flow characteristics of the resulting supersonic co-flowing under-expanded jet. Then the chemistry of chlorine is studied off-line using a complex chemistry solver applied on trajectories extracted from the LES time-averaged flow-field. Finally, the online chemistry is analyzed by means of the multi-species version of the LES solver using a reduced chemical scheme. To the best of our knowledge, this represents one of the first LES of a reactive supersonic jet, including nozzle geometry, performed over such a long computational domain. By capturing the effect of mixing of the exhaust plume with ambient air and the interactions between turbulence and combustion, LES offers an evaluation of chemical species distribution in the SRM plume with an unprecedented accuracy. These results can be used to initialize atmospheric simulations on larger domains, in order to model the chemical reactions between active chlorine and ozone and to quantify the ozone loss in SRM plumes.

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.

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<p>The present dissertation focuses on the numerical solution of compressible flows with an emphasis on simulations of transitional hypersonic boundary layers. Initially, general concepts such as the governing equations, numerical approximations and theoretical modeling strategies are addressed. These are used as a basis to introduce two innovative techniques, the Quasi-Spectral Viscosity (QSV) method, applied to high-order finite difference settings and the Legendre Spectral Viscosity (LSV) approach, used in high-order flux reconstruction schemes. Such techniques are derived based on the mathematical formalism of the filtered compressible Navier-Stokes equations. While the latter perspective is only typically used for turbulence modeling in the context of Large-Eddy Simulations (LES), both the QSV and LSV subfilter scale (SFS) closure models are capable of performing simulations in the presence of shock-discontinuities. On top of that, the QSV approach is also shown to support dynamic subfilter turbulence modeling capabilities.</p> <p>QSV’s innovation lies in the introduction of a physical-space implementation of a spectral-like subfilter scale (SFS) dissipation term by leveraging residuals of filter operations, achiev- ing two goals: (1) estimating the energy of the resolved solution near the grid cutoff; (2) imposing a plateau-cusp shape to the spectral distribution of the added dissipation. The QSV approach was tested in a variety of flows to showcase its capability to act interchangeably as a shock capturing method or as a SFS turbulence closure. QSV performs well compared to previous eddy-viscosity closures and shock capturing methods. In a supersonic TGV flow, a case which exhibits shock/turbulence interactions, QSV alone outperforms the simple super- position of separate numerical treatments for SFS turbulence and shocks. QSV’s combined capability of simulating shocks and turbulence independently, as well as simultaneously, effectively achieves the unification of shock capturing and Large-Eddy Simulation.</p> <p>The LSV method extends the QSV idea to discontinuous numerical schemes making it suitable for unstructured solvers. LSV exploits the set of hierarchical basis functions formed by the Legendre polynomials to extract the information on the energy content near the resolution limit and estimate the overall magnitude of the required SFS dissipative terms, resulting in a scheme that dynamically activates only in cells where nonlinear behavior is important. Additionally, the modulation of such terms in the Legendre spectral space allows for the concentration of the dissipative action at small scales. The proposed method is tested in canonical shock-dominated flow setups in both one and two dimensions. These include the 1D Burgers’ problem, a 1D shock tube, a 1D shock-entropy wave interaction, a 2D inviscid shock-vortex interaction and a 2D double Mach reflection. Results showcase a high-degree of resolution power, achieving accurate results with a small number of degrees of freedom, and robustness, being able to capture shocks associated with the Burgers’ equation and the 1D shock tube within a single cell with discretization orders 120 and higher.</p> <p>After the introduction of these methods, the QSV-LES approach is leveraged to perform numerical simulations of hypersonic boundary layer transition delay on a 7^◦-half-angle cone for both sharp and 2.5 mm-nose tip radii due to porosity representative of carbon-fibre-reinforced carbon-matrix ceramics (C/C) in the Reynolds number range Re_m = 2.43 · 106 – 6.40 · 10⁶ m⁻¹ at the freestream Mach number of M_∞ = 7.4. A low-order impedance model was fitted through experimental measurements of acoustic absorption taken at discrete frequencies yielding a continuous representation in the frequency domain that was imposed in the simulations via a broadband time domain impedance boundary condition (TDIBC). The stability of the base flow is studied over impermeable and porous walls via pulse-perturbed axisymmetric simulations with second-mode spatial growth rates matching linear predictions. This shows that the QSV-LES approach is able to dynamically deactivate its dissipative action in laminar portions of the flow making it possible to accurately capture the boundary layer’s instability dynamics. Three-dimensional transitional LES were then performed with the introduction of grid independent pseudorandom pressure perturbations. Comparison against previous experiments were made regarding the frequency content of the disturbances in the transitional region with fairly good agreement capturing the shift to lower frequencies. Such shift is caused by the formation of near-wall low-temperature streaks that concentrate the pressure disturbances at locations with locally thicker boundary layers forming trapped wavetrains that can persist into the turbulent region. Additionally, it is shown that the presence of a porous wall representative of a C/C material does not affect turbulence significantly and simply shifts its onset downstream.</p>