This thesis proposes a large eddy simulation reduced order model (LES-ROM) framework for the numerical simulation of realistic flows. In this LES-ROM framework, the proper orthogonal decomposition (POD) is used to define the ROM basis and a POD differential filter is used to define the large ROM structures. An approximate deconvolution (AD) approach is used to solve the ROM closure problem and develop a new AD-ROM. This AD-ROM is tested in the numerical simulation of the one-dimensional Burgers equation with a small diffusion coefficient ( ν= 10⁻³). / Master of Science

<p dir="ltr">The computational campaign reported in this thesis focuses on a series of experiments at Mach 2.85 carried out in the 1980s at NASA Ames Research Center on a set of cylinder / skewed flare configurations designed to produce highly three-dimensional shockwave / boundary-layer interactions in the absence of end-wall effects. Computations carried out in that era were unable to match the experimental results using the numerical techniques, turbulence models, and grid resolution available at the time. In the present work, newer Reynolds-averaged Navier-Stokes and detached eddy simulation methods have been applied to these flows, and relatively good agreement has been obtained with the experimental data. Difficulty in capturing the correct separation bubble size was encountered with initial detached eddy simulations, but the introduction of resolved turbulence via a boundary layer trip produced much better results. This thesis reports on results obtained for four inclination angles (0 deg, 5 deg, 10 deg, and 23 deg) of the skewed flare. Detached eddy simulation is seen to be an economical alternative to large eddy simulation for capturing many features of large-scale separation unsteadiness over long time intervals at true Reynolds number.</p>

According to the United Nations, 55% of the world's population currently lives in urban areas and which is projected to increase to 67% by 2050. Thus, it is imperative that effective strategies are developed to mitigate urban pollution. Complementing field experiments, computational fluid dynamics (CFD) analyses are becoming an effective strategy for identifying critical factors that influence urban pollution and its mitigation. This thesis focuses on two scales of the urban micro-climate environment: (i) evaluation of LES simulations with a simplified grid for modeling pollution dispersion in a street canyon and (ii) investigation of the effects of leaf surface micro-characteristics, wind speed, and particle sizes on the dry deposition of fine particulate matter (PM2.5). The first of these studies focuses on reproducing the pollution dispersion in a street canyon measured in a wind tunnel at Karlsruhe Institute of Technology (KIT), Germany. A simplified grid with the Large Eddy Simulations (LES) approach for canyon ratio W/H = 1 is proposed with the goal to reduce the computational cost by eliminating the need to model the entire canyon while striving to preserve the mixing induced by individual jets used to model vehicle emission in the experiment. LES is also capable of providing transient flow field and pollution concentration data not available with widely-used steady approaches such as RANS. The time-dependent information is crucial for pollution mitigation since pedestrians are usually exposed to pollution on a short-time basis. The predictions are in satisfactory agreement with the experiment for W/H = 1, yielding the Pearson correlation coefficient R = 0.81, with better performance near the leeward wall. Due to the small span modeled, three-dimensional instabilities fail to develop which could probably explain the overprediction of pollution concentration near ground level. However, other LES investigations where the full canyon was modeled also observed over-predictions. The use of a discrete emission source was not observed to provide benefits. The current model could be further improved by using a larger spanwise domain with a continuous line source to allow large wavelength instabilities to develop and increase turbulent diffusion. The second part of this thesis investigates the impact of trichome morphology and wind speed on the deposition of 0.3 μm and 1.0 μm particles on leaves. Using the one-way coupling approach to predict the fluid-particle interactions with the assumption that all particles that impact the leaf or trichome surface deposit, trichomes of 5 μm and 20 μm in diameter are modeled as equally spaced and uniform cylinders on an infinitely large plane. The results show that trichome diameter, density, and wind speed have a favorable impact on deposition velocity. Comparing to the smooth leaf, the presence of the thicker 20 μm hairs increases the deposition velocity by 1.5 – 4 times, whereas, the presence of short 5um trichomes reduces the deposition by 15 - 45%. Increasing trichome height from H/D = 20 to 30 shows benefits for the thinner trichomes but lowers the deposition for the densely packed thicker trichomes. Less aerosol deposition is also observed when the particle diameter increases from 0.3 μm to 1.0 μm. Due to the non-uniform contributions of these various traits, a non-dimensional ratio Rhp is proposed to model the aerosol deposition on leaf surface at wind speed of 1 m/s which yields a satisfactory linear correlation coefficient of 0.89 for 0 < R_hp < 0.3. Comparing to other published field and wind tunnel experiments conducted on a much larger scale, the deposition velocities predicted are at the lower end (U_dep^* = 0.002 to 0.012 cm/s) because of the idealized conditions. Nonetheless, the results still offer valuable insight into the effects of trichome morphology on pollutant deposition in isolation from other macro-factors. / Master of Science / According to the United Nations, 55% of the world’s population currently lives in urban areas and which is projected to increase to 67% by 2050. Thus, it is imperative that effective strategies are developed to mitigate urban pollution. Complementing field experiments, computational fluid dynamics (CFD) analyses are becoming an effective strategy for identifying critical factors that influence urban pollution and its mitigation. This thesis focuses on two scales of the urban micro-climate environment: (i) evaluation of Large Eddy Simulation (LES) with a simplified method for modeling pollution dispersion in a street canyon and (ii) investigation of the effects of leaf surface micro-characteristics, wind speed, and particle sizes on the dry deposition of fine particulate matter (PM2.5). The first of these studies focuses on reproducing the pollution dispersion in a street canyon measured in a wind tunnel at Karlsruhe Institute of Technology (KIT), Germany. A simplified grid with the LES approach for canyon ratio W/H = 1 is proposed. The goal of this study is to reduce the computational cost by modelling the canyon with a very thin span instead of the entire canyon while providing time-dependent information which is crucial for pollution mitigation since pedestrians are usually exposed to pollution on a short-time basis. The predictions are in satisfactory agreement with the experiment for W/H = 1 with better performance near the leeward wall (i.e. the left wall) and overprediction of pollution concentration near ground level – as observed by other LES investigations. The current model could be further improved by using a larger spanwise domain with a continuous line source to allow instabilities to develop, thus improve prediction accuracy. The second part of this thesis investigates the impact of trichome (i.e. a hair or an outgrowth from leaf surface) morphology and wind speed on the deposition of 0.3 mm and 1.0 mm particles on leaves. The results show that trichome diameter, density, and wind speed have a favorable impact on deposition velocity. Less aerosol deposition is also observed when the particle diameter increases from 0.3 mm to 1.0 mm. No clear effects is observed by altering the trichome height. Due to the non-uniform contributions of these various traits, a non-dimensional ratio D∗ �D∗ �2 Rhp = hair hair is proposed to model the aerosol deposition on leaf surface at wind speed of D∗ H∗ S∗ p hair hair 1 m/s which yields a satisfactory linear correlation coefficient of 0.89 for 0 < Rhp < 0.3. This ratio includes trichome diameter (D∗ ), height (H∗ ), spacing (S∗ ) as well as the ratio of hair hair hair trichome diameter to particle diameter (D∗ /D∗ ). The results offer valuable insight into the hair p effects of trichome morphology on pollutant deposition in isolation from other macro-factors.

The work presented in this thesis attempts to provide an understanding of the physics behind the dynamic stall process by simulating the flow past pitching NACA-0012 airfoil at 100,000 and 1 million Reynolds number based on the chord length of the airfoil and at different reduced frequencies of 0.188 and 0.25 respectively in a three dimensional flow field. The mean angles of attack are 12 deg. and 15 deg. and the amplitudes of pitching are 6 deg. and 10 deg. respectively. The turbulence in the flow field is resolved using large eddy simulations with dynamic Smagorinsky model at the sub grid scale. The lift hysteresis plots of this simulation for both the configurations are compared with the corresponding experiments. The development of dynamic stall vortex, vortex shedding and reattachment as predicted by the present study are discussed in detail. There is a fairly good match between the predicted and experimentally measured lift coefficient during the upstroke for both cases. The net lift coefficient for the Re = 100,000 case during downstroke matches with the corresponding experimental data, the present study under-predicts the lift coefficient as compared to the experimental values at the start of downstroke and over-estimates for the remaining part of the downstroke. The trend of the lift coefficient hysteresis plot with the experimental data for the Re = 1 million case is also similar. This present simulations have shown that the downstroke phase of the pitching motion is strongly three dimensional and is highly complex, whereas the flow is practically two dimensional during the upstroke. / Master of Science

Large-Eddy Simulations (LES or wall-resolved LES, WRLES) has been used extensively in capturing the physics of anisotropic turbulent flows. However, near wall turbulent scales in the inner layer in wall bounded flows makes it unfeasible for large Reynolds numbers due to grid requirements. This study evaluates the use of a wall model for LES (WMLES) on a channel with rotation at ã Reã _b = 34,000 from ã Roã _b = 0 to 0.38, non-staggered 90° ribbed duct with rotation at ã Reã _b = 20,000 from ã Roã _b = 0 to 0.70, stationary 45° staggered ribbed duct at ã Reã _b = 49,000, and two-pass smooth duct with a U-bend at ã Reã _b = 25,000 for ã Roã _b = 0 to 0.238 against WRLES and experimental data. In addition, for the two-pass smooth duct with a U-bend simulations, the synthetic eddy method (SEM) is used to artificially generate eddies at the inlet based on given flow characteristics. It is presented that WMLES captures the effects of Coriolis forces and predicts mean heat transfer augmentation ratios reasonably well for all simulations. The alleviated grid resolution for these simulations indicates significant reductions in resources, specifically, by a factor of 10-20 in non-staggered 90° ribbed duct simulations. The combined effects of density ratio, Coriolis forces, with SEM for the inlet turbulence, capture the general trends in heat transfer in and after the bend. / Master of Science

Gas turbines for military applications, when operating in harsh environments like deserts often encounter unexpected operation faults. Such performance deterioration of the gas turbine decreases the mission readiness of the Air Force and simultaneously increases the maintenance costs. Some of the major factors responsible for the reduced performance are ingestion of debris during take off and landing, distorted intake flows during low altitude maneuvers, and hot gas ingestion during artillery firing. The focus of this thesis is to study ingestion of debris; specifically sand. The region of interest being the internal cooling ribbed duct of the turbine blade. The presence of serpentine passages and strong localized cross flow components makes this region prone to deposition, erosion, and corrosion (DEC) by sand particles. A Lagrangian particle tracking technique was implemented in a generalized coordinate multi-block Navier-Stokes solver in a distributed parallel framework. The developed algorithm was validated by comparing the computed particle statistics for 28 microns lycopodium, 50 microns glass, and 70 microns copper with available data [2] for a turbulent channel flow at Ret=180. Computations were performed for a particle-laden turbulent flow through a stationary ribbed square duct (rib pitch / rib height = 10, rib height / hydraulic diameter = 0.1) using an Eulerian-Lagrangian framework. Particle sizes of 10, 50, and 100 microns with response times (normalized by friction velocity and hydraulic diameter) of 0.06875, 1.71875, and 6.875 respectively are considered. The calculations are performed for a nominal bulk Reynolds number of 20,000 under fully developed conditions. The carrier phase was solved using Large Eddy Simulation (LES) with Dynamic Smagorinsky Model [1]. Due to low volume fraction of the particles, one-way fluid-particle coupling was assumed. It is found that at any given instant in time about 40% of the total number of 10 micron particles are concentrated in the vicinity (within 0.05 Dh) of the duct surfaces, compared to 26% of the 50 and 100 micron particles. The 10 micron particles are more sensitive to the flow features and are prone to preferential concentration more so than the larger particles. At the side walls of the duct, the 10 micron particles exhibit a high potential to erode the region in the vicinity of the rib due to secondary flow impingement. The larger particles are more prone to eroding the area between the ribs and towards the center of the duct. At the ribbed walls, while the 10 micron particles exhibit a fairly uniform propensity for erosion, the 100 micron particles show a much higher tendency to erode the surface in the vicinity of the reattachment region. The rib face facing the flow is by far the most susceptible to erosion and deposition for all particle sizes. While the top of the rib does not exhibit a large propensity to be eroded, the back of the rib is as susceptible as the other duct surfaces because of particles which are entrained into the recirculation zone behind the rib. / Master of Science

Yes / The bed slope and the tailwater depth are two important ones among the factors that affect the propagation of the dam-break flood and Favre waves. Most previous studies have only focused on the macroscopic characteristics of the dam-break flows or Favre waves under the condition of horizontal bed, rather than the internal movement characteristics in sloped channel. The present study applies two numerical models, namely, large eddy simulation (LES) and shallow water equations (SWEs) models embedded in the CFD software package FLOW-3D to analyze the internal movement characteristics of the dam-break flows and Favre waves, such as water level, the velocity distribution, the fluid particles acceleration and the bed shear stress, under the different bed slopes and water depth ratios. The results under the conditions considered in this study show that there is a flow state transition in the flow evolution for the steep bed slope even in water depth ratio α = 0.1 (α is the ratio of the tailwater depth to the reservoir water depth). The flow state transition shows that the wavefront changes from a breaking state to undular. Such flow transition is not observed for the horizontal slope and mild bed slope. The existence of the Favre waves leads to a significant increase of the vertical velocity and the vertical acceleration. In this situation, the SWEs model has poor prediction. Analysis reveals that the variation of the maximum bed shear stress is affected by both the bed slope and tailwater depth. Under the same bed slope (e.g., S0 = 0.02), the maximum bed shear stress position develops downstream of the dam when α = 0.1, while it develops towards the end of the reservoir when α = 0.7. For the same water depth ratio (e.g., α = 0.7), the maximum bed shear stress position always locates within the reservoir at S0 = 0.02, while it appears in the downstream of the dam for S0 = 0 and 0.003 after the flow evolves for a while. The comparison between the numerical simulation and experimental measurements shows that the LES model can predict the internal movement characteristics with satisfactory accuracy. This study improves the understanding of the effect of both the bed slope and the tailwater depth on the internal movement characteristics of the dam-break flows and Favre waves, which also provides a valuable reference for determining the flood embankment height and designing the channel bed anti-scouring facility. / National Natural Science Foundation of China (Grant No: 51879179, 52079081), the Open Fund from the State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University (SKHL1809) and the Sichuan Science and Technology Program (No. 2019JDTD0007)

京都大学 / 新制・課程博士 / 博士(工学) / 甲第25278号 / 工博第5237号 / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 黒瀬 良一, 教授 長田 孝二, 教授 岩井 裕 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Die vorliegende Doktorarbeit beschäftigt sich mit der numerischen Ausbreitungssimulation von Luftschadstoffen im urbanen Raum. Als Strömungshindernisse prägen Gebäude maßgeblich das kleinskalige Windfeld und damit den horizontalen Transport und die Verteilung von urbanen Luftschadstoffen. Die häufig im Rahmen von Großwirbelsimulationen (engl.: Large Eddy Simulation, LES) zur expliziten Darstellung von Gebäuden verwendeten hohen räumlichen Auflösungen in Bereich von wenigen Metern erforden einen hohen Rechenaufwand und beschränken daher die Anwendung meist auf wenige Gebäudeblöcke. Mittels eines neuen Diskretisierungsansatzes können Gebäude auch bei deutlich geringeren Auflösungen in Bereich der sogenannten urbanen Grauzone (horizontale Gitterweite > 10m) mittels diffuser Gebäudekanten explizit dargestellt werden. Im Rahmen dieser Arbeit wurde das urbane Dispersionsmodells CAIRDIO (v1.0) mit diesem neuen Diskretisierungsansatz entwickelt. Das Modell wurde zunächst mit einem Windkanalexperiment, welches die turbulente Ausbreitung eines Test-Tracers in einer Modellstadt beinhaltet, validiert. Dabei konnte bis zu einer horizontalen Auflösung von 40m noch eine hinreichend gute Übereinstimmung des Modells mit Konzentrationsmessungen erzielt werden, es wurde jedoch nur ein Bruchteil der Rechenzeit einer Referenzsimulation mit 5m Gitterweite benötigt. Der Ansatz mit diffusen Gebäuden erlaubt daher den Anwendungsbereich mikroskaliger Simulationen im Vergleich zu klassischen LES Simulationen erheblich zu erweitern, z.B. auf die Simulation ganzer Städte. In einer ersten realistischen Fallstudie wurde das Modell CAIRDIO auf die Stadt Leipzig in Mitteldeutschland angewandt. Dabei wurde die Ausbreitung von realistischen Feinstaub und Ruß-Emissionen bei einer horizontalen Auflösung von 40m für das gesamte Stadtgebiet über einen Zeitraum von 2 Tagen (Anfang März 2020) berechnet und mit Messungen validiert. Es konnten die Auswirkungen der variablen Grenzschichtdynamik auf den urbanen Luftschadstofftransport mit dem Modell qualitativ korrekt wiedergegeben werden. Nur für verkehrsnahe Standorte führte eine höhere räumliche Auflösung von bis zu 5m noch zu einer besseren Übereinstimmung mit den Messungen. Insgesamt stellten jedoch bereits bei 40m Auflösung die Unsicherheiten in den Emissionen und den Randbedingungen den größten Anteil am Modellfehler dar. In einer zweiten Fallstudie wurde eine weiter entwickelte Version des Modells CAIRDIO auf die Stadt Dresden im Elbtal angewandt. Der Fokus dieser Studie lag diesmal auf der den urbanen Raum umgebenden Orographie, welche erhebliche Auswirkungen auf die lokale Luftqualität bei Inversionswetterlagen haben kann. Dazu wurde das Dresdner Becken mittels geländefolgender Koordinaten zusammen mit den nach wie vor diffus repräsentierten Stadtgebäuden auf der Skala der urbanen Grauzone im Modell dargestellt. Neben der Dispersion von Ruß-Emissionen wurde auch die Ruß-Alterskonzentration, welche sich besonders gut zur Identifikation von orographisch bedingten Akkumulation von Luftschadstoffen eignet, über einen Zeitraum von insgesamt 24 Tagen modelliert. Die räumliche Verteilung der Alterskonzentrations-Hotspots wurde bei stabilen Wetterlagen neben den Gebäudeeffekten maßgeblich durch die Orographie beeinflusst, während bei neutraler Schichtung orographische Effekte nur gering ausgeprägt waren.

This thesis presents a computational framework that models the coupled behavior between sand deposits and their surrounding fluid. Particle deposits that form in gas turbine engines and industrial burners, can change flow dynamics and heat transfer, leading to performance degradation and impacting durability. The proposed coupled framework allows insight into the coupled behavior of sand deposits at high temperatures with the flow, which has not been available previously. The coupling is done by using a CFD-DEM framework in which a physics based collision model is used to predict the post-collision state-of-the-sand-particle. The collision model is sensitive to temperature dependent material properties of sand. Particle deposition is determined by the particle's softening temperature and the calculated coefficient of restitution of the collision. The multiphase treatment facilitates conduction through the porous deposit and the coupling between the deposit and the fluid field. The coupled framework was first used to model the behavior of softened sand particles in a laminar impinging jet flow field. The temperature of the jet and the impact surface were varied(T^* = 1000 – 1600 K), to observe particle behavior under different temperature conditions. The Reynolds number(Rejet = 20, 75, 100) and particle Stokes numbers (Stp = 0.53, 0.85, 2.66, 3.19) were also varied to observe any effects the particles' responsiveness had on deposition and the flow field. The coupled framework was found to increase or decrease capture efficiency, when compared to an uncoupled simulation, by as much as 10% depending on the temperature field. Deposits that formed on the impact surface, using the coupled framework, altered the velocity field by as much as 130% but had a limited effect on the temperature field. Simulations were also done that looked at the formation of an equilibrium deposit when a cold jet impinged on a relatively hotter surface, under continuous particle injection. An equilibrium deposit was found to form as deposited particles created a heat barrier on the high temperature surface, limiting more particle deposition. However, due to the transient nature of the system, the deposit temperature increased once deposition was halted. Further particle injection was not performed, but it can be predicted that the formed deposit would begin to grow again. Additionally, a Large-Eddy Simulation (LES) simulation, with the inclusion of the Smagorinsky subgrid model, was performed to observe particle deposition in a turbulent flow field. Deposition of sand particles was observed as a turbulent jet (Re jet=23000,T_jet^*= 1200 K) impinged on a hotter surface(T_surf^*= 1600 K). Differences between the simulated flow field and relevant experiments were attributed to differing jet exit conditions and impact surface thermal conditions. The deposit was not substantive enough to have a significant effect on the flow field. With no difference in the flow field, no difference was found in the capture efficiency between the coupled and decoupled frameworks. / Master of Science / Particle deposits can form in a wide range of environments leading to altered performance. In applications, such as jet engines, particles are heated to critically high temperatures. At these high temperatures, the particles can soften, and begin to exhibit characteristics of both a liquid and a solid. Overtime as these softened particles aggregate on a wall, a deposit will begin to form. These deposits alter the geometry resulting in changes in fluid temperature and velocity. This change in fluid behavior will affect the rate of particle deposition that happens in the future. There has been limited work that has looked at the coupled behavior between a deposit and its surrounding fluid, experimentally or computationally. The purpose of this research was to develop a framework that models the deposition of softened particles, and the coupled behavior between deposits and the fluid. This research was able to show that the presence of a deposit could change its surrounding fluid’s velocity and temperature significantly. Differences in the rate of particle deposition also occurred when a deposit had formed on a surface. These results show the importance of capturing the relationship between deposits and the surrounding fluid. With further development, this proposed framework can provide insight into altered gas turbine performance and can lead to improved maintenance plans.