Studying the effects of free stream turbulence on noise, vibration, and heat transfer on structures is very important in engineering applications. The problem of the interaction of large scale turbulence with a flat-plate cascade is a model of important problems in propulsion systems. Addressing the problem of large scale turbulence interacting with a flat plate cascade requires flow simulation over a large number of plates (6-12 plates) in order to be able to represent numerically integral length scales on the order of blade-to-blade spacing. Having such a large number of solid surfaces in the simulation requires very large computational grid points to resolve the boundary layers on the plates, and that is not possible with the current computing resources. In this thesis we develop a computational technique to predict the distortion of homogeneous isotropic turbulence as it passes through a cascade of thin flat plates. We use Large-Eddy Simulation (LES) to capture the spatial development of the incident turbulence and its interaction with the plates which are assumed to be inviscid walls. The LES is conducted for a linear cascade composed of six plates. Because suppression of the normal component of velocity is the main mechanism of distortion, we neglect the presence of mean shear in the boundary layers and wakes, and allow slip velocity on the plate surfaces. We enforce the zero normal velocity condition on the plates. This boundary condition treatment is motivated by rapid distortion theory (RDT) in which viscous effects are neglected, however, the present LES approach accounts for nonlinear and turbulence diffusion effects by a sub-grid scale model. We refer to this type of turbulence-blade interaction as shear-free interaction. To validate our calculations, we computed the unsteady loading and radiated acoustic pressure field from flat plates interacting with vortical structures. We consider two fundamental problems: (1) A linear cascade of flat plates excited by a vortical wave (gust) given by a 2D Fourier mode, and (2) The parallel interaction of a finite-core vortex with a single plate. We solve the nonlinear Euler equations by a high-order finite-differece method. We use nonreflecting boundary conditions at the inflow and outflow boundaries. For the gust problem, we found that the cascade response depends sensitively on the frequency of the convicted gust. The unsteady surface pressure distribution and radiated pressure field agree very well with predictions of the linear theory for the tested range of reduced frequency. We have also investigated the effects of the incident gust frequency on the undesirable wave reflection at the inflow and outflow boundaries. For the vortex-plate interaction problem, we investigate the effects of the internal structure of the vortex on the strength and directivity of radiated sound. Then we solved the turbulence cascade interaction problem. The normal Reynolds stresses and velocity spectra are analyzed ahead, within, and downstream of the cascade. Good agreement with predictions of rapid distortion theory in the region of its validity is obtained. Also, the normal Reynolds stress profiles are found to be in qualitative agreement with available experimental data. As such, this dissertation presents a viable computational alternative to rapid distortion theory (RDT) for the prediction of noise radiation due to the interaction of free stream turbulence with structures. / Ph. D.

The issue of the effects of free stream turbulence on the flow field over a surface-mounted prism is examined through experimental and numerical investigations. In the experimental studies, particle image velocimetry measurements are conducted in the ESM water tunnel at Reynolds number of $9,600$ and under two cases of turbulent inflow conditions. The results show that the mean flow separation, reattachment and parameters such as mean velocity, root mean square, Reynolds stresses and turbulent kinetic energy are affected by the turbulence characteristics of the incident flow. The instantaneous dynamics of the interactions between the separating shear layer and the solid wall and between the shear layer and the turbulence in the incident flow are detailed. In the numerical studies, large eddy simulations of the flow over a surface-mounted prism under two inflow conditions, namely, smooth inflow and isotropic homogeneous turbulence inflow, are performed. The use of a fifth-order scheme (CUD-II-5), which is a member of a family of Compact Upwind Difference schemes, in large eddy simulations of this flow is assessed. The performance of this scheme is validated by comparing the rate of temporal decay of isotropic turbulence with available experimental measurements for grid-generated turbulence. The results show that the spectra are sensitive to the method of flux vector splitting needed for the implementation of the upwind scheme. With van Leer splitting, the CUD-II-5 scheme is found to be too dissipative. On the other hand, using the Lax-Friedrichs vector splitting yields good agreement with experiments by controlling the level of artificial dissipation. This led us to recommend a new procedure, we denote by C6CUD5 scheme, that combines a compact sixth-order scheme with the CUD-II-5 scheme for large eddy simulation of complex flows. The simulation results, including flow patterns, pressure fields and turbulence statistics show that the CUD-II-5 scheme, with Lax-Friedricks flux vector splitting, provides high resolution of local flow structures. The results present new physical aspects of the flow topology over surface-mounted prisms. The effects of the incident homogeneous turbulence on the size of the separation region and suction pressures are determined by pointing out differences in the flow topologies between the two incident flow cases. / Ph. D.

This dissertation investigates the flow physics, turbulence structure, and particle classification process in hydrocyclones using large-eddy simulations of turbulent multiphase flow. Two types of hydrocyclones are considered. The first is a classifying hydrocyclone, and the second is a mineral flotation hydrocyclone, also known as an air-sparged hydrocyclone (ASH). Large-eddy simulations (LES) are conducted for multi-phase flow (air, water, and sand particles) so that the complex anisotropic turbulence of a swirling flow is computed correctly. The effects of mesh refinements on the mean flow and turbulence stresses are investigated, and (LES) results are validated by comparisons with experimental data for classifying hydrocyclone. The two-phase flow in air-sparged hydrocyclone has not been analyzed before. ANSYS CFX software V17.2 has been used to conduct the simulations. Firstly, large-eddy simulations have been conducted for two-phase flow (water and air) in a conventional hydrocyclone using the Eulerian two-fluid (Eulerian-Eulerian) and Volume-of- Fluid (VOF) models. Subgrid stresses are modeled using a dynamic eddy–viscosity model, and results are compared to those using the Smagorinsky model. The effects of grid resolutions on the mean flow and turbulence statistics have been thoroughly investigated. Five block-structured grids of 0.72, 1.47, 2.4, 3.81, and 7.38 million elements have been used for the simulations of a typical conventional hydrocyclone designed and tested by Hsieh (75 mm hydrocyclone) [1]. Mean velocity profiles and normal Reynolds stresses have been compared with experimental data. The results of the Eulerian two-fluid model agree with those of the VOF model. A fine mesh in the axial and radial directions is necessary for capturing the turbulent vortical structures. Turbulence structures in the hydrocyclone are dominated by helical vortices around the air core. Energy spectra are analyzed at different points in the hydrocyclone, and regions of low turbulent kinetic energy are identified and attributed to stabilizing effects of the swirling velocity component. Turbulent energy spectra in the different regions of the hydrocyclone have been analyzed. The energy spectra are calculated at two points near the air-water interface. They show a short inertial subrange where energy decays as f−5/3, followed by viscous damping where energy drops as f−7, where f is frequency. However, for the points located near the boundary where high turbulent kinetic energy is found, the energy spectra exhibit f^(−4) decay. Secondly, the two-fluid (Eulerian two-fluid) model and large-eddy simulation are used to compute the turbulent two-phase flow of air and water in a cyclonic flotation device known as an Air-Sparged Hydrocyclone (ASH). In the operation of ASH, the air is injected through a porous cylindrical wall. The study considers a 48-mm diameter hydrocyclone and uses a block-structured fine mesh of 10.5 million hexahedral elements. The air-to-water injection ratio is 4, and a uniform air bubble diameter of 0.5 mm has been specified. The flow field in ASH has been investigated for the inlet flow rate of water of 30.6 L/min at different values of underflow exit pressure. The present simulations show that the value of static pressure imposed at the underflow section strongly affects the distribution of air volume fraction, water axial velocity, tangential velocity, and swirling layer thickness in ASH. The loci of zero-axial velocity surfaces have been determined for different exit pressures. The water split ratio through the overflow opening varies with underflow exit pressure as 6%, 8%, 16%, and 26% for 3, 4, 5, and 6 kPa, respectively. These results indicate that regulating the pressure at the underflow exit can be used to optimize ASH's performance. Turbulent energy spectra in different regions of the hydrocyclone have been analyzed. Small-scale turbulence spectra at near-wall points exhibit f^(−4) law, where f is frequency. Whereas for points at the air-column interface, the energy spectra show an inertial subrange f^(−5/3) followed by a dissipative range of f^(−7) law. Thirdly, large-eddy simulation (LES) has been used to investigate the flow separation in multi-phase flow (gas, liquid, and solid) in a classifying hydrocyclone using the multi-fluid (Eulerian multi-fluid) model. The results of the CFD simulation are compared with the Hsieh [1] experimental data. The water phase is considered a continuous phase, while air and solid particles are considered dispersed phases. Drag between water-air and water-sand is the only considered interfacial force. The Schiller-Naumann and Wen-Yu models are used to model the drag, and the Gidaspow model is used to calculate the solid pressure term. Various particle sizes are tested in the hydrocyclone to investigate the underflow recovery percentages. The results agree with the experimental data for the particles of a diameter smaller than 20 μm, while the results vary based on the model for the large particles. Therefore, using the Wen Yu and Schiller-Naumann model for the drag model and the Gidaspow model for the solid pressure in the three-fluid model could give acceptable results for the small particles underflow recovery and volume fraction distribution. However, the models failed for large particles. Finally, the large particle size separation needs more investigation. / Doctor of Philosophy / Hydrocyclones are widely used in mining and chemical industries. They can be used as separation devices to separate solid or fluid particles based on their size or/and weight. They can also be used as flotation devices to capture certain mineral particles from a slurry of water and solid particles. The flow field within a hydrocyclone is complex as it involves flow of different phases of matter (liquid, gas, and solid). It is also a turbulent flow in which the velocity and pressure fluctuate in time with many frequencies. The efficiency of the hydrocyclone depends on its geometry and distribution of the velocity. Computer simulations are very efficient tools to predict and study the flow field in hydrocyclones. This dissertation used a computer simulations to explain how turbulence could affect the particle separation from the slurry inside the hydrocyclones. The water's velocity fields, swirling flow, air behavior, pressure distribution and turbulence statistics are analysed. Understanding the turbulence structure and statistics in hydrocyclones is important for particle tracking and dispersion. Also, turbulent structure affects the motion of the air bubbles and solid particles in the flow field, which eventually will affect the hydrocyclone's performance. In short, a more comprehensive understanding of the behavior of turbulence of hydrocyclones represents an important tool that can guide the design of hydrocyclones according to their use goals and will help engineers who model these processes to develop a better model.

For the past several years there has been great effort in the analysis of internal duct cooling. The steady increase in power output and thermal efficiency requirements for gas turbine engines has called for significant advancement in turbine blade internal duct cooling technology. Numerical analysis of turbulent duct flow has been largely limited to Reynolds Averaged Navier-Stokes (RANS) simulations. This is because of the low computational requirements of such calculations relative to Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS). However, the tides have started to turn in favor of LES, partly because of the exponential increase in computer hardware performance in recent years. Three conference papers make up the contents of this thesis. LES is performed for fully developed flow and heat transfer in a staggered 45º ribbed duct in the first paper. The rib pitch-toheight ratio P / e is 10 and a rib height to hydraulic diameter ratio h e / D is 0.1. The Reynolds numberbased on the bulk flow rate and hydraulic diameter is 47,300. The overall heat transfer enhancement obtained was a factor of 2.3, which matched experimental data within 2%. The surfaces of highest heat transfer enhancement were the ribbed walls and the outer wall. Results from LES of an orthogonally rotating 90º ribbed duct are presented in the second paper for rotation numbers: Ro = 0.18, 0.35 and 0.67. The Reynolds number is 20,000. The P / e and h e / D were the same as in the first paper. Turbulence and heat transfer are augmented on the trailing surface and reduced at the leading surface. Secondary flows induced by Coriolis forces, increase heat transfer augmentation on the smooth walls. Finally, the third paper studies the same flow conditions of the second paper and goes further by including effects of centrifugal buoyancy forces using LES. Two buoyancy numbers are studied: Bo = 0.12 and 0.29. Centrifugal buoyancy does not have a large effect on leading side augmentation ratios for all rotation numbers, but increases heat transfer significantly on the trailing side. In all papers, mean flow and heat transfer results compare well with experimental data. / 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

京都大学 / 新制・課程博士 / 博士(工学) / 甲第25278号 / 工博第5237号 / 新制||工||1998(附属図書館) / 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 Agricultural Science / Kyoto University / DFAM

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

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

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.