PhD, University of Cincinnati, 2013, Engineering and Applied Science: Aerospace Engineering

A numerical study is conducted to examine the vortex structure and aerodynamic loading about a revolving wing in quiescent flow. The high-fidelity, implicit large eddy simulation (ILES) technique is employed to simulate a revolving wing configuration consisting of a rectangular plate extended out half a chord from the rotational axis at a fixed geometric angle relative to this axis. Shortly after the onset of the motion, the rotating wing generates a coherent vortex system along the leading-edge. This vortex system remains attached throughout the motion for the range of Reynolds numbers explored despite the unsteadiness and vortex breakdown observed at higher Reynolds numbers. The average and instantaneous wing loading also increases with Reynolds number. At a fixed Reynolds number, the attachment of the leading-edge vortex (LEV) is shown to be insensitive to geometric orientation of the wing. Additionally, the flow structure and forcing generated by a purely translating wing is investigated and compared with that of the revolving wing. Similar features are present at the inception of the motion; however, the two flows evolve very differently for the remainder of the maneuver. The role of aspect ratio is also examined, and the span-wise evolution of the leading-edge vortex is analyzed. The mean lift and drag both increase with aspect ratio until the chord-wise growth of the leading-edge vortex becomes constrained by the trailing edge causing a saturation of the aerodynamic loads. Additional LEV substructures form with increased aspect ratio from the increase in local span-based Reynolds number. The genesis of these substructures has been traced to the eruption of secondary, wall-induced vorticity under the LEV that penetrates the LEV feeding sheet. The disrupted shear layer then rolls-up under self-induction to form discrete substructures. Vortex breakdown of the vortex core occurs around mid-span despite aspect ratio of the wing indicating that it (open full item for complete abstract)

Committee: Paul Orkwis Ph.D. (Committee Chair); Miguel Visbal Ph.D. (Committee Member); Shaaban Abdallah Ph.D. (Committee Member); Mark Turner Sc.D. (Committee Member) Subjects: Aerospace Materials

Doctor of Philosophy (PhD), Wright State University, 2024, Engineering PhD

It is generally accepted that there exist two types of laminar separation bubbles (LSBs): short and long. The process by which a short LSB transitions to a long LSB is known as bursting. In this research, large eddy simulations (LES) are used to study the evolution of an LSB that develops along the suction surface of the L3FHW-LS at low Reynolds numbers. The L3FHW-LS is a new high-lift, high-work low-pressure turbine (LPT) blade designed at the Air Force Research Laboratory. The LSB is shown to burst over a critical range of Reynolds numbers. Bursting is discussed at length and its effect on transition, vortex shedding, and profile loss development are analyzed in depth. The results of these analyses make one point very clear: the effects of bursting are non-trivial. That is, long LSBs are not just longer versions of short LSBs. They are phenomena unto themselves, distinct from short LSBs in terms of their vortex dynamics, profile loss footprint, time-averaged topology, etc. This work culminates in a demonstration of how, with the aid of unsupervised machine learning, these differences can be leveraged to reduce the energy requirements of steady vortex generator jets (VGJs). Relative to pulsed VGJs, steady VGJs require significantly more energy to be effective but are more realistic to implement in actual application. By tailoring VGJ actuation to LSB type (i.e., actuating differently in response to a long LSB than to a short LSB), it is shown that significant energy savings can be realized.

Committee: Mitch Wolff Ph.D. (Advisor); George Huang Ph.D. (Committee Member); John Clark Ph.D. (Committee Member); Christopher Marks Ph.D. (Committee Member) Subjects: Fluid Dynamics

PhD, University of Cincinnati, 2023, Engineering and Applied Science: Aerospace Engineering

This thesis focuses on the use of Large Eddy Simulations (LES) to investigate non-reacting and reacting multiphase flows. In the first part, the study explores the impact of increasing Weber number on spray and flow characteristics in a jet in crossflow configuration. The results demonstrate that as the Weber number increases, finer droplets are formed, leading to increased droplet velocities and adjustments to the freestream more quickly. The analysis also reveals a linear decline in droplet size with increasing Weber number, shedding light on the influence of this parameter on spray characteristics. Moving on to the combustion simulation, the thesis examines the combustion of a liquid fuel jet injected into crossflowing air, utilizing a v-gutter bluff-body for flame stabilization. Surprisingly, the study finds that a majority of droplets impinging upon the surface of the v-gutter get deposited, forming a thin film that eventually breaks up downstream. This unexpected behavior results in the formation of a nearly stoichiometric fuel-air mixture, indicating the high efficiency of the current configuration for achieving clean combustion. Detailed analysis of mean profiles reveals asymmetric temperature distribution, with higher temperatures closer to the bottom wall of the test section due to concentrated dispersed droplets trapped in the wake of the v-gutter. However, the velocity profiles become nearly symmetric at the exit of the combustor, indicating sufficient residence time for velocity gradients to readjust. In the last part of the thesis, the focus shifts to the simulation of non-reacting flow in a full-scale FAA Nexgen burner. The study provides insights into the unsteady flowfield, identification of flow structures, and quantification of swirl generation and turbulence caused by the flow's unsteadiness within the turbulator. It reveals the transition from laminar to turbulent flow, with maximum turbulence production occurring within the pocket (open full item for complete abstract)

Committee: Prashant Khare Ph.D. (Committee Chair); Jongguen Lee Ph.D. (Committee Member); Luis Bravo Ph.D. (Committee Member); Shaaban Abdallah Ph.D. (Committee Member) Subjects: Aerospace Engineering

MS, University of Cincinnati, 2021, Engineering and Applied Science: Mechanical Engineering

Gas turbine engines are one of the most widely used propulsion systems and are commonly encountered in commercial and defense aircraft.Increasing the turbine inlet temperatures is one of the convenient approaches to increase the efficiency of the engine, but this process is restricted by the material threshold temperatures and the structural integrity of the turbine blade. In order to keep the blade temperatures within limits for seamless working of the engine, various cooling techniques are employed. One such technique is film cooling using holes wherein the air from the compressor is bled and fed into the film cooling holes. A part of this computational study on film cooling involves the numerical validation of the LES computational technique with Dynamic Smagorinsky model to predict the adiabatic mixing phenomenon of the cold air exiting from the 7-7-7 baseline shaped hole with the mainstream hot air for the BR and DR values of 1.5 against the experimental data provided by PSU. A single hole from the experimental array of 5 holes is considered with periodic boundary conditions and the parameters of comparison are the centerline and lateral averaged adiabatic effectiveness. The non-dimensional velocity parameter shows good comparison with the PIV measurements in the boundary layer at the leading and the trailing edges of the hole and the flat plate adiabatic effectiveness after compound averaging show decent comparison with the Infrared image data. The CRVP structure is weak in this case as expected for a laid-back fan shaped hole and the centerline and lateral averaged adiabatic effectiveness as compared to the experimental data lies within 25% deviation. The overall accuracy of the solution based on the lateral average adiabatic effectiveness is calculated to be 74.46% for this perfectly adiabatic case. The second part of this computational study involved employing the RANS technique with the K-Omega turbulence model to simulate the multiple perforation cy (open full item for complete abstract)

Committee: Mark Turner Sc.D. (Committee Chair); Milind Jog Ph.D. (Committee Member); Jay Kim Ph.D. (Committee Member); Paul Orkwis Ph.D. (Committee Member) Subjects: Aerospace Materials

Master of Science in Mechanical Engineering (MSME), Wright State University, 2021, Mechanical Engineering

This study characterizes the functional dependence of drag on Reynolds number for a deformed vacuum lighter-than-air vehicle. The commercial computational fluid dynamics (CFD) code, FLUENT, is used to preform large eddy simulations (LES) over a range of Reynolds numbers; only Reynolds numbers less than 310,000 are considered. While the overarching goal is drag characterization, general flow-field physics are also discussed, including basic turbulence spectra. All large eddy simulations are preceded by a Reynolds-averaged Navier-Stokes (RANS) simulation using Menter's shear stress transport (SST) model. The precursor RANS simulation serves to (1) provide realistic initial conditions, (2) decrease the time needed to achieve a statistically averageable state, (3) assess near-wall mesh resolution, and (4) provide an estimate of the integral length scale. After achieving a statistically averageable state, each LES is integrated for at least 5 through-flow times. For sub-grid scale modeling, turbulence kinetic energy (TKE) transport is enabled, as it is the only model which allows for direct assessment of TKE resolution; all simulations resolve at least 80% of the total TKE at every point in the computational domain. To validate this study, all calculated drag coefficients are compared with experimental wind tunnel data.

Committee: Mitch Wolff Ph.D. (Committee Chair); Anthony N. Palazotto Ph.D., P.E. (Committee Co-Chair); George P. Huang Ph.D. (Committee Member) Subjects: Aerospace Engineering; Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2021, Aero/Astro Engineering

Shock wave/boundary-layer interactions (SBLIs) are ubiquitous to both the external vehicle body and internal propulsion flowpath. The external surfaces include for example the nose, wings, and tail, while internal surfaces include bounding walls of mixed compression inlets, diffusers, and isolators. To obtain predictions of these flowfields, Reynolds-averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) solvers are preferred over computationally expensive scale-resolving methods like large-eddy simulation (LES), hybrid RANS/LES, and direct numerical simulation (DNS). However, RANS predictions are highly dependent on the choice of turbulence model. Reasons include the fundamental assumption (Boussinesq eddy-viscosity approximation) made in the development of turbulence models and the estimation of the terms that govern turbulence transport. In this work, key mechanisms in the exact Reynolds stress transport equation are examined with a view towards identifying important flow phenomena and improving prediction of flows with SBLIs. Upon linking the unsteady dynamics back to the Reynolds stress transport, significant mechanisms can be isolated, whose relative importance varies across the SBLI region. By discovering the importance of these mechanisms to SBLIs, current models can be improved and mechanisms generally assumed negligible due to the lack of experimental or high-fidelity data can be modeled. With this purpose in mind, wall-resolved LES (WRLES) of SBLI are considered at several Reynolds numbers. The flow conditions and geometry are based on the experiments performed at the Institut Universitaire des Systemes Thermiques Industriel (IUSTI) in Marseille, France at Mach 2.29 for an 8 degree deflection angle shock wave impinging on a turbulent boundary layer developing on the opposite surface. Due to a closely coupled relationship between the corner flow and centerline separation in small- to medium-aspect-ratio configurations, the full tunnel span (open full item for complete abstract)

Committee: Datta Gaitonde (Advisor); Mohammad Samimy (Committee Member); Jen-Ping Chen (Committee Member); Yoder Dennis (Committee Member) Subjects: Aerospace Engineering; Fluid Dynamics

MS, University of Cincinnati, 2021, Engineering and Applied Science: Mechanical Engineering

Motivated by recent experiments on the dustiness of nanoscale powders, this research addresses the modeling of powder aerosolization within the Venturi Dustiness Test (VDT) apparatus. In particular, we study the effect of vortex shedding on the aerosolization of a test particle from a powder hill at Reynolds number of 20,000 in the powder holding device attached to VDT. The powder holding tube is represented as a cylindrical tube with a hemispherical (powder) hill situated within. As the first step in such modeling, we investigate the flow over a hemispherical (powder) hill, at the operating Reynolds number Re ~ 20,000, in the powder holding tube attached to the VDT. The powder holding tube is idealized as a cylindrical tube, obstructed by a hemispherical (powder) hill. The upstream flow field is characterized by the presence of a horse-shoe vortex, formed due to the separation of the boundary layer from the wall of the tube. A stagnation point occurs at the lower front surface of the bump. Strong near-wall vorticity is generated midway up the bump, and the flow is separated near the top of the bump. Kelvin-Helmholtz vortices are produced due to the strong shear-layer vorticity, and these then travel downstream with the flow. The upstream near-wall flow is dominated by a horseshoe vortex forming a necklace pushed out into the wake area. The flow detaches from the bump at the separation line, leading to vortex `roll-up'. These rolled-up vortices merge with the horseshoe vortices to form a large, entangled hairpin vortex. The arch-type vortices shed with a frequency consistent with the Strouhal estimate fSt ~ 7670 Hz. After this study, a test particle is positioned at various locations on the hill. We study the flow behavior past the hemispherical hill in the tube and its influence on the aerodynamic forces experienced by a test particle located at various positions on the hill. The frequency of the vortex shedding from the bump yields a Strouhal number St = (open full item for complete abstract)

Committee: Milind Jog Ph.D. (Committee Chair); Urmila Ghia Ph.D. (Committee Member); Leonid Turkevich Ph.D. (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2019, Aero/Astro Engineering

Recent experimental and computational investigations have provided a comprehensive phenomenological description of unsteadiness in nominally two-dimensional (2-D), spanwise homogeneous shock-wave/turbulent-boundary-layer interactions (STBLIs), including both the impinging-shock and compression-ramp configurations. However, a complete mechanistic description of unsteadiness from an objective dynamical systems perspective has been lacking. Furthermore, the STBLIs encountered in many aerospace applications are fundamentally three-dimensional (3-D), in which the separation structure and topology are profoundly different from those exhibited by 2-D interactions, rendering many of the conclusions derived for the latter inapplicable. This dissertation addresses the main knowledge gaps and advances the understanding of STBLI unsteadiness by providing: (1) an objective mechanistic description of unsteadiness in 2-D interactions, (2) a phenomenological description of unsteadiness in nominally 3-D interactions (swept interactions that are inherently not spanwise homogeneous), including the sharp-fin and swept-compression-ramp configurations, (3) an objective mechanistic description of unsteadiness in these 3-D interactions, and (4) a phenomenological description of unsteadiness in a representative compound 3-D interaction (a double-fin inlet/isolator configuration). The approach employs high-fidelity large-eddy simulations of various 2-D and 3-D STBLIs in the high-supersonic (Mach 2-4) speed regime. Simulation accuracy is ensured through extensive comparison with experimental data obtained from concurrent experimental campaigns at partner institutions. The phenomenological description of unsteadiness is then compiled from various analyses of the resulting, spatiotemporally varying, turbulent flow. These include spectra of the unsteady fluctuations, band-isolated fluctuation dynamics obtained through temporal filtering, reduced-order representations, correlations, and other (open full item for complete abstract)

Committee: Datta Gaitonde PhD (Advisor); Mohammad Samimy PhD (Committee Member); Jen-Ping Chen PhD (Committee Member) Subjects: Aerospace Engineering

Doctor of Philosophy, The Ohio State University, 2017, Aero/Astro Engineering

The exact mechanism by which turbulent fluctuations in jets are converted into acoustic energy remains unexplained. The current work aims to improve our understanding of this problem by localizing acoustic sources and identifying its causal dynamics. We use Large-Eddy Simulations of a Mach 1.3 turbulent cold jet for this purpose. The localization question is resolved by using a novel technique, termed Synchronous Large-Eddy Simulations (SLES), that tracks the non-linear evolution of small perturbations from any region (window) in a time-varying base flow. This provides superior insights into the generation of intermittency and directivity compared to traditional approaches that use linear stability analyses based on steady basic states or backward correlations between different regions of the flow. In SLES, two simulations are performed in a lock-step manner. At each step, native fluctuations from a desired spatial window in the first (or baseline simulation) are scaled to small values and then injected into the second (or twin simulation) to provide a forcing in the targeted region. At subsequent times, the difference between the two simulations provides a snapshot of the evolution of the perturbation field associated with the continuous forcing in the chosen spatial window. The perturbation field, which is equivalent to the solution of the forced Navier-Stokes equations linearized about the time-evolving base flow, is then statistically analyzed to identify its modulation by the turbulent region of the jet. Results are distilled by examining forcing at lipline and centerline locations in detail. The end of the potential core is found to be a sensitive zone where perturbations are amplified and lead to secondary sources. Perturbations within the shear layer on the other hand, are initially channeled toward the core and undergo higher amplification compared to those originating from the centerline, before propagating outward. Stati (open full item for complete abstract)

Committee: Datta Gaitonde Dr. (Advisor); Jen-Ping Chen Dr. (Committee Member); Brenda Henderson Dr. (Committee Member); Sandip Mazumder Dr. (Committee Member); Mo Samimy Dr. (Committee Member) Subjects: Aerospace Engineering; Mechanical Engineering

MS, University of Cincinnati, 2016, Engineering and Applied Science: Aerospace Engineering

In an aircraft engine at high altitude, the low-pressure turbine (LPT) section can experience low-Reynolds number (Re) flows making the turbine blades susceptible to large separation losses. These losses are detrimental to the performance of the turbine and lead to a roadblock for “higher-lift” blade designs. Accurate prediction of the separation characteristics and an understanding of mitigation techniques are of the utmost importance. The current study conducts simulations of flow control techniques for the Air Force Research Laboratory (AFRL) L2A turbine blade at low-Re of 10,000 based on inlet velocity and blade axial chord. This blade was selected for its “high-lift” characteristics coupled with massive separation on the blade at low-Re which provides an excellent test blade for flow control techniques. Flow control techniques involved various configurations of vortex generator jets (VGJs) using momentum injection (i.e. jet blowing). All computations were executed on dual-topology, multi-block, structured meshes and incorporated the use of a parallel computing platform using the message passing interface (MPI) communications. A high-order implicit large eddy simulation (ILES) approach was used in the simulations allowing for a seamless transition between laminar, transitional, and turbulent flow without changing flow solver parameters. A validation study was conducted involving an AFRL L1A turbine blade which showed good agreement with experimental trends for cases which controlled separation in the experiments. The same cases showed good agreement between different grid sizes. The differences between experimental and numerical results are largely attributed to differences in the setup. That is, the simulation did not include freestream turbulence or wind-tunnel wall effects. The flow control study conducted for the L2A blade showed a small degree of separation control for jets placed just downstream (DS) of the separation point. A limited study was cond (open full item for complete abstract)

Committee: Kirti Ghia Ph.D. (Committee Chair); Rolf Sondergaard Ph.D. (Committee Member); Shaaban Abdallah Ph.D. (Committee Member); Urmila Ghia Ph.D. (Committee Member) Subjects: Aerospace Materials

Doctor of Philosophy, The Ohio State University, 2015, Aero/Astro Engineering

This work uses Large-Eddy Simulations to examine the effect of actuator parameters and jet exit properties on the evolution of coherent structures and their impact on the near-acoustic field without and with control. For the controlled cases, Localized Arc Filament Plasma Actuators (LAFPAs) are considered, and modeled with a simple heating approach that successfully reproduces the main observations and trends of experiments. A parametric study is first conducted, using the flapping mode (m=±1), to investigate the sensitivity of the results to various actuator parameters including: actuator model temperature, actuator duty cycle, and excitation frequency. It is shown by considering a Mach 1.3 jet at Reynolds number of 1 ×106 that the response of the jet is relatively insensitive to actuator model temperature within the limits of the experimentally measured temperature values. Furthermore, duty cycles in the range of 20%−90% were observed to be effective in reproducing the characteristic coherent structures of the flapping mode. The 90% duty cycle exhibits strengthened coherent structures and slightly higher jet growth along the flapping plane, but the overall dynamics remain almost identical to the lower duty cycle cases. However, a 100% duty cycle had no perceptible effect on the jet. Therefore, increasing the energy inserted into the flow via actuator temperature or duty cycle does not significantly alter the flow dynamics. The largest sensitivity was associated with excitation frequency, with the most significant effect associated with the column mode instability frequency (St ≅0.3), which results in alternating vortex rings for this mode. Higher and lower frequencies reduced the rate of decay of the centerline velocity. Although the higher frequencies increased the number of features observed in the phase-averaged data, their prominence is reduced due to their breakdown into smaller structures. These actuator parameter results confirm that the flow re (open full item for complete abstract)

Committee: Datta Gaitonde (Advisor); Mo Samimy (Committee Member); Mei Zhuang (Committee Member); Jen-Ping Chen (Committee Member) Subjects: Acoustics; Aerospace Engineering

MS, University of Cincinnati, 2010, Engineering : Aerospace Engineering

Presented is a high-fidelity, computational study of transitional flow over an airfoil as it is pitched up from an initial zero incidence to 40 degrees at a nominally constant pitch rate, held, and then returned in a similar manner. The Reynolds numbers were chosen to bracket the regions of laminar and transitional flows applicable to prototypical micro air vehicle conditions, 5000 < Re < 40,000. A high-order, implicit large eddy simulation technique was employed to show the degree of fidelity required to capture these highly transitional flows. Two-dimensional analyses examining the effects of Reynolds number and pitch rate were conducted and a discussion is provided. Additionally, the impact of transition and spanwise extent on the flowfield and force histories were explored through three-dimensional, spanwise periodic simulations. These simulations were shown qualitatively to compare extremely well with available experimental observations.

Committee: Paul Orkwis PhD (Committee Chair); Mark Turner ScD (Committee Member); Miguel Visbal PhD (Committee Member) Subjects: Aerospace Materials

MS, University of Cincinnati, 2009, Engineering : Aerospace Engineering

A laminar separation bubble is known to be detrimental to the performance of airfoils operating at low Reynolds numbers (Re < 105). With increasing interest in Micro Air Vehicles (MAV), a clear understanding of the formation and subsequent turbulent breakdown of laminar separation bubbles is required for improved handling, stability, and endurance of MAV's. A computational investigation of flow past the SD7003 airfoil over the Reynolds number range 104 < Re < 9x104 is presented. This airfoil was selected due to its robust laminar separating bubble and the availability of high-resolution experimental data. A high-order implicit large-eddy simulation (ILES) approach capable of capturing the laminar separation and subsequent three-dimensional breakdown is shown. The ILES methodology also predicts, without change in parameters, the passage into full airfoil stall at high incidence. In addition, computed separation, reattachment, and transition locations, as well as aerodynamic loads generally agree well with experimental data. Finally, a blowing/suction slot positioned near the leading edge was shown to energize the two-dimensional mode and reduced spanwise instabilities of the shear layer. This caused transition to occur further downstream and effectively eliminated the time mean laminar separation bubble.

Committee: Paul Orkwis Prof (Committee Chair); Mark Turner Prof (Committee Member); Shaaban Abdallah Prof (Committee Member); Miguel Visbal Dr (Committee Member) Subjects: Aerospace Materials; Engineering

PhD, University of Cincinnati, 2008, Engineering : Mechanical Engineering

The operating Reynolds numbers (Re) for a low-pressure turbine (LPT) in an aircraft engine can drop below 25,000 during high-altitude cruise conditions, resulting in massive separation and subsequent transition on the blade suction surface. This separation causes a significant loss in the engine efficiency. Hence, accurate prediction of the flow physics at these low-Re conditions is required to effectively implement flow control techniques which can help mitigate separation-induced losses. The present work investigated this low-Re transitional flow through a LPT cascade comprised of the generic Pratt & Whitney “PAKB” blades, using high-order accurate compact numerical schemes in conjunction with large-eddy simulation (LES), with and without subgrid-scale (SGS) models. The study examined the predictive capability of the explicit Smagorinsky and dynamic Smagorinsky SGS models, as compared to the Implicit LES (ILES) technique (LES without an explicit SGS model). The research also implemented active flow control on the LPT blades using momentum injection via surface blowing. All simulations utilized a dual-topology, multi-block, structured grid, and computations were performed on a massively parallel computing platform using MPI-based communications. The baseline cases (without control) were simulated at Re ~ 10,000, 25,000 and 50,000. The computed numerical results for all three cases showed good agreement with available experimental data. The Smagorinsky and dynamic Smagorinsky SGS model results provided no significant improvement over the ILES results because of the low level of energy in the subgrid-scales for the present low-Re flow conditions investigated, and hence, the ILES technique was used for all subsequent flow-control simulations. Separation control of the LPT flow was implemented using synthetic normal jets, synthetic vortex-generator jets, and pulsed vortex-generator jets (VGJs) at Re ~ 10,000, for four blowing ratios ranging from 0.5 to 4.7, where the b (open full item for complete abstract)

Committee: Urmila Ghia (Advisor) Subjects: Engineering, Mechanical

MS, University of Cincinnati, 2006, Engineering : Mechanical Engineering

Combustion in a lean pre-mixed (LPM) combustor may become unstable due to small changes in geometry and the manner in which reactants are introduced. This may lead to excessive thermal loads and possible off-design operation. A comprehensive understanding of combustion instability is therefore needed. The present study aims to analyze the flow and flame dynamics in a model LPM gas turbine combustor in LPM combustion. Fluent is used as the flow solver for the present study. The 3-D Navier-Stokes equations are solved along with finite-rate chemical reaction equations and variable thermo-physical properties. Large-eddy-simulation (LES) technique is used to model turbulence. The dynamic version of the Smagorinsky-Lilly model is employed to describe subgrid-scale turbulent motions and their effect on large-scale structures. At first a non-reactive LES was performed in model round and LM6000 combustor. The results for time averaged mean velocity are compared with the previous LES work by Grinstein et al. and Kim et al. Using non-reacting case for LM6000, reactive simulation was initiated, with lean methane-air mixture with equivalence ratio 0.56. Species transport equation is solved for global methane-air two-step reaction with six volumetric species to predict the local mass fraction of each species. The reaction rates that appear as source terms in the species transport equation are computed using finite-rate/eddy-dissipation model, which computes both, the Arrhenius rate and the mixing rate and uses the smaller of the two. It is observed that as the flow enters the chamber, it bifurcates in two shear layers forming a prong like structure. The layers further tend to reattach to the wall at a distance approximately equal to 3D. Counter-clockwise recirculation zones are formed in the corners, whereas clock-wise toroidal vortex structure is formed in the center. The flame is located in between these vortex structures and thus experiences shear-layer instabilities. It is al (open full item for complete abstract)

Committee: Dr. Urmila Ghia (Advisor) Subjects:

Doctor of Philosophy in Engineering, University of Toledo, 2004, Engineering

In chemical processing industries, heating, cooling and other thermal processing of viscous fluids are an integral part of the unit operations. Consequences of improper mixing include non-reproducible processing conditions and lowered product quality. Static mixers economically promote the mixing of flowing fluid streams. One typical static mixer, the helical static mixer, consists of left- and right-twisting helical elements placed at right angles to each other. The range of Reynolds numbers of practical flows for helical static mixers in industry is usually from very small values to not very large values (e.g., Re = 5,000). This thesis describes how static mixing processes of single-phase Newtonian and also non-Newtonian liquids can be simulated numerically and provides useful information that can be extracted from the simulation results. The Turbulent flow case is solved using the most common Reynolds Averaged Navier-Stocks (RANS) models as well as Large-Eddy Simulation (LES) turbulent flow model. The numerical simulation of the mixing in the helical static mixer has been performed via a two-step procedure. In the first step, the flow velocity (and the pressure) is computed. These values are then used as input to the next step. In the second step the particle trajectory in the flow field is calculated. At the entry of the pipe inlet, a large number of marker particles are uniformly distributed over half of the flow field. This represents a simplified model for diametrical feeding of the mixer with two liquids. Using different measurement tools, such as Residence Time Distribution (RTD) and Particles Distribution Uniformity (PDU), the performance of a six-element helical static mixer is studied. It is shown that the Reynolds number has a major impact on the performance of a static mixer. It is also shown that the performance of a helical static mixer is different for Newtonian and non-Newtonian fluids in non-creeping flows. Finally, heat transfer within a helical (open full item for complete abstract)

Committee: Theo Keith (Advisor) Subjects: Engineering, Mechanical

Master of Science, The Ohio State University, 2012, Aero/Astro Engineering

This effort investigates numerical and physical parameters influencing an ideally-expanded Mach 1.3 jet excited by the m=+/-1 flapping mode. The excitation is imposed by eight Localized Arc Filament Plasma Actuators (LAFPA) placed around the periphery of the circular nozzle exit. The devices are modeled with a proven surface heating approach. The reference case considers the most amplified (jet column mode) frequency corresponding to a Strouhal number of 0.3, based on the diameter of the nozzle and the jet velocity, with an actuator-imposed temperature of 1500K and a duty cycle of 20%. Relative to this reference, the effects of changing frequency, duty cycle and actuator model temperature are explored. In some cases, e.g., actuator temperature, experimental data is not available, but for frequency, there is. The results are analyzed with several different quantitative and qualitative metrics, including time-averaged centerline decay and jet half width as well as phase-averaged coherent structures. Raising the frequency affects the dynamics in several ways. The number of vortical features observed in the phase-averaged data increases and the rate of decay of the centerline velocity is reduced. Furthermore, the alternating vortex ring interactions observed in the reference case are not distinct but are rather replaced by smaller structures, trends which are also observed in experiment. The flow mixes the fastest around the jet column mode (St~0.22). The higher duty cycles exhibits strengthened coherent structures and slightly higher jet growth along the flapping plane, but the overall dynamics remain the same. The response of the jet is relatively insensitive to actuator temperature model within reasonable limits. The latter two studies, with different duty cycles and actuator temperatures, are consistent with previous analyses demonstrating that instability manipulation, rather than heat deposition is the primary mechanism of control.

Committee: Datta Gaitonde PhD (Advisor); Mo Samimy PhD (Committee Member); Mei Zhuang PhD (Committee Member) Subjects: Aerospace Engineering; Engineering; Mechanical Engineering

Master of Science, The Ohio State University, 2011, Civil Engineering

We use a recently developed high-resolution atmospheric large eddy simulation to determine the potential optimal locations for wind-powered turbines on OSU property. The Ohio State University (OSU) can increase its energy efficiency and reduce its carbon footprint by self producing some of the power it consumes with wind energy. The locations of future wind-powered turbines will have to fit within the existing constraints of building locations and forested park areas. Trees and building modify wind patterns and affect the efficiency of near-by wind turbines. In planning the optimal locations for future wind energy production, it is essential to have a tool that can resolve the effects of natural and manmade structures within a small, restricted-space property. Such a tool will be useful on the OSU campus and has broad commercial potential for middle-scale wind-energy produces. We used county mapping and LIDAR data to generate a 3-D map of the campus, which includes topography, land-use, buildings and vegetation at 3m resolution. The Regional-atmospheric-modeling-based Forest Large Eddy Simulation (RAFLES) resolves air flow and turbulence inside and above 3-D spatially explicit heterogeneous forest canopies and structures. We used a set of RAFLES simulations to develop a high resolution map of mean wind speed and turbulence on the OSU campus including the effects of current and planned campus buildings and green areas. These variables, combined with 30 years of weather data for Ohio were converted into "wind potential" maps that will be used to determine the optimal locations of wind-turbines, which are most efficient with a combination of strong wind with low turbulence. An ecological survey of vegetation and bird activity completes the picture by determining the potential impacts of turbine-induced micro-climatic changes.

Committee: Gil Bohrer (Advisor); Alper Yilmaz (Committee Member); Ethan Kubatko (Committee Member) Subjects: Civil Engineering; Environmental Engineering