Doctor of Philosophy with a Major in Aerospace Engineering

Series

Doctor of Philosophy with a Major in Aerospace Engineering

Permanent Link

https://hdl.handle.net/1853/72952

Series Type

Degree Series

Associated Organization(s)

Organizational Unit

Daniel Guggenheim School of Aerospace Engineering

Full item page

Publication Search Results

Now showing 1 - 10 of 39

Modeling moderately dense to dilute multiphase reacting flows

(Georgia Institute of Technology, 2022-03-14) Panchal, Achyut

Computational modeling of multiphase flows consists of two broader sets of methods: resolved approaches where the multiphase entities (MPE) are larger and resolved on the computational grid, and dispersed approaches where the MPEs are relatively small and not resolved on the grid but they are treated as point-particles that interact with the background continuum. A consistent multiphase formulation is developed in this work that can be used to model either resolved or unresolved MPEs over a complete range of volume fractions. One phase is always modeled as a continuum Eulerian phase, whereas the other phase is modeled either as a continuum Eulerian phase, a dispersed Eulerian phase, or a dispersed Lagrangian phase. In the dispersed phase limit, a hybrid EE-EL formulation is developed from first principles, which asymptotes to well-established EE and EL methods in limiting conditions. A smooth and dynamic transition criterion and a corresponding algorithm for conversion between EE and EL are developed. To use EE and EL in their respective regions of effectiveness (dense and dilute, respectively), the transition criterion is designed as a function of the local volume fraction and the local kinetic energy of random uncorrelated motion of particles. Simulations of particle evolution in turbulence, particle dispersion in sector blast, and reactive spray jet show the method’s validity and practical relevance. In the resolved multiphase limit, the formulation limits to a compressible seven-equation diffused interface method (DIM). Novel extensions are developed for modeling surface tension, viscous effects, arbitrary EOS, multi-species, and reactions. The use of a discrete equations method (DEM) relieves the need to use conventional stiff relaxation solvers. Shock propagation through a material interface, surface tension-driven oscillating droplet, droplet acceleration in a viscous medium, and shock/detonation interaction with a deforming droplet are simulated to validate various part of the computational framework and demonstrate its applicability.
Entrainment, Mixing, and Ignition in Single and Multiple Jets in a Supersonic Crossflow

(Georgia Institute of Technology, 2020-08-19) Fries, Dan

Jets in crossflow are a canonical example for three-dimensional turbulent mixing. Here, non-reacting and reacting sonic jets in a supersonic crossflow are studied. The influence of injectant properties on turbulent mixing is investigated. Using pure gases, the molecular weight and specific heat ratio is varied between 4-44 g/mol and 1.24-1.66, respectively. The jets are injected into a Mach 1.71 crossflow with a stagnation temperature ~600 K. Two single jet injectors and two staged jet injectors are designed to characterize potential enhancements in turbulent mixing and combustion processes. Mixture fraction and velocity fields are determined via Mie-scattering off solid particles. Velocity vectors are obtained by processing Mie-scattering image pairs with a correlation technique (particle image velocimetry). To ignite the flow field and enable systematic variation of the ignition location a traversable laser spark system is employed. The reacting flow is probed via CH* chemiluminescence and OH planar laser induced fluorescence visualizing regions containing hot combustion products. A new trajectory scaling improves correlation between all data sets considered, suggesting that the bow shock, boundary layer and momentum flux ratio are the dominant controlling factors. Turbulent mixing rates are highest for injectants with higher molecular weight and lower specific heat ratio. The larger of two jet spacings tested yields the greater enhancement of turbulent mixing rates. Ignition locations on the symmetry plane of the flow field are evaluated for their ability to sustain chemical reactions/heat release. Most favorable ignition locations lie in the windward jet shear layer away from the regions of highest flow strain. The smallest diameter single jet with presumably more boundary layer interaction and moderate strain rates provides the best results with regard to thermal energy release after spark deposition. Trends suggest that moderate compressible strain rates and no flow expansion are advantageous to sustain thermal energy release. Implications for future research directions and opportunities are discussed.
Numerical simulations of real-gas flows with phase-equilibrium thermodynamics

(Georgia Institute of Technology, 2020-07-20) Tudisco, Principio

Motivated by the complex physics of multi-component mixtures in strongly non-ideal, real-gas (RG) conditions reported in the field of chemical engineering, this work aims to address the behavior of multi-phase thermodynamics from a broader point of view. The focus is to evaluate the differences, as well as the possible sources of errors that would arise in a computational fluid-dynamics (CFD) simulation when conventional single-phase and multi-phase equilibrium RG thermodynamics are employed: an area of research that despite the active interest in many communities (especially CFD), has not been completely understood. Knowledge of the effects that multi-phase RG thermodynamics with the assumption of vapor-liquid equilibrium (VLE) can have on a flow dynamics is important because it establishes the relevance of the fully coupled CFD-VLE solver. In fact, this relevance may go beyond the stand-alone calculation of a multi-phase state, providing important insights about the physics that may not be captured if the single-phase assumption is invoked. This work provides an extensive study of RG mixtures from a physical and numerical point of view. The difficulties associated with their modeling are discussed in detail and solutions are provided accordingly. Emphasis is given to the occurrence and suppression of numerical noise in form of pressure oscillations that can pollute the simulation to the point that it cannot be performed. Extension of existing models to eliminate such problem is achieved by incorporating the effects of VLE thermodynamics in a consistent manner, ultimately forming a new and robust tool to investigate the physics further. The resulting model is applied to non-reacting and reacting flows in canonical setups where emphasis is devoted to the discussion of the differences and sources of errors that would occur if this multi-phase behavior is not taken into account. Results show that the different thermodynamic states reached by this advanced model can have an impact on the flow physics, especially in a non-reacting (or more in general cold) regime. In particular, the strong non-linear coupling between the VLE thermodynamics and the transport properties is identified as a key element of difference with respect to the single-phase model counterpart. These differences manifest into the occurrence of localized changes in the fluid properties (such as density) that affect the flow-field in their vicinity, causing visible discrepancies even when time-averaging is performed. Concurrently, results obtained on the reacting side and carried out (for the first time) with finite-rate kinetics suggest that any VLE formation between the products and the reactants may be considered of minor importance. The latter conclusion is supported by the analysis conducted on the multi-phase field which appears to be largely composed of the vapor solution, as expected, hence limiting the analogous effect observed the non-reacting system where a broader range of phase-separation appears instead.
Ignition, topology, and growth of turbulent premixed flames in supersonic flows

(Georgia Institute of Technology, 2019-11-12) Ochs, Bradley Alan

Supersonic combustion ramjets (scramjets) are currently the most efficient combustor technology for air breathing hypersonic flight, however, lack of fundamental understanding and numerous engineering challenges hinder regular deployment of these devices. This work addresses scramjet-relevant knowledge gaps in supersonic turbulent premixed combustion, including laser ignition, numerical modeling, and flame-compressibility interaction. One of the main contributions of this work is introduction of a new turbulent premixed flame arrangement where flame-compressibility interaction can be systematically explored: flame kernels in an expanding flow field. The scramjet flow path is replaced by a simplified channel geometry with a well characterized mean flow acceleration that mimics flow field expansion typically imposed on scramjet combustors to avoid thermal choking. Spherically expanding flames are created via laser ignition and subsequent flame growth and morphology are investigated using combined physical and numerical experiments. Pressure-density misalignment due to flame-compressibility interaction produces vorticity at the flame surface through baroclinic torque, i.e. flame-compressibility interaction acts like a turbulence source. The flame ultimately evolves into a reacting vortex ring that increases the flame speed and enhances reactant consumption. To explore the relative importance of turbulence and compressibility on flame dynamics, the Mach number (M=1.5,1.75,2), equivalence ratio (φ= 1.0,0.9,0.8,0.7), and root-mean-squared turbulent velocity (u'=3.98,4.14,4.45 m/s) are varied systematically. This work also introduces flame kernels in an expanding flow field as a canonical numerical validation test case for flame-compressibility interaction. Inaccuracies in simulation results are easily identified due to high flow velocity and simplicity of the problem. The numerical setup and models are scrutinized to minimize errors. Using the appropriately verified numerical models, simulation results show very reasonable agreement with experimental data. Validated simulations are instrumental in enhancing understanding of the underlying physics of supersonic flame kernels. Laser ignition studies in supersonic flows have historically focused on ignition of non-premixed fuels within cavity flame holders. This work introduces a far simpler and more tractable problem: laser ignition of a fully premixed supersonic gas. Ignition experiments with a range of laser settings are performed to determine supersonic breakdown and ignition probabilities, length of time the ignition event influences flame growth, and Mach number influence on the ignition process. The ignition event has a long-lasting effect on kernel growth, but the influence can be minimized by properly selecting the laser energy. Mach number has a minimal impact on the ignition process, but does affect the initial kernel shape due to flow field variations with Mach number. Kernel growth matches low speed studies closely at early times, but deviates at later times due to vortex ring topology. It is not obvious how the turbulent flame speed will scale for flows with mean compressibility. Therefore, the combined physical and numerical experiments are leveraged to explore this question. The vortex ring causes significant errors in the line of sight-measured burned volume, hence correction factors to convert from line of sight to volumetric measurements are presented. Conditions for displacement and consumption speed equivalency are shown to depend heavily on the particular diagnostic used; which progress variable isocontour is measured and where it is measured within the flame brush must be considered carefully during interpretation of experimental data. Scaling with the RMS turbulent velocity cannot collapse these flame speed data, i.e. previously established flame speed scalings are inappropriate for flames interacting with compressibility. Drawing motivation from vortex ring literature, a new flame speed scaling based on the ring propagation velocity is proposed. The proposed scaling collapses the data and produces a nearly linear scaling regime, which suggests turbulence plays a secondary role to the hydrodynamic instability created by flame-compressibility interaction. In summary, flame kernels are a new and effective canonical configuration for exploring flame-compressibility interactions in supersonic flows.
A generalized Maccormack scheme for low Mach number, chemically-reacting large-eddy simulations

(Georgia Institute of Technology, 2017-06-16) Gallagher, Timothy Patrick

Chemically reacting flows contain a wide range of regimes with many velocity and time scales. The increasing access to computational resources enables higher-fidelity simulations of these flows. In order to take advantage of these capabilities, numerical schemes must be robust, efficient and accurate in all of the regimes present in the flow. Pressure-based schemes are suitable for many low Mach number flows, but are limited to low velocities and relatively small temperature variations. Density-based schemes struggle to converge in low-speed flows due to the time-step restrictions imposed by the acoustic velocity, which may be orders of magnitude larger than the convective velocity. Furthermore, such codes may exhibit excessive numerical dissipation due improper scaling of the dissipative properties of the scheme. Chemical reactions introduce another set of temporal scales associated with the kinetics mechanism used to model the system. These scales are often much smaller than the convective or acoustic scales and impose additional restrictions on the time-step. This disparity requires numerical schemes designed to handle the challenges that occur in low Mach number, chemically reacting flows. Analysis of density-based schemes at the low Mach number limit suggests that the development of improved, robust preconditioning with suitable operator splitting techniques leads to improved solution fidelity. In this work, a dual-time framework with low-Mach preconditioning is developed for complex, chemically reacting large-eddy simulations. A new version of the well-known MacCormack scheme is proposed and the resulting scheme improves the solution quality significantly at low Mach numbers. An established ordinary differential equation solver for stiff systems treats the stiffness associated with the chemical source terms. Methods to couple the PDE and ODE solvers in both pseudo-time and in physical time are proposed and analyzed. Validation of the non-reacting scheme and the coupled reacting scheme using canonical test cases demonstrates the improved solution fidelity and simulations of representative industrial applications demonstrate the combined scheme.
An embedded boundary approach for simulation of reacting flow problems in complex geometries with moving and stationary boundaries

(Georgia Institute of Technology, 2017-05-16) Muralidharan, Balaji

Many useful engineering devices involve moving boundaries interacting with a reacting compressible flow. Examples of such applications include propulsion systems with moving components such as Internal Combustion (IC) engines, hypersonic propulsive devices such as Oblique Detonation Wave (ODW) engines and solid rocket motors involving regressing propellant surfaces. Computational Fluid Dynamics (CFD) can be effectively employed to study these systems. However, conventional numerical methods face several difficulties related to grid generation, treatment of moving boundaries, lack of adequate grid resolution at an affordable computational cost, and shortcomings in closure models required for Large Eddy Simulation (LES). This thesis demonstrates new accurate numerical models and subgrid closures for LES of problems in non-trivial geometries with moving boundaries. A new high-order adaptive cut-cell based embedded boundary method is developed for viscous flows, which can provide a smooth and accurate reconstruction to predict the near-wall shear stress and pressure distribution. The method can achieve a high order of accuracy even under adverse geometrical constraints such as narrow gaps and sharp corners due to a novel and robust cell clustering algorithm. This algorithm also enforces the stability of the numerical scheme in the presence of arbitrary low volume cells formed in the cell cutting process. Additionally, an extended cell clustering approach, which can achieve exact conservation of mass, momentum, and energy is proposed for moving boundaries. The embedded boundary method is built on a massively parallel framework that performs block structured Adaptive Mesh Refinement (AMR) by interfacing with the BoxLib open source library. This modeling framework is then applied to study fundamental physics in high-speed propulsion systems, for example, shock-turbulence interactions, flame-turbulence interaction, and flame/detonation stabilization in a reacting system. LES using the multilevel subgrid closure for flow and chemistry is used to study flame anchoring in a transverse reacting jet in cross flow. Important mechanisms that stabilize the flame are identified and shown to be consistent with past observations from experiments and using direct numerical simulations (DNS) but obtained here using much coarser grid LES. Finally, to demonstrate the ability of the methodology to simulate moving bodies in a reactive system, DNS of a hypersonic projectile fired into a reacting flow is performed to reveal key effects of pressure on the stabilization of detonation ahead of the projectile.
Simulations of vitiated bluff body stabilized flames

(Georgia Institute of Technology, 2016-05-17) Smith, Andrew Gerard

Bluff bodies have a wide range of applications where low-cost, light weight methods are needed to stabilize flames in high-speed flow. The principles of bluff body flame stabilization are straightforward, but many details are not understood; this is especially true in vitiated environments where measurements are difficult to obtain. Most work has focused on premixed flames but changing application requirements are now driving studies on non-premixed gaseous and spray flames. This thesis aims to improve the understanding of vitiated, bluff body stabilized flames, specifically on non-premixed, spray flames, through the use of Large Eddy Simulation (LES). The single flameholder facility at Georgia Tech was chosen as the basis for the simulations in this thesis. The flameholder was a rectangular bluff body with an aerodynamic leading edge with discrete liquid fuel injectors embedded just upstream of the trailing edge in a configuration described as “close-coupled.” The liquid phase was modeled using a Lagrangian particle approach where discrete fuel droplets were injected into the domain. Experimental data was used to tune model parameters as well as the stripped droplet velocities and sizes. The discharge coefficient needed to be taken into account to achieve the correct fuel jet penetration. The experiments were conducted over a range of global equivalence ratios; lean equivalence ratios, φ global ≈ 0.5, exhibited symmetric flame shedding and conversely large scale sinusoidal B ́ernard/von-K ́arm ́an shedding was observed when the equiva- lence ratio was near unity. Reacting flow LES were computed at these two fuel flow rates to improve understanding of the different flame dynamics. LES were first com- pleted using a quasi-laminar subgrid turbulence-chemistry interaction model. Span- wise averaging of instantaneous and time-averaged LES results were compared with experimental high- and low-speed imaging and showed the LES was in qualitative agreement at both fuel flow rates. At phi_global ≈ 0.5, the fuel jet did not penetrate as far into the crossflow compared to phi_global ≈ 0.95 and thus more fuel was delivered to the shear layers of the bluff body resulting in higher heat release in the shear layers for the low fuel flow rate. The heat release damped the large sinusoidal structures via gas expansion and baroclinic torque generation. Higher fuel jet penetration in the phi_global ≈ 0.95 case meant less fuel was delivered to the shear layers and so less heat release occurred directly behind the bluff body so the large scale sinusoidal shedding was not damped. The impact of the subgrid turbulence-chemistry interaction model on the flame dynamics was tested by comparing the quasi-laminar LES with LES using the subgrid linear eddy model (LEMLES). The flame structure predicted with LEMLES matched that of the quasi-laminar LES, at both fuel flow rates in the near- field behind the bluff body but deviated farther downstream. A flame edge analysis showed little sensitivity to the choice of subgrid model in the region x < 4D. A high-order hybrid finite-difference solver with consisting of a WENO upwind method and compact central scheme was implemented to assess the effects of the numerical method. A series of test cases was used to verify, validate and compare several of the available spatial and temporal methods before the high fuel flow rate bluff body case was run. For the simple test cases the higher-order methods were clearly more efficient but for more complex cases the differences between the second- order and high-order methods are smaller. To test the hypothesis that the fuel jet penetration was the main factor in the flame dynamics another configuration with a modified fuel injector diameter was simulated. The injector size was chosen to match the spray penetration of phi_global ≈ 0.5 case while maintaining the fuel flow rate of the phi_global ≈ 0.95 case. The results confirmed the hypothesis as the flame dynamics of this configuration match the original low fuel flow rate case.
Hybrid RANS-LES closure for separated flows in the transitional regime

(Georgia Institute of Technology, 2016-04-04) Hodara, Joachim

The aerodynamics of modern rotorcraft is highly complex and has proven to be an arduous challenge for computational fluid dynamics (CFD). Flow features such as massively separated boundary layers or transition to turbulence are common in engineering applications and need to be accurately captured in order to predict the vehicle performance. The recent advances in numerical methods and turbulence modeling have resolved each of these issues independent of the other. First, state-of-the-art hybrid RANS-LES turbulence closures have shown great promise in capturing the unsteady flow details and integrated performance quantities for stalled flows. Similarly, the correlation-based transition model of Langtry and Menter has been successfully applied to a wide range of applications involving attached or mildly separated flows. However, there still lacks a unified approach that can tackle massively separated flows in the transitional flow region. In this effort, the two approaches have been combined and expended to yield a methodology capable of accurately predicting the features in these highly complex unsteady turbulent flows at a reasonable computational cost. Comparisons are evaluated on several cases, including a transitional flat plate, circular cylinder in crossflow and NACA 63-415 wing. Cost and accuracy correlations with URANS and prior hybrid URANS-LES approaches with and without transition modeling indicate that this new method can capture both separation and transition more accurately and cost effectively. This new turbulence approach has been applied to the study of wings in the reverse flow regime. The flight envelope of modern helicopters has increased significantly over the last few decades, with design concepts now reaching advance ratios up to μ = 1. In these extreme conditions, the freestream velocity exceeds the rotational speed of the blades, and a large region of the retreating side of the rotor disk experiences reverse flow. For a conventional airfoil with a sharp trailing edge, the reverse flow regime is generally characterized by massive boundary layer separation and bluff body vortex shedding. This complex aerodynamic environment has been utilized to evaluate the new hybrid transitional approach. The assessment has proven the efficiency of the new hybrid model, and it has provided a transformative advancement to the modeling of dynamic stall.
A study of magnetoplasmadynamic effects in turbulent supersonic flows with application to detonation and explosion

(Georgia Institute of Technology, 2015-07-28) Schulz, Joseph C.

Explosions are a common phenomena in the Universe. Beginning with the Big Bang, one could say the history of the Universe is narrated by a series of explosions. Yet no matter how large, small, or complex, all explosions occur through a series of similar physical processes beginning with their initiation to their dynamical interaction with the environment. Of particular interest to this study is how these processes are modified in a magnetized medium. The role of the magnetic field is investigated in two scenarios. The first scenario addresses how a magnetic field alters the propagation of a gaseous detonation where the application of interest is the modification of a condensed-phase explosion. The second scenario is focused on the aftermath of the explosion event and addresses how fluid mixing changes in a magnetized medium. A primary focus of this thesis is the development of a numerical tool capable of simulating explosive phenomenon in a magnetized medium. While the magnetohydrodynamic (MHD) equations share many of the mathematical characteristics of the hydrodynamic equations, numerical methods developed for the conservation equations of a magnetized plasma are complicated by the requirement that the magnetic field must be divergent free. The advantages and disadvantages of the proposed method are discussed in relation to explosion applications.
A study of dispersion and combustion of particle clouds in post-detonation flows

(Georgia Institute of Technology, 2015-07-24) Gottiparthi, Kalyana Chakravarthi

Augmentation of the impact of an explosive is routinely achieved by packing metal particles in the explosive charge. When detonated, the particles in the charge are ejected and dispersed. The ejecta influences the post-detonation combustion processes that bolster the blast wave and determines the total impact of the explosive. Thus, it is vital to understand the dispersal and the combustion of the particles in the post-detonation flow, and numerical simulations have been indispensable in developing important insights. Because of the accuracy of Eulerian-Lagrangian (EL) methods in capturing the particle interaction with the post-detonation mixing zone, EL methods have been preferred over Eulerian-Eulerian (EE) methods. However, in most cases, the number of particles in the flow renders simulations using an EL method unfeasible. To overcome this problem, a combined EE-EL approach is developed by coupling a massively parallel EL approach with an EE approach for granular flows. The overall simulation strategy is employed to simulate the interaction of ambient particle clouds with homogenous explosions and the dispersal of particles after detonation of heterogeneous explosives. Explosives packed with aluminum particles are also considered and the aluminum particle combustion in the post-detonation flow is simulated. The effect of particles, both reactive and inert, on the combustion processes is analyzed. The challenging task of solving for clouds of micron and sub-micron particles in complex post-detonation flows is successfully addressed in this thesis.