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Daniel Guggenheim School of Aerospace Engineering

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https://hdl.handle.net/1853/70742

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College of Engineering

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Aerospace Design Group

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Aerospace Systems Design Laboratory (ASDL)

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Air Transportation Laboratory

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Cognitive Engineering Center

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Space Systems Design Laboratory (SSDL)

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Publication Search Results

Now showing 1 - 10 of 90

A robust multi-objective statistical improvement approach to electric power portfolio selection

(Georgia Institute of Technology, 2012-11-13) Murphy, Jonathan Rodgers

Motivated by an electric power portfolio selection problem, a sampling method is developed for simulation-based robust design that builds on existing multi-objective statistical improvement methods. It uses a Bayesian surrogate model regressed on both design and noise variables, and makes use of methods for estimating epistemic model uncertainty in environmental uncertainty metrics. Regions of the design space are sequentially sampled in a manner that balances exploration of unknown designs and exploitation of designs thought to be Pareto optimal, while regions of the noise space are sampled to improve knowledge of the environmental uncertainty. A scalable test problem is used to compare the method with design of experiments (DoE) and crossed array methods, and the method is found to be more efficient for restrictive sample budgets. Experiments with the same test problem are used to study the sensitivity of the methods to numbers of design and noise variables. Lastly, the method is demonstrated on an electric power portfolio simulation code.
Optimal allocation of thermodynamic irreversibility for the integrated design of propulsion and thermal management systems

(Georgia Institute of Technology, 2012-11-13) Maser, Adam Charles

More electric aircraft systems, high power avionics, and a reduction in heat sink capacity have placed a larger emphasis on correctly satisfying aircraft thermal management requirements during conceptual design. Thermal management systems must be capable of dealing with these rising heat loads, while simultaneously meeting mission performance. Since all subsystem power and cooling requirements are ultimately traced back to the engine, the growing interactions between the propulsion and thermal management systems are becoming more significant. As a result, it is necessary to consider their integrated performance during the conceptual design of the aircraft gas turbine engine cycle to ensure that thermal requirements are met. This can be accomplished by using thermodynamic modeling and simulation to investigate the subsystem interactions while conducting the necessary design trades to establish the engine cycle. As the foundation for this research, a parsimonious, transparent thermodynamic model of propulsion and thermal management systems performance was created with a focus on capturing the physics that have the largest impact on propulsion design choices. A key aspect of this approach is the incorporation of physics-based formulations involving the concurrent usage of the first and second laws of thermodynamics to achieve a clearer view of the component-level losses. This is facilitated by the direct prediction of the exergy destruction distribution throughout the integrated system and the resulting quantification of available work losses over the time history of the mission. The characterization of the thermodynamic irreversibility distribution helps give the designer an absolute and consistent view of the tradeoffs associated with the design of the system. Consequently, this leads directly to the question of the optimal allocation of irreversibility across each of the components. An irreversibility allocation approach based on the economic concept of resource allocation is demonstrated for a canonical propulsion and thermal management systems architecture. By posing the problem in economic terms, exergy destruction is treated as a true common currency to barter for improved efficiency, cost, and performance. This then enables the propulsion systems designer to better fulfill system-level requirements and to create a system more robust to future requirements.
Premixed flame kinematics in a harmonically oscillating velocity field

(Georgia Institute of Technology, 2012-11-13) Shin, Dong-hyuk

Air pollution regulations have driven modern power generation systems to move from diffusion to premixed combustion. However, these premixed combustion systems are prone to combustion instability, causing high fluctuations in pressure and temperature. This results in shortening of component life, system failure, or even catastrophic disasters. A large number of studies have been performed to understand and quantify the onset of combustion instability and the limit cycle amplitude. However, much work remains due to the complexity of the process associated with flow dynamics and chemistry. This thesis focuses on identifying, quantifying and predicting mechanisms of flame response subject to disturbances. A promising tool for predicting combustion instability is a flame transfer function. The flame transfer function is obtained by integrating unsteady heat release over the combustor domain. Thus, the better understanding of spatio-temporal characteristics of flame is required to better predict the flame transfer function. The spatio-temporal flame response is analyzed by the flame kinematic equation, so called G-equation. The flame is assumed to be a thin interface separating products and reactant, and the interface is governed by the local flow and the flame propagation. Much of the efforts were done to the flame response subject to the harmonic velocity disturbance. A key assumption allowing for analytic solutions is that the velocity is prescribed. For the mathematical tools, small perturbation theory, Hopf-Lax formula and numerical simulation were used. Solutions indicated that the flame response can be divided into three regions, referred to here as the near-field, mid-field, and farfield. In each regime, analytical expressions were derived, and those results were compared with numerical and experimental data. In the near field, it was shown that the flame response grows linearly with the normal component of the velocity disturbance. In the mid field, the flame response shows peaks in gain, and the axial location of these peaks can be predicted by the interference pattern by two characteristic waves. Lastly, in the far field where the flame response decreases, three mechanisms are studied; they are kinematic restoration, flame stretch, and turbulent flow effects. For each mechanism, key parameters are identified and their relative significances are compared.
Viscous hypersonic flow physics predictions using unstructured Cartesian grid techniques

(Georgia Institute of Technology, 2012-11-12) Sekhar, Susheel Kumar

Aerothermodynamics is an integral component in the design and implementation of hypersonic transport systems. Accurate estimates of the aerodynamic forces and heat transfer rates are critical in trajectory analysis and for payload weight considerations. The present work seeks to investigate the ability of an unstructured Cartesian grid framework in modeling hypersonic viscous flows. The effectiveness of modeling viscous phenomena in hypersonic flows using the immersed boundary ghost cell methodology of this solver is analyzed. The capacity of this framework to predict the surface physics in a hypersonic non-reacting environment is investigated. High velocity argon gas flows past a 2-D cylinder are simulated for a set of freestream conditions (Reynolds numbers), and impact of the grid cell sizes on the quality of the solution is evaluated. Additionally, the formulation is verified over a series of hypersonic Mach numbers for the flow past a hemisphere, and compared to experimental results and empirical estimates. Next, a test case that involves flow separation and the interaction between a hypersonic shock wave and a boundary layer, and a separation bubble is investigated using various adaptive mesh refinement strategies. The immersed boundary ghost cell approach is tested with two temperature clipping strategies, and their impact on the overall solution accuracy and smoothness of the surface property predictions are compared. Finally, species diffusion terms in the conservation equations, and collision cross-section based transport coefficients are installed, and hypersonic flows in thermochemical nonequilibrium environments are studied, and comparisons of the off-surface flow properties and the surface physics predictions are evaluated. First, a 2-D cylinder in a hypersonic reacting air flow is tested with an adiabatic wall boundary condition. Next, the same geometry is tested to evaluate the viscous chemistry prediction capability of the solver with an isothermal wall boundary condition, and to identify the strengths and weaknesses of the immersed boundary ghost cell methodology in computing convective heating rates in such an environment.
Reliability, multi-state failures and survivability of spacecraft and space-based networks

(Georgia Institute of Technology, 2012-10-30) Castet, Jean-François

Spacecraft fulfill a myriad of critical functions on orbit, from defense and intelligence to science, navigation, and telecommunication. Spacecraft can also cost several hundred millions of dollars to design and launch, and given that physical access for maintenance remains difficult if not impossible to date, designing high reliability and survivability into these systems is an engineering and financial imperative. While reliability is recognized as an essential attribute for spacecraft, little analysis has been done pertaining to actual field reliability of spacecraft and their subsystems. This thesis consists of two parts. The first part fills the gap in the current understanding of spacecraft failure behavior on orbit through extensive statistical analysis and modeling of anomaly and failure data of Earth-orbiting spacecraft. The second part builds on these results to develop a novel theoretical basis (interdependent multi-layer network approach) and algorithmic tools for the analysis of survivability of spacecraft and space-based networks. Space-based networks (SBNs) allow the sharing of on-orbit resources, such as data storage, processing, and downlink. Results indicate and quantify the incremental survivability improvement of the SBN over the traditional monolith architecture. A trade-space analysis is then conducted using non-descriptive networkable subsystems/technologies to explore survivability characteristics of space-based networks and help guide design choices.
A matched-harmonic confluence approach to rotor loads prediction with comprehensive application to flight test

(Georgia Institute of Technology, 2012-09-18) McColl, Chance C.

Future management of helicopter fleets will be more heavily based on individual component damage tracking and less on legacy usage monitoring (flight parameter-based) methods. This enhances health assessment capabilities by taking into account the actual loads on a component-by-component basis. However, accurate loads prediction in rotating frame components remains a challenge. Even with advanced computational fluid dynamics (CFD) techniques, prediction of the unsteady aerodynamic loads acting on the rotor blades is computationally intensive and problematic in terms of accurate loads prediction across the entire flight regime of the helicopter. High-speed flight can potentially introduce both shock and near-stall effects within a given rotor rotation. Low-speed flight can include blade-vortex interaction effects, wherein flow from a given blade (vorticity loading from tip vortices) impinges upon the preceding blade, causing unsteady aerodynamic loading that is difficult to quantity and predict numerically. Vehicle maneuvering can produce significantly higher blade pitching moments than steady flight. All of these regimes combine to represent the loading history of the rotor system. Therefore, accurate loads prediction methods, in terms of matching peak-to-peak, magnitude, phase, as well as vibratory/harmonic content, are required that capture all flight regimes for all critical structural components. This research focuses on the development of a loads prediction method, known as the Load Confluence Algorithm (LCA), and its application to the analysis of a large set of flight test data from the NASA/US Army UH-60A Airloads Program. The LCA combines measured response at a prescribed set of locations with a numerical model of the rotor system. For a given flight condition (steady flight, maneuvers, etc.) the numerical simulation's predicted loads distribution is iteratively incremented (by harmonic) until convergence with measured loads is reached at the prescribed locations (control points). Predicted loads response at non-instrumented locations is shown to be improved as well, thus enhancing fatigue lifing methods for these components. The procedure specifically investigates the harmonic content of the applied loads and the improved prediction of the harmonic components. The impact of the enhanced accuracy on loads predictions on component structural fatigue is illustrated by way of an example. Results show that, for a limited sensor set (two 3-axis sensors per blade), blade loads are accurately predicted across a full range of flight regimes. Hub loads are best modeled using the pushrod as the control point. Results also show that load magnitude has a tremendous influence on damage, with a 25% over-estimation of vibratory load resulting in a damage factor of nearly 3. This research highlights the importance of accurate loads prediction for a rotorcraft life tracking program. Small inaccuracies in loads lead to dramatic errors in damage assessment.
Numerical investigation of static and dynamic stall of single and flapped airfoils

(Georgia Institute of Technology, 2012-08-30) Liggett, Nicholas Dwayne

Separated flows about single and multi-element airfoils are featured in many scenarios of practical interest, including: stall of fixed wing aircraft, dynamic stall of rotorcraft blades, and stall of compressor and turbine elements within jet engines. In each case, static and/or dynamic stall can lead to losses in performance. More importantly, modeling and analysis tools for stalled flows are relatively poorly evolved and designs must completely avoid stall due to a lack of understanding. The underlying argument is that advancements are necessary to facilitate understanding of and applications involving static and dynamic stall. The state-of-the-art in modeling stall involves numerical solutions to the governing equations of fluids. These tools often either lack fidelity or are prohibitively expensive. Ever-increasing computational power will likely lead to increased application of numerical solutions. The focus of this thesis is improvements in numerical modeling of stall, the need of which arises from poorly evolved analysis tools and the spread of numerical approaches. Technical barriers have included ensuring unsteady flow field and vorticity reproduction, transition modeling, non-linear effects such as viscosity, and convergence of predictions. Contributions to static and dynamic stall analysis have been been made. A hybrid Reynolds-Averaged Navier-Stokes/Large-Eddy-Simulation turbulence technique was demonstrated to predict the unsteadiness and acoustics within a cavity with accuracy approaching Large-Eddy-Simulation. Practices to model separated flows were developed and applied to stalled airfoils. Convergence was characterized to allow computational resources to be focused only as needed. Techniques were established for estimation of integrated coefficients, onset of stall, and reattachment from unconverged data. Separation and stall onset were governed by turbulent transport, while the location of reattachment depended on the mean flow. Application of these methodologies to oscillating flapped airfoils revealed flow through the gap was dominated by the flap angle for low angles of attack. Lag between the aerodynamic response and input flap scheduling was associated with increased oscillation frequency and airfoil/flap gap size. Massively separated flow structures were also examined.
Multi-phase flows using discontinuous Galerkin methods

(Georgia Institute of Technology, 2012-08-28) Gryngarten, Leandro Damian

This thesis is concerned with the development of numerical techniques to simulate compressible multi-phase flows, in particular a high-accuracy numerical approach with mesh adaptivity. The Discontinuous Galerkin (DG) method was chosen as the framework for this work for being characterized for its high-order of accuracy -thus low numerical diffusion- and being compatible with mesh adaptivity due to its locality. A DG solver named DiGGIT (Discontinuous Galerkin at the Georgia Institute of Technology) has been developed and several aspects of the method have been studied. The Local Discontinuous Galerkin (LDG) method -an extension of DG for equations with high-order derivatives- was extended to solve multiphase flows using Diffused Interface Methods (DIM). This multi-phase model includes the convection of the volume fraction, which is treated as a Hamilton-Jacobi equation. This is the first study, to the author's knowledge, in which the volume fraction of a DIM is solved using the DG and the LDG methods. The formulation is independent of the Equation of State (EOS) and it can differ for each phase. This allows for a more accurate representation of the different fluids by using cubic EOSs, like the Peng-Robinson and the van der Waals models. Surface tension is modeled with a new numerical technique appropriate for LDG. Spurious oscillations due to surface tension are common to all the capturing schemes, and this new approach presents oscillations comparable in magnitude to the most common schemes. The moment limiter (ML) was generalized for non-uniform grids with hanging nodes that result from adaptive mesh refinement (AMR). The effect of characteristic, primitive, or conservative decomposition in the limiting stage was studied. The characteristic option cannot be used with the ML in multi-dimensions. In general, primitive variable decomposition is a better option than with conservative variables, particularly for multiphase flows, since the former type of decomposition reduces the numerical oscillations at material discontinuities. An additional limiting technique was introduced for DIM to preserve positivity while minimizing the numerical diffusion, which is especially important at the interface. The accuracy-preserving total variation diminishing (AP-TVD) marker for ``troubled-cell' detection, which uses an averaged-derivative basis, was modified to use the Legendre polynomial basis. Given that the latest basis is generally used for DG, the new approach avoids transforming to the averaged-derivative basis, what results in a more efficient technique. Furthermore, a new error estimator was proposed to determine where to refine or coarsen the grid. This estimator was compared against other estimator used in the literature and it showed an improved performance. In order to provide equal order of accuracy in time as in space, the commonly used 3rd-order TVD Runge-Kutta (RK) scheme in the DG method was replaced in some cases by the Spectral Deferred Correction (SDC) technique. High orders in time were shown to only be required when the error in time is significant. For instance, convection-dominated compressible flows require for stability a time step much smaller than is required for accuracy, so in such cases 3rd-order TVD RK resulted to be more efficient than SDC with higher orders. All these new capabilities were included in DiGGIT and have provided a generalized approach capable of solving sub- and super-critical flows at sub- and super-sonic speeds, using a high-order scheme in space and time, and with AMR. Canonical test cases are presented to verify and validate the formulation in one, two, and three dimensions. Finally, the solver is applied to practical applications. Shock-bubble interaction is studied and the effect of the different thermodynamic closures is assessed. Interaction between single-drops and a wall is simulated. Sticking and the onset of splashing are observed. In addition, the solver is used to simulate turbulent flows, where the high-order of accuracy clearly shows its benefits. Finally, the methodology is challenged with the simulation of a liquid jet in cross flow.
Multiscale analysis of wave propagation in heterogeneous structures

(Georgia Institute of Technology, 2012-07-02) Casadei, Filippo

The analysis of wave propagation in solids with complex microstructures, and local heterogeneities finds extensive applications in areas such as material characterization, structural health monitoring (SHM), and metamaterial design. Within continuum mechanics, sources of heterogeneities are typically associated to localized defects in structural components, or to periodic microstructures in phononic crystals and acoustic metamaterials. Numerical analysis often requires computational meshes which are refined enough to resolve the wavelengths of deformation and to properly capture the fine geometrical features of the heterogeneities. It is common for the size of the microstructure to be small compared to the dimensions of the structural component under investigation, which suggests multiscale analysis as an effective approach to minimize computational costs while retaining predictive accuracy. This research proposes a multiscale framework for the efficient analysis of the dynamic behavior of heterogeneous solids. The developed methodology, called Geometric Multiscale Finite Element Method (GMsFEM), is based on the formulation of multi-node elements with numerically computed shape functions. Such shape functions are capable to explicitly model the geometry of heterogeneities at sub-elemental length scales, and are computed to automatically satisfy compatibility of the solution across the boundaries of adjacent elements. Numerical examples illustrate the approach and validate it through comparison with available analytical and numerical solutions. The developed methodology is then applied to the analysis of periodic media, structural lattices, and phononic crystal structures. Finally, GMsFEM is exploited to study the interaction of guided elastic waves and defects in plate structures.
Rule-based graph theory to enable exploration of the space system architecture design space

(Georgia Institute of Technology, 2012-06-29) Arney, Dale Curtis

NASA's current plans for human spaceflight include an evolutionary series of missions based on a steady increase in capability to explore cis-lunar space, the Moon, near-Earth asteroids, and eventually Mars. Although the system architecture definition has the greatest impact on the eventual performance and cost of an exploration program, selecting an optimal architecture is a difficult task due to the lack of methods to adequately explore the architecture design space and the resource-intensive nature of architecture analysis. This research presents a modeling framework to mathematically represent and analyze the space system architecture design space using graph theory. The framework enables rapid exploration of the design space without the need to limit trade options or the need for user interaction during the exploration process. The architecture design space for three missions in a notional evolutionary exploration program, which includes staging locations, vehicle implementation, and system functionality, for each mission destination is explored. Using relative net present value of various system architecture options, the design space exploration reveals that the launch vehicle selection is the primary driver in reducing cost, and other options, such as propellant type, staging location, and aggregation strategy, provide less impact.