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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 11

A numerical study on non-equilibrium multi-temperature thermo-chemistry

(Georgia Institute of Technology, 2019-03-29) Raghunandan, Pratibha

The accurate computation of hypersonic flowfields is an ongoing endeavor and is important for the accurate prediction of heat transfer and space vehicle design. The governing equations for hypersonic flowfields have been evolving from multi-temperature modeling, state-to-state modeling to the more recent reduced order modeling. Of the various models existing till date of varying levels of complexity, multi-temperature modeling continues to be the most widely implemented and computationally least expensive form of modeling hypersonic flows. This thesis explores the resulting physics from various forms of multi-temperature modeling. The non-equilibrium flows at typical hypersonic re-entry conditions can be modeled by considering varying extents of non-equilibrium: chemical, thermo-chemical with the typical two temperature formalism, and thermo-chemical with more than two temperatures used to represent the gas under consideration. Most initial numerical verification studies examine non-equilibrium relaxation rates using zero-dimensional heat bath systems. Literature abounds with heat bath relaxation rate studies at isothermal conditions, and consequently for very dilute systems with negligible chemical non-equilibrium. Nonetheless, the same verified representative equations, including the Landau-Teller translation-vibration energy exchange, are used to compute multi-dimensional flows involving high degrees of chemical non-equilibrium. Thus, despite a well-established understanding of the temperature limitations of Park's empirical two-temperature model, and the original form of the Landau-Teller formulation, the effects of system dilution levels on the resulting non-equilibrium characteristics have not been well understood. The present work focuses on the effects of such dilution on the thermo-chemical non-equilibrium characteristics of isochoric, finite heat baths represented by varying resolutions of multi-temperature models. The non-equilibrium characteristics for such heating systems reveal non-linear effects in attaining thermal non-equilibrium which are enhanced by the mixing-type Millikan-White relaxation time empirical curve fit. Multi-vibrational, single translational temperature modeling leads to significantly altered time-scales of non-equilibrium chemistry relative to a simple two-temperature model representation for internal energy. This was further confirmed during the two-dimensional flows studied where thermo-chemical modeling with first order effects exhibited altered shock stand-off, near-surface temperatures, and flow field chemistry with multi-vibrational, single translational temperature modeling. The implementation of an increased resolution of thermal non-equilibrium representation for any weakly ionized flows present in the system closely followed the trends exhibited by a system in which electron and heavy particle translational temperatures were assumed to be in equilibrium with each other. However, the shock structure showed an enhanced sensitivity to a more complete representation of the underlying chemical kinetics. For the aerothermodynamics community, this work contributes to understanding the resultant thermo-chemical non-equilibrium rate effects for various forms of representing the effective temperature of a reaction, within a multi-temperature modeling perspective. In particular, it emphasizes the temporal and spatial sensitivity of non-equilibrium energy modeling approaches for various zero-dimensional and two-dimensional flow fields. Such intermediate non-equilibrium chemistry effects could be leveraged for aerodynamic flow control and controlled heat transfer applications.
A time accurate fluid-structure interaction framework using a Cartesian grid CFD solver

(Georgia Institute of Technology, 2017-11-15) Bopp, Matthew Scott

The landing of the Mars Science Laboratory (MSL) in 2012 demonstrated the limits of supersonic planetary entry technology through the use of a disk-gap-band parachute deployed from behind the aeroshell capsule. With the eventual goal of sending humans to Mars, the payload requirements are estimated to increase by a factor of 40, far outside the current technological envelope. With a density of less than 1% of Earth's, the Martian atmosphere makes the task of generating aerodynamic drag very challenging. Larger aeroshells produce more drag, but the vehicle is then too large to fit as payload inside a rocket. By utilizing inflatable aerodynamic decelerators, the drag area can be significantly increased, while the pre-deployed configuration has high packing efficiency. New technologies bring with them the requirement to study their behavior, and characterize their flight limits. Wind tunnel tests are difficult due scaling concerns, and flight tests are costly and time consuming. Thus, accurate computational modeling of the fluid-structure interactions (FSI) is critical in the development of aerodynamic decelerators. Much of the current research in FSI focuses on high fidelity analysis, which is often very computationally expensive, and requires significant user intervention. The current work fills a niche where the analysis time and human interaction is reduced, by utilizing an adaptive, Cartesian grid framework for solving the computational fluid dynamics (CFD). A time accurate, partitioned coupling strategy is employed to study FSI applied to flexible materials under high dynamic pressure loads. The structural dynamics is solved using LS-DYNA, and care must be taken at the interface boundary conditions to reduce numerical errors. The development of this tool has relied on a complete re-write of the in-house CFD code, NASCART-GT, where significant improvements have been made in computational efficiency and scalability. CFD simulations with prescribed motions are studied in order to validate the fluid dynamics of high speed flows with non-stationary boundary conditions, and to study the effects of solution-based grid adaption for these simulations. The interaction with rigid body dynamics is presented in simulations of the free flight dynamics of the MSL capsule. FSI simulations are then presented for a series of test cases, where the physics is validated for the unsteady, time accurate coupling of 1-D piston motion and 2-D beam deformation. Finally, steady state and time accurate simulations of an inflatable aerodynamic decelerator demonstrate the effectiveness of the current methodology in furthering the development of decelerator technologies.
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.
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.
Numerical study of innovative scramjet inlets coupled to combustors using hydrocarbon-air mixture

(Georgia Institute of Technology, 2010-04-06) Malo-Molina, Faure Joel

To advance the design of hypersonic vehicles, high-fidelity multi-physics CFD is used to characterize 3-D scramjet flow-fields in two novel streamline traced configurations. The two inlets, Jaws and Scoop, are analyzed and compared to a traditional rectangular inlet used as a baseline for on/off-design conditions. The flight trajectory conditions selected are Mach 6 and a dynamic pressure of 1,500 psf (71.82 kPa). Analysis of these hypersonic inlets is performed to investigate distortion effects downstream with multiple single cavity combustors acting as flame holders, and several fuel injection strategies. The best integrated scramjet inlet/combustor design is identified. The flow physics is investigated and the integrated performance impact of the two innovative scramjet inlet designs is quantified. Frozen and finite rate chemistry is simulated with 13 gaseous species and 20 reactions for an Ethylene/air finite-rate chemical model. In addition, URANS and LES modeling are compared to explore overall flow structure and to contrast individual numerical methods. The flow distortion in Jaws and Scoop is similar to some of the distortion in the traditional rectangular inlet, despite design differences. The baseline and Jaws performance attributes are stronger than Scoop, but Jaws accomplishes this while eradicating the cowl lip interaction, and lessening the total drag and spillage penalties. The innovative inlets work best on-design, whereas for off-design, the traditional inlet is best. Early pressure losses and flow distortions in the isolator aid the mixing of air and fuel, and improve the overall efficiency of the system. Although the trends observed with and without chemical reactions are similar, the former yields roughly 10% higher mixing efficiency and upstream reactions are present. These show a significant impact on downstream development. Unsteadiness in the combustor increases the mixing efficiency, varying the flame anchoring and combustion pressure effects upstream of the step.
Parallelized Cartesian Grid Methodology for Non-Equilibrium Hypersonic Flow Analysis of Ballutes

(Georgia Institute of Technology, 2007-07-09) Lee, Jin Wook

Hypersonic flow analysis is performed on an inflatable aerocapture device called a "Ballute" for Titan's Mission. An existing unstructured Cartesian grid methodology is used as a starting point by taking advantage of its ability to automatically generate grids over any deformed shape of the flexible ballute. The major effort for this thesis work is focused on advancing the existing unstructured Cartesian grid methodology. This includes implementing thermochemical nonequilibrium capability and porting it to a parallel computing environment using a Space-Filling-Curve (SFC) based domain decomposition technique. The implemented two temperature thermochemical nonequilibrium solver governs the finite rate chemical reactions and vibrational relaxation in the high temperature regimes of hypersonic flow. In order to avoid the stiffness problem in the explicit chemical solver, a point implicit method is adopted to calculate the chemical reaction source term. The AUSMPW+ scheme with MUSCL data reconstruction is adopted as the numerical scheme to avoid non-physical oscillations and the carbuncle phenomenon. The results for five species air model and for thirteen species N2-CH4-Ar model to simulate Titan entry are included for verification against DPLR (NASA Ames' structured grid hypersonic flow solver). The efficient parallel computation of any unstructured grid flow solver requires an adequate grid decomposition strategy because of its complex spatial data structure. The difficulties of even and block-contiguous partitioning in frequently adapting unstructured Cartesian grids are overcome by implementing the 3D Hilbert SFC. Grids constructed by the SFC for parallel environment promise short inter-CPU communication time while maintaining perfect load balancing between CPUs. The load imbalance due to the local solution adaption is simply apportioned by re-segmenting the curve into even pieces. The detailed structure of the 3D Hilbert SFC and parallel computing efficiency results based on this grid partition method are also presented. Finally, a structural dynamics tool (LS-DYNA) is loosely coupled with the present parallel thermochemical nonequilibrium flow solver to obtain the deformed surface definition of the ballute.
Development of an Efficient Viscous Approach in a Cartesian Grid Framework and Application to Rotor-Fuselage Interaction

(Georgia Institute of Technology, 2006-05-18) Lee, Jae-doo

Despite the high cost of memory and CPU time required to resolve the boundary layer, a viscous unstructured grid solver has many advantages over a structured grid solver such as the convenience in automated grid generation and shock or vortex capturing by solution adaption. Since the geometry and flow phenomenon of a helicopter are very complex, unstructured grid-based methods are well-suited to model properly the rotor-fuselage interaction than the structured grid solver. In present study, an unstructured Cartesian grid solver is developed on the basis of the existing solver, NASCART-GT. Instead of cut-cell approach, immersed boundary approach is applied with ghost cell boundary condition, which increases the accuracy and minimizes unphysical fluctuations of the flow properties. The standard k-epsilon model by Launder and Spalding is employed for the turbulence modeling, and a new wall function approach is devised for the unstructured Cartesian grid solver. It is quite challenging and has never done before to apply wall function approach to immersed Cartesian grid. The difficulty lies in the inability to acquire smooth variation of y+ in the desired range due to the non-body-fitted cells near the solid wall. The wall function boundary condition developed in this work yields stable and reasonable solution within the accuracy of the turbulence model. The grid efficiency is also improved with respect to the conventional method. The turbulence modeling is validated and the efficiency of the developed boundary condition is tested in 2-D flow field around a flat plate, NACA0012 airfoil, axisymmetric hemispheroid, and rotorcraft applications. For rotor modeling, an actuator disk model is chosen, since it is efficient and is widely verified in the study of the rotor-fuselage interaction. This model considers the rotor as an infinitely thin disk, which carries pressure jump across the disk and allows flow to pass through it. The full three dimensional calculations of Euler and RANS equations are performed for the GT rotor model and ROBIN configuration to test implemented actuator disk model along with the developed turbulence modeling. Finally, the characteristics of the rotor-fuselage interaction are investigated by comparing the numerical solutions with the experiments.
Extending the functionalities of Cartesian grid solvers : viscous effects modeling and MPI parallelization

(Georgia Institute of Technology, 2002-05) Marshall, David D.
Development of a solution adaptive cartesian-grid solver for 2-D thermochemical nonequilibrium flows

(Georgia Institute of Technology, 2001-12) Tu, Shuangzhang
The artificially blunted leading edge concept for aerothermodynamic performance enhancement

(Georgia Institute of Technology, 1999-08) Gupta, Anurag