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Daniel Guggenheim School of Aerospace Engineering
Daniel Guggenheim School of Aerospace Engineering
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ItemModeling moderately dense to dilute multiphase reacting flows(Georgia Institute of Technology, 2022-03-14) Panchal, AchyutComputational 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.
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ItemNumerical simulations of real-gas flows with phase-equilibrium thermodynamics(Georgia Institute of Technology, 2020-07-20) Tudisco, PrincipioMotivated 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.
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ItemA 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.
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ItemArtificial neural networks based subgrid chemistry model for turbulent reactive flow simulations(Georgia Institute of Technology, 2009-08-17) Sen, Baris AliTwo new models to calculate the species instantaneous and filtered reaction rates for multi-step, multi-species chemical kinetics mechanisms are developed based on the artificial neural networks (ANN) approach. The proposed methodologies depend on training the ANNs off-line on a thermo-chemical database representative of the actual composition and turbulence level of interest. The thermo-chemical database is constructed by stand-alone linear eddy mixing (LEM) model simulations under both premixed and non-premixed conditions, where the unsteady interaction of turbulence with chemical kinetics is included as a part of the training database. In this approach, the information regarding the actual geometry of interest is not needed within the LEM computations. The developed models are validated extensively on the large eddy simulations (LES) of (i) premixed laminar-flame-vortex-turbulence interaction, (ii) temporally mixing non-premixed flame with extinction-reignition characteristics, and (iii) stagnation point reverse flow combustor, which utilizes exhaust gas re-circulation technique. Results in general are satisfactory, and it is shown that the ANN provides considerable amount of memory saving and speed-up with reasonable and reliable accuracy. The speed-up is strongly affected by the stiffness of the reduced mechanism used for the computations, whereas the memory saving is considerable regardless.
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ItemSimulation of magnetohydrodynamics turbulence with application to plasma-assisted supersonic combustion(Georgia Institute of Technology, 2009-01-14) Miki, KenjiThe main objective of this thesis is to develop a comprehensive model with the capability of modeling both a high Reynolds number and high magnetic Reynolds number turbulent flow for application to supersonic combustor. The development of this model can be divided into three categories: one, the development of a self-consistent MHD numerical model capable of modeling magnetic turbulence in high magnetic Reynolds number applications. Second, the development of a gas discharge model which models the interaction of externally applied fields in conductive medium. Third, the development of models necessary for studying supersonic combustion applications with plasma-assistance such the extension of chemical kinetics models to extremely high temperature and non-equilibrium phenomenon.
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ItemWall-models for large eddy simulation based on a generic additive-filter formulation(Georgia Institute of Technology, 2008-12-19) Sánchez Rocha, MartínIn this work, the mathematical implications of merging two different turbulence modeling approaches are addressed by deriving the exact hybrid RANS/LES Navier-Stokes equations. These equations are derived by introducing an additive-filter, which linearly combines the RANS and LES operators with a blending function. The equations derived predict additional hybrid terms, which represent the interactions between RANS and LES formulations. Theoretically, the prediction of the hybrid terms demonstrates that the hybridization of the two approaches cannot be accomplished only by the turbulence model equations, as it is claimed in current hybrid RANS/LES models. The importance of the exact hybrid RANS/LES equations is demonstrated by conducting numerical calculations on a turbulent flat-plate boundary layer. Results indicate that the hybrid terms help to maintain an equilibrated model transition when the hybrid formulation switches from RANS to LES. Results also indicate, that when the hybrid terms are not included, the accuracy of the calculations strongly relies on the blending function implemented in the additive-filter. On the other hand, if the exact equations are resolved, results are only weakly affected by the characteristics of the blending function. Unfortunately, for practical applications the hybrid terms cannot be exactly computed. Consequently, a reconstruction procedure is proposed to approximate these terms. Results show, that the model proposed is able to mimic the exact hybrid terms, enhancing the accuracy of current hybrid RANS/LES approaches. In a second effort, the Two Level Simulation (TLS) approach is proposed as a near-wall model for LES. Here, TLS is first extended to compressible flows by deriving the small-scale equations required by the model. The full compressible TLS formulation and the hybrid TLS/LES approach is validated simulating the flow over a flat-plate turbulent boundary layer. Overall, results are found in reasonable agreement with experimental data and LES calculations.
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ItemA New Two-Scale Decomposition Approach for Large-Eddy Simulation of Turbulent Flows(Georgia Institute of Technology, 2006-06-22) Kemenov, Konstantin A.A novel computational approach, Two Level Simulation (TLS), was developed based on the explicit reconstruction of the small-scale velocity by solving the small-scale governing equations on the domain with reduced dimension representing a collection of one-dimensional lines embedded in the three-dimensional flow domain. A coupled system of equations, that is not based on an eddy-viscosity hypothesis, was derived based on the decomposition of flow variables into the large-scale and the small-scale components without introducing the concept of filtering. Simplified treatment of the small-scale equations was proposed based on modeling of the small-scale advective derivatives and the small-scale dissipative terms in the directions orthogonal to the lines. TLS approach was tested to simulate benchmark cases of turbulent flows, including forced isotropic turbulence, mixing layers and well-developed channel flow, and demonstrated good capabilities to capture turbulent flow features using relatively coarse grids.