Browsing by Subject "Hypersonics"

Now showing 1 - 8 of 8
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    Air-Carbon Ablation for Hypersonic Flow Environments
    (2022-05) Prata, Krishna Sandeep
    Recent molecular beam experiments of high-velocity O, N, and O2 impacting carbon material at high temperatures produced detailed surface chemistry data relevant for carbon ablation processes. New data on O and N reactions with carbon has been published using a continuous molecular beam with lower velocity (2000 m/s) and approximately 500 times higher beam flux than previous pulsed-beam experiments. This data is interpreted to construct a new air-carbon ablation (ACA) model for use in modeling carbon heat shield ablation. The new model comprises 20 reaction mechanisms describing reactions between impinging O, N, and O2 species with carbon and producing scattered products including desorbed O and N, O2, and N2 formed by surface-catalyzed recombination, as well as CO, CO2, and CN. The new model includes surface-coverage-dependent reactions and exhibits non-Arrhenius reaction probability in agreement with experimental observations. All reaction mechanisms and rate coefficients are described in detail and each is supported by experimental evidence or theory. The model predicts pressure effects and is tested for a wide range of temperatures and pressures relevant to hypersonic flight. Model results are shown to agree well with available data and are shown to have significant differences compared to other models from the literature. A preliminary step towards validating the ACA model required simulating the plasma flows in the plasma chamber of von Karman Institute's (VKI's) Inductively Coupled Plasma (ICP) facility called Plasmatron using US3D, a 3D unstructured Navier-Stokes equations solver developed at the University of Minnesota. First, a parameter study of transport properties and the wall-catalycity of a catalytic probe used to characterize the plasma flow was conducted. It was found that the Gupta-Yos mixing rule with collision cross-section data performed better than Wilke's mixing rule with Blottner curve fits and Eucken relation to compute mixture viscosity and thermal conductivity. Also, the wall-catalycity had a strong effect on the boundary layer edge properties along the stagnation line for lower pressure flows in the Plasmatron. It was also found that the Self Consistent Effective Binary Diffusion (SCEBD) model predicted higher stagnation line enthalpy at the boundary layer edge for a flow over a non-catalytic wall when compared to the Fickian diffusion model that attributes a single diffusion coefficient to all the species in the mixture. Then, a series of iterative US3D simulations were performed to characterize the plasma flows over a fully-catalytic wall for seven air-plasma experiments in the Plasmatron. The simulations matched the experimentally measured cold wall heat flux and agreed relatively well with the boundary layer edge properties predicted by VKI's own analysis, giving confidence that the plasma freestream was well characterized in the seven experiments. Then, preliminary simulations of carbon ablation using the seven plasma flows were performed. Two ablation models called the ZA model and the MURI model gave comparable carbon mass loss rates with the experiments. However, the ZA model predicted lower surface heat flux than the MURI model due to the presence of spurious gas-surface reactions. Further experiments of carbon ablation measuring the surface heat flux are suggested in addition to mass-loss measurements. Finally, an analytical framework was developed that characterizes a flight mission or an experimental condition as reaction-limited or diffusion-limited with respect to carbon ablation. The framework uses the flight conditions such as the velocity, altitude, nose radius of the vehicle, and the surface temperature of the ablating heat shield to calculate time scales for diffusion and gas-surface chemical reactions. A new Damkohler number for ablation, defined as the ratio of diffusion time scale to the time scale for gas-surface chemical reactions was proposed. The framework was applied to several flight conditions in the existing literature. It was found that ablation of larger heat-shields like the Stardust re-entry capsule and Orion space-crew vehicle falls under a diffusion-limited regime, while the ablation of smaller objects like the nose tip of the Re-entry F vehicle falls under a reaction-limited regime. Future CFD simulations of ablation on various heat shields using the existing ablation models in the literature are recommended to establish a reference Damkohler number for the classification of the ablation regimes.
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    Atomistic Simulations of High Temperature Air Chemistry Including Three-Body Collisions and Recombination
    (2023-05) Geistfeld, Eric
    This dissertation studies chemical reactions relevant to modeling high-speed flight through Earth’s atmosphere. Shock-heated mixtures of nitrogen and oxygen undergo gas phase chemical reactions that control the concentrations of O2, O, N2, N, and NO (aptly named five-species air) that impact the heat shielding of hypersonic craft. Predicting the extreme gas state surrounding the heat shield material is extremely complex because the time scales of fluid flow, internal energy relaxation, and chemical reactions all become similar. Experiments studying this process are difficult to perform, expensive, do not exactly reproduce flight conditions, and require assumptions to interpret the measured data. First-principles computational simulations are now being used to understand these chemical processes. Advances in computational chemistry techniques and high-performance parallel computing have made the comprehensive study of these processes in dilute gases possible. To date, these computational studies have focused heavily on reactions of oxygen and nitrogen and have focused on the dissociation of diatomic molecules. Because it is highly reactive, atomic oxygen is incredibly important to model correctly. The formation and destruction of nitric oxide (NO) is also important to understand because NO is liable to emit energy as radiation. This thesis uses first-principles molecular simulation in several ways to complete the study of reactions in five-species air. Potential Energy Surfaces (PESs) describing interatomic forces in oxygen are used to simulate reactions of oxygen atoms and diatoms, and the predictions are validated against new molecular beam experimental data. This study shows that the new PESs used to describe these reactions do a better job of reproducing experimental results than earlier efforts, and that this improvement results from a more complete description of the relevant chemical states of the oxygen system. A QuasiClassical Trajectory (QCT) study of reactions that form and destroy nitric oxide is performed using new PESs and compared to Direct Molecular Simulation (DMS). These comparisons show that dissociation reactions in air mixtures are biased towards high energy vibrational states of reactant diatoms, but exchange reactions (referred to as Zeldovic reactions) are not as strongly biased. This suggests that while reduced-order chemistry models for hypersonic flow simulations should account for vibrational bias in dissociation, they do not need to account for any vibrational bias in the Zeldovic reactions. A major contribution of this thesis is the development of a new QCT theory and simulation framework to study recombination processes that involve the collisions of three neutral heavy particles. This ternary kinetic approach is based on a definition of the lifetime of binary collisions that is consistent with hard-sphere models in the limit of instantaneous reactions, and does not require an explicit appeal to the principle of detailed balance. The aspects of different sampling strategies for this method and the convergence of recombination rate constants are investigated. The results of this method are compared to the predictions of detailed balance, the results of dissociation QCT simulations, and a reduced-order chemistry model formulated using dissociation data combined with detailed balance. These comparisons show that the ternary kinetic method reproduces similar trends to those seen in dissociation studies and a reduced-order model built from them. Further analysis shows that while long-lived binary collisions are more likely to be hit by a third particle when they occur, they occur extremely rarely, are not any more likely to cause recombinations than short-lived binary collisions, and do not produce molecules in vastly different states than those predicted by dissociation data and the principle of detailed balance.
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    Characterizing Stratospheric Aerosol Particles and Particle Effects on Hypersonic Flight Vehicles
    (2023-12) Habeck, Joseph
    Micro-particles in the form of aerosols are present at cruise altitudes of operating hypersonic vehicles. As these particles traverse the flow-field around a vehicle, they generate small-scale disturbances. These disturbances can interact with the boundary layer, potentially leading to laminar-turbulent transition and a substantial increase in surfaceheating. Even if the boundary layer is not receptive to particle-induced disturbances, the thermal protection system may still sustain damage from particles impacting the surface. A notable challenge to addressing these issues is that particle sizes and concentrations in the atmosphere are not well-characterized, particularly at the high altitudes where hypersonic flight vehicles operate. This lack of characterization partly prohibits an accurate assessment of the implications of aerosol particles on future hypersonic missions. The first part of this dissertation presents in-situ measurements of particle size distributions and concentrations in the lower stratosphere obtained through various weather balloon campaigns. The second part utilizes data from these campaigns in numerical simulations of hypersonic flows to investigate their influence on boundary layer transition and potential damage to the vehicle surface.
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    Consistent Chemical Kinetics and Continuum Models for High Temperature Air
    (2020-02) Singh, Narendra
    High-fidelity nonequilibrium reaction models for hypersonic air flow are developed. Hypersonic flows create shock waves, which compress and heat the surrounding gas to high-temperatures. Strong shock waves cause dissociation of nitrogen and oxygen molecules. Predicting the extent of dissociation and recombination of atomic species is important since the state of the gas near the vehicle surface determines heating rates and gas-surface chemistry that damages the heat shield. Since experimental data is difficult to obtain under such extreme conditions, numerical simulation plays an important role. Predictive numerical simulations require accurate reaction chemistry models. Computational models developed thus far range from simple empirical models fit to limited experimental data to models with millions of input parameters that track individual quantized energy state transitions. The level of model fidelity required for accurate engineering analysis remains an open question of active research. Models coupling internal energy and dissociation chemistry tend to be developed at either the kinetic scale or the continuum scale. In this dissertation, we develop new nonequilibrium models for shock heated flows that are analytically consistent between kinetic and continuum formulations, and are based on recent ab-initio data.
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    Development of a Thermal-Vacuum Chamber for Extending Calibration of Optical Particle Detectors to Stratospheric Conditions
    (2019) Meyer, Jacob J; Flaten, James A; Candler, Graham V
    Hypersonic flight dynamics are complex and certain important issues, such as what can trigger laminar hypersonic flows to become turbulent, are not well understood; thus new advances in science need to be made before aircraft can be designed that can safely and routinely fly at many times the speed of sound [1]. A team of undergraduate and graduate student researchers in the Aerospace Engineering and Mechanics (AEM) Department, working on a recently-funded MURI grant [2], are using optical particle detectors flown on weather balloon missions to characterize the particulate content of the stratosphere. Data collected will be used in computer simulations that study the onset of turbulence in hypersonic flows. The purpose of this project is to build and test a thermal/vac chamber to assist in the calibration of optical particle detectors such as the modest-cost Alphasense OPC-N2 detector [3] and the much-higher-cost LOAC detector [4] for use on stratospheric balloon flights. The University of Minnesota is one of the three main institutional players in the MURI grant and our role is to characterize particulate content in the stratosphere at altitudes from 80,000 to 120,000 feet – the altitude range at which hypersonic vehicles fly. Particle detectors are characterized by the manufacturer at STP conditions, yet the MURI project requires understanding detector output in the low-temperature, low-pressure conditions of the stratosphere. The MURI team is working with the Particle Calibration Lab (PCL) in the Mechanical Engineering (ME) Department to help calibrate the Alphasense particle detectors at STP in various airflow environments. However the current PCL calibration set-up is unable to duplicate the extreme temperature and pressure conditions encountered on stratospheric balloon flights; hence the need for a way to extend STP calibration results to stratospheric conditions.
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    Direct Molecular Simulation of Nitrogen and Oxygen at Hypersonic Conditions
    (2018-02) Grover, Maninder
    The objective of this thesis is to characterize the gas-phase thermochemical non-equilibrium that occurs during hypersonic flight for nitrogen and oxygen gases. This thesis uses the direct molecular simulation (DMS) method in conjunction with potential energy surfaces (PESs) to provide an in-depth molecular level analysis of internal energy excitation and dissociation of molecular nitrogen due to $N_2+N_2$ and $N_2+N$ interactions. Characteristic vibrational excitation times and non-equilibrium dissociation rate coefficients are calculated using the $ab-initio$ PESs developed at NASA Ames Research Center. Comparison of these rate coefficients and non-equilibrium vibrational energy distributions is carried out against prior work done with nitrogen using an independently developed $ab-initio$ PES at the University of Minnesota. Good agreement was found between properties predicted by the two PESs. Furthermore, comparative studies were carried out for the nitrogen system between the DMS method and the state-to-state method. The results obtained by the two different methods, are found to be in good agreement. The DMS method is used to calculate benchmark data for vibrational energy excitation and non-equilibrium dissociation due to $O_2+O$ interactions. $O_2+O$ interactions are modeled using nine PESs corresponding to. $1^1A'$, $2^1A'$, $1^1A''$, $1^3A'$, $2^3A'$, $1^3A''$ $1^5A'$, $2^5A'$ and $1^5A''$ states, which govern electronically adiabatic (ground-electronic-state) collisions of diatomic oxygen with atomic oxygen. This is the first data set in the aerospace community that incorporates all nine PESs for the $O_2+O$ system and fully describes the dynamics of ground state interactions of diatomic oxygen with atomic oxygen. Characteristic vibrational excitation times are calculated over a temperature range of $T=3000K$ to $T=15000K$. It is observed that the characteristic vibrational excitation time for $O_2+O$ interactions is weakly dependent on temperature and increases slightly with increasing temperature. Vibrational excitation is slowest for interactions in the quintet spin state, with the $1^5A''$ state having the slowest excitation rate, and vibrational excitation is fastest on the $1^1A'$ potential energy surface. Non-equilibrium dissociation rate coefficients are calculated over a temperature range of $T=6000K$ to $T=15000K$ during quasi-steady state (QSS) dissociation, and the results agree well with experimental data. For the $O_2+O_2$ system interactions can occur over singlet, quintet and triplet spin states. An in-depth analysis of excitation and dissociation on the quintet and singlet surfaces is provided and bench-mark data for excitation using all three PESs for $O_2+O_2$ interactions is presented for a temperature range of $T=5000K$ to $T=12000K$ . Finally, this thesis explores internal energy exchange processes in oxygen and nitrogen. Probability distribution functions for vibrational energy change during collisions are presented (due to $N_2+N_2$ non-reactive collisions, $N_2+N_2$ exchange reactions, $N_2+N$ non-reactive collisions, $N_2+N$ exchange reactions, $O_2+O$ non-reactive collisions, and $O_2+O$ exchange reactions). It is shown that non-reactive collisions are less efficient in vibrational energy redistribution when compared to exchange reactions. Furthermore, it is observed that the probability distribution functions for vibrational energy change (for both oxygen and nitrogen) are self-similar and may be modeled by simplified functional forms.
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    Modeling and Analysis of Chemical Kinetics for Hypersonic Flows in Air
    (2018-11) Chaudhry, Ross
    Gas-phase chemical kinetics are relevant for hypersonic flows, but they are currently modeled in CFD using empirical assumptions and decades-old experimental data. Recent advances in quantum chemistry have enabled the construction of accurate potential energy surfaces (PESs) for diatom-diatom interactions in air. Using these PESs, a database of simulated interactions is generated and analyzed; N2 + N2, N2 + N, N2 + O2, O2 + O2, and O2 + O reactions are considered. The conditions studied range from 4000 K to 30,000 K and include thermal equilibrium and nonequilibrium test sets. The nitrogen dissociation rate is found to be similar for collision partners N2, N, and O2. The oxygen dissociation rate, in contrast, is moderately dependent on partner species; O2 is approximately 2 to 3 times more effective than partner N2. Oxygen dissociation with partner N2 is therefore found to be substantially overpredicted by current CFD models, which is consistent with the limited experimental data available for this reaction. The presence of N is known from experiments to promote nitrogen dissociation; this augmentation is found to be due to increased vibrational relaxation, rather than an increased dissociation rate as described by current CFD models. Similar observations are made for oxygen dissociation with partner O, due to a combination of vibrational and electronic energy relaxation. Using only the shock tube data that informed popular CFD models, it was impossible to isolate the effect of increased relaxation from increased dissociation. The change in vibrational energy per dissociation, a necessary input to CFD, is found to be very sensitive to the degree of thermal nonequilibrium. This dependence is not well predicted by any existing chemical kinetics models; correctly describing this term fundamentally changes the thermal evolution of a gas in CFD. The mechanics of dissociation are similar for all reactions studied, so a series of aggregate analyses on all dissociation reactions is performed. Vibration is found to have a more pronounced effect on dissociation than rotation, due to rotation increasing the centrifugal barrier. The classic Marrone-Treanor preferential dissociation model is found to accurately describe all data in the nonequilibrium test sets, but it neglects the effect of rotational energy on dissociation. A modified model is proposed that describes rates to within 22% and vibrational energy changes to within 4% of the dissociation energy, for all dissociation reactions and conditions. For this work, we have considered Boltzmann or approximately Boltzmann distributions, but the population of high-energy molecules is known to be depleted in a dissociating gas ensemble. Various kinetics models based on Boltzmann distributions are implemented in US3D, a production CFD solver designed for hypersonic flows. As expected, the dissociation rate is overpredicted compared to the benchmark data. Work remains, therefore, to account for the non-Boltzmann distributions that exist in reality. These data and insights about dissociation can form the basis for next-generation chemical kinetics models for CFD.
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    Numerical Study of High-Speed Transition due to Passive and Active Trips
    (2019-04) Shrestha, Prakash
    Transitional hypersonic boundary layers due to passive and active trips on a flat plate are studied using direct numerical simulations (DNS). In the case of passive trips (diamond- shaped and cylindrical), three dynamically prominent flow structures are consistently observed in both their isolated and distributed configurations. These flow structures are the upstream vortex system, the shock system, and the shear layers and the counter-rotating streamwise vortices from the wake of the trips. Analysis of the power spectral density (PSD) reveals the dominant source of instability due to the diamond-shaped trips as a coupled system of the shear layers and the counter-rotating streamwise vortices irrespective of spanwise trip-spacing. However, the dominant source of instability due to an array of cylindrical trips (Williams et al. 2018) is observed to be the upstream vortex system similar to Subbareddy et al. 2014 who used an isolated cylindrical trip. Therefore, the shape of a roughness element plays an essential role in the instability mechanism. Furthermore, dynamic mode decomposition (DMD) of three-dimensional snapshots of pressure fluctuations unveil globally dominant modes consistent with the PSD analysis in all the trip configurations. Higher peak-amplitude frequencies and amplitudes characterize dominant instabilities in higher freestream Reynolds number flows. When the trip heights are reduced, the source of instability has been observed to be unchanged, while peak-amplitude frequencies, the mean upstream recirculation zone, the mean instability-onset location, and the maximum turbulent kinetic energy is found to be reduced. When the trip spacing is greater than three times the trip width, each trip of the trip array becomes isolated. In the case of active trips, a two-dimensional (2-D) sonic jet from a straight slot is injected into Mach-10 three-dimensional (3-D) laminar boundary layers (Berry et al. 2004). The dynamically dominant flow structures observed in the vicinity of the jet correspond to upstream and downstream separation bubbles, where the number and the size of these bubbles vary with the injector pressure. A higher injector pressure leads to the formation of larger bubbles that cause the flow to become more unstable, resulting in a sequence of three successive bifurcations: (1) steady 2-D bubble formation, (2) transition from 2-D steady to 3-D quasi-unsteady bubble, and (3) transition from 3-D quasi-unsteady to 3-D unsteady bubble. This finding indicates that specific injector pressures are required to control the onset of transition in the laminar boundary layers. Streamwise streaks with a dominant spanwise wavelength are observed in both 3-D quasi-steady and 3-D unsteady flows. DMD of spanwise velocity reveals that the streamwise streaks originate from the upstream bubbles. In particular, the streaks arise from the coupled undulation of a primary upstream bubble and the upstream secondary bubble, which causes the flow to bifurcate from 2-D steady to 3-D quasi-unsteady. It is proposed that the source of the unsteadiness observed is generated by high-pressure fluctuations present between the secondary bubble and the jet. The unsteady interaction between the secondary bubble and the jet selects a specific wavelength of the spanwise undulation of the secondary bubble, which then modulates the primary bubble across span with the same wavelength. These two bubbles emanate two flow structures that have opposite spanwise velocities. These flow structures then travel to the top of the downstream bubbles to form a streamwise streak. The spanwise wavelength of the dominant DMD mode agrees with that of the streaks observed in the DNS. The simulation data in all cases agree well with their corresponding experiment. No effect of real gas has been found in this current study. The source of instability is observed to be independent of the thermal nature of the wall (isothermal or adiabatic). The angle of injection is observed to play a significant role in flow unsteadiness downstream of the jet. The mean Mach-disk height and the mean upstream recirculation length are compared to existing models in order to access their accuracy under the present flow and jet configurations.