Browsing by Subject "Fluid Dynamics"

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    Development and Validation of a Turbulence Wall Model for Compressible Flows with Heat Transfer
    (2016-08) Komives, Jeffrey
    The computational cost to model high Reynolds number flows of engineering interest scales poorly with problem size and is excessively expensive. This fact motivates the development of turbulence wall models to lessen the computational burden. These models aim to provide accurate wall flux quantification on computational meshes that would otherwise be unable to accurately estimate these quantities. The benefit of using such an approximation is that the height of the wall-adjacent computational elements can be increased by one to two orders of magnitude, allowing for comparable increases in stable explicit timestep. This increase in timestep is critically necessary for the large eddy simulation of high Reynolds number turbulent flows. To date, most research in the application of wall models has focused on incompressible flows or flows with very weak compressibility. Very few studies examine the applicability of wall models to flows with significant compressibility and heat transfer. The present work details the derivation of a wall model appropriate for compressible flows with heat transfer. The model framework allows for the inclusion of non-equilibrium terms in the determination of wall shear and heat transfer. The model is applied to a variety of supersonic and hypersonic flows, and is studied in both Reynolds-averaged simulations and large eddy simulations. The impact of several modeling approaches and model terms is examined. The wall-modeled calculations show excellent agreement with wall-resolved calculations and experimental data. For time accurate calculations, the use of the wall model allows for explicit timesteps more than 20 times larger than that of the wall-resolved calculation, significantly reducing both the cost of the calculation and the time required converge the solution.
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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.