[Show abstract][Hide abstract] ABSTRACT: This dissertation presents a detailed, computational study quantifying the effects of nonequilibrium on the surface properties of a hypersonic vehicle by comparing Navier-Stokes-based Computational Fluid Dynamics (CFD) and direct simulation Monte Carlo (DSMC) simulation results for the flow about a cylinder and a wedge. Physical submodels contained in both computational methods are ensured to be as equivalent as possible. Translational nonequilibrium effects are isolated by considering a monatomic gas, argon. Thermal nonequilibrium effects are included by considering a diatomic gas, nitrogen. Several different flow regimes are considered, from the continuum into the transitional (freestream Knudsen numbers are 0.002, 0.01, 0.05 and 0.25), with Mach numbers of 10 and 25. Effects on surface properties (total drag and peak heat transfer rate) are quantified at each flow condition. Flow field properties are also compared. Continuum breakdown parameter values are compared with other flow and surface properties. The effectiveness of several types of CFD slip boundary conditions is evaluated, and the velocity slip and temperature jump (including vibrational temperature jump) values are compared with those extracted from DSMC simulation results. The slip conditions of Gokcen (AIAA Paper 1989-0461) most accurately predict surface properties, while the slip conditions of Lockerby et al. (AIAA J. 43(6) (June 2005), 1391-1393) agree best with DSMC slip values. For flows of argon and nitrogen about a cylinder, CFD total drag predictions remain within 6% of DSMC predictions, and heat flux agreement is 8% or better. For flows about a wedge, total drag differences range between 2% and 34%, mostly due to friction force differences. Peak heating differences are between 70% and 100%; DSMC predicts a much higher temperature near the leading edge than CFD. Flow property differences near the wall surface are shown to be concentrated primarily in the Knudsen layer. Validation of the CFD code, as well as the effect of various levels of surface accommodation, are shown by considering a nitrogen flow over a flat plate and comparing the simulation results with experimental data. Ph.D. Aerospace Engineering University of Michigan, Horace H. Rackham School of Graduate Studies http://deepblue.lib.umich.edu/bitstream/2027.42/58370/1/ajloft_1.pdf
[Show abstract][Hide abstract] ABSTRACT: The behaviour of pressureless sintered HfC and HfB2 ceramics, when exposed to high enthalpy plasma flows typical of atmospheric re-entry environment, was investigated with an arc-jet facility at temperatures exceeding 2000 °C. The surface temperature and emissivity of the materials were evaluated during the test. The microstructure modifications were analysed after exposure. Fluid dynamic numerical simulations were carried out to evaluate the catalytic atom recombination efficiencies of the materials at the experimental conditions. Surface and cross sections of the samples showed the formation of scales mainly consisting of HfO2 and SiO2. For the HfB2-based composite numerical results correlated quite well with experimental ones assuming a low catalytic surface behaviour. For the HfC-based material the surface behaviour changed from low catalytic to partially catalytic as the temperature increased. The post-test analyses confirm the potential of these composites to endure re-entry conditions with temperature approaching 2000 °C or even higher.
Journal of the European Ceramic Society 28(9):1899–1907. · 2.36 Impact Factor
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