A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
Browse
2 results
Search Results
Item Computational Fluid Dynamic Solutions of Optimized Heat Shields Designed for Earth Entry(2010) Meeroff, Jamie Gabriel; Lewis, Mark J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Computational fluid dynamic solutions are obtained for heat shields optimized aerothermodynamically using Newtonian impact theory. Aerodynamically, the low-order approach matches computational simulations within 10%. Benchmark Apollo 4 solutions show that predicted heat fluxes under-predict convective heating by 30% and over-predict radiative heating by 16% compared to computational results. Parametric studies display a power law reliance of convective heat flux on edge radius. A slender heat shield optimized for a single design point produces heat fluxes 1.8 times what was predicted using the Newtonian approach. Here, maximum heating decreases with the inverse cube of the base sharpness. Coupled vehicle/trajectory optimized designs are examined for lunar return (11 km/s) and Mars return (12.5 km/s) and show possible discrepancies for eccentric shapes using low-order empirical correlations. Ultimately, gains suggested by the low-order approach using complex geometries are not reflected in high-fidelity simulations. In some respects, the simpler shape is the ideal oneItem Analysis of Air Quality with Numerical Simulation (CMAQ), and Observations of Trace Gases(2009) Castellanos, Patricia; Ehrman, Sheryl H; Dickerson, Russell R; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ozone, a secondary pollutant, is a strong oxidant that can pose a risk to human health. It is formed from a complex set of photochemical reactions involving nitrogen oxides (NOx) and volatile organic compounds (VOCs). Ambient measurements and air quality modeling of ozone and its precursors are important tools for support of regulatory decisions, and analyzing atmospheric chemical and physical processes. I worked on three methods to improve our understanding of photochemical ozone production in the Eastern U.S.: a new detector for NO2, a numerical experiment to test the sensitivity to the timing to emissions, and comparison of modeled and observed vertical profiles of CO and ozone. A small, commercially available cavity ring-down spectroscopy (CRDS) NO2 detector suitable for surface and aircraft monitoring was modified and characterized. The CRDS detector was run in parallel to an ozone chemiluminescence device with photolytic conversion of NO2 to NO. The two instruments measured ambient air in suburban Maryland. A linear least- squares fit to a direct comparison of the data resulted in a slope of 0.960±0.002 and R of 0.995, showing agreement between two measurement techniques within experimental uncertainty. The sensitivity of the Community Multiscale Air Quality (CMAQ) model to the temporal variation of four emissions sectors was investigated to understand the effect of emissions' daily variability on modeled ozone. Decreasing the variability of mobile source emissions changed the 8-hour maximum ozone concentration by ±7 parts per billion by volume (ppbv). Increasing the variability of point source emissions affected ozone concentrations by ±6 ppbv, but only in areas close to the source. CO is an ideal tracer for analyzing pollutant transport in AQMs because the atmospheric lifetime is longer than the timescale of bound- ary layer mixing. CO can be used as a tracer if model performance of CO is well understood. An evaluation of CO model performance in CMAQ was carried out using aircraft observations taken for the Regional Atmospheric Measurement, Mod- eling and Prediction Program (RAMMPP) in the summer of 2002. Comparison of modeled and observed CO total columns were generally in agreement within 5-10%. There is little evidence that the CO emissions inventory is grossly overestimated. CMAQ predicts the same vertical profile shape for all of the observations, i.e. CO is well mixed throughout the boundary layer. However, the majority of observations have poorly mixed air below 500 m, and well mixed air above. CMAQ appears to be transporting CO away from the surface more quickly than what is observed. Turbulent mixing in the model is represented with K-theory. A minimum Kz that scales with fractional urban land use is imposed in order to account for subgrid scale obstacles in urban areas and the urban heat island effect. Micrometeorological observations suggest that the minimum Kz is somewhat high. A sensitivity case where the minimum Kz was reduced from 0.5 m2/s to 0.1 m2/s was carried out. Model performance of surface ozone observations at night increased significantly. The model better captures the observed ozone minimum with slower mixing, and increases ozone concentrations in the residual layer. Model performance of CO and ozone morning vertical profiles improves, but the effect is not large enough to bring the model and measurements into agreement. Comparison of modeled CO and O3 vertical profiles shows that turbulent mixing (as represented by eddy diffusivity) appears to be too fast, while convective mixing may be too slow.