https://aurora.auburn.edu/handle/11200/44253 2026-04-24T09:17:18Z https://aurora.auburn.edu/handle/11200/50765 Secondary size distributions for single drop impacts at high wall superheat The impingement of liquid sprays on hot walls is used extensively in both spray-cooling systems and in combustor fuel injection applications. At low and moderate wall temperatures, the secondary size distributions have been reported in the literature. For high wall superheat conditions, particularly for real multicomponent fuels, this secondary size distribution has received less attention. Understanding the resultant size distribution for a spray-wall impact is key to capturing vaporization and local mixture for fuel-spray impingement. In this study, single drop impacts for a range of single-component (n-decane) and multicomponent jet fuel (F-24) are characterized through dual-view imaging. Secondary droplets are captured for impact Weber numbers of 100–600 and wall temperatures spanning the nucleate and film boiling (Leidenfrost) regimes. Imaging through a transparent sapphire substrate is used to capture the impact phenomena and impact-induced breakup of impacting drops. We report empirical correlations for the secondary droplet size for single-component (n-decane) and multicomponent (F-24) liquid fuels with varying wall temperature to provide validation datasets for spray-wall simulations. https://aurora.auburn.edu/handle/11200/50481 The Role of Breccia Lenses in Regolith Generation From the Formation of Small, Simple Craters: Application to the Apollo 15 Landing Site Impact cratering is likely a primary agent of regolith generation on airless bodies. Regolith production via impact cratering has long been a key topic of study since the Apollo era. The evolution of regolith due to impact cratering, however, is not well understood. A better formulation is needed to help quantify the formation mechanism and timescale of regolith evolution. Here we propose an analytically derived stochastic model that describes the evolution of regolith generated by small, simple craters. We account for ejecta blanketing as well as regolith infilling of the transient crater cavity. Our results show that the regolith infilling plays a key role in producing regolith. Our model demonstrates that because of the stochastic nature of impact cratering, the regolith thickness varies laterally, which is consistent with earlier work. We apply this analytical model to the regolith evolution at the Apollo 15 site. The regolith thickness is computed considering the observed crater size-frequency distribution of small, simple lunar craters (< 381m in radius for ejecta blanketing and <100m in radius for the regolith infilling). Allowing for some amount of regolith coming from the outside of the area, our result is consistent with an empirical result from the Apollo 15 seismic experiment. Finally, we find that the timescale of regolith growth is longer than that of crater equilibrium, implying that even if crater equilibrium is observed on a cratered surface, it is likely that the regolith thickness is still evolving due to additional impact craters. Plain Language Summary Impact cratering likely generates much of the regolith (the surface layer made up of a mixture of rocks, rock fragments, sand, and dust) observed on airless planetary surfaces. However, the way that the regolith layer evolves and thickens over time due to impact cratering events is not well understood. When a small, simple crater forms into hard rock, regolith is produced by fracturing the target rock and is deposited in the crater's ejecta blanket and within its transient crater cavity. Here we discuss an analytically derived stochastic model that describes the evolution of regolith developed by simple craters. Our results indicate that the regolith deposited on crater interiors is particularly important to consider when describing the distribution of regolith. Our model also indicates that the regolith thickness varies from one location to another. We apply this model to the regolith at the Apollo 15 landing site by considering the size distribution of observed small, simple lunar craters. Allowing for some regolith coming from outside of the area of the landing site, our result is consistent with an empirical result from the Apollo 15 seismic experiment. https://aurora.auburn.edu/handle/11200/49827 The Role of Breccia Lenses in Regolith Generation From the Formation of Small, Simple Craters: Application to the Apollo 15 Landing Site Impact cratering is likely a primary agent of regolith generation on airless bodies. Regolith production via impact cratering has long been a key topic of study since the Apollo era. The evolution of regolith due to impact cratering, however, is not well understood. A better formulation is needed to help quantify the formation mechanism and timescale of regolith evolution. Here we propose an analytically derived stochastic model that describes the evolution of regolith generated by small, simple craters. We account for ejecta blanketing as well as regolith infilling of the transient crater cavity. Our results show that the regolith infilling plays a key role in producing regolith. Our model demonstrates that because of the stochastic nature of impact cratering, the regolith thickness varies laterally, which is consistent with earlier work. We apply this analytical model to the regolith evolution at the Apollo 15 site. The regolith thickness is computed considering the observed crater size-frequency distribution of small, simple lunar craters (< 381m in radius for ejecta blanketing and <100m in radius for the regolith infilling). Allowing for some amount of regolith coming from the outside of the area, our result is consistent with an empirical result from the Apollo 15 seismic experiment. Finally, we find that the timescale of regolith growth is longer than that of crater equilibrium, implying that even if crater equilibrium is observed on a cratered surface, it is likely that the regolith thickness is still evolving due to additional impact craters. Plain Language Summary Impact cratering likely generates much of the regolith (the surface layer made up of a mixture of rocks, rock fragments, sand, and dust) observed on airless planetary surfaces. However, the way that the regolith layer evolves and thickens over time due to impact cratering events is not well understood. When a small, simple crater forms into hard rock, regolith is produced by fracturing the target rock and is deposited in the crater's ejecta blanket and within its transient crater cavity. Here we discuss an analytically derived stochastic model that describes the evolution of regolith developed by simple craters. Our results indicate that the regolith deposited on crater interiors is particularly important to consider when describing the distribution of regolith. Our model also indicates that the regolith thickness varies from one location to another. We apply this model to the regolith at the Apollo 15 landing site by considering the size distribution of observed small, simple lunar craters. Allowing for some regolith coming from outside of the area of the landing site, our result is consistent with an empirical result from the Apollo 15 seismic experiment. https://aurora.auburn.edu/handle/11200/49821 No Change in the Recent Lunar Impact Flux Required Based on Modeling of Impact Glass Spherule Age Distributions The distributions of Ar-40/Ar-39-derived ages of impact glass spherules in lunar regolith samples show an excess at <500 Ma relative to older ages. It has not been well understood whether this excess of young ages reflects an increase in the recent lunar impact flux or is due to a bias in the samples. We developed a model to simulate the production, transport, destruction, and sampling of lunar glass spherules. A modeled bias is seen when either (1) the simulated sampling depth is 10 cm, consistent with the typical depth from which Apollo soil samples were taken, or (2) when glass occurrence in the ejecta is limited to >10 crater radii from the crater, consistent with terrestrial microtektite observations. We suggest that the observed excess of young ages for lunar impact glasses is likely due to limitations of the regolith sampling strategy of the Apollo program, rather than reflecting a change in the lunar impact rate.