Wonyong Jang

Concentration evolution of gas species within a collapsing bubble in a liquid medium

In this study numerical methods are used to investigate the relationship between chemical concentration of gas species within a cavitating bubble, equilibrium radius of the gas bubble and pressure variations in the ambient liquid. For this purpose, governing equations are developed to describe the dynamic equilibrium of a bubble in a flowing fluid and mass transfer between gas and liquid phases, where it was assumed that gases undergo isothermal compression, obey the ideal gas law, Henry law. It is further assumed that the concentration of each phase within the bubble is uniform. The resulting nonlinear equations are solved using implicit Trapezoidal method with Newton iteration. Four gas species are modeled under various initial and ambient pressure variation conditions. These conditions maybe considered to represent typical cavitation events. The numerical results obtained are presented in terms of dimensionless numbers. These results indicate that chemical damage maybe an important component of cavitation surface damage, since high concentration profiles may develop within a collapsing bubble. Proposed formulation and numerical solutions are simple and cost effective to implement. The results presented in this study maybe used to benchmark experimental investigations or other more complex solutions, which are outside the scope of this study.

Historical reconstruction of groundwater contamination at contaminated sites and uncertainty analysis.

Outline Background Mathematical Model Methodology Numerical Applications Conclusions Background US Marine Corps Base Camp Lejeune, NC Background  Water-distribution systems at the site were contaminated with volatile organic compounds (VOCs)  Tarawa Terrace Area was contaminated with PCE.  Hadnot Point Area was contaminated with TCE and other chemicals.  Purpose of the study is to determine if there is an association between exposure to contaminated drinking water and birth defects and childhood cancers in children born to women who were pregnant while living in base house during the period 1968 – 1985. Background  ATSDR and Georgia Tech have been involved in water modeling activities to support an epidemiological study conducted by ATSDR at Camp Lejeune Base.  Modeling techniques are used to reconstruct historical conditions of groundwater flow, contaminant fate and transport, and the distribution of contaminated drinking water within the water distribution system which may have delivered contaminants to the family housing area. Background  Hadnot Point area of the site is the ongoing study area.  In this presentation the data available at the Tarawa Terrace area is used to test the applicability of the method.  Groundwater in the vicinity of the Tarawa Terrace was potentially contaminated by seepage from the dry cleaners;  Detailed geohydrologic information on aquifer properties are limited;  Limited concentration records are available at some observation sites after 1994; and,  Pumping operation schedules for pumping wells at the site are available before 1994.  The goal is to reconstruct contamination history at several observation sites before 1994.

Modeling of Chlorinated VOCs Transport under Dual Bioreactions

Once chlorinated volatile organic compounds (CVOCs) are released into the subsurface they are spread out through the porous soil matrix by complicated processes such as advection, dispersion, and sorption. CVOCs can be also biodegraded by indigenous microorganism via aerobic and anaerobic bioreactions, which generate distinct benign or harmful by-products. Heterogeneous subsurface characteristics and non-uniform distribution and consumption of oxygen in the subsurface may allow the dual bioreactions to coexist within a representative subsurface volume (RSV). The portion of each bioreaction within a RSV will depend on multiple factors such as oxygen, contaminant, and microorganism levels. Oxygen can be supplied into the contaminated zone through the dispersive and advective transport of oxygen in gas phase as well as the flow of the groundwater containing dissolved oxygen. As the dual bioreactions are coupled with multiple fluid flows (groundwater and gas) and multispecies transport, the analysis of the bioreactions is very complicated. However, the enhanced understanding of the dual bioreactions of CVOCs is required in accurately predicting the fate and transport of CVOCs at the contaminated sites. This study numerically investigates: the effects of dual bioreactions on the fate and transport of CVOCs in the variably saturated zone; and, the temporal and spatial evolution of bioreaction zones in the domain. Trichloroethylene is selected as a primary contaminant, and its sequential aerobic/anaerobic bioreactions are considered.