Many aquatic systems, including oceans, often have regions where the water density varies with depth owing to variations in temperature and salinity. These regions of water density stratification are associated with formation of algal blooms and accumulation of marine snow particles. This project comprises computational and experimental studies to test the hypothesis that density stratification leads to enhanced horizontal clustering of particles and organisms, especially those with elongated shapes. Furthermore, the project will investigate whether density stratification suppresses convection plumes that can disperse algal cells, leading to accumulation of cells. The equations that govern the motion of fluid and particles will be solved numerically and results will be compared with experiments to visualize the motion of algal cells and particles in water with density stratification. Some algal blooms are toxic, and they pose serious problems in coastal waters, including reduced oxygen content, increased marine animal mortality, and increased fouling of marine equipment. This project will help uncover the mechanisms of algal bloom formation, which could lead to new strategies to control it. The project will provide opportunities for students at various academic levels to participate in research, and results of the project will be incorporated in fluid mechanics courses taught by the investigators.
The lead investigator for this project has recently shown that water density stratification affects not only the dynamics of suspensions of algal cells, but also the motion of individual cells and marine particles. The goal of the proposed research is to quantify effects of density stratification, turbulence and shear, motility and shape of microorganisms on microstructure formation and accumulation of planktonic species by developing a large-scale computational framework to fully resolve the corresponding flows. Three-dimensional Direct Numerical Simulations of the nonlinear equations governing organism and particle transport will be implemented to gain new insights into the mechanisms governing the distribution of planktonic organisms and particles at pycnoclines. Experiments involving particle image velocimetry of algal cells and particles in density stratified liquids with an imposed turbulent flow will be used to obtain parameters needed for the computations and to validate computational results.
