This collaborative project uses experiments and numerical simulation to investigate the collective motion of small swimming organisms in water. The hypothesis underlying the project is that the collective motion of many small organisms can induce fluid motion over length scales that are much larger than the organism itself. The investigators will focus on motion in stratified fluid layers, which are commonly found in the ocean, where the top fluid layer usually is warmer and saltier than deeper fluid layers. The stratification can lead to an instability that induces fluid motion, but the presence of the swimming organisms may change conditions for the onset of the motion and affect detailed flow patterns and fluid mixing that result from the motion. Although the research focuses on motion in the ocean as an example, the insights gained from the project will apply to other particulate and multiphase systems. Both investigators will involve high school students, undergraduates and graduate students in the project. Results from the project will be used in outreach efforts to engage in the research students from groups that are traditionally underrepresented in science and engineering.
The objective of the proposal is to address experimentally and numerically the fundamental questions of multi-scale, many-body, fluid-structure interactions in stratified layers. The experiments will use a unique system that can produce on-demand vertical migrations of plankton via phototaxis in a controlled laboratory setting. A laser-guidance system will control swimming speed and swimmer spacing in collective motions. Two-dimensional particle imaging velocimetry will be used to measure flows generated by the swimmers' motion. The numerical methods implemented in the project are capable of simulating large numbers of swimming organisms with fidelity sufficient for studying details of both the near- and far-field flows. Numerical simulations will help test the possibility that collective swimming motions drive large-scale convection by modifying effective diffusivities of heat and salinity so as to induce double-diffusive convection.
