Collaborative Research: How small is too small? On the Minimum Swimmer Size Required to Generate Sustained Biogenic Turbulence

University Of Pittsburgh, Pittsburgh PA

Investigators

Abstract

In order to predict the future evolution of Earth’s climate, it is important to understand and quantify the processes that govern vertical mixing of heat and CO2 in the ocean, such as tides, internal waves and others. The degree to which the motion of small organisms such as zooplankton can contribute to this mixing is currently unknown, although several hypotheses have been forwarded that arrive at very different conclusions. The current project aims to improve the understanding of such biogenic mixing via a collaborative investigation that combines theoretical analysis with laboratory experiments and detailed computer simulations. In this way, the project results will enable more accurate predictions of future climate trends. The project will involve significant educational and outreach activities, including research by undergraduate and high school students. While it has been proposed that biologically generated turbulence plays an important role in oceanic turbulence, the range of zooplankton swimmer sizes that can contribute to such mixing is currently unknown. Recent research indicates that the minimum swimmer required depends on the nature of the flow field in which the swimmer moves, so that the capability of a swimmer to produce sustainable biogenic turbulence is not an inherent and static characteristic, but rather, it is modulated by the swimmer’s orientation in relation to the local shear and the intensity of the ambient hydrodynamic shear. The objective of the proposed research is to employ both laboratory experiments and direct numerical simulations (DNS) to reveal the minimum size and the corresponding biogenic turbulence production mechanism in a space spanned by the strength of the background shear, the orientation of the swimmer with regard to this shear, and the swimmer size. On this basis, models for the incorporation of these effects into ocean simulation tools will be developed. The experiments will use a unique system that can produce accurate on-demand migrations of zooplankton via phototaxis in a background shear in a controlled laboratory setting. The computational methodology is based on a well-validated immersed boundary method approach, and it employs an established squirmer model to represent the individual organisms. The proposed research will reveal how the swimmer’s agitation produces turbulence and dissipation. It will be the first systematic experimental and numerical study of biogenic turbulence considering both swimmer and background flow. Although the research is motivated by ocean flows, the insights gained from the project will deepen our understanding of how physical perturbations affect turbulent flows in general. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.