Abstract
In turbulent flows, energy flux refers to the transfer of kinetic energy across different scales of motion, a concept that is a cornerstone of turbulence theory. The direction of net energy flux is prescribed by the dimensionality of the fluid system. According to Kolmogorov's 1941 scaling theory, three-dimensional turbulence has a net energy flux toward smaller length scales, while in two-dimensional turbulence, energy transfers toward larger scales, as described in Kraichnan and Batchelor's seminal works. Manipulating energy flux across different scales with localized physical perturbations in flow systems is a formidable task because the energy at any scale is not localized in physical space. In this talk, I will present a theoretical framework that enables the manipulation of energy flux direction in turbulent flows. Based on this framework, we have successfully demonstrated the manipulation of a flow system to achieve the desired directions of net energy flux through both electromagnetically driven thin-layer flow experiments and direct numerical simulations in two dimensions. Notably, we generated a type of turbulent flow that has never been produced before—two-dimensional Navier-Stokes turbulence with a net forward energy flux. Apart from theoretical interest, we will discuss how our theoretical framework can have profound applications and implications in natural and engineered systems across length scale ranges from millimeters to hundreds of kilometers (10^{-3} to 10^{6} m), including enhanced mixing of microfluidic devices, biologically generated turbulence for a better understanding of the biogeochemical structure of water columns, breaking persistent coastal transport barriers for better coastal ecological health, and ocean energy budget in facing of climate change.
About the Speaker
Lei Fang received a Ph.D. from Stanford University in the Civil and Environmental Engineering Department with a Ph.D. minor in Computational and Mathematical Engineering in 2020. After that, he joined Civil and Environmental Engineering at the University of Pittsburgh as an Assistant Professor with secondary appointments in Mechanical and Materials Science Engineering and Bioengineering. His research focuses on turbulence dynamics and transport and mixing problems with particular emphasis on topics relevant to biology, environment, and health. Current interests include turbulence dynamics, two-way coupling between swimmers and background flows, biologically generated turbulence, and the development of new experimental methods. He is always looking for outstanding Ph.D. students and welcomes CVs for potential discussions.
You are welcome to visit his research website: http://www.leifang.pitt.edu/
Host: Professor Leonardo Chamorro