Probing Fundamental Bounds in Hydrodynamics Using Variational Optimization Methods

Bartosz Protas, McMaster University SCAIM Seminar
February 26, 2019 12:30 pm ESB 4133

In the presentation we will discuss our research program concerning the study of extreme vortex events in viscous incompressible flows. These vortex states arise as the flows saturating certain fundamental mathematical estimates, such as the bounds on the maximum enstrophy growth in 3D (Lu & Doering, 2008). They are therefore intimately related to the question of singularity formation in the 3D Navier-Stokes system, known as the hydrodynamic blow-up problem. We demonstrate how new insights concerning such questions can be obtained by formulating them as variational PDE optimization problems which can be solved computationally using suitable discrete gradient flows. More specifically, such an optimization formulation allows one to identify “extreme” initial data which, subject to certain constraints, leads to the most singular flow evolution. In offering a systematic approach to finding flow solutions which may saturate known estimates, the proposed paradigm provides a bridge between mathematical analysis and scientific computation. In particular, it makes it possible to determine whether or not certain mathematical estimates are “sharp”, in the sense that they can be realized by actual vector fields, or if these estimates may still be improved. In the presentation we will review a number of results concerning 2D and 3D flows characterized by the maximum possible growth of different Sobolev norms of the solutions. Even when extreme initial data is used, high-resolution computations for the 3D Navier-Stokes system reveals no tendency for singularity formation in finite time.

[Joint work with Diego Ayala, Dongfang Yun and Di Kang]

Bartosz Protas is a professor in the Department of Mathematics and Statistics, McMaster University.