Finite-Time Braiding Exponents

Abstract

Topological entropy is a common measure of the rate of mixing in a flow. It can be computed by partition methods, or by estimating the growth rate of material lines or other material elements. This requires detailed knowledge of the velocity field, which is not always available, such as when we only know a few particle trajectories (ocean float data, for example). We propose an alternative approximation to topological entropy,applicable to two-dimensional flows, which uses only a finite number of trajectories as input data. To represent these sparse data sets, we use braids, algebraic objects that record how strands, i.e., trajectories, exchange positions with respect to a projection axis. Material curves advected by the flow are represented as simplified loop coordinates. The exponential rate at which a braid deforms loops over a finite time interval as the strands exchange places is the Finite-Time Braiding Exponent (FTBE) and serves as a proxy for topological entropy of the two-dimensional flow. We demonstrate that FTBEs are robust with respect to the value of numerical time step,details of braid representation, and choice of initial conditions inside the mixing region. We also explore how closely the FTBEs approximate topological entropy depending on the number and length of trajectories used.