Spatiotemporal evolution of particle puffs in transitional channel flow

Abstract

We study the shape evolution of puffs composed of tracers advected in a transitional channel flow. We perform a direct numerical simulation of a spatially evolving channel flow, where the inflow condition is given by a solution of the Orr–Sommerfeld equation and the flow evolves through all stages of transition up to fully developed turbulence. In this setting, we release spherical puffs of particles and track their evolution using measures derived from their approximation as ellipsoids. By varying the initial position of puffs, we characterize the spatial non-homogeneity of the flow, with respect to both the distance from the wall and, most importantly, the streamwise coordinate along which the flow evolves. Furthermore, we assess the influence of scale-dependent phenomena on puff shapes by varying the initial size of the clouds of particles. The present approach explores the interaction between flow features and advected Lagrangian structures. Additionally, it reveals the interplay between flow scales and how their balance changes during transition, where the intermittency causes large puffs to be much less elongated than smaller puffs independently of the distance from the walls.