Lagrangian coherent structures in flows within arrays of wall-mounted emergent cylinders.

Ana Margarida Ricardo, Ricardo Canelas, Rui M.L. Ferreira

Thursday 2 july 2015

9:00 - 9:15h at Amazon (level 1)

Themes: (T) Water engineering, (ST) River and coastal engineering

Parallel session: 10B. Engineering - River

The vegetation that is often present in water bodies impacts the geomorphologic processes, affects the fluxes of sediment, nutrients and contaminants, promotes water quality, maintains biodiversity and improves landscape integrity. One of the processes of paramount importance in flows populated with vegetation is the dispersion and mixing of passive tracers which has been linked to the presence of Lagrangian coherent structures. Coherent structures can be defined as vortex structures that separate the fluid field to distinct regions, which are rotating relative to each other and can be of any shape. Lagrangian coherent structure boundaries, in steady flows, are typically identified as local minimizers or maximizers of the stability or instability of the time field by stable and unstable material lines or surfaces that stretch and fold blobs of initial conditions. The goal of the present work is to detect boundaries of coherent Lagrangian structures in flows with arrays of emergent cylinders, simulating rigid and emergent vegetation conditions. A 2D experimental and a 3D numerical databases are explored. The experimental data consisted in instantaneous velocities maps acquired with a Particle Image Velocimetry system. The numerical data is obtained by Smoothed Particle Hydrodynamics simulations, a Lagrangian meshless method, closed with a LES model for the turbulent stresses. The methodology employed defines Lagrangian coherent structures as ridges in the field of the finite-time Lyapunov exponents. The Finite-Time Lyapunov Exponent field of a dynamical system, which characterizes the rate of separation of neighboring trajectories over a finite time interval, is computed by seeding a velocity field with massless particles and integrating their positions forward or backward in time. The results show that vortexes travel downstream following preferred paths through the array of cylinders. There are regions acting as barriers that are not crossed by the coherent structures. The present work brings hints to the understanding of the processes of dispersion of scalars within vegetated flows.