MEASUREMENTS OF 3D TRANSPORT BOUNDARIES WITH URBAN CANOPIES USING MAGNETIC RESONANCE IMAGING

Measurements of mean flow transport boundaries in a realistic urban canopy model are conducted using magnetic resonance imaging (MRI) techniques. A transport boundary is defined as a ridge of the Finite Time Lyapunov Exponent (FTLE) field computed using the mean velocity. It is found that ridges of the backwards FTLE field, which identify hyperbolic manifolds exhibiting exponential convergence of fluid trajectories, act as barriers to scalar transport. The MRI measurements enable the three-dimensional characterization of transport boundary topology and explain observed cross-wind and vertical scalar transport behaviour. Despite the fact that the FTLE field only guarantees negligibly small mean scalar flux perpendicular to the ridge, the structures appear remarkably robust to turbulent mixing. By contrast, the forwards FTLE field, which describes exponentially separating trajectories, does not inhibit turbulent mixing. This observation is explained by analyzing a simple cellular flow field with both stable and unstable manifolds.