The role of intermittency in scalar transport by turbulent flows
Mark Rast
Department of Astrophysical and Planetary Sciences, and Laboratory for Atmospheric and Space Physics (LASP),University of Colorado Boulder
Date and time:
Thursday, February 13, 2014 - 2:00pm
Location:
ECCR 257
Abstract:
Scalar transport by turbulent flows is best described in terms of Lagrangian parcel motions. We examine a simplified point vortex flow, measuring the Eulerian distance travel along Lagrangian trajectories to determine the probabilistic impulse response function in the absence of molecular diffusion. As expected, the mean squared Eulerian displacement scales ballistically for times shorter than the integral time and diffusively for longer times. However, the displacement distribution at any given time is only approximately that of a random walk with Gaussianly distributed step sizes, with significant deviations of the distribution from that of anomalous or normal diffusion found. The probability of long distance transport is reduced at inertial time scales by spatial and temporal intermittency in the flow. This can be statistically mimicked by a series of trapping events with durations uniformly distributed between the Kolmogorov and integral time scales. The probability of long distance transport is enhanced beyond that of the random walk for times shorter than the Kolmogorov or longer than the integral time, likely because of the superposition of vortex contributions. These findings have implications for turbulent transport modeling beyond the simplified flow studied.