The dispersion of a scalar quantity from point sources located in a Direct Numerical Simulation of turbulent channel flow is studied. An algorithm for tracking fluid particles or molecular (or thermal) markers is developed and tested. Accurate estimates of a number of Lagrangian characteristics of the flow, necessary for the description of the diffusion process, are reported for the case of a point source in the center of the channel. The consequences of molecular diffusivity on the effectiveness of the turbulence to disperse a foreign substance (or heat) are also explored. A new method is proposed for calculating the effect of Peclet number on the Lagrangian property autocorrelation in isotropic turbulence. Computed property autocorrelations, from a simulated experiment of point source diffusion in the center of the channel, are also reported. Finally, results for the diffusion from point sources located at the channel walls are presented and discussed.

This book contains a collection of the main contributions from the first five workshops held by Ercoftac Special Interest Group on Synthetic Turbulence Models (SIG42. It is intended as an illustration of the sig’s activities and of the latest developments in the field. This volume investigates the use of Kinematic Simulation (KS) and other synthetic turbulence models for the particular application to environmental flows. This volume offers the best syntheses on the research status in KS, which is widely used in various domains, including Lagrangian aspects in turbulence mixing/stirring, particle dispersion/clustering, and last but not least, aeroacoustics. Flow realizations with complete spatial, and sometime spatio-temporal, dependency, are generated via superposition of random modes (mostly spatial, and sometime spatial and temporal, Fourier modes), with prescribed constraints such as: strict incompressibility (divergence-free velocity field at each point), high Reynolds energy spectrum. Recent improvements consisted in incorporating linear dynamics, for instance in rotating and/or stably-stratified flows, with possible easy generalization to MHD flows, and perhaps to plasmas. KS for channel flows have also been validated. However, the absence of "sweeping effects" in present conventional KS versions is identified as a major drawback in very different applications: inertial particle clustering as well as in aeroacoustics. Nevertheless, this issue was addressed in some reference papers, and merits to be revisited in the light of new studies in progress.