Book Description

Direct numerical simulation techniques are used to study wall turbulence and turbulent dispersion. Turbulent dispersion using a Lagrangian approach is studied. Heat or mass markers are released from sources located at the wall of the channel and their trajectories are monitored. The Lagrangian point of view is that scalar transport is the result of the behavior of a sum of these sources. This approach allows the study of turbulent transport at different Prandtl (or Schmidt) numbers, Pr = 0.1 (liquid metals), Pr = 0.7 (air), Pr = 10 (water) to Pr = 100 and 2400 (heavy oils). It also allows the study of different configurations such as dispersion from a line source, heat transfer from a heated plate, heat transfer between a hot and a cold wall and heat transfer between two heated walls. The most interesting result is that the behavior of a single instantaneous point source at the wall can be used as a building block to describe a variety of problems. An Eulerian simulation at Pr = 10 was also carried out. A comparison between results from existing Eulerian DNS with the Lagrangian results shows excellent agreement. Eulerian DNS simulations for Pr $>$ 10 are not currently available. However, Lagrangian simulations at high Pr are possible and offer unique opportunities. In order to study the coherent structures of turbulence in the logarithmic layer, a plane Couette flow simulation is developed. The total stress is constant throughout the Couette channel, providing a logarithmic layer that extends to the center of the channel. A very important result is that backscattering of turbulent kinetic energy feeds energy from small scale turbulence to large scale structures. One implication of this finding is that subgrid modeling in large eddy simulations should take into account backscattering even at simple geometries. A channel flow simulation at double the Reynolds number (Re = 5750) of that used for the Couette flow simulation is documented. The scope is to obtain a thicker logarithmic layer and to compare with results at half Reynolds number. The effect of Re on turbulence statistics and the structure of turbulence is investigated as well as the scaling of turbulence quantities in the inner and outer flow regions.