Transport Processes in a Direct Numerical Simulation of Turbulent Channel Flow

A direct numerical simulation of fully-developed, time-dependent, three-dimensional turbulent flow in a channel is used to investigate turbulent transport processes. Detailed properties of the turbulent velocity field are presented. Three different transport processes are explored using this extensive data base. The first is the identification of the origin and fate of flow-oriented structures responsible for transporting momentum close to the wall. An important finding is that they regenerate themselves by a process that appears to be weakly dependent on the outer flow. This involves the enhancement of streamwise vorticity at the wall, of opposite sign, at a location where a stress-producing eddy lifts from the wall. Another area of exploration is the analysis of how small, dense particles move in a carrier fluid and deposit on a boundary. A Stokes drag force is used in the equation of motion for the aerosol and the particles are assumed to have no influence on the flow field. It is shown that these particles accumulate in the near wall region by turbophoresis and by free-flight. They deposit due to their own inertia. A new method for identifying free-flight particles is presented and a prediction of free-flight deposition is made using fluid velocity distributions. The third subject involves the effect of Prandtl number on the transport of heat in turbulent flow between a hot wall and a cold wall. The effects of Prandtl number on the turbulent diffusivity of heat and on the dissipation of temperature fluctuations are presented. A prediction of the Nusselt number based upon the Reynolds analogy, which relates the turbulent temperature field to the turbulent velocity field, is also presented.