LAGRANGIAN STATISTICS IN TURBULENT CHANNEL FLOW

Lagrangian statistics have been obtained from large eddy simulations o f fully developed turbulent channel flow, Calculations were performed at Reynolds numbers of 3200 and 21,900 (based on centerline velocity a nd channel half-width); statistics of the Eulerian velocity field are in good agreement with both direct numerical simulation data and exper imental measurements. Single-particle Lagrangian velocity autocorrelat ions and particle mean-square dispersion were obtained from trajectori es measured for 5000 fluid elements initially in either the viscous su blayer, buffer layer, or logarithmic region. The Lagrangian velocity a utocorrelation of particles initially located in the log region decrea ses less rapidly than for particles initially in the buffer layer, whi ch in turn decreases more slowly than for particles initially in the v iscous sublayer. The ratio of the Lagrangian to Eulerian integral time scales were found to be proportional to the inverse of the turbulence intensity, in agreement with theoretical predictions and atmospheric m easurements. Growth of particle mean-square dispersion at long diffusi on times is propertional to time and in agreement with theory (with th e exception of the surface-normal coordinate in which the presence of the channel wall limits dispersion). However, extremely long transport times are required to achieve the asymptotic state for the dispersion .