Effect of particle size on velocity correlations in turbulent channel flow

The response of glass beads was examined in a fully developed turbulent channel flow of air with Reynolds number Re_h=4500 where h is the channel half width. Five sets of monodisperse particles were tested with integral Stokes numbers in the range 0.2-10 at the channel centerplane. Particle-to-air mass loadings were of order 10%. Laser sheets were aligned in streamwise-spanwise flow planes, and seeded, particle-laden flow was imaged with a dual-frame camera. A separation algorithm was used to obtain simultaneous gas and particle velocity measurements of homogeneous flow planes over a range of wall-normal distances. Profiles of single-point velocity statistics including mean and rms slip velocity, particle drift velocity, and gas-particle correlation were computed. In addition, two-point gas-particle and particle-particle velocity correlations were obtained. Near the wall, the gas-gas correlations yielded long, narrow patterns related to near-wall low-speed streaks. At the centerplane, the correlations were broader in the spanwise direction. Gas-particle and particle-particle correlations resembled the gas correlations in shape and size, but the values were smaller. The gas-particle covariance tended toward zero for St = 10. Particle-particle correlations were still significant for St = 4.