The interactions between dispersed particles and fluid turbulence have been investigated experimentally and numerically in a fully developed downflow channel in water. Particle velocities and fluid velocities in the presence of 340 μm and 500 μm glass beads and 420 μm cellulose particles were measured by digital particle image velocimetry. In the presence of particles slightly greater than the Kolmogorov lengthscale of flow and with a large particle time constant, turbulence energy was augmented in the streamwise direction, which means that the energy was transported by means of particle concentration fluctuations. The dissipation rate in the equation of streamwise component of the Reynolds stress was also increased. Cellulose particles with a small particle time constant dissipated the turbulence energy in the streamwise and transverse directions due to particle drag. A multiple time-scale model in which the effect of particle concentration fluctuations on large scales of turbulent motion was reflected can present both turbulence attenuation and augmentation by particles. The particle concentration fluctuations increase the turbulence energy in the production range, which induces increasing the energy transport from the production range to the transfer range and yields finally enhancing the dissipation.
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