Particle-fluid interactions in a plane near-wall turbulent flow

The role of particles heavier than the fluid (glass spheres in water) in a turbulent open channel flow over a smooth bed is examined at moderately high concentration. The present work focuses on the dynamical interaction between the particles (solid phase) and the fluid phase in the near-wall region. Experimental measurements have been performed by means of Phase Doppler Anemometry to achieve two velocity components, particle size and concentration simultaneously. It is observed that particles increasingly smooth the vertical profiles of streamwise mean velocity as particle size increases. Turbulence intensities of the streamwise and especially of the vertical velocity are also damped in the outer wall-region, while enhanced in the near-wall region. By analyzing the different contributions to the Reynolds stress, the mechanism of entrainment and deposition of particles within the fluid is found to depend on the dynamics of the large-scale coherent structures. The measurements reveal that the presence of particles lead to a re-equilibrium between the injection of external flow towards the wall (“sweeps”) and the ejection of wall flow into the outer region (“bursts”) in comparison to the clear water conditions.