Turbulence generation in homogeneous dilute particle-laden flows

Homogeneous turbulence generated by the motion of particles in dispersed multiphase flows was studied both theoretically and experimentally, motivated by applications to sprays, particle-laden jets, bubble plumes and rainstorms, among others. The experiments involved uniform fluxes of monodisperse spherical particles falling through a slow upflow of air. Particle fluxes and phase velocities were measured by sampling and phase-discriminating laser Doppler velocimetry (LDV), respectively. Measured particle velocities included mean and fluctuating streamwise and cross-stream velocities and probability density functions (PDF's). Measured continuous-phase velocities included mean and fluctuating streamwise and cross-stream velocities, PDF's and the higher moments of velocity fluctuations such as skewness and kurtosis, energy spectra of velocity fluctuations and integral length scales based on streamwise velocity fluctuations. Continuous-phase velocity measurements included conditional averages for particle wake disturbances and the turbulent inter-wake region surrounding these disturbances as well as overall flow properties. Present and earlier results in the literature provided particle Reynolds numbers of 38–990, particle volume fractions less than 0.01% and turbulence intensities (normalized by mean particle relative velocities) of 0.1–10.0%. Theory included characterization of particle wake disturbances as laminar-like turbulent wakes observed for intermediate particle Reynolds numbers in turbulent environments, characterization of the turbulent inter-wake region by analogy to grid-generated isotropic turbulence, and estimation of overall flow properties by conditional averaging of the properties of the wake disturbances and the turbulent inter-wake region.; Present measurements showed that particle wake disturbances during turbulence generation were properly characterized by the properties of laminar-like turbulent wakes. The turbulent inter-wake region was homogeneous and isotropic with properties that could be scaled by the mean particle spacing normalized by the particle momentum diameter. For present conditions, and probably for most practical dispersed flows, the turbulent inter-wake region exhibited very small turbulence Reynolds numbers and was in the final decay period where turbulence becomes disconnected turbulent spots and scaling of turbulence properties differs considerably from conventional isotropic turbulence at large turbulence Reynolds numbers. Finally, conditional averaging of the properties of wake disturbances and the turbulent inter-wake region provided good estimations of the properties of the overall flow.