High resolution measurements of turbulence modification by particles

Previous studies have shown that a dilute dispersion of fine particles in a turbulent gas flow can either augment or attenuate the gas-phase turbulent kinetic energy (TKE). However, the underlying physical mechanisms of the turbulence modification are not understood. The main objective of the present study is to investigate experimentally the small scale turbulence structures which affect the overall turbulence modification to improve understanding and prediction of the macroscopic turbulence modification.; A high resolution PIV/PTV system was developed using conventional optical components that provided two-dimensional velocity field measurements with a sub-Kolmogorov vector-spacing of 60mum at a working distance of 0.25m. A new PIV data analysis technique was developed for measurement of the TKE dissipation rate that solves the longstanding problem of noise contamination of PIV-based dissipation measurements. The technique was validated using a set of synthetic PIV images generated using a novel benchmark, Random Oseen vortices Synthetic image Test (ROST).; High resolution measurements of particle-laden isotropic turbulence were performed in the facility developed by Hwang and Eaton (2004) in the presence of 500mum glass, 250mum glass or 250mum polystyrene particles at mass loading ratio up to 0.45. The Reynolds number based on the Taylor microscale was 130 for the unladen case. The TKE was attenuated by up to 25% in the presence of particles. Both the measured dissipation rate and its analysis for the case of impulsive insertion of particles indicate that changes to the dissipation rate should be smaller than changes to the TKE for the range of parameters of the present experiments. Analysis of a large set of PIV images allowed calculation of the average turbulence distortion around particles of each size class. The measurements show strong damping of the TKE and strong augmentation of the dissipation rate in a roughly spherical region surrounding the particles. The turbulence damping was strongest in the front of the falling particles, suggesting that turbulence generation in the particle wake may partially counteract the damping effects.