Numerical simulations of two-way coupling effects in a particle-laden turbulent pipe flow, and, Evaluation of the equilibrium Eulerian approach for the evolution of particle concentration in isotropic turbulence

Turbulence modulation in a fully developed downward pipe flow by dense particles at a Reynolds number of 360 based on friction velocity and pipe diameter is studied using direct numerical simulations. Particles are smaller than the Kolmogorov scale of turbulence and are treated using the Lagrangian approach. The effects of varying particle response time, volume fraction and settling velocity on fluid turbulence are investigated.; In the absence of gravity, variation of either the particle volume fraction or the response time has negligible effects on the fluid streamwise mean velocity profile. However, an increase in either the volume fraction or the response time augments the streamwise RMS velocities and attenuates the radial and azimuthal RMS velocities. Particles with positive settling velocity lead to a marginal increase in the mean streamwise fluid velocities and a substantial increase in the streamwise RMS velocities. The fluid radial and azimuthal RMS velocities, in the presence of gravity, are higher than those for the no-gravity case. Turbulence augmentation at the smaller dissipative scales can be seen in the energy spectra at certain radial locations. The longitudinal energy spectra show greatest turbulence augmentation close to the pipe center and the pipe wall. In case of azimuthal energy spectra, augmentation at medium and high wavenumbers is seen close to the pipe center. Particles with non-zero settling velocity provide some augmentation in the azimuthal energy spectra near the wall. In the presence of gravity, due to the reverse cascade, turbulence augmentation at low wavenumbers is observed. Particle collisions reduce the degree of particle migration to the wall. Their effects on fluid turbulence are also presented.; The second part of this thesis concerns the application of the equilibrium Eulerian approach to study particle preferential concentration and settling velocity in isotropic turbulence. The equilibrium Eulerian approach is extended to evolve the particle concentration for varying particle response time and still-fluid settling velocity. Over the entire range of particle parameters considered, there is good agreement between the Eulerian and the Lagrangian statistics. The equilibrium Eulerian approach tends to overpredict preferential concentration, compared to the Lagrangian particles, at higher response times.