Fully Resolved Direct Numerical Simulations Of Interactions Between Big Particles And Turbulence In Particle-laden Pipe And Channel Flows

The particulate flows have extensive industrial applications, e.g. the oil transportation in the pipe. Till present, numerous works have been dedicated to the study of this subject. However some fundamental principles embedded in this subject are still unclear, for example the effect of particles on the transition to turbulence and the interactions between particles and turbulence structures. In this article, the Direct Forcing/Fictitious Domain method (DF/FD) is employed to perform the direct numerical simulation (DNS) of particulate pipe and channel flows at low and moderate Reynolds numbers.This thesis primarily consists of three part work:(1) We performed the particle-laden pipe flow at low Reynolds number (Re <2300) by the fully-resolved direct numerical simulation and investigated the influence of particles to the transition to turbulence under different particle concentrations and particle sizes. The results indicate that:when the particle concentration is dilute, the critical Reynolds number decreases with the increase of particle fraction. When the particle concentration is high, the increase of particle concentration leads to the increase of critical Reynolds number.(2) The fully-resolved direct numerical simulation method was employed to perform the particulate turbulent flow in a horizontal pipe at Reynolds number being 4900 and studied the effect of large neutral particles on the turbulent structures. Our results indicate that the presence of large particles decreases the maximum root-of-mean-square (rms) of the streamwise velocity fluctuation near the wall by weakening the intensity of large-scale streamwise vortices, although in the region very close to the wall the particles increase the rms of streamwise velocity fluctuation. On the other hand, the particles induce small-scale vortices in the near wall region, resulting in the enhancement of the rms of radial and circumferential velocity fluctuations there.(3) We performed the particulate turbulent flow in a horizontal channel at Reynolds number being 5000 by the fully-resolved direct numerical simulation and investigated the influence of settling particles to the near-wall turbulent coherent structures. The results show that:(a) When the particle sedimentation effect is negligible (i.e. neutrally-buoyant), the presence of particles decreases the maximum rms of streamwise velocity fluctuation near the wall, while increasing the rms of streamwise fluctuating velocity in the region very close to the wall and in the center region. On the other hand, the particles increase the rms of transverse and spanwise fluctuating velocities in the near wall region. (b) When the particle settling effect is so substantial that most particles settle to the bottom wall and form particle sediment layer (SL). the SL plays a role of a rough wall and parts of vortex structures shedding from the SL ascend into the core region and increase substantially the turbulence intensity there. (c) When the particle settling effect is moderate, the effects of particles on the turbulence are a combination of the former two situations.