MetadataShow full item record
Near-wall turbulent flows are frequently encountered in environmental and engineering applications. Surface roughness is often present, frequently with an inhomogeneous distribution. One classical example of such a flow is the internal boundary layer (IBL) formed downstream of a step change from a smooth to a rough surface on a flat plate. Computational fluid dynamics modeling of turbulent flow over rough surfaces remains a significant challenge, especially for more complex roughness configurations. In this thesis, a numerical study was carried out to assess the ability of the Reynolds-Averaged Navier-Stokes, or RANS-based two-layer k-ε model developed by Durbin et al. (2001) to predict two different rough-wall flows. The two-layer model introduces a hydrodynamic roughness length, y_0, to implement the effects of roughness. Firstly, fully developed turbulent flow in a vertical pipe was simulated to benchmark the model, and investigate the predictions of the mean velocity and turbulence fields in the region very close to the wall. The second simulation considered the IBL created by an abrupt transition from a smooth to a rough surface on a flat plate, with the focus of the study being the effects of roughness on the mean velocity and turbulence fields as they develop downstream of the step. For the turbulent pipe flow, the model correctly predicted the downward shift of the mean velocity profiles for both the transitionally-rough and fully-rough flow cases compared to that of the smooth flow case. The value of the eddy viscosity for the fully-rough flow was finite at the wall and close to the value of the molecular viscosity. The profiles of Reynolds shear stress and turbulence kinetic energy for the fully-rough flow both exhibited two peaks in the near-wall region, with one located at the wall. The turbulence kinetic energy budget for the fully rough flow exhibited a peak value in the dissipation at the wall that was much larger than the production. As such, in the model formulation the roughness dramatically changed the mean velocity and turbulence properties at the wall. For case of the IBL flow, a transition and equilibrium zone were predicted downstream of the step. In the transition zone, the mean velocity profile in the lower region exhibited the effects of surface roughness, whereas the mean velocity profile further away from the plate retained the characteristics of the smooth wall boundary layer upstream of the step. For the equilibrium zone, the flow was in equilibrium with the rough surface throughout the entire boundary layer. A novel method was used to determine the thickness of the IBL based on the turbulence kinetic energy profile in the transition zone. The model predicted a growth rate for the IBL that agreed well with other studies in the literature. For the transition zone, both the Reynolds shear stress and turbulence kinetic energy profiles showed a collapse and recovery cycle. Just as for the pipe flow, the roughness significantly modified the Reynolds shear stress and turbulence kinetic energy very close to the roughness elements.
DegreeMaster of Science (M.Sc.)
CommitteeBugg, Jim; Helgason, Warren; Scott, Noble
Copyright DateOctober 2018
Internal boundary layer
turbulence, roughness, two layer k-epsilon model