Experimental Investigation of Highly Irregular Roughness Effects in Wall Turbulence

322 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008.Finally, the spatial structure of flow over the RF1 surface is contrasted with that over the smooth wall, both within and outside the roughness sublayer, to further assess the possibility of structural modifications due to roughness. Hairpin vortex packets, commonly observed in smooth-wall turbulence, are observed often in the outer layer of the rough-wall flow. As in smooth-wall flow, these large-scale features are also found to contribute heavily to the Reynolds shear stress. However, while qualitative consistency is observed between the structure of smooth- and rough-wall flow, some quantitative differences are observed. For example, a slight reduction in the streamwise extent of two-point correlations of streamwise velocity, rhouu, is noted both within the roughness sublayer and the log layer of the flow. This reduction could be indicative of roughness-induced modifications of outer-layer vortex organization. This reduction in streamwise extent is also noted in spatial correlations of swirling strength. However, spatial correlations of the other velocity components, which are more representative of the smaller-scale features of outer-layer organization, show little sensitivity to roughness as they collapse well with smooth-wall results. Proper orthogonal decomposition (POD) is also employed to aid in studying the large- and small-scale features of these flows and this analysis reveals that the larger scales structures of the flow are most sensitive to roughness. Despite this sensitivity, the contributions of larger- and smaller-scales to the turbulent stresses are quite similar in the smooth- and rough-wall cases. Finally, conditional averaging reveals the importance of hairpin-like vortices and their organization into larger-scale packets to the production of Reynolds shear stress in both flows, particularly through the generation of intense ejection and sweep events.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD