Open-Channel Flow With Roughness Elements of Different Spanwise Aspect Ratios: Turbulence Structure and Numerical Modeling

310 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1997.Under rough-wall conditions the length scales governing the flow dynamics in the inner region of boundary layers are no longer determined by the flow itself (i.e. $\nu$/$u\sb*$ for smooth walls), but rather independently specified by the real dimensions of the elements. In particular, the ratio between the largest spanwise distance obstructing the flow divided by the characteristic height of the roughness element (i.e. the spanwise aspect ratio $\lambda\sb{y}$) is regarded as an important parameter that determines the turbulence structure of the so-called roughness sublayer through the associated eddy-shedding process. The present work makes use of acoustic Doppler anemometry and traditional hot-film sensors to investigate the structure of the turbulence induced by roughness elements with three different spanwise aspect ratios: $\lambda\sb{y} \ll$ 1 (rod-like elements simulating vegetation), $\lambda\sb{y}$ = O(1) (natural cobbles), and $\lambda\sb{y} \gg$ 1 (transverse square strips). Special emphasis is placed on the ability of cumulant expansions of low order to approximate joint probability density distributions of streamwise and vertical velocity fluctuations, and thus to predict some turbulence statistics, like vertical fluxes of turbulent kinetic energy, net momentum, relative contributions of coherent events to the total turbulent momentum transport, etc. The characteristics and extent of the roughness sublayer and the inner region are also studied. Wall similarity arguments in the outer region have been tested as well, and results further compared to observations under different types of boundary-layer flows with both smooth and rough walls. The experimental information on mean flow and turbulence structure gathered with cylindrical roughness elements simulating vegetation is used to calibrate two different types of two-equation turbulence model (a k-$\varepsilon$ and a k-$\omega$ closure scheme) which are further employed, together with dimensional analysis, to study suspended sediment transport processes in vegetated channels.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD