Scalar and momentum transport over complex surfaces

Abstract

Understanding and modeling of scalar and momentum exchanges between rough surfaces and the atmosphere form the overarching goals of this dissertation. Two types of complex surfaces are considered in this study, namely rough surfaces consisting of large bluff-bodies and water surfaces with surface waves. Chapter 1 presents a general review of the theoretical, numerical and experimental studies over the complex surfaces. Chapter 2 outlines the development and improvement of computational method for high-Reynolds number flow over complex-shaped objects. A computationally efficient method has been developed and tested to reduce the Gibbs phenomenon in spectral method used in the context of immersed boundary method. Chapter 3 examines the quality and reliability of wall-modeled large-eddy simulation (LES). The results underline the importance of conducting experimental or numerical studies for convective scalar transfer problems at a Reynolds number commensurate with the flow of interest, and support the use of wall-modeled LES as a technique for that can already capture important aspects of the physics. Chapter 4 studies the similarities and differences of momentum and passive scalar exchanges over large three-dimensional roughness elements of variable geometries in a turbulent channel flow using LES. The turbulent transports of momentum and scalar are similar but strong dissimilarity is noted between the dispersive momentum and scalar fluxes. Increasing frontal density induces a general transition in the flow from a rough boundary layer type to a mixed-layer-like type. This transition results in an increase in the efficiency of turbulent momentum transport, but the reverse occurs for scalars due

to reduced contributions of large scale motions in the roughness sublayer. The geometric dependence of momentum and scalar roughness lengths is studied in chapter 5. The scalar roughness length depends on both the geometric parameters and the spatially variable surface scalar transfer coefficients, which cannot be easily determined a priori for a given geometry. Revisiting the surface renewal theory, we derive a general scaling relation between the logarithmic ratio log(z0m/z0s) and Re∗, which explains the results computed from LES. Chapter 6 examines air-wave interactions using the Lake-Atmosphere Turbulent EXchange (LATEX) dataset. The wind waves and swells present in the lake are found to impact the coupling between surface and the air differently. A new relative wind velocity for surface layer similarity formulations is constructed and tested using the data. Finally, conclusions and outlook are presented in the last chapter.