Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Siddiqui, Kamran

Abstract

The hydrodynamic and thermal boundary layers are known to be key regulators of the interfacial transport of mass, momentum and heat, which are crucial in a wide range of engineering and environmental applications. The boundary layers encountered in these applications are often turbulent in nature and characterized by the presence of three-dimensional motion and non-linear dissipative phenomena. The presence of heat transfer between the bulk fluid and the solid wall increases flow complexity due to the interaction of the buoyant force with flow inertia and non-linear coupling between thermo-fluid variables. As a key contributor to multiple engineering systems and environmental phenomena, advancement of the current knowledge on turbulent boundary layer dynamical behaviors is crucial.

In the present study, turbulent boundary layer flow over a heated horizontal smooth wall was investigated utilizing an experimental approach. The current state-of-the-art techniques for 3D flow characterization are often limited in their broad applicability. The present knowledge is improved upon with the development of a novel technique based on volumetric illumination with a multi-color pattern. In the absence of heat transfer, the turbulent boundary layer is known to contain a wide range of dynamical phenomena whose behaviors still lack a comprehensive understanding. The present study investigated the unheated turbulent boundary layer utilizing a unique implementation of the Particle Image Velocimetry (PIV) technique to characterize the three-dimensional (3D) nature of the flow and reported new findings on near-wall turbulent flow behavior. In the presence of heat transfer, once the buoyant force magnitude is sufficiently large, thermals detach and rise from the heated wall. The characteristics of thermals in a heated turbulent boundary layer was investigated in 3D utilizing PIV. A novel image processing algorithm was developed to detect thermals. The modification to the turbulent boundary layer velocity field by wall heating was studied utilizing PIV data. Results indicate that boundary layer behavior is influenced by the buoyant force via modification to the turbulent velocity field and associated velocity statistics. This study provides multiple new contributions on flow characterization techniques and the behaviors of the turbulent boundary layer in the presence and absence of heat transfer.

Summary for Lay Audience

Fluid mechanics is one of the broadest areas of physics with a wide variety of practical applications. Often topics of interest in fluid mechanics feature the interaction of fluid and solid object where mass and energy are exchanged. As fluid passes over a solid, a thin region of fluid adjacent to the solid forms known as the boundary layer. The most well-known boundary layer is the atmospheric boundary layer (ABL) produced by air in the Earth’s atmosphere passing over the Earth’s surface. Every form of life on Earth is influenced by the behavior of the atmospheric boundary layer. The energy exchange between the ABL and Earth’s surface governs the wind loading experienced by structures such as buildings and bridges. Thermal energy, i.e. heat, and mass exchange (e.g. evaporation) strongly influence the strength of tropical storms and winter blizzards. The distribution of emissions, greenhouse gases, and particulate matter are all governed by ABL behaviors. In light of the far-reaching impact and near ubiquity of boundary layer flows, many physical processes that determine boundary layer behavior are unknown or not well-understood.

In the present study, boundary layer behavior in the presence of heating from the solid surface was investigated. As boundary layer flow often features highly three- motion, a new technique was developed to describe this fluid flow. Next, experiments were performed in the boundary layer to investigate and characterize some boundary layer phenomena that were not well-understood in the past. The findings of this study show that heat transfer from the solid drives unique fluid phenomena that modify overall boundary layer behavior in a non-linear manner. The conclusions of this study can be used by scientists and engineers to improve engineering systems and produce more advanced predictive models of the atmospheric boundary layer.

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