| dc.description.abstract | Turbulent wall-bounded flows form a cornerstone of modern fluid mechanics research and are ubiquitous in diverse natural and industrial settings. In practical scenarios, turbulent wall-bounded flows are commonly exposed to incoming disturbances, which can significantly influence their characteristics. This thesis examines the impact of varying incoming turbulence intensities on two types of turbulent wall-bounded flows. Conducting systematic planar particle image velocimetry measurements, internal flow is investigated in an air channel facility, while external flow is examined in a free-surface water channel facility, both equipped with active grids to generate varied inflow turbulence intensities. At the core of this thesis are four studies, each with specific objectives and methodologies, providing essential insights into the influence of incoming turbulence intensity on various aspects of these flows.
The first investigation examines how inlet turbulence affects turbulent channel flow, especially its quiescent core. In contrast to similar experiments performed in a zero-pressure-gradient boundary layer, the study reveals that the inflow turbulence intensity has a negligible effect on the friction velocity, wall-normal Reynolds stress, and Reynolds shear stress. The core region of the channel is identified using an improved technique, revealing significant effects of increased inlet turbulence intensity, e.g., making the core more discontinuous and thicker, as well as creating new core states with different momentum content. These new core states are associated with large-scale structures of low- or high-momentum fluid passing through the measurement domain.
The second study introduces a novel approach based on vorticity distributions to identify the interface between a turbulent boundary layer (TBL) and freestream turbulence (FST). Unlike traditional fixed-threshold methods, this dynamic ap proach delineates the interface by identifying individual velocity thresholds for each instantaneous field based on the fundamental difference between the vortical structures of the TBL and FST. It offers a key advancement in distinguishing instantaneous TBL and FST flows, which is essential for further exploration of their interactions.
The third study, building upon the method introduced in the second investigation, assesses the impact of varied FST intensities on key properties of a TBL and their streamwise evolution. Higher FST intensities lead to increased interface fluctuations and move the interface’s mean location closer to the wall, hence compressing uniform momentum zones within the TBL. Conditional averaging across the interface indicates intensified mean vorticity and turbulent fluctuations on the boundary layer side of the interface with increasing FST intensity; nonetheless, the dynamics of the flow remain robust to the FST perturbations across all the test cases. Downstream measurements indicate that FST decays while the TBL develops, resulting in a recovery of the interface location, conditional averages, and uniform momentum zones towards their natural undisturbed state.
The fourth study explores the interplay of inlet turbulence and localized injection in turbulent channel flow, a scenario crucial in diverse engineering contexts. The results reveal that localized injection alters flow properties throughout a layer within the boundary layer, which moves away from the wall moving downstream, with inlet turbulence slightly enhancing the transport of this layer. Notably, the combined effects of inlet turbulence and localized injection can be approximated as a linear superposition. Turbulence structure analysis shows that the channel’s nearwall structures are resilient to inlet disturbances, while localized injection effects are observed throughout the affected layer, such as increased coherent structure inclination angles for higher injection rates above the injection zone.
Through these studies, this thesis advances our understanding of turbulent wallbounded flows in the presence of incoming turbulence. Furthermore, the developed methodologies offer enhanced tools for core identification and interface detection. In essence, the findings significantly contribute to our fundamental understanding of turbulent wall-bounded flows, provide a vital foundation for further studies, and pave the way for more accurate modeling and prediction of these complex turbulence-turbulence interactions in both natural and industrial contexts. | en_US |
