Flow Controls of Rough Wall Turbulent Boundary Layers
| dc.contributor.advisor | Chin, Rey | |
| dc.contributor.advisor | Bennetts, Luke (The University of Melbourne) | |
| dc.contributor.advisor | Nugroho, Bagus (The University of Melbourne) | |
| dc.contributor.author | Kong, Jiahao | |
| dc.contributor.school | School of Electrical and Mechanical Engineering | |
| dc.date.issued | 2025 | |
| dc.description.abstract | Turbulent boundary layers (TBLs) are essential in fluid dynamics, significantly impacting various engineering fields, such as aerospace, maritime, and automotive industries. The chaotic and irregular nature of turbulent flows leads to substantial velocity and pressure fluctuations, resulting in increased drag and reduced energy efficiency. While extensive research has delved into understanding and controlling turbulence in smooth-wall TBLs, the effects of surface roughness—a prevalent feature in real-world applications like aircraft rivets, ship hull biofouling, and vehicle underbodies—remain less explored. Surface roughness can significantly modify TBL characteristics by amplifying turbulent intensity and elevating drag, driving the development of advanced flow control strategies to counteract these impacts. Enhancing our ability to understand and control turbulence in TBLs, especially over rough surfaces, is crucial for improving performance and sustainability in the aforementioned applications. This thesis addresses this research gap by investigating the interaction between turbulent flows, surface roughness and passive flow control devices, with a particular focus on TBL control using miniature vortex generators (MVGs). MVGs are known for their ability to control flow separation by generating beneficial vortex dynamics. However, their potential for reducing drag in both smooth- and rough-wall TBLs has not been examined. The thesis comprises four interconnected experimental studies that enhance our understanding of how MVGs affect turbulence structures and drag characteristics in various flow conditions. The thesis begins by examining two-dimensional (2D) rough-wall TBLs characterized by spanwise square bar roughness elements. By systematically varying the Reynolds number (Reτ = 1840–7500) and pitch ratio (px/k = 8), the research validates the outerlayer similarity hypothesis across a wide range of friction Reynolds numbers. A key finding reveals that a pitch ratio of px/k = 8 is the value that generates maximum drag. The drag coefficient converges at higher Reynolds numbers and larger boundary layer-to-roughness height ratios, and its sensitivity to pitch ratio diminishes. These results highlight the intricate interactions between turbulence and surface roughness, emphasizing the complexity of turbulence-roughness dynamics and providing a foundation for subsequent flow control studies. Building on this foundational understanding, the thesis explores how MVGs modulate turbulence structures in smooth-wall TBLs. Before characterizing the MVG influences, accurately determining friction velocity Uτ is essential for characterizing turbulence and assessing drag reduction effects. Various wall-similarity techniques were evaluated for determining Uτ in smooth-wall TBLs modified by MVGs. Traditional methods, including the defect profile, modified Clauser chart (MCC), and least-squares log-law (LLS), were found inadequate in estimating Uτ from the TBL profiles altered by MVGs. In contrast, the Musker function utilizes the near-wall reference profile, demonstrating higher accuracy and reliability. The Musker method consistently maintained uncertainty levels below 3% across various datasets, demonstrating itself as the most robust technique for determining friction velocity in flows influenced by MVGs. Hot-wire anemometry measurements reveal that MVGs induce vortices that create alternating high- and low-momentum regions, leading to attenuation of smaller-scale turbulence near the wall and large-scale turbulence structures. These modulated structures migrate downstream to the outer region, establishing persistent outer energy peaks associated with MVG-induced vortex cores. At higher Reynolds numbers and increased MVG height ratios, the effects of MVGs become more pronounced. They further attenuate near-wall turbulence and promote longer, more energetic motions in the outer boundary layer. These findings illustrate the dual role of MVGs in suppressing undesirable large-scale turbulence, offering a potential mechanism for turbulence control and drag reduction. The investigation extends to TBLs with rough walls, focusing on how MVGs interact with surface roughness to affect turbulence structures and drag characteristics. Experimental results demonstrate that MVGs can effectively reduce total drag by up to 16% at a Reynolds number of Reτ = 2500, representing a significant advancement in controlling flow over rough surfaces. Unlike smooth-wall scenarios, the impact of MVGs in rough-wall TBLs declines more quickly downstream, mainly due to the increased upward motions created by the roughness elements. However, MVGs still manage to produce distinct turbulence intensity peaks in both inner and outer regions, which helps reduce overall turbulence and suppress large-scale structures linked to surface roughness. This dual modulation, enhancing small-scale motions while diminishing large-scale turbulence, positions MVGs as a promising passive control strategy for reducing drag in rough-wall TBLs. This thesis examines the intricate interactions between surface roughness, turbulence structures, and flow control devices in TBLs. The research establishes a framework for analyzing the effects of MVGs by validating the outer-layer similarity hypothesis in rough-wall TBLs. The development and validation of reliable friction velocity determination methods were crucial for accurately quantifying turbulence modifications induced by MVGs, ensuring the reliability of velocity measurements used in the analysis. MVGs were found to attenuate large-scale turbulence while enhancing small-scale motions, representing a notable advancement in passive flow control technologies. Although they did not achieve drag reduction on smooth surfaces, their effectiveness in rough-wall TBLs underscores their adaptability and potential for applications where energy efficiency and flow optimization are critical. Future research will further validate and optimize MVG configurations, explore their applicability to more complex roughness profiles, and transition MVGs from passive to active flow control systems for enhanced drag reduction across diverse industrial applications. | |
| dc.description.dissertation | Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Mechanical Engineering, 2025 | en |
| dc.identifier.uri | https://hdl.handle.net/2440/146221 | |
| dc.language.iso | en | |
| dc.provenance | This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legals | en |
| dc.subject | Flow controls | |
| dc.subject | tubulent boudary layer | |
| dc.subject | vortex generator | |
| dc.title | Flow Controls of Rough Wall Turbulent Boundary Layers | |
| dc.type | Thesis | en |
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