| Author: | Zeng, Lingwei |
| Title: | Flow control of bluff body |
| Advisors: | Tang, Hui (ME) Liu, Yang (ME) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Fluid dynamics Unsteady flow (Fluid dynamics) Fluid-structure interaction Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xxxiv, 193 pages : color illustrations |
| Language: | English |
| Abstract: | Bluff bodies are ubiquitous in nature and engineering applications, such as bridges, buildings, and marine structures, to name a few. Fluid flow around these bluff bodies can usually lead to various phenomena such as vortex shedding, flow separation, and flow-induced vibration (FIV). These phenomena may have detrimental effects on engineering structures, significantly compromising their safety and reliability. Historical incidents, such as the collapse of Tacoma Bridge and Ferrybridge Power Station and the very large vibration of structures like the Humen Bridge and SEG Building, have emphasized the importance of effective control mechanisms for these bluff bodies. For decades, various technologies have been applied to control the flow around bluff bodies. These technologies can be classified into passive control, such as surface modifications by spiral wire, splitter plate, and attached fins et al., and active control, such as body motion, blowing/suction, and synthetic jets et al. However, some unsolved issues still need to be addressed. For example, passive control of FIV of bluff bodies with high mass ratios has seldom been studied. In addition, the implementation of nature-inspired shapes like cactus structures can be a novel approach for the control. These research gaps motivated us to conduct the present research. This study aims to address several important issues in passive and active flow control of bluff bodies. These issues are related to the use of trailing-edge splitter plate, attached fins, biomimetic surface, and oscillatory morphing surface and its variants. Here, we mainly demonstrate the control ideas by using a circular cylinder, which is the most representative and simplified shape to stand for bluff bodies. First, a cylinder attached by a trailing edge splitter plate was evaluated to assess its impact on the resulting FIV. Five different vibration modes have been found with the increase of splitter length (L), i.e., typical vortex-induced-vibration (VIV when L/D = 0,0.125, where D is the diameter of the cylinder), Transition I (L/D = 0.25), Galloping (L/D = 0.5, 0.75), Transition Ⅱ (L/D = 1.0) and Suppression regions (L/D = 1.5 ~ 3.5). These observations supplement the research gap at high mass ratio cases and may offer guidelines for engineering applications. Next, a cylinder attached with fins was investigated on its FIV and energy harvesting performance. A new innovative device for harvesting bi-directional flow energy has been created by attaching four fins on both the windward and leeward sides of a cylinder. This device surpasses the performance of a plain cylinder by producing greater vibration amplitudes and functioning efficiently over a broader range of velocities. This new bi-directional flow-energy harvester is an appropriate candidate to work at sites where the flow periodically changes its directions, for example, in tidal flows. Then, we examined the FIV performance of a nature-inspired cylinder equipped with three or four ribs. The findings revealed that the three ribs suppress the cylinder’s oscillation at lower angles of attack (AOAs at 0° ~ 30°) while promoting galloping at higher AOAs (45° ~ 60°) compared to the normal cylinder. In comparison, the four-rib cases at lower AOAs (0° ~ 15°) exhibit a typical VIV response, accompanied by a symmetry break, while remarkably mitigating the cylinder’s oscillation at higher AOAs (30° ~ 45°). This offers new potential avenues for FIV control of bluff bodies. Last, the wake of a cylinder was actively controlled by the cylinder's oscillatory morphing surface. It was found that, compared to a normal cylinder, oscillatory morphing surface results in a smaller vortex formation length Lf, especially at intermediate frequency perturbations. Beyond this, Lf for the smaller or higher frequency perturbations will increase. For these intermediate frequency oscillatory morphing cases, the shear layers transition and roll up earlier due to the significantly enhanced flow instability. For higher perturbation case, small vortices will form regularly along and superimpose upon the separated shear layers. To further explore the feasibility of using morphing surface for drag reduction, CFD simulation has been conducted based on the variants of the morphing surface, namely oscillating surface, and anti-phase jets. It was found that, for Reynolds number Re = 1,000, using an oscillating surface can effectively manipulate the wake of the cylinder and reduce the drag. Anti-phase jets can also achieve similar control performance (drag reduction of about 16.6%). A similar control effect has also been achieved in a three-dimensional control case; through lock-on, the jet can stabilize the spanwise flow and delay the occurrence of three-dimensional flow, forming a quasi-two dimensional one. The findings from this study can provide more physical insights into the flow control of bluff bodies, which may be useful in the realm of engineering applications, including civil engineering, aerospace engineering, mechanical engineering, and marine engineering. |
| Rights: | All rights reserved |
| Access: | open access |
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