Hydrodynamics of flapping foils and flags: scaling analysis and experiments

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dc.description학위논문 (박사)-- 서울대학교 대학원 : 기계항공공학부, 2016. 2. 김호영.-
dc.description.abstractIn the present research, we conducted the fluid dynamic study of flapping foils for bio locomotion of swimming and flying animals and the fluttering flags adjacent to rigid plate. The scaling laws for the force produced by the flapping foils are constructed by considering the momentum imparted to the vortical structures generated by the foil motion. The novel concept of the wind energy generator is developed using the contact electrification of the fluttering flag and the plate.
We first studied the hydrodynamics of the angularly reciprocating plate without a free stream velocity as an elementary mode of the flapping locomotion. We visualize the flow field around the flat plate to find that two independent vortical structures are formed per half-cycle, resulting in the separation of two distinct vortex pairs at sharp edges. Based on our observations, we derive a scaling law to predict the thrust of the flapping plate considering the momentum imparted to the vortical structures. The scaling law is in good agreement with the experimental observations.
The study on the angularly reciprocating plate is extended to find a physical explanation for the relatively stubby fins of small aquatic animals. The thrust and flow field around the angularly reciprocating plate is examined varying the tail shapes and aspect ratio over two order of magnitudes. The thrust of a flapping tail can be predicted based on a universal scaling law regardless of the tail shapes, which considers the momentum imparted to the surrounding flows. The thrust in the given tail area and kinematics is maximized at the low aspect ratio of 0.7, whereas the efficiency continuously increases along with the aspect ratio. Combination of the current mathematical modeling and biological observations suggests that small aquatic animals selected a caudal fin with a low aspect ratio for the purpose of added thrust but at the expense of efficiency.
We next construct a scaling law for the lift of hovering insects that elucidates a unified mechanism enabling the insects to be airborne through relatively simple scaling arguments of the strength of the leading edge vortex and the momentum induced by the vortical structure. Comparison of our theory with the measurement data of 33 species of insects confirms that the scaling law captures the essential physics of lift generation of hovering insects. Our results offer a simple yet powerful guideline for biologists who seek the evolutionary direction of the shape and kinematics of insect wings, and for engineers who design flapping-based micro air vehicles.
Next, the dynamics and energy transfer of fluttering motions of flags placed adjacent to a plate are investigated systematically by varying the inter-space distance, material, and dimensions of the flag, and incoming velocity. The stability condition of the flag adjacent to the plate is almost identical to that in the case of when the flag is not placed adjacent to a plate. However, when flutter occurs, it exhibits asymmetric dynamic behavior with respect to the centreline because of the influence of the plate, even when no contact is made. The energy transfer process is characterized by three distinct stages for each half cycle: energy absorption at the middle of the flag, transition, and energy release at the end part. Flag-plate contact modes, which can be classified as single contact, double contact, contact-propagation, and chaotic contact, are illustrated in their regime plot based on the mass ratio and non-dimensional velocity. Furthermore, the reduced frequency and the Strouhal number of the flag flutter adjacent to the plate are examined by varying the flag length and incoming velocity. Reduced frequency is confined in the narrow range from 0.3 to 0.6, which corresponds to the single flag flutter. The Strouhal number is shown to converge to the optimal value of 0.3 as the incoming velocity increases, implying self-optimization.
Based on the study of the fluttering dynamics of the fluttering flag adjacent to the plate, we developed a novel and powerful wind energy harvesting system, namely, flutter-driven triboelectric generator (FTEG) using the self-sustained oscillation of flags as a mechanical energy, which has the potential use for wireless electronics. The flutter-driven triboelectric generator having dual plate configuration with dimensions of 7.5 cm × 5 cm under the incident air flow velocity of 15 m/s can produce outstanding power performances such as the instantaneous output voltage and current of 200 V and 60 μA, respectively, especially with the extremely high frequency of 158 Hz. Our novel system can fully charge a 100 μF capacitor within 245 seconds, corresponding to an average power density of approximately 0.86 mW. In addition, the flutter-driven triboelectric generator can be easily scaled up toward large energy generating system by employing the parallel devices in wind-rich place. We expect that the flutter-driven triboelectric generator can not only be an ideal source for the small scale energy harvesting system for wireless electronics, but also it can provide a novel pathway to the electrical energy generation using wind.
dc.description.tableofcontents1. Introduction 1
1.1 Outline 1
1.2 Background for the study of the flapping foils 2
1.3 Background for the study of the fluttering flags 7

2. Wake and thrust of an angularly reciprocating plate 11
2.1 Introduction 11
2.2 Experimental apparatus 13
2.3 Flow visualisation results 14
2.4 Scaling laws 19
2.5 Conclusions 14

3. Hydrodynamic principles of the evolutioinary selection of fish caudal fin shapes 26
3.1 Introduction 26
3.2 Experimental apparatus 27
3.3 Results 30
3.3.1 Flow measurements 30
3.3.2 Scaling laws 30
3.3.3 Optimal aspect-ratio 34
3.4 Discussion 37

4. A scaling law for the lift of hovering insects 41
4.1 Introduction 41
4.2 Experimental appratus 43
4.3 Results and discussion 44
4.3.1 Estimation of leading edge vortex strength 44
4.3.2 A model for lift 45
4.3.3 Scaling law based on the conventional steady potential-flow theory 49
4.4 Conclusion 50

5. Experimental study on the fluttering flag-plate interaction 53
5.1 Introduction 53
5.2 Materials and methods 55
5.2.1 Experimental appratus 55
5.2.2 Hydrodynamic modelling 56
5.2.3 Proper orthogonal decomposition 58
5.3 Results and disccusion 58
5.3.1 Stability condition 58
5.3.2 Dynamics and energy transfer process 61
5.3.3 Flag-plate contact modes 65
5.4 Conclusion 65

6. Flutter-driven triboelectrification for harvesting wind energy 71
6.1 Introduction 71
6.2 Materials and methods 74
6.2.1 Experimental set-up for characterization 74
6.2.2 Fabrication of a FTEG 74
6.3 Results 76
6.3.1 Flutter-driven triboelectric generator (FTEG) set-up 76
6.3.2 Analysis of fluttering behavior 76
6.3.3 Working principle 80
6.3.4 Electrical characterization of FTEG 81
6.3.5 Performance of a FTEG as a function of flow velocity 83
6.3.6 Durability and Efficiency 85
6.3.7 Application of multiple array of FTEGs 86
6.3.8 Demonstration in an open environment 88
6.4 Discussion 89

7. Conclusions 91

A Comparison of the scaling law with high St computational results 93

B Influence of the chordwise vortices on the thrust of an angularly reciprocating plate 96

References 98

Abstract (in Korean) 112
dc.format.extent33962529 bytes-
dc.publisher서울대학교 대학원-
dc.subjectflapping locomotion-
dc.subjectvortex flows-
dc.subjectflag flutter-
dc.subjectenergy harvesting-
dc.titleHydrodynamics of flapping foils and flags: scaling analysis and experiments-
dc.citation.pagesxvii, 115-
dc.contributor.affiliation공과대학 기계항공공학부-
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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Mechanical Aerospace Engineering (기계항공공학부)Theses (Ph.D. / Sc.D._기계항공공학부)
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