흐름이 성게 수정에 미치는 영향

학위논문(석사)--서울대학교 대학원 :공과대학 건설환경공학부,2019. 8. 황진환.Complex hydrodynamic structures inside the benthic boundary layer influence various biological process of marine ethology. The external fertilization of sea urchins is one of the biological mechanism which highly depend on flow conditions of boundary layer. To explain fertilization process from perspective of mechanics, the analysis of fluid structure near sea urchins should be preceded. In general, turbulent boundary layers are developed near rough walls and behind streamline objects, and variations of flow structure generate complex hydrodynamic structures. Gametes released from sea urchins are transported via surrounding flow to reach collision and fertilization with different sex through complex interaction with hydrodynamic structures. The main objective of corresponding study is to analyze the flow effects on sea urchin fertilization. Fertilization rate is explored for uni-directional flow with free stream velocity ranging from 0.025 to 0.2 m/s to analyze the effect of velocity on fertilization and relative contribution of different locations (aboral, wake, substrate, and water column). As velocity increases, fertilization decreases and wake had most influence on total fertilization in general. Corresponding study is progressed in three steps. First, hydrodynamic characteristics near sea urchin and trajectory of gametes are simulated through Lagrangian Particle Tracking. Second, actual eggs being fertilized are computed based on relative distance between gametes of different sex and nearby sperm concentration. Last, contribution of each location on fertilization is computed and fertilization process is explained by hydrodynamic structures around sea urchin. Corresponding study suggest mechanical perspective to understand biological process and Computational Fluid Dynamics (CFD) as possible tool to simulate fertilization process of sea urchins. The methods applied to this study can help analyze various engineering or scientific problems that are driven by turbulent mixing and reaction between particles. It suggests wider perspective and possibilities to study complex processes which requires analysis of both particles and background flow.해저 경계층 내부의 복잡한 유체역학적 구조는 해양 생태계의 다양한 생물학적 현상을 지배하는 주요 인자이다. 성게의 번식 과정은 체외수정을 통해 이루어지며 경계층 내의 흐름 조건에 의존한다. 따라서 성게의 수정 과정을 역학적으로 설명하기 위해서는 주변 유체에 대한 이해가 선행되어야 한다. 일반적으로 거친 바닥과 유선형 물체 뒤에서는 난류 경계층이 형성되며, 경계층 내부의 급격한 유체 구조 변화는 난류 혼합을 발생시킨다. 성게의 몸체에서 방류된 생식 세포는 이러한 난류 혼합에 의해 서로 만나고 수정된다. 본 연구의 목적은 성게 수정에 위치가 미치는 영향을 흐름 조건에 따라 파악하는 것이다. 단일 방향 흐름 조건에서 유속 범위가 0.025 – 0.05 m/s 일 때, 수정률을 계산하였으며 소 영역별로 (Aboral, Wake, Substrate, Water Column) 전체 수정률에 미치는 상대적 영향력을 파악하였다. 속도가 증가할수록, 수정률은 감소하는 형상을 보였으며, 전반적으로 Wake지역에서 가장 많은 수정이 일어났다. 연구는 3단계로 진행되며 순서는 다음과 같다. 첫 번째, 수치 모의를 통해 구조물 후류의 구조를 파악하고 입자 확산 궤적을 Lagrangian Particle Tracking (LPT)을 통해 구현한다. 두 번째, 난자와 정자 사이의 거리와 정자의 밀도를 바탕으로 수정된 생식세포를 파악한다. 마지막 단계는 유체 구조를 바탕으로 총 4개로 나뉜 소 영역이 전체 수정에 미치는 상대적 영향력을 분석한다. 본 연구는 선행연구에서 실험으로 모의한 성게의 수정 과정을 수치 모델링을 통해 구현하고 역학적 관점으로 확장하여 분석하는데 의의가 있다. 본 연구에 적용된 방법론을 통해 난류 확산과 입자 사이의 반응 작용에 의해 발생하는 다양한 공학적 자연과학적 현상을 연구 할 수 있으며 입자의 궤적과 유속 장을 동시에 계산해야하는 복잡한 문제에 대한 해결책을 제시한다.ABSTRACT i TABLE OF CONTENTS iii List of Figures v List of Tables vii List of Symbol viii CHAPTER 1. INTRODUCTION 1 1.1 General introduction 1 1.2 Objectives 4 CHAPTER 2. THEORETICAL BACKGROUNDS 6 2.1 Fertilization process of sea urchin 6 2.1.1 External fertilization of sea urchin 6 2.2.2 Analytical models to explain external fertilization 7 2.2.3 Precedent and present research 8 2.2 Flow structure in the benthic boundary layer 11 2.2.1 Boundary boundary layer 11 2.2.2 Flow structures behind a sea urchin 12 2.2.3 Turbulent motions 13 2.3 Particle-laden flow 16 2.3.1 Introduction to particle-laden flown 16 2.3.2 Quantification of particle distribution 20 CHAPTER 3. METHODOLOGY 23 3.1 Numerical model description 23 3.1.1 Governing equations for fluid motion 24 3.1.2 Lagrangian particle tracking 25 3.1.3 Contact and fertilization detection algorithm 27 3.2 Simulation setup 31 3.3 Model validation 34 3.3.1 Model convergence 34 3.3.2 Comparison with laboratory experiment 38 CHAPTER 4. Results and discussion 43 4.1 Collision and fertilization 43 4.2 Flow structure analysis 49 4.3 Analysis of sea urchin fertilization process 57 4.3.1 Distribution of sea urchin gametes 57 4.3.2 Sperm density 67 4.3.3 Analysis of contact rate and fertilization rate 71 4.4 Relative contribution of different locations on fertilization 72 CHAPTER 5. Conclusion 77 5.1 Summary 77 5.2 Recommendation and future works 79 REFERENCES 80 국문초록 85Maste

Quantifying the Response of Grass Carp Larvae to Acoustic Stimuli Using Particle-Tracking Velocimetry

Acoustic deterrents are recognized as a promising method to prevent the spread of invasive grass carp, Ctenopharyngodon idella (Valenciennes, 1844) and the negative ecological impacts caused by them. As the efficacy of sound barriers depends on the hearing capabilities of carp, it is important to identify whether carps can recognize acoustic signals and alter their swimming behavior. Our study focuses on quantifying the response of grass carp larvae when exposed to out-of-water acoustic signals within the range of 100–1000 Hz, by capturing their movement using particle-tracking velocimetry (PTV), a quantitative imaging tool often used for hydrodynamic studies. The number of responsive larvae is counted to compute response ratio at each frequency, to quantify the influence of sound on larval behavior. While the highest response occurred at 700 Hz, we did not observe any clear functional relation between frequency of sound and response ratio. Overall, 20–30% of larvae were consistently reacting to sound stimuli regardless of the frequency. In this study, we emphasize that larval behaviors when exposed to acoustic signals vary by individual, and thus a sufficient number of larvae should be surveyed at the same time under identical conditions, to better quantify their sensitivity to sound rather than repeating the experiment with individual specimens. Since bulk quantification, such as mean or quantile velocities of multiple specimens, can misrepresent larval behavior, our study finds that including the response ratio can more effectively reflect the larval response