Hydrodynamic Analysis of Fish-Like Locomotion

Abstract

The current scenario, research in the underwater exploration has reached to a peak. Various scientific studies and principles are applied in order to explore the behaviour of Automated Underwater Vehicle. The following work and a small research contribute to the efforts made in analyzing the behaviour of such vehicles at a very interim level, which includes the analysis of airfoil by depicting it as a fish. The need of such analysis was recognized from the fact that the effect of wavelength and frequency of undulation of a real fish could not be identified or could be measured. Moreover, the experimental approach to build a robot-fish and carry out analysis was an option, however it put limitations when it came to the flexibility criteria. Thus, a need of numerical hydrodynamic analysis of fish like locomotion arises in order to study behaviour of an actual fish achieved by undulating airfoil NACA 0012. The main aim of the study is to determine the behaviour of flow past undulating fish. This is accomplished by studying the variation in flow past undulating fish for different amplitude and frequency of undulation. To depict the NACA 0012 as a fish having flexibility a “User Define Function (UDF)” is incorporated to calculate change in Y co-ordinates of the fish-represented by ΔY. Non dimensional governing parameters used in the present report are Reynolds number(Re= ρ ν C μ), frequency of undulation (St= fC 𝑈∞)(Strouhal number), maximum amplitude of undulation (A max), and wavelength of undulation (λ), flexure amplitude of undulation (A(X)) and equation of motion. Initial part of the study represents the basic understanding of the fluid structure inter-action (FSI) and various methods available for solving the FSI problem. Later it is followed by the modelling of geometry, the meshing technique adopted for grid generation within the computational domain, analysis of airfoil using UDF developed to undulate the airfoil with the help of ANSYS Fluent 14.5 version and finally the study of effect of various parameters on flow behaviour. A constant value of Re = 500 and A max = 0.2, at various non-dimensional frequencies, wavelengths and Moving Head Linear Motion (MHLM) equation is used for undulation. The optimum range of Strouhal number and wavelength (λ) is 0.2 to 0.8 and 0.8 to 1.2 respectively. It is found that fishes travel with wavelength λ=1 and have zero thrust at St=0.5 The flow regimes are discussed with the help of instantaneous as well as time averaged velocity, vorticity and pressure contours. It is found that moving head linear motion (MHLM) yields higher thrust and propulsive efficiency. In present work somehow thrust force and negative vortices is not observed. Other parameters such as Pressure contour, velocity contour, streamlines and flow pattern is justified the literature. The effect of frequency is found to be more pronounced than wavelength. Increasing in frequency and wavelength (λ) leads to decreasing the drag force. The relative difference between the magnitude of pressure in various cases increases with increasing St and wavelength λ. The variation of magnitude of PPZ (Positive Pressure Zone) and NPZ (Negative Pressure Zone) on the surface of the hydrofoil results in drag at smaller frequency. Furthermore a relatively large magnitude of NPZ is seen near the head.