Hydrodynamic Analysis of Fish-Like Locomotion

Abstract

The Automated Underwater Vehicles (AUVs) are being developed for the research to be carried out in the field of underwater exploration. But it has been found that the efficiency of these AUVs is very less. The fish are called to be the best swimmers and hence their motion can be mimicked in the AUVs. Therefore the idea or the motivation of this present work comes out from this requirement to make necessary developments in the AUVs to make them as efficient as fish. Experimental investigations can be made by developing the Robo-fish but can turn out to be an expensive method. Moreover it becomes difficult to incorporate accurate flexibility and undulatory motion in the Robo-fish. Hence the present work includes the modeling of the AUV as a fish using a NACA 0012 hydrofoil and to carry out the numerical analysis of the model. The present work involves the method to study hydrodynamics of fish like locomotion for single as well as school of fish like arrangements. Furthermore the propulsive efficiency of single and in group of fish are compared with each other. The study of in-group of fish like locomotion involves different types of arrangements of the hydrofoils such as: Tandem Fish (one behind the other), Side by Side (one above the other) and ten hydrofoils in school of fish like arrangement (Triangular formation). The objective of the present work is to carry out numerical investigation of the hydrofoil with a detail parametric study in order to have optimum combination of parameters which yield maximum propulsive efficiency. The undulation of the hydrofoil is modelled using a UDF (User Define Function) which includes various parameters such as fish speed, Strouhal number, amplitude, wavelength, Reynolds number, spacing between hydrofoils (Sh for horizontal & Sv for vertical) etc. The Moving Mesh Method is used for the simulation in which a computational domain is created. Multi-block grid system is adopted for generating the grid. Near the fish very fine grid is adopted to capture flow physics effectively. The rest of the domain consists of sufficiently coarser grid. There are six different equation for the motion of the hydrofoil: two with static head and four with moving head; of which two are linear, three are quadratic and one is exponential. Among which Moving Head Linear Motion equation yields the maximum propulsive efficiency thereby is used in this present work for the simulations. In the first problem of this present work the simulations are carried out for the Single Fish with different Strouhal number varying from 0.2 to 1.2. The Reynolds number is kept to be 500 and wavelength is varied from 0.87 to 1.2. The constant amplitude A_max=0.1 is kept for the simulation. Numerical results obtained in the present study are validated with the published numerical results and are found in the acceptable range. The numerical investigation are carried out to evaluate various engineering parameters such as drag force, lift force and propulsive efficiency. The flow regime map showing drag as well as thrust produced regimes are obtained also the flow regimes are discussed with the instantaneous as well as time averaged velocity, pressure and vorticity contours. It is observed from the contours that the maximum thrust is obtained when the tail reaches its peak position. It is also found that the effect of frequency is superior than that of the wavelength. The final objective of the present study is to obtain the optimized set of the variables which yields maximum propulsive efficiency. After discussing about the Single Fish the second problem of this present work deals with the Group of fish like arrangement in which first is Tandem Fish like arrangement. Two hydrofoils are arranged one behind the other. The distance between two heads of the hydrofoil is given by non-dimensional number Sh. For the present work the simulations are been carried out with different Sh ranging from 1.2 to 2, with different frequency of undulation of leading/ trailing hydrofoil ( 〖St〗_1/〖 St〗_2=0.4 to 0.8), with different wavelength like 0.87 and 1, at a constant amplitude A_max=0.1 and Reynolds number Re = 500. The thrust co-efficient of leading hydrofoil is found always larger than that of single hydrofoil, whereas for the trailing hydrofoil the advantage in the thrust co-efficient (compared to the leading hydrofoil) is found only at lower values of undulation and frequency of the leading hydrofoil. Our results shows savings in energy in tandem arrangement compared to the single fish and are shown by the instantaneous as well as time averaged velocity, pressure and vorticity contours. The third problem deals with the Side by Side configuration in which one hydrofoil is above the other. The spacing between two hydrofoils ( Sv = 0.5 to 1 ). The study is carried out with in-phase as well as anti-phase undulating side with Reynold number as 500, at a constant amplitude A_max=0.1 and Strouhal number of the hydrofoils (St=0.4 to 0.8). The results are shown by the instantaneous as well as time averaged velocity, pressure and vorticity contours and is found that anti-phase as compared to in-phase side yields more propulsive efficiency.

The final problem of this present work corresponds to the school of fish; the group of ten fishes arranged in the triangular formation as commonly observed in nature. The objective of the study is to find the appropriate formation of the fishes which results in best propulsive efficiency. The study is done for various frequency (St=0.4 to 0.8), wavelength as 1, Reynold number 500, transverse spacing (Sv = 0.5 to 1) and at a constant amplitude A_max=0.1. The results obtained from the simulations carried out in this present work helps to optimize the motion of the Automated Underwater Vehicles. In future a more detailed parametric study can be carried out for the single as well as group of fish like locomotion.