Experimental investigation of hydrodynamic force coefficients over AUV hull form
Introduction
In view of increasing the importance of deep ocean resources, application of underwater vehicle (UWV) is extended to a wide range of areas such as exploration and exploitation of seafloor minerals, environmental monitoring and protection and deep sea exploration of hydrocarbons up to 6000 m. This emphasizes the need for better understanding of the hydrodynamic forces acting on the underwater bodies under various conditions. Such understanding will lead to more efficient powering systems for the UWV. The present investigation sets out to address this aspect, there by aiming to investigate the hydrodynamic force coefficients—drag, lift and pitching moment coefficients acting over a typical underwater body at various angles of attack and speed. While measurements provide valuable data, most of the experimental investigations on axisymmetric underwater bodies have been conducted in a wind tunnel (Gertler, 1950; Granville, 1953; Nakayama and Patel, 1974; Patel and Lee, 1977; Huang et al., 1978; Roddy, 1990; Anil Dash et al., 1996; Hackett, 2000). Zedan and Dalton (1979) made a critical comparison between the drag characteristics based on volume, surface area and frontal area for different axisymmetric bodies. Sayer (1996) measured drag and added mass coefficients on a ROV and a solid box for various depths of submergence from the free surface in a towing tank. However, hydrodynamic coefficients derived from the towing tank experimental investigations are limited in literature. Recently, this is overcome by applying computational fluid dynamics (CFD) techniques. Several authors (Patel and Chen, 1986; Choi and Ching, 1991; Sung et al., 1993, Sung et al., 1995; Sarkar et al., 1997a, Sarkar et al., 1997b; Ananthakrishnan and Zhang, 1998; Mulvany et al., 2004; Jagadeesh and Murali, 2006) investigated various issues related to the application of CFD to underwater hydrodynamics. Selection of turbulence models, grid generation and boundary resolution techniques, influence of boundary conditions on CFD solutions, etc. were investigated over axisymmetric bodies. The review of literature also revealed that Reynolds number (Re) for practical applications existed in the transition regime (1×105<Re=ρUL/μ<1×106) and hence there is a need for low-Re k-ε turbulence models and related knowledge. The low-Re models are expected to give better results as they are superior in prediction of entire boundary layer over underwater hull form. Therefore, finding a suitable turbulence model for three-dimensional (3D) simulation of hydrodynamic characteristics on Afterbody1 for autonomous underwater vehicle (AUV) operating conditions will have paramount importance for the future hydrodynamic investigations at practical conditions.
Hence, in the current investigations forces on conducting towing tank experimental studies on Afterbody1 to determine hydrodynamic characteristics for AUV operating speeds, ranging from 0.4 m/s (Rev=1.05×105) to 1.4 m/s (Rev=3.67×105) for different angles of attack. Following a two pronged approach, numerical studies are conducted using low-Re k-ε turbulence model for reproducing the experimentally measured quantities in CFD. The CFD investigations have been carried out by implementing the low-Re models in the commercial flow solver FLUENT. The measured and computed forces coefficients are compared in the range of Rev=ρU∇1/3/μ (1.05×105–3.67×105) for various angles of attack (α). The angles of attack considered are 0°, 5°, 10° and 15°. The studies suggest that the force coefficients very significantly as a function of angle of attack, α. On the otherhand, the CFD procedure is shown to be capable of reproducing the experimental investigations quite well.
Section snippets
Experimental setup
The AUV hull form considered for the current investigation is Afterbody1 by Huang et al. (1978). The length to diameter (L/D) ratio for Afterbody1 is 10.9745. A model scale ratio of 1:2 was selected for the experimental body using Froude scaling. The total length and maximum diameter of the model was 1.4 and 0.14 m (L/D=10.0), respectively. The tail portion of the model was slightly adjusted in order to accommodate sting/L-frame for a captive towing test along with force balance arrangement
Numerical modelling
Authors selected k-ε AKN (Abe et al., 1994) turbulence model for numerical investigation based on their previous experience in two-dimensional (2D)-axisymmetric studies for similar type of bodies (Jagadeesh and Murali, 2005, Jagadeesh and Murali, 2006) in order to validate application of low-Re turbulence models for 3D underwater hydrodynamic applications.
Stud effect on measurements
The influence of stud on hydrodynamic force coefficients is assessed by the numerical estimation of stud effect on drag coefficient. Numerical studies are performed with and without stud on Afterbody1 at α=0° corresponding to a maximum speed of 1.4 m/s (Rev=3.67×105). Results from numerical studies are revealed that the drag coefficient is over predicted by 2% with stud when compared to a no stud condition. Henceforth, the pressure correction at tail end of the model due to stud is not
Conclusions
The axial, normal, drag, lift and pitching moment coefficients are determined from towing tank experimental study on Afterbody1 for AUV operating conditions for different angles of attack. Results from this study reveals a maximum increase that is observed in normal force coefficient compared to axial force coefficient for the highest speed and angle of attack. Similarly, an increase with descending order of magnitude is observed for pitching moment, lift and drag coefficients, respectively,
Acknowledgement
The first author expresses their gratitude towards the authorities of Indian Institute of Technology Madras, Chennai, for providing necessary facilities.
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