ABSTRACT

The estimation of the shape of an AUV and its drag using semi-empirical equations is an important part of its preliminary design. Most of the drag is due to the main body which is often axisymmetric. The drag of axisymmetric bodies can be approximately determined by using Hoerner's semi-empirical equation for the coefficient of drag. The expression is independent of the shape and therefore not useful for optimizing the shape. In this paper, the drags of two families of AUVs are computed using the Siemens STAR-CCM+ package at zero Angle of Attack. Then, a semi-empirical equation for the drag that is a function of the diameter to length ration and the shape of the nose and tail is determined by using the CFD results.

INTRODUCTION

Autonomous underwater Vehicles (AUVs) are underwater systems used for undertaking long underwater missions for civilian, military, and scientific applications. It is primarily used for defense purposes. Estimation of the shape of an AUV and its resistance through semi-empirical equations is crucial during its initial design phase. AUVs typically have an axisymmetric main body which is the main part that generates drag. Hoerner's semi-empirical equation is commonly applied to approximate the drag coefficient of such axisymmetric bodies [Hoerner, 1965].

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Hoerner considered various expressions for Cf and noted that the Schoenherr equation log10(ReCf) = 0.242Cf−0.5 is widely used. The factor used to normalize the drag is 0.5ρSU2 where ρ is the density of the fluid, S is the wetted surface area, and U is the free-stream velocity. The expression is independent of the shape and therefore not useful for optimizing the shape.

The Myring profile is used to systematically change the shape of an axisymmetric AUV and study the effect on a function of interest. The profile is in the form of equations for the nose and the tail. The mid-body is a circular cylinder. The radius of the axisymmetric body, r(x), is a function of the distance, x, from the nose. For fixed lengths of the nose, mid-body, and the tail, the parameters n and θ are used to control the shape of the nose and the tail, respectively. In this paper, the drags of two families of AUVs are computed using the Siemens STAR-CCM+ package at zero Angle of Attack. The AUVs have length to diameter ratios of 5 and 10.69, respectively. The diameter is 0.533 m and the lengths of the nose and the tail are 0.6 m and 1.4 m, respectively. For each family, the drag is computed for n = 1, 1.5,2, and 3 and θ = 15, 20, 25, and 30 degrees. Then, a semi-empirical equation for the drag that is a function of d/L, n and θ is determined by using the CFD results.

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