Drag reduction and power optimization due to an innovative, toroidal hull form of an AUV

Abstract

The use of unconventional design has become a norm for mission-specific underwater autonomous craft. Geometrical characteristics and drag optimized shapes of single-hulled underwater vehicles are known since 1970s and a few significant improvements have been possible since then. These are mostly related to the efficiency of a wake adapted propeller, positioned at the stern in wake of the hull. In a novel effort, when the propeller is integrated into the craft at center-line, and the hull in its entirety is designed to act as the duct, the drag of the hull is further reduced. This craft uses a toroidal hull form that distributes payload axi-symmetrically around an internal ducted thruster. The craft is designed to operate at an economic cruise speed of 2kn. The power consumption is optimized to achieve an endurance of over 500 miles through this implementation of a ducted thruster. The hydrodynamic characteristics of the new toroidal hull are compared to those of a reference design which is slender and conventionally shaped. This conventional design is intended for the same volume and dimensions of payload. A reduction of 13% in drag from the first to final design iteration is achieved. This reduction in drag is due to a higheer effective length to thickness ratio of the ducted AUV, quantified in terms of a new parameter defined as Ih . The reduction of the wetted surface area also contributes to the drag optimization. The article shows that the shape of the leading edge of the toroidal hull becomes important to achieve low drag. The process of design and optimization using CFD, with the most appropriate turbulence models is detailed. The beneficial effect of the acceleration induced in the flow at the leading edge and trailing edge of the hull by the integrated thruster is studied and explained with the help of CFD results.