Simulations of a self propelled autonomous underwater vehicle

Abstract

The missions being proposed for autonomous underwater vehicles (AUVs), by both marine scientists and industry, are becoming increasingly complex and challenging. In order to meet these demands the next generation of AUVs will need to be faster, operate for longer and be more manoeuvrable than existing vehicles. It is therefore vital that the hydrodynamic forces and moments acting on a self propelled manoeuvring AUV can be predicted accurately at the initial design stage. The flow around a typical AUV is both turbulent and three dimensional with significant interactions between the hull, propeller and control surfaces. An unsteady computational fluid dynamics analysis based on the Reynolds Averaged Navier Stokes (RANS) equations is too expensive for AUV design. In order to capture the action of the propeller at an acceptable computational cost, a novel method of coupling a commercial RANS solver with a body force propeller model based on blade element momentum theory has been developed. This discretises the propeller plane into a series of radial and circumferential sectors. The local axial and tangential inflow conditions at each sector of the propeller plane can then be considered. This allows analysis of non-uniform propeller inflow conditions due to the interaction of hull, propeller and control surfaces. During a manoeuvre the hull boundary layer may separate due to the adverse pressure gradient, resulting in free vortex sheets which roll up to form a pair of body vortices. An adaptive mesh strategy is required to ensure a suitable mesh structure and density to capture these flow features. Modifications to a vortex capture algorithm (VORTFIND) are proposed, optimising it as a tool for identifying the path of vortex structures. This enables it to be used as part of an iterative meshing strategy, capturing vortical flow features more accurately and consequently their influence on the pressure loading experienced by the hull. To demonstrate the pertinence of the numerical methods developed in this work a series of case studies has been analysed. These include: determining the hydrodynamic derivatives of an AUV, propeller-rudder interaction studies, steady state manoeuvring performance of the self propelled KVLCC2, and in-service straight line performance prediction of Autosub 3. These highlight the roles of the numerical methodologies in the design process for future AUVs. The techniques developed in this work enable the designer to accurately predict the hydrodynamic loading acting on a self propelled manoeuvring AUV

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Identifiers

ORCID for Alexander Brian Phillips: orcid.org/0000-0003-3234-8506

ORCID for S.R. Turnock: orcid.org/0000-0001-6288-0400

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