Numerical investigation of the drag of twin prolate spheroid hulls in various longitudinal and transverse configurations

The purpose of this paper is to provide guidance for operators on suitable spacings for multiple vehicle missions. This paper then investigates the combined drag of a pair of towed prolate spheroids for the length-Reynolds Number of 3.2×106. The model has a length-diameter ratio of 6:1. A series of configuration of a pair of spheroids is simulated by varying both longitudinal and transverse spacing. Three-dimensional simulations are performed using a commercial Reynolds Averaged Navier Stokes (RANS) Computational Fluid Dynamics code ANSYS CFX 12.1 with the SST turbulence closure model. In each case, the fluid domain has a mesh size of approximately nine million cells including inflated prism layers to capture the boundary layer. Mesh convergence is tested and then validated with wind tunnel test results. The drag of each spheroid is compared against the benchmark drag of a single hull. The results show that the transverse separations and longitudinal offsets determine the interaction drag between both hulls. Increasing of spacing results in lower the interference drag. Five zones have been suggested based on the characteristics of the combined drag and individual drags. These are Parallel Region, Echelon Region, Low Interaction Region, Push Region and Drafting Region. Based on the results, operators can determine the optimal configurations based on energy considerations.