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A computational fluid dynamics approach to study drag reduction of axisymmetric underwater bodies by air jet injection in the boundary layer is presented. The well-known 'mixture' model is used to capture the multiphase flow and the SST k-ω (shear stress transport) turbulence closure model has been used in the computations. Well-studied Afterbody1 (Huang et al., 1978) which has a tapered and smooth stern profile is considered. A companion shape of Afterbody1, which has a blunt stern profile, is also studied. The numerical study is carried out with different air jet velocity to body velocity ratios, various angles of air jet and various angles of attack of the body. Effects of these parameters on drag reduction are reported. The effect of tapered vs. blunt aft shape of Afterbody1 has been found to have significant effect on drag reduction performance.

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Autonomous Underwater Vehicles (AUV) are slow-moving small unmanned robots capable of swimming independently below the water surface on pre-defined mission paths and are commonly used for oceanographic exploration, bathymetric surveys and military applications. With the use of appropriate sensors and equipment, AUVs can perform underwater object recognition and obstacle avoidance. Amogh is a miniature AUV developed at Centre For Innovation (CFI), IIT Madras for AUVSI RoboSub competition. The vehicle has a non-conventional dual hull heavy bottom hydrodynamic design equipped with six thrusters which allow for motion control in 4 degrees of freedom. This paper presents various aspects of the unique design of the vehicle. The performance of a simple PID controller for steady depth and heading control has been discussed. Simulations performed on a decoupled mathematical model of the vehicle are compared against experimental results.