Numerical investigation of an influence of square cylinder crossovers on twin bare hulls in close proximity

This paper investigates the influence of two crossovers on twin bare spheroids in close proximity. Firstly, to examine the impact of the crossovers to the flow behaviour and overall drag coefficient of spheroids. Secondly, to compare the drag coefficient for various speeds. The CFD RANS-SST with a commercial code ANSYS CFX simulation is performed for the fully submerged twin spheroids with transverse separation (S/D) of 1.02; where S is the distance between centreline to centreline and D is the maximum diameter of a spheroid. The Reynolds Numbers used are 2 × 106, 3 × 106, and 4 × 106. The results show that each spheroids experience an additional 20% drag which is dominated by crossovers. The drag coefficient of small volume crossovers between spheroids is 10 times higher than the drag of each spheroids, consequently, the total drag of system is increased by 11 times compares to twin bare spheroids system. Increasing speed results in the drag reduction. At the Reynolds Number 2 × 106 shows the highest drag coefficient of twin hulls for both cases (with or without crossovers). The result suggests the use of twin bare hulls without crossovers in the fleet, an application; for example, a fleet of small autonomous underwater vehicles

Simple drag prediction strategies for an Autonomous Underwater Vehicle’s hull shape

The range of an AUV is dictated by its finite energy source and minimising the energy consumption is required to maximise its endurance. One option to extend the endurance is by obtaining the optimum hydrodynamic hull shape with balancing the trade-off between computational cost and fluid dynamic fidelity. An AUV hull form has been optimised to obtain low resistance hull. Hydrodynamic optimisation of hull form has been carried out by employing five parametric geometry models with a streamlined constraint. Three Genetic Algorithm optimisation procedures are applied by three simple drag predictions which are based on the potential flow method. The results highlight the effectiveness of considering the proposed hull shape optimisation procedure for the early stage of AUV hull desig

Optimisation of a fleet of autonomous underwater vehicles to minimise energy dissipation

The range of an AUV is dictated by its finite energy source and minimising the energy consumption is required to maximise its endurance. For an individual AUV, this may be achieved by obtaining the optimum hydrodynamic hull shape design. For a fleet of multiple AUVs, this may be targeted for both individuals and the entire fleet. The purpose of this work is, firstly, to develop a rational approach to find an optimal hull shape, secondly, to provide guidance for operators on suitable configuration for multiple AUVs' missions, finally, to investigate the influence of the propeller on the drag of twin self-propelled AUVs.An AUV hull form has been optimised to obtain low resistance hull. Hydrodynamic optimisation of hull form has been carried out by employing five parametric geometry models with a streamlined constraint. Three Genetic Algorithm optimisation procedures are applied by three simple drag predictions which are based on the potential flow method. The results highlight the effectiveness of considering the proposed hull shape optimisation procedure for the early stage of AUV hull design. The influence on the drag of the fleet of multiple towed prolate spheroids is investigated with various configurations and spacings. A series of three-dimensional simulations are performed using a commercial RANS-CFD code ANSYS CFX 12.1 with the SST turbulence closure model at the length Reynolds Number of 3:2 x 106. The results show that the spacing between two hulls determines the drags. Seven zones based on the drag characteristic of twin towed models are classified. Both the multi vehicle vee and echelon configurations show limited influence against that of the entire fleet's energy budget. Then the investigation extended to determine the combined drag of a pair of propelled prolate spheroids at various longitudinal offsets and transverse separations.The RANS-HO propeller models are selected to estimate the time averaged thrust and torque of the propeller. The results show that the self-propelled vehicles experience an additional drag which is dominated by the thrust distribution of the propeller rather than torque. The drag of the following AUV is increased due to the upstream propeller, defined as a propeller race deduction. The two sources of self-propelled drag increment are the viscous interaction and a direct result of proximity to the propeller race upstream. The result highlights the importance of both thrust deduction and propeller race deductions when calculating the propulsive power consumption. Based on this optimisation procedure and this numerical data, operators can design the optimal hull shape of an individual AUV including the determination of the optimal configurations in transverse separation and longitudinal offset based on energy considerations of fleets of multiple AUVs, it can be very effective at the early design stage

Mathematical formulation of body of revolution applied to AUV's shape

This paper summarises the investigation of the mathematical shape formulation that can be applied to the AUV’s hull-form. Firstly, a shape with rounded nose and rounded end is presented. The influence of independent variables towards a meridian shape is discussed. Body shapes of various parameters’ change are addressed. Secondly, the design of a hull shape can be divided into three sections, namely, forebody, midbody and afterboday. This allows to modelling a variety of hull’s shapes. It is a combination of different curves with different constraints. There are four choices of curves: rounded curve, pointed curve, cusped curve and flat face curve. The hull-form shaping by those lines’ equations later can be adopted for optimisation the low-drag hull shapes. This study can give a preliminary idea for the efficient design for AUV

Numerical investigation of the influence of propeller to the interference drag of twin prolate spheroids at various longitudinal offsets and transverse separations

The purpose of this paper is to provide guidance for operators on suitable spacing for multiple vehicle missions. This paper investigates the combined drag of a pair of propelled prolate spheroids and compared to the towed models for the 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 at various longitudinal offsets and transverse separations. 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 three-dimensional cylinder is modelled to simulate the thrust distribution of propeller. The drag of the propelled model is compared against the single bare hull model. The results show that the transverse separations and longitudinal offsets determine the interaction drag between both hulls. The increasing of separation results in lower interference drag. The decreasing of offset results in higher drag reduction. By implementing the body force propeller, the combined drag and drag of the follower is interfered by the accelerated flow. Based on the numerical information, operators can determine the optimal configurations in transvers separation and longitudinal offset based on energy considerations

Numerical investigation of a fleet of towed AUVs

This paper investigates the influence on the fleet of the drag of multiple towed prolate spheroids to determine the hydrodynamic effect of the viscous interaction between hulls and to study the influence of the configuration?s shape of multiple hulls in the vee and echelon formations. A series of CFD RANS-SST simulations has been performed at the Reynolds Number 3.2×106 by a commercial code ANSYS CFX 12.1. Mesh convergence is tested and then validated with experimental and empirical results. The drag of each spheroid is compared against the benchmark drag of a single hull. The results show that the spacing between two hulls determines the individual drag and combined drag. The dominant spacing has been classified into seven zones based on the drag characteristic of twin towed models. Regions are characterised to parallel, echelon, no gain, push, drafting, low interaction, and no interaction. Both the multi-vehicle vee and echelon configurations show limited influence against that of the entire fleet?s energy budget. For an individual spheroid where a lower propulsion cost is required, then the use of three/four in vee or echelon formation should be considered. Based on this numerical information, operators can determine the optimal fleet configuration based on energy considerations

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 combineddrag of a pair of towed prolate spheroids for the length-Reynolds Number of 3.2×106. The model has a length diameterratio of 6:1. A series of configuration of a pairof spheroids is simulated by varying both longitudinaland transverse spacing. Three-dimensional simulationsare performed using a commercial Reynolds AveragedNavier Stokes (RANS) Computational Fluid Dynamicscode ANSYS CFX 12.1 with the SST turbulence closuremodel. In each case, the fluid domain has a mesh size ofapproximately nine million cells including inflated prismlayers to capture the boundary layer. Mesh convergenceis tested and then validated with wind tunnel test results.The drag of each spheroid is compared against thebenchmark 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 DraftingRegion. Based on the results, operators can determine theoptimal configurations based on energy considerations

Numerical investigation of a pair of self-propelled AUVs operating in tandem

This paper investigates the influence of the propeller race on upstream and downstream self propelled AUVs. Initially simulations of a self-propelled hull are performed at the Reynolds Number 3.2x10^6 with software commercial RANS code ANSYS CFX 12.1, utilising a body force model to replicate the impact of the propeller utilising momentum source terms. This is then extended to consider a fleet of two self-propelled vehicles operating at a range of longitudinal offset and transverse separations. The results highlight that operation in close proximity to another self-propelled vessel has a significant impact of both the flow around the hull and drag experienced by the vehicle. A propeller race deduction is proposed to account for the increase in vehicle drag due to the propulsors of other vehicles. The propeller race deduction is dependent upon both longitudinal and transverse separation. From a vehicle or mission design perspective, it is important to correctly understand the true propulsive energy budget of the vehicle and its impact on both range and endurance. This study highlights the importance of considering both thrust deduction and any propeller race deductions when calculating the propulsive power consumption of an individual or fleet of vehicle