3,133 research outputs found
Ship wake field analysis using a coupled BEMt-RANS approach
No full textThe prediction of a ship’s wake field and self-propulsion capabilities has traditionally been centered on experiments; however with the advancement in modern computing power, this can be achieved through the use of computational methods. An advantage with the use of CFD is its ability to provide insight into flow characteristics close to the wall, which are difficult to obtain through experiments. The most interesting and challenging aspect of using CFD in this analysis, is the influence of the propeller action and the unsteady hydrodynamic of the rudder working in the propeller wake. One approach to address the problem is to discretize the ship, propulsor and the rudder using unsteady RANS computations (Carrica et al., 2011). Due to the small time steps and high computational cost involved, simulations are often performed using representative propeller models or body force method. The level of complexities in the body force propeller approach varies from prescribing the body forces, Badoe et al., (2012), Phillips et al., (2010), through to coupling a more complex propeller performance code which accounts for the non-uniform inflow at the propeller plane, Phillips et al., (2009). There are several self-propulsion computations using body force propeller models reported in the literature. Banks et al., (2010) performed a RANS simulation of multiphase flow around the KCS hull form using a propeller model with force distribution based on the Hough and Ordway thrust and torque distribution (Hough and Ordway, 1965). Simonsen and Stern, (2003) coupled a body force propeller model based on potential theory formulation in which the propeller was represented by bound vortex sheets on the propeller disk and free vortices shed from the downstream of the propeller to a RANS code to simulate the manoeuvring characteristic of the Esso Osaka with a rudder. In the present work an investigation is carried out into the sensitivity with which the wakefield of a container ship in calm water is resolved using a coupled BEMt-RANS sectorial approach.<br/
Simple drag prediction strategies for an Autonomous Underwater Vehicle’s hull shape
No full textThe 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
Numerical propeller rudder interaction studies to assist fuel efficient shipping
No full textReducing the fuel consumption of shipping presents opportunities for both economic and environmental gain. From a resistance and propulsion standpoint, a more holistic propeller/hull/rudder interaction strategy has the potential to reduce fuel consumption, and minimise the risk of cavitation. The goal of this paper is to demonstrate that powering requirements can be reduced by optimizing the interaction between a ship’s rudder and propeller. In this paper, ongoing investigation regarding the design of an energy efficient rudder by adapting the local rudder incidence across the span to the effective inflow angle due to propeller swirl is presented. Numerical simulations are performed using an open-source RANS CFD code, Open FOAM, due to its ease with complex topology. Propeller effects are simulated using a body force model approach with special emphasis on ensuring the correct inflow to the rudde
The use of computational fluid dynamics to assess the hull resistance of concept autonomous underwater vehicles
No full textAutonomous Underwater Vehicles (AUV’s) provide an important tool for collecting detailed scientific information from the oceans depths. The hull resistance of an AUV is an important factor in determining the powering requirements and range of the vehicle. This paper discusses the use of Computational Fluid Dynamics (CFD) to determine the hull resistance of three existing AUV’s, of differing shape and size. The predictions are compared with available experimental data and good agreement found. This work has demonstrated that with use of suitable shape parameterisation it is possible to carry out concept design evaluation using a RANS flow solver
Virtual planar motion mechanism tests of the autonomous underwater vehicle autosub
No full textHydrodynamic derivatives are used to model the manoeuvring performance of proposed and existing hull forms. A simple robust method, using unsteady RANS simulations is presented to numerically replicate the experimental PMM tests performed on a scale model of the Autonomous Underwater Vehicle (AUV) Autosub. The method uses a body fitted inner domain to capture the unsteady flow. This body fitted mesh moves relative to a fixed outer domain via stretching/compressing cells at the interface. Detailed results for pure sway motion are presented and show good agreement for a relatively low computational cost. It is estimated that at the initial design stage a full set of manoeuvring derivatives could be found for an axis-symmetric AUV or submarine in under two days of simulation time using a desktop pc
Grid-based GA path planning with improved cost function for an over-actuated hover-capable AUV
No full textFor an AUV to perform a long-range mission with its maximum endurance, its energy consumption during transit must be kept to a minimum. This paper presents an improved cost function for a grid-based genetic algorithm (GA) path planning in 2D static environments. The proposed function consists of energy consumption terms that are estimated according to dynamics of a hover-capable AUV - notably Delphin2 AUV. It seeks for a path that requires least effort for the vehicle to move along. A simulation was written in Matlab and the outcomes of the GA with the improved cost function are compared with the ones of a GA with an optimal distance approach as well as an A* approach. It is found that outcomes of an improved cost function require less energy compared with the other technique
The simulation of free surface flows with Computational Fluid Dynamics
No full textComputational fluid dynamics is a powerful and versatile tool for the analysis of flow problems encountered in themaritime environment. The University of Southampton Fluid-Structure Interactions research group use ANSYS CFX tomodel a wide variety of flow problems; to gain insight into flow physics, improve designs and increase the efficiencyand safety of marine vehicles. A series of three case studies from on-going research looks at: loads applied on liquefiednatural gas tanks due to sloshing, slamming pressures experienced by high speed craft as well as the influence ofpropellers on the resistance characteristics of autonomous underwater vehicles. The presence of the free surface,complex shapes and the unsteady nature of these applications make their simulation with computational fluid dynamicsparticularly challenging. The successful validation of the computational models has resulted in the development of aselection process for suitable multiphase models as well as cost-effective meshing strategies
Experimental Verification of a Depth Controller using Model Predictive Control with Constraints onboard a Thruster Actuated AUV
No full textIn this work a depth and pitch controller for an autonomous underwater vehicle (AUV) is developed. This controller uses the model predictive control method to manoeuvre the vehicle whilst operating within the defined constraints of the AUV actuators. Experimental results are given for the AUV performing a step change in depth whilst maintaining zero pitch
Comparison of various approaches to numerical simulation of ship resistance and propulsion
No full textThe operation of a marine propeller dominates the flow interaction effects and alters the resistance on an upstream hull and the forces on a downstream rudder. A study is carried out into how these effects can be resolved by comparing four different methods. A classical prescribed body force approach in which an averaged nominal wake is used as input for the propeller model with prescribed thrust and torque; Two coupled BEMt-RANS solver which accounts for the non-uniform inflow into the propeller and a time resolved discretize propeller approach employing the use of an Arbitrary Mesh Interface model (AMI). The main differences between these four methods are also outlined quantitatively. The accurate results obtained using the two coupled BEMt-RANS approaches makes them fast and robust methods which can be used for ship resistance and self-propulsion estimation in the initial design phas
Modelling tidal current turbine wakes using a coupled RANS-BEMT approach as a tool for analysing power capture of arrays of turbines
No full textThe downstream evolution of the wake generated by a rotating tidal energy conversion device influences the performance of the device itself as well as the performance of any downstream device. An improved method is proposed for coupling a blade element momentum theory inner solution for a horizontal axis tidal turbine with an outer domain flow solved using a commercial finite volume computational Fluid Dynamics solver. A mesh sensitivity study is carried out and shows that for wake evolution of distance 10 diameters a high resolution mesh (>10M cells) is required. The importance of swirl is shown in retarding the wake spreading and that the inclusion of a suitableturbulence intensity term is also required to capture the spread of the near wake. A final section demonstrates the use of such a technique for analysing the energy capture of an array of multiple turbines distributed over 1km2 of seabed. The use of staggered lateral position between longitudinal arrays demonstrates a potential for more effective power capture
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