Feature 1 | CFD AND HYDRODYNAMICS


shapes. It will be seen that there is up to a 3% difference between the calculated efficiencies and it is concluded for a hull such as this it is more important to use the correct capture area than for a more conventional waterjet hull form. Based on the preliminary CFD results,


with a non-optimised, first iteration design, it was found that the AWJ-design starts to be more efficient than propellers around 30knots. However, there is room for optimisation by for instance incorporating better fairing of the waterjet/hull integration and the AWJ intake geometry. Since in the speed range


from 25-30knots the difference between the propeller design and the first iteration AWJ design is only small, it is believed that with such further optimisation the crossover point may shiſt down to 25knots, which may make the AWJ a viable alternative propulsor. Further CFD studies are required to study the benefits of AWJ propulsion in more detail. NA


REFERENCES


1) Will Giles, Tom Dinham-Peren, Shane Amaratunga, Arthur Vrijdag, Richard Partridge, ‘The Advanced WaterJet:


Propulsor Performance and Effect on Ship Design, International Naval Engineering Conference, May 2010.


2) Dinham-Peren, T A, Craddock, Lebas, C, Ganguly A, Use of CFD for Hull Form and Appendage Design Assessment on an Offshore Patrol Vessel and the Identification of a Wake Focussing Effect, RINA Marine CFD 2008 Conference, Southampton, UK.


3) ITTC, Report of the Waterjets Group, Proceedings of the 21st ITTC, 1996.


4) ITTC, Report of the Specialist Committee on Validation of Waterjet Test Procedures, 23rd ITTC, 2002.


Simulation for tomorrow’s world


UK-based design consultancy Frazer-Nash looks to push design studies further using the latest developments in computational fluid dynamics (CFD) and hydrodynamics calculations, which Henry Gordon-Wright, engineer, Frazer-Nash Consultancy explains further.


M


arine manufacturers are being driven to provide cost effective, timely solutions to the engineering


challenges posed by ever increasing customer demands and expectations in a competitive industry. In addressing these mounting pressures, designers are finding that computer aided simulation techniques, and notably CFD, are invaluable in enabling them to rapidly prototype and evaluate engineering solutions. Computational Fluid Dynamics began


life in simple panel codes developed by the aerospace industry as far back as the 1960s. Today, CFD is being applied across a broad spectrum of disciplines, ranging from medicine and chemistry, to sports and renewable power. CFD has become an established tool in the marine industry, and recent technological and process developments are allowing CFD to be applied to an increasingly wide range of problems. CFD in the marine industry has moved


beyond hydrodynamic simulation and is being used by Frazer-Nash to evaluate and improve external aerodynamics, airwake and thermal signatures, internal ventilation flows, cooling and gas dispersion, and in the prediction of flow-induced noise.


36


Hydrodynamics Hydrodynamic simulation using CFD has become standard practice across a diverse range of marine applications from hard chine planing craſt to displacement hulls and more sophisticated multi-hulled or unmanned configurations, and the evaluation of vessel seakeeping performance is as much a part of new build as refit or conversion projects. With rising operating costs, any performance gains, however small, that can be found through optimisation of vessel geometry, propulsion, and operation are relentlessly pursued. However, CFD is still as much an art as a


science, requiring experienced users and not inconsiderable effort to produce useful and reliable results. As a result, designers must consider a hierarchy of methods to evaluate hydrodynamic performance. Selection of an appropriate method depends on the required fidelity of the solution as well as time and cost constraints. It is for this reason that hand calculations and design codes are still in many cases the solution of choice early in the design process. As the design matures and develops the required solution fidelity increases so that designers can make informed decisions regarding relatively small scale changes to hull design, and generate more accurate predictions of vessel performance. It is at


this stage that more complex methods such as linear and non-linear strip theory, panel methods and ultimately full CFD come into their own. For example, Frazer-Nash has used its in


house solver, HydroDyna, to demonstrate that reducing the length of a large naval platform by 8% would result in only a minimal reduction in crew availability due to motion sickness, but would dramatically reduce cost and allow the vessel to be constructed in a much wider selection of shipyards. Recent developments in CFD soſtware


codes have focussed on attempting to bridge the gap in the solver cascade (the contrast between simplistic, but computationally efficient low fidelity solutions and complex, high fidelity, computationally intensive solutions) by improving usability, increasing automation, and through integration with design soſtware. Tis has produced a step change in the throughput of CFD simulations giving designers a unique and detailed perspective on vessel performance and seakeeping at an early stage in the design process. Te effects of changes to vessel topology on seakeeping performance can now be evaluated in hours or days rather than weeks or months as previously.


The Naval Architect July/August 2010


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