The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community
Issue link: http://sea-technology.epubxp.com/i/605550
www.sea-technology.com November 2015 / st 37 W hen naval architects and hydrodynamics specialists at Finnish ship design frm Foreship examined the re- sults of a recent computational fuid dynamics (CFD) analy- sis of different bow designs, they were startled at what they found. The CFD software, developed to simulate real sea conditions rather than calm seas, turned conventional wis- dom on its head. Contrary to expectations, the design experts discovered that a bulbous bow was actually less effective than a vertical stem across a much broader range of speeds than previously thought. Comparing Hull Structures Foreship had been working on developing the methodol- ogy for CFD analysis to be based on real conditions, rather than calm water, for some time. The opportunity to test the latest CFD advances came in a project for one of the frm's major cruise clients. The cruise operator contracted Foreship to compare the performance of two different bow confgurations for a new cruise vessel—a bulbous bow and a vertical stem—with a view to achieving the minimum resistance in waves. The 300-m cruise vessel was to be optimized for a speed of 18 kt. Other variables, including beam, draft, block coeffcient and submerged length, were all constant. Foreship's head of hydrodynamics, Janne Niittymäki, ex- plained what his team had been aiming to fnd out: "It is received wisdom that the bulbous bow is the optimal solu- tion in calm water, but above a certain threshold condition (wave height and period) the vertical stem becomes the op- timal solution," he said. "But what is the threshold?" Using the latest CFD software, the analysis demonstrated that the bulbous bow was indeed slightly more effective in calm seas at speeds ranging from 14 to 18 kt. However, at 22 kt., the vertical stem generated 2.7 percent less resis- tance. What the development team was not expecting, however, was that using CFD to simulate the types of sea conditions which the cruise operator expected the ship to encounter in real-life operation showed that the vertical stem performed more effectively at lower speeds, too. In conditions simu- lated to include head waves with a height of 1.75 m and a period of 8 s, modeled in intervals of 20 s in calm seas and 25 s in waves, the vertical stem demonstrated consistently better performance. In fact, it achieved a reduction in hull resistance of 2 percent even in small regular waves across 14, 18 and 22- kt. test speeds. And while this margin, in itself, is not a large one, the fndings are important because a whole range of variables can now be simulated for the frst time. This has signifcant implications both for naval architects and ship operators in other sectors. Niittymäki candidly admits the results were entirely un- expected. "When the frst estimates came through," he said, "I have to say that I did not believe them. I believed the ver- tical stem would be seen as the better option at some point, but not at such low wave heights. The resistance in waves was shown to be clearly better across the whole speed range in the case of the vertical stem version. "We knew that even small waves might affect how the optimal hull form was decided on, but it is only now that we have the new methodology that we can measure precisely what the effect will be," he added. Progress via Computers The breakthrough is another example of how advances in computer software and faster computer processing speeds are underpinning advances in both ship design and op- eration. CFD helps naval architects to optimize hull forms while real-time remote and shipboard monitoring can sup- port operational decision making for parameters, including speed and trim. Until now, constraints on processing speeds have meant that ship designers must rely on CFD analysis based on calm sea conditions. But everyone knows that few ships of any type operate consistently in calm seas, and that is why Fore- ship's new approach, simulating real-life sea states, is such an important development. "Actual conditions at sea are very rarely completely calm, and the optimal hull form for real operational con- ditions is likely to differ signifcantly from one optimized for calm seas," Mattias Jörgensen, Foreship's vice president Latest Advance In CFD Analysis Hydrodynamic Breakthrough Could Lead to Super-Optimized Hull Forms By Markus Aarnio