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Computational Methods for More Fuel-Efficient Shipsby Barry Koren The flow of water around a ship powered by a combustion engine is a key factor in the ship's fuel consumption. The simulation of flow patterns around ship hulls is therefore an important aspect of ship design. While lengthy computations are required for such simulations, research by Jeroen Wackers and Barry Koren has shown that these computations can be conducted with much greater speed. Turbulent water flow reduces the efficiency of a ship's propeller. The propulsion system works best when the water is undisturbed, but that certainly isn't the case in a ship's wake. The stern of most ships is therefore designed to ensure a relatively regular flow pattern. The bow wave is also a key factor in energy consumption. Energy is expended in creating waves: a smaller bow wave therefore results in less energy loss. Ship designers study these phenomena in order to design better ships. The ultimate goal is to calculate the ship geometry that will best meet the various demands placed on the vessel, not only in terms of fuel efficiency, but also with respect to stability, safety and logistical efficiency. To study the properties of ships, designers need software to simulate wave and wake patterns around the hull. However, the computation of such flow patterns tends to be a time-consuming process, because waves take a long time to dissipate. During simulation, the waves wash around in the computer for a long time before a steady wave pattern emerges. Numerous computational steps are required before a usable result is obtained. Moreover, the simulations must be repeated for different sailing conditions and ship speeds. Hence there is a need for computational methods that produce a steady wave pattern more quickly. The Ever-Changing Surface Jeroen Wackers and Barry Koren found an algorithm to quickly calculate the steady flow pattern without having to progress through each successive time phase. They explicitly factored in the air above the water surface. Air is a fluid with very low density. Several good algorithms exist to describe the interplay of two fluids, allowing rapid computation of a steady wave pattern in which all forces are in equilibrium. With air above it, the water surface no longer constitutes the boundary of the problem, it lies in the interior of the computational domain, and boundary conditions need not be specified for it. The entire simulation area – water and air – can be efficiently treated in the same manner. Much Greater Speed  Figure 1: Two ships at cruising conditions. The upper ship generates a water wave with quite a high amplitude (see halfway hull). The wave contains a lot of energy, which has been transferred to the water by the ship, and is not returned to the ship. The hull of the left ship has been redesigned, at the Maritime Research Institute Netherlands (MARIN), to lower the aforementioned wave. The result is clearly visible: the right ship has a significantly lower wave drag. CWI cooperates with MARIN on the development of computational methods for the optimization of ship-hull shapes. The study of Wackers and Koren will enable a reliable and efficient method of calculating flow around ships, allowing the prediction of performance, stability and control under various sailing conditions. This will give rise to better design tools, and ultimately to better ship designs. Wackers conducted the main part of his research at CWI as part of the BRICKS programme, and some additional work was performed at the National Maritime Research Institute in Japan. In November 2007, he obtained his PhD cum laude at TU Delft. Currently, he is a post-doc in an internationally leading research group in ship hydrodynamics at Ecole Centrale de Nantes, France. Links: Please contact: |
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