NEWSWAVE - HSVA

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CALCULATION OF HYDRODYNAMIC BODY FORCE COEFFICIENTS FOR TThe motion of a ship can be predicted if the hydrodynamic forces on the hull, rudder and propeller are known. The forces are a function of ship velocity and acceleration (higher order time-derivatives can usually be neglected). With a polynomial approach like: the time dependent forces can be calculated. The unknown coefficients are determined either by model-testing or calculations. A sufficiently accurate set of these coefficients is vital for a reliable prediction of the manoeuvring capabilities and thus for the design of the manoeuvring devices like rudder and thrusters in an early state of the design process. Since 1 st January 2005 the European Integrated Project (IP) VIRTUE, “The Virtual Tank Utility in Europe” works on advanced numerical simulation tools. “The Numerical Manoeuvring Tank”, being one of the 4 different CFD development fields in the project, focuses on numerical manoeuvring predictions. Within this work package a major task is a substantial improvement in accuracy, efficiency and consistency of computations for simple modes of motion like steady drift and rotation. Fig. 1 Forces and Moments on the “Hamburg Test Case” at 1.05 m/s model speed 4 NEWSWAVE 2006/2 News from the VIRTUE labs MANOEUVRING PREDICTIONS by Marco Schneider X=Xu u+X 2 u u u2 u+Xu u+Xu 2u 2 +Xυ 2υ 2 +Xr 2r 2 • • • • +... Fig. 2 Forces and Moments on the “Hamburg Test Case” at 1.89 m/s model speed Wake field of the “Hamburg Test Case” at 30° drift angle Comprehensive investigations on numerical discretisation, turbulence modelling, free surface flow and scale effects are performed that yield recommendations for practical applications. Based on these developments HSVA sets out to deliver CFD based manoeuvring coefficients. Figure1 shows the comparison of the calculated non-dimensional forces and moments at different drift angles and velocities of the “Hamburg Test Case” with experiments. The calculations were performed with the Reynolds-Averaged Navier-Stokes Equations (RANSE) solver Comet. In a first approach the deformation of the free surface at speed was neglected. A double body model that uses a symmetry plane located at the undisturbed free surface has been employed. At lower ship speeds the double body model gives good agreements between experiment and simulation for the X and Y forces and the N moment. At higher speed the hydrostatic pressure generated by the bow wave (shown in Figure3) has a significant influence on the forces and thus cannot be neglected any more. Here the volume of fluid (VOF) free-surface model implemented in Comet was used to predict the forces and moments more accurately (shown in Figure2) at the cost of a significant increase of the computation time.
Further investigations on turbulence modelling and scale effects and the investigation of unsteady ship motions are scheduled. The promising results achieved so far will soon allow to integrate numerical manoeuvring computations into the portfolio of HSVA’s CFD services. MINIMISE OPERATING COSTS BY TRIM VARIATION TESTS RoRo-Vessel Heavy-Lift-Vessel Full Scantling Draught 18 kn 21 kn 23 kn 15 kn 17 kn 18 kn Trimming by head 97% 98% 95% 106% 105% 105% Trimming to the stern 105% 102% 99% 105% 97% 95% Design Draught 18 kn 21 kn 23 kn 15 kn 17 kn 18 kn Trimming by head 89% 95% 98% 105% 104% 104% Trimming to the stern 110% 106% 104% 105% 94% 90% Ballast Draught 18 kn 22 kn 24 kn 15 kn 17 kn 18 kn Trimming by head 100% 96% 103% 99% 103% 106% Trimming to the stern 111% 104% 103% 108% 99% 91% Table 1 – Results of Trim Variation Tests for two Newbuildings Fig. 3 Bow wave in the experiment by Uwe Hollenbach In the past three years costs for fuel oil have almost doubled. To face this problem more and more ship owners order numerical calculations, or take the opportunity to use an existing ship model of their newbuildings for additional model tests. The aim of these tests and calculations is to provide decision support to their nautical officers on board their ships to minimise operating costs. In addition to manoeuvring tests to check the fulfilment of IMO manoeuvring criteria, sea-keeping tests to determine the speed loss for contracted environmental conditions, tests and numerical investigations to determine the squat effect and to find out the economical speed in shallow water, some ship owners order trim variation tests for their actual newbuildings in order to quantify the effect of trim on the power demand and the fuel oil consumption. Most recently such tests have been ordered for newbuildings of a RoRoand a Heavy-Lift-Vessel. When compared to the even keel condition, remarkable differences in power demand have been found for calm water conditions on various draughts and different ship speeds. Some results are given below, with the even keel condition as reference (100%). While for the RoRo-Vessel the largest power saving (-11%) has been found when the vessel is trimmed by the head, the largest power saving (-9%) of the Heavy-Lift-Vessel has been found when trimming by the stern. The largest differences in power demand between the best and the most unfavourable trim condition are more than 20% for the RoRo-Vessel and about 15% for the Heavy-Lift-Vessel. It should be mentioned that the differences in the power demand may be lower for other size and type of ships. 2006/2 NEWSWAVE 5
Page 1 and 2: NEWSWAVE T HE H AMBURG S HIP M ODEL
Page 3: MANOEUVRING CHARACTERISTICS General
Page 7 and 8: During the entire test run, the shi
Page 9 and 10: The oil produced will be transferre
Page 11 and 12: Fig. 5 Non bursting vortex, smooth

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