u/ Vs 1


Speed Trials per ship type model Figure 1: Number of speed trials per ship type in the MARIN database ship


0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0


Figure 2: PARNASSOS calculation of the axial velocity model and full-scale in a plane near the propeller


measured model values. These methods account for various scale effects between model and full-scale. William Froude was the first to develop a scientific approach to this problem of extrapolation. Surprisingly, the assumption of the mutual independence of the wave and viscous resistance com- ponents as adopted by William Froude has worked wonderfully for more than a century.


PARNASSOS Only recently, the interaction of components has been recognised as a possible refinement in the extrapolation of model experiments. Raven et al. (Marine CFD 2008) showed that the numerical tool PARNASSOS, which is used extensively for viscous hull flow computations, can also be used to study scale effects on various resistance components. An interesting vis- cous scale effect on the wave resistance


2006


Model testing for winning ABN-AMRO Volvo Ocean yacht


was found, contrary to the assumptions in current extrapolation methods. PARNASSOS can be used to determine form factors by means of flow calculations on a double model ignoring free-surface effects on both model and full-scale. This was validated last year by comparing form factors derived from model measurements with those calculated with PARNASSOS. Figure 2 shows the model and full-scale PARNASSOS calculated axial velocities in a plane just in front of the propeller of a high- block tanker, with a scale factor of around 30. The dark-blue region at the hull above the propeller indicates separated flow with (almost) zero or negative speed. This is only present in the model calculation and not at full-scale. The example suggests the use of large ship models, as MARIN does as stand- ard, because the scale effect induced by the flow separation is uncertain and not known


beforehand. Moreover, it strongly depends on the scale factor, making the power prediction progressively more uncertain for smaller models in this case.


Future Extrapolation JIP Although the uncertainty in the extrapolation is (usually) only a modest contributor to the total uncertainty, it is important to have a trans- parent and physics-based method. Such a method preferably makes use of the form factor concept in combination with a good friction line. Especially when testing new concepts, or ships with deviating dimen- sions or speed, modern extrapolation methods should give more reliable predic- tions. Initiatives for an Extrapolation JIP are underway which aims to introduce and evaluate modern extrapolation methods in combination with CFD tools such as PARNASSOS.


2006


Research on sinking MS Estonia


report


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