In-depth | hyDroDynamiCS
Figure 4 Prediction of inSEan 2340 model (DTmB 5415 variant); Figure 5 Prediction of nPL C3 model; automated correlation.
automated correlation.
scale-corrected lift-drag coefficients, are For example, a hybrid Bare-Hull methods are very cost-effective and
then applied to more accurately predict Drag module for HydroComp’s accessible to any practicing naval
quantitative performance. NavCad software is built upon a novel architect. They are strong solutions
The software can be applied for implementation of thin-ship theory for quantitative problems, but offer
full wake-adapted design of optimal that avoids the use of hull offsets (as little in the way of localised qualitative
characteristics or off-design analysis would be needed for a Michell Integral analysis. On the other hand, complex
of performance and KT-KQ curves. A calculation, for example). In place of three-dimensional codes, such as
variety of design choices are available, offset definition, a simplified approach CFD, are uniquely qualified for
including different optimal circulation was adopted that used only longitudinal analyses requiring visualisation of
methodologies and tip or root sectional properties. This insures flow and comparative optimisation.
unloading. more well-behaved and reliable code, Unfortunately, they are expensive to
The empirical correlation functions and protects the user from significant acquire and maintain, and required
also extend to ducted propeller inaccuracies that occur with irregular an advanced level of training and
performance. Development of this changes in surface offsets for things like experience to insure that they produce
tool included data analysis and fitting tunnel thrusters or propeller pockets (as reliable results. New hybrid tools, based
of nozzle-propeller thrust ratios, and shown in Figure 3). on well-established “relational element”
prediction of the nozzle’s influence Of course, wave-making drag is techniques, are emerging that bridge
on induced velocity. A validation important only in so far as it leads to the gap between the empirical and
plot is shown below for the Kaplan correct prediction of total bare-hull computational. These tools are available
4.55 propeller in the 19A nozzle, with drag. Extensive effort has been put to any naval architect, and should be
excellent results (even near bollard). into reliable prediction of frictional considered when both quantitative
drag and form factor, and also into and qualitative answers for distributed
Example – Bare-hull drag empirical correlation functions for the geometries are needed. NA
Calculation of bare-hull resistance can contributions of immersed transom
also be accomplished in a similar manner stern, sinkage and trim, pressure drag, references
by slicing the hull onto longitudinal and viscous effects. These correlation ITTC Resistance Committee, Final
elements of sectional area and beam. functions are automated in the code, Report and Recommendations to the
This provides greater definition of the and it is important to point out that 25th ITTC, International Towing Tank
geometry than you have with a purely no special modelling decisions were Conference, Proceedings Vol. I, 2008.
statistical series (that might use only the made in the creation of the following Brizzolara, S., Gaggero, S., and
prismatic coefficient, for example, to validation plots (Figures 4 and 5). Grasso, A., Parametric Optimisation of
define the sectional area curve). While Open and Ducted Propellers, SNAME
you do not have full three-dimensional Summary Propellers/Shafting Symposium,
definition of the hull surface that you As stated at the outset, the proper tool 2009.
get with complex codes, you will have a for hydrodynamic design and analysis Donald MacPherson,
practical and meaningful distribution of depends on the nature of the problem to VP Technical Director,
the hull’s shape. be solved. Empirically based statistical HydroComp, Inc.
34 The Naval Architect January 2010
NA Jan 10 - p32+33+34.indd 34 12/01/2010 09:56:05
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