UNIVERSIDAD
POLITÉCNICA MADRID ETSI NAVALES MODEL BASIN |
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Potential Flow Calculation |
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Tdynlin |
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Calculation of wave resistance and potential flow. The computer program Tdynlin calculates the steady inviscid flow around a ship hull, the wave pattern and the wave resistance. It solves the linearised potential flow problem by a Dawson type panel method. Tdynlin has been developed by the ETSIN towing Tank CFD group in the period 1992 - 2001, and has been routinely applied in practical ship hull design since. The entire procedure of running the code, analysing the results and recommending hull form improvements forms an integral part of the ETSIN towing tank services. Tdynlin is commercialised by COMPASS http://www.compassis.com/ in conjunction with the pre-postprocessor GiD, which is used by the system Tdynlin. Free version available for academic purposes |
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Applications
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Primary
purpose is the minimisation of wave making and wave resistance. Visualisation
of the computed results gives a clear view of all features of the wave
pattern, hull pressure distribution and streamline direction over the
hull. Analysis and expert judgement then indicate which modifications
of the hull form will reduce the wavemaking; which subsequently is checked
by running Tdynlin for the modified design. In a few steps a hull
form can thus be optimised efficiently and quickly. Any subsequent model
testing thus normally can be limited to just one or few final designs.
Additionally, the predicted pressure distribution may indicate possible
improvements from the viscous resistance point of view (e.g. reduction
of flow separation). The flow direction on the hull can be used for aligning
bilge keels or knuckle lines with the local flow. Predicted far-field
wave heights are relevant for wash. Tdynlin is applicable to the
great majority of vessels, ranging from tankers to frigates, from sailing
yachts to ferries. The code can handle monohulls or multihulls, deep or
shallow water.
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Accuracy
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Extensive
validations have been carried out by means of Bajel
project. The predicted flow and wave pattern have been found to be
accurate, and consistently indicate the quality of a design and possibilities
for improvement. The predicted wave resistance is generally realistic
but not always quite precise, see below.
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Restrictions
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The
method is based on inviscid flow theory, which excludes the effect of
boundary layers, dead water zones behind a transom, or flow separation.
Consequently, the amplitude of the stern wave system is usually overestimated;
very little for slender transom stern vessels, more for fuller hull forms.
The wave resistance prediction obviously is affected by this, and for
fuller hull forms is not quantitatively accurate, but still good for ranking
different designs. Wave breaking or spray are not modelled.
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Input
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The
geometry of the hull is represented by a panel distribution, prepared
by directly digitizing a body plan or generated from a hull surface representation
in a CAD system. Tdynlin works as a module of the pre and postprocessor
program GID. Calculations
can then be made for a range of speeds, displacements, LCG positions and
water depths, to be specified by the user. The same graphical interface
GiD that helps preparing the input and running the code is available for
analysing the results.
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The output consists of the velocity and pressure distribution on the hull, the wave pattern, the wave profile along the hull, the wave resistance, the actual wetted surface area at speed, the far-field wave spectrum, etc. All results can be simultaneously visualized on a colour graphics PC, thus giving detailed insight in the character of the flow, the origin of dominant wave components and possibilities for optimising the hull form. Click on the figures for some examples and comments: |
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Computational
approach
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The
flow is calculated by means of a panel method That solves a linearised
free surface problems. Source panels are distributed over the hull and
at the free surface. The hull and free-surface boundary conditions lead
to a large system of equations, that is solved using an efficient algorithm.
A complete calculation with e.g. 5000 panels per symmetric half typically
takes half an hour on a Pentium PC.
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