# Free surface

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 March 24, 2007, 12:41 Free surface #1 Dimitri N. Guest   Posts: n/a I am conducting a review of available solutions to model boat hulls moving through the water including the generation of the wave by the motion in order to predict accurately the drag. So far I have been using AcuSolve with some success and before launching a new cycle of develoment I would like to understand better what is available on the market for this kind of simulations. I am looking forward to rank solutions that differ in the order of 1-3% between each others. Thnaks in advance for your input.

 March 25, 2007, 08:13 Re: Free surface #2 Leo Lazauskas Guest   Posts: n/a It depends on whether you are interested in the far-field or the near-field, or both. "Michlet" is a free code that allows calculation of free-surface wave elevations of thin monohulls and multihulls with up to five demihulls in any arrangement. It can also "solve" the inverse problem: i.e. given a user-specified wave pattern it will attempt to find the hull (or hulls) that made that pattern. SWPE and Flotilla are two other codes that can calculate both near-field and far-field wave elevations of thin ships and travelling pressure distributions. Flotilla can also calculate wave elevations and wave resistance of Surface Effect Ships. Other codes I have helped develop can also solve inverse problems such as finding the "free wave spectrum" and wave resistance using a longitudinal cut (or cuts) through the wave pattern. There are many other codes, linear and non-linear, that you can examine. Apart from the linear codes mentioned above there is also Shipflow, SWAN, Uncle, CFD-Ship, etc. The predictions of several codes were compared at the Gothenburg 2000 Workshop which would be an excellent start for your investigations. All in all, the predictions were pretty poor. Some codes couldn't even estimate the surface area of the test cases to within 5%! Hopefully the CFD types here will point you towards their favoured computational beasts. There is at present an extensive experimental program being conducted at about 30 towing tanks around the world. This work stems from great dissatisfaction with the claimed accuracy of many CFD codes which, as you know, can often be tweaked by users to supply a desired result. (It's no surprise that some commercial codes don't like being tested on ships for which experimental results are not known in advance). These tests are due to be completed late in 2008. Good luck! Leo.

 March 25, 2007, 13:14 Re: Free surface #3 Dimitri N Guest   Posts: n/a Leo, thanks for your answer. My experience is that I hve been abe to simulate the waves reasonnably well compared to the towing tank but still far from the accuacy I need to be able to rank my hulls (that differ from as little as 1% in drag). Could you give me more details on the ongoing study performed by worldwide tanks you are talking about?

 March 26, 2007, 02:16 Re: Free surface #4 Leo Lazauskas Guest   Posts: n/a Google around for "ITTC" and "DTMB 5415". The hull that is being tested is a destroyer hull with a largish sonar dome. Two geosims, one with L=5.72m and one smaller hull are being tested. Some towing tanks can't handle the 5.72m hull. I think that more than half of the tests have already been done but results will only be published after all tests have been completed and collated. I can certainly sympathise with your problem. I have recently been assessing several candidate rowing shells and the differences can be of the order of 1-2%. I have found that it is very important to include dynamic sinkage and trim to get a better indication of the relative performance. Good luck! Leo.

 March 26, 2007, 10:13 Re: Free surface #5 ubik Guest   Posts: n/a you can use Comet or try to use Star CCM+ by

 March 26, 2007, 11:49 Re: Free surface #6 Leo Lazauskas Guest   Posts: n/a Comet didn't perform very well on the DTMB 5415 hull at the Gothenburg 2000 Workshop on Numerical Ship Hydrodynamics. See, for example, Section D4 of the Proceedings. Do you have any references to more recent comparisons? Regards, Leo.

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