Delft Hull Series

Discussion in 'Hydrodynamics and Aerodynamics' started by Remmlinger, Aug 20, 2014.

  1. Mikko Brummer
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    Mikko Brummer Senior Member

    A nice paper on the subject coming up at HPYD 2005, discussing the aft overhang/transom effect:

    Hydrodynamic aspects of transom stern optimization
    Michal Orych1,2, Lars Larsson1,2

    1 Chalmers University of Technology, Department of Shipping and Marine Technology, SE-412 96 Göteborg, Sweden
    2 Flowtech International AB, Chalmers Tvärgata 10, SE-400 22 Göteborg, Sweden

    Abstract
    This project is a continuation of a study described in the paper titled Sailing Yacht Transom Sterns – A Systematic CFD Investigation by Allroth et al, also proposed for the HPYD5. A need for a better understanding of the stern flow was the primary reason to extend the work and deepen the subject.

    It is well known (see for instance Larsson and Raven, 2010) that the optimum transom size increases with Froude number, Fn. A common explanation for the increase suggests that it helps to extend the hollow on the water surface that the hull leaves behind - a virtual extension of the hull - which causes the effective Fn to become lower. This can be understood as favourable before reaching the strong hump at a critical speed in the wave resistance coefficient curve that can be observed for most fast monohulls. However, it would also imply that after the hump the opposite is true i.e. reducing the Fn would increase the wave resistance coefficient. To avoid that, the length of the extended hull should be smaller, and this would require a smaller transom, not a larger one.

    In this work an explanation is proposed which takes into account the hull shape between the midship and the transom. A systematic stern modification is performed to study the influence of the shape of waterlines and buttocks on the resistance components.

    The aft body of a hull is designed in a way that gives a possibility to separate the effects of waterline and buttock curvature and to study their effect on the flow. A section shape with a relatively small bilge radius is used to emphasize the importance of the transom aspect ratio. For a specific parameter combination, the shape of the waterline and keel line can be varied simultaneously without changing the volume distribution to decouple the effects of the longitudinal centre of buoyancy and prismatic coefficient. In order to simplify the hull generation procedure only two parameters are used which control the transom width and the transom immersion respectively. The Froude numbers are chosen to provide information for wetted- as well as dry- transom conditions.

    To evaluate the performance of the hulls the SHIPFLOW steady state Reynolds Averaged Navier- Stokes (RANS) code with a VOF surface capturing method is used in combination with the k- SST turbulence model. The code was first thoroughly validated for a transom stern hull.

    The results show a very strong effect of the waterline shape both on trim and resistance. On the contrary, in the same range of transom sizes, the rocker that controls the transom submergence has much smaller influence on the resistance at the higher Fn when the transom clears. It can be observed that the optimum transom size is also dependent on its aspect ratio.

    The paper focuses on illustrating and discussing changes in the wave pattern and the hydrodynamic and hydrostatic resistance components of the transom and of the rest of the hull. Physical explanations are given for the effects of the aft body hull lines. The transom size, waterline curvature and rocker effects are analysed.

    Reference: Larsson, L. and Raven, H., (2010), “Ship Resistance and Flow”, PNA Series, SNAME, USA

    http://hpyd.org.nz/site/Home1/filesystem/documents/HPYD5_abstracts.pdf
     
  2. Remmlinger
    Joined: Jan 2011
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    Remmlinger engineer

    Some of the results in this paper were already discussed in
    http://www.boatdesign.net/forums/hy...-shapes-cruising-yachts-51845.html#post712464
    Allroth's paper can be downloaded, access is free.
    The HPYD papers are expensive, accessible only for a small audience.
     
  3. Mikko Brummer
    Joined: May 2006
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    Mikko Brummer Senior Member

    I have all the CDs HPYD 1-4, if there's an interesting paper just give me a note and I can post it.

    http://www.hpyd.org.nz/default.aspx
     
  4. Nick_D
    Joined: Jan 2015
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    Nick_D Junior Member

    Uli,
    It strikes me that the most fundamental part of the whole study is your different (more accurate!) treatment of the skin friction component, as this leads to a completely different CR curve and regression.
    I would like to learn more, and attempt to recreate what you have done for the friction components. From reading your various papers it appears that you treat the hull form as half of an axisymmetric body. Is this correct?
    Could you tell me which part of the book (M&CoBLF) you got the integral method from for calculation of Cf and CF?
    Many thanks,
    Nick
     
  5. Remmlinger
    Joined: Jan 2011
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    Location: Germany

    Remmlinger engineer

    Yes, this is correct. The equivalent diameter is described in the regression-paper.

    Cebeci & Cousteix recommend and describe in 2005 a method that is a slightly simplified version of Green's lag-entrainment method (ARC R&M No. 3791): http://naca.central.cranfield.ac.uk/reports/arc/rm/3791.pdf
    Since this paper was written in 1977, it is advisable to replace the estimates for the friction coefficient at the flat plate with more recent test results, as e.g. published by Österlund: http://www.diva-portal.org/smash/get/diva2:8624/FULLTEXT01.pdf
    For the laminar part of the hull I recommend to use Eppler's code. It is applicable to 2-D as well as axisymmetric flows:
    http://ntrs.nasa.gov/search.jsp?R=19800020753
    Alternatively you can use Thwaites method for laminar flow.
     

  6. Nick_D
    Joined: Jan 2015
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    Location: UK

    Nick_D Junior Member

    Uli,

    Thank you very much indeed. I fear that I have a steep learning curve ahead of me! Many thanks for at least starting me off in the right direction!

    Kind regards,
    Nick
     
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