Unusual concept for analysis

Discussion in 'Hydrodynamics and Aerodynamics' started by Windmilljohn, Apr 30, 2011.

  1. Windmilljohn
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    Windmilljohn New Member

    Dear all

    My company has designed a tidal turbine support platform that you can see in model form in the attached images and on www.tidalstream.co.uk. Basically, the device is towed to site as a catamaran holding three turbines, then is attached to the seabed by a rigid pivoted twin tether arm, then ballasted into a mostly submerged position where the turbines can operate in a tidal flow and generated power. The wheels do not touch the seabed in operation - the buoyancy and lift from the hydrofoil crossarms is enough to balance the rotor thrust in all cases. One image shows the device floating on the surface; the other shows the actual 1-10th scale model sitting on the riverbed in its operating position. The wheels are just for convenience on the model. It seems to work!

    We would like to analyse the sea-keeping characteristics of a the full-size turbine in its floating position when anchored as shown to the seabed. My question is, what would be the best software to analyse its characteristics - in a current flow, sea from any quarter, etc.....? Is there any software that would do this? Could some be adapted? We have some funding to spend on getting this right, so are not looking necessarily for the cheapest cost solution.

    Thanks for reading this through, and look forward to your replies...

    John Armstrong
    Tidalstream Ltd
    London, UK
     

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  2. Squidly-Diddly
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    Squidly-Diddly Senior Member

    seen the cover of current Popular Mechanics?

    similar looking contraption but a kite wind generator
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    John

    The closest would be FPSO and similar moored buoy arrangements. The software used is not cheap and not easy to use either. The software would also be "tailored" for such operations. It would perhaps be "indicative", but only as a basic trend. However, it would be difficult to get some kind of validation, since FSPO’s etc are not multihulls.

    Best way would be to tank test the arrangement and then match the ROAs to a half decent software more suited for multihulls that you can then use to manipulate the arrangement for sensitivity analysis.
     
  4. Eric Sponberg
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    Eric Sponberg Senior Member

    I second Ad Hoc's comment that you might get further with tank testing. Your device is very unusual with lots of different parts all creating a bunch of different forces and moments. Who knows what will happen without actually trying it in real, but controlled conditions. I highly recommend you do your testing at The Institute for Marine Dynamics at the Memorial University of Newfoundland in St. Johns, Newfoundland; they have amongst their facilities a huge flume tank, which is like a towing tank without a towing carriage. The water flows through the flume tank while the observers watch the model in the tank through the glass-sided tank walls. This would likely be ideal for your application. All commercial operations for the towing facilities at IMD and MofN are handled through Oceanic Consulting Corp., also in St. Johns, Newfoundland. Link: http://www.oceaniccorp.com/. (If you look at the photo in the upper right of the first page, you will see my model of the Moloka'i Strait motoryachts that we had tested there.)

    Good luck,

    Eric
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I also think you need to do some form of tank testing but, as you are in the UK, I would first try the Uni of Strathclyde. They are (or were) working on similar methods of energy extraction.

    http://www.esru.strath.ac.uk/EandE/Web_sites/01-02/RE_info/Tidal Power.htm#currentb2

    It's good to see that you are already testing your devices in real streams. It will be interesting to see what grows on different parts of the structure and nacelles.

    Good luck!
    Leo.
     
  6. BYDE
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    BYDE Junior Member

    a CFD program (RANSE solver) would be able to simulate this, but it requires experience to use it as it's a complex problem. Moreover the computational time would be 1 day at least, or more, with an average cluster.
    However the accuracy is not bad overall. We recently analysed an offshore platform in waves and comparison with experiments was ok.
    But rather than the sea-keeping (intended as motions in waves), the focus was more on the loads experienced by the structure due to the wave impact.
     
  7. BYDE
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    BYDE Junior Member

    if you check the website of that company
    http://www.tidalstream.co.uk/html/development.html
    you'll see that they already did rather extensive experiments, also in Glasgow as suggested by Leo L.
    I think he's strictly asking for a computational tool
     
  8. DCockey
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    DCockey Senior Member

    The rotors complicate the analysis. The forces and moments from them will be a significant portion of the total forces and moments on the system. Modeling rotors/propellors in an unsteady enviorment is not casual and not a simple extension of steady state models. One place to start for the rotor effects would be analysis methods and tools developed for windmills. Combine that with analysis methods and tools for moored buoy systems. I suspect added-mass and viscous drag may be more significant than surface wave making by the portion of the system near the surface.
     
  9. Windmilljohn
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    Windmilljohn New Member

    Thank you for all your input. We did indeed test one of our models at Strathclyde, and another at Ifremer in France. The problem is not so much in the fully submerged position with the rotors operating - the device is very well behaved as the rotors produce a lot of damping. We got all we could out of the tank testing with the rotors operational.

    The real question is around the behaviour while floating on the surface, with the rotors stationary and out of the water, when there can be a combination of current, beam sea and possibly breaking waves. That can only be tested for real in the sea, and we need to get as good an idea of what may happen computationally first.

    So what we really want to analyse is a twin-hulled cat with a very long keel tethered with a pivot to the seabed. Ad Hoc's idea of a package adapted to FPSO's sounds plausible - any ideas of suitable software?

    Cheers, John
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

  11. Windmilljohn
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    Windmilljohn New Member

    Thanks Ad Hoc. We want to get others to do this for us - I know people at both BMT and Frazer Nash so will start there. What is your own background? -can you mail me at johnarmstrong@tidalstream.co.uk?
     
  12. BYDE
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    BYDE Junior Member

    I don't think that the linear strip theory can go very far for such a complex simulation.
    At least I would use a 3-d panel method such as ANSYS AQWA for example
    http://www.ansys.com/Products/Other Products/ANSYS AQWA
    You may want to read the article 'Harnessing the Power of Ocean Waves', in the link above, it seems similar to your problem though I haven't read it all.

    I'll repeat myself, but in this case a CFD program with VOF solution is likely the only choice to have reliable and useful results.
    We use STAR-CCM+ and OpenFOAM for these problems with 6 DOF
     
  13. Windmilljohn
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    Windmilljohn New Member

    BYDE, sorry to show my ignorance here (I am a wind-turbine designer not a naval architect), but what are the differences in the approaches you describe here, eg linear strip theory vs 3d panel? VOF? Can you recommend somewhere I can read this up?
     
  14. BYDE
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    BYDE Junior Member

    strip theory and panel method are both based on potential flow, therefore all the viscous and rotational effects of the fluid flow are completely neglected. Though that's not necessarily too bad when you analyse waves and related problems.
    In the strip theory, the hull (or any floating body) is approximated by several 2d cross sections of the underwater part. Then the 2d potential flow solution is calculated for each of these sections along the hull.
    For 3d panel methods the hull is discretized by a proper 3d surface mesh, normally quadrilaterals. So strip methods do a greater simplification.
    Then there's a difference in the theory used, which can be divided in linear, weakly non linear and non linear. It's difficult to go into the details here, but in general linear theory will be suitable for the 'easiest' cases, such as motions of a ship in long waves with moderate accelerations.
    Linear strip theory is often used for the initial design stage of a ship, when you want to compare different designs. For your problem probably isn't enough in my opinion.

    Those are methods based on potential flow. Then there are programs based on the Navier-Stokes equations and turbulence models (RANSE). Anyway surely you know them already (Fluent, Star, CFX, etc).
    VOF stands for volume of fluid, it's the most common model used to solve two-phase flows. You can simulate very steep waves, breaking waves, spray, vorticity,... many things that potential flow cannot, plus a better physical model also for what potential flow does.

    The accuracy and versatility of these programs is much higher than for potential flow-based codes, but also take ~1-2 days computation for each case, while the panel methods will take less than 1 hour (just some minutes often). So it all depends on the accuracy you need and time available. Also if you outsource the work, the price and time required will be higher for a RANSE study.

    For a better explanation of panel methods for this type of problem you can check the WAMIT website:
    http://www.wamit.com/techdescription.htm
    there's quite a lot of technical info about their code (which is one of the best)

    For VOF:
    http://en.wikipedia.org/wiki/Volume_of_fluid_method
    (just a general description)
    if you try to google some keywords like 'volume of fluid, ship, seakeeping' etc., I'm sure you'll find a lot as it's the most popular method.

    or just ask here, I'm glad to give my opinion!
     

  15. Ad Hoc
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    Ad Hoc Naval Architect


    The problem i have with many of these codes, is just that, accuracy.

    How many codes out there have been used and validated and verified to provide accurate results of an arrangement such as being requested here? I'll go out on a limb and say none.

    The current state of play is not bad, for "simple" models. What John is requesting here is extremely complex, far far too many variables to consider.

    Thus any code that can provide an accurate result is highly questionable given the lack of data and verification.

    A code of sorts could be used for basic sensitivity analysis, but any more than that, is putting far too much faith in a code that is operating way beyond its current limits.

    Despite what many would have you believe, the current state of play cannot even provide accurate results of simple hulls shapes running at high Fns, or planing hulls in irregular seas etc. These are "simple" models compared to what John is looking for.

    Don’t try and crack a walnut with a nuclear warhead!
     
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