Ship rudder sections and XFOIL or alternatives

Discussion in 'Hydrodynamics and Aerodynamics' started by TTTP, Sep 10, 2013.

  1. TTTP
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    TTTP Junior Member

    I am trying to evaluate different rudder sections for a cargo ship. The Reynolds numbers are above 10M and the flow is quite turbulent because most of the rudder is in the wake of the hull.

    I tried simulating some NACA 00XX profiles with XFOIL and used parameters Re = 10M and N_cr = 4. What I found out was that even NACA 0006 reached large lift coefficients like 1.56 at 15 degrees. This seems dubious to me.

    I would like to know how to get useful results for high Reynolds numbers rudder sections in XFOIL or if some other software would work better for me.
     
    Last edited: Sep 11, 2013
  2. MikeJohns
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    MikeJohns Senior Member

    A ships rudder would normally be operating not just in the wake field but in the prop wash.

    Perhaps you should obtain and read Mollands book on Marine rudders, it is a good reference.

    But firstly is this a student exercise or real world ? Then we need to move on to the vessel type/parameters.
     
  3. DCockey
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    DCockey Senior Member

    How did you do the simulations? What software or analytical models did you use?

    What are your concerns about the Reynolds Number being 10x10^6? Are you assuming a turbulent free stream? If so what are your assumptions?
     
  4. TTTP
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    TTTP Junior Member

    I know. But often engineering requires simplifications so I would like to have a way for analyzing rudder sections.

    I have it beside me right now and it has limited information about sections.

    This is a real world case.

    I made the simulation with XFOIL and used the parameters that I mentioned.
     
  5. MikeJohns
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    MikeJohns Senior Member

    Just be careful
    Presuming your cargo ship is reasonably standard you will not gain anything over the empirical based design guides currently described by books such as Molland's. That includes section recommendations. You certainly won't improve on the game of 'follow the leader' using lift drag predictors like xfoil in ideal flow.

    The rudder and the prop slipstream interaction is important, not only does it increase the CL considerably before stall but the rudder recovers much of the rotational energy from the prop slipstream and the inflow is skewed along the span by up to 10 degrees either side from top to bottom. Analysis of foil sections in an ideal free flow are not particularly valid in that environment.

    Also consider a principal design concern will be cavitation at that Reynolds number (10E6).
     
  6. daiquiri
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    daiquiri Engineering and Design

  7. tspeer
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    tspeer Senior Member

    I think XFOIL tends to over-estimate maximum lift. Here are some examples:
    http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA532502
    http://proceedings.ewea.org/ewec2009/allfiles2/302_EWEC2009presentation.pdf
    http://www.lr.tudelft.nl/fileadmin/..._1995/doc/Timmer_Thick_airfoils_for_hawts.pdf

    Mark Drela points out the effect of compressibility on wind tunnel data in this post: http://www.homebuiltairplanes.com/f...-new-technology/4154-maximum-lift-values.html
    So one has to look at wind tunnel data, especially the older data, somewhat skeptically, too.

    I realize compressibility is not a factor for a ship's rudder, but the fact that high local velocities can be encountered at the leading edge of thin sections brings its own problems in water. The leading edge can easily be below the incipient cavitation pressure at high lift.

    MSES is a more capable 2D code, but it uses a similar integral boundary layer method to XFOIL. For a single element section at high Reynolds number, I think the XFOIL and MSES results will be pretty similar.

    Given the practical constraints of ambient turbulence and biofouling, I don't think the results of any 2D calculation are going to be very representative of maximum lift. I think the 2D results can be useful upper bounds for the purpose of calculating structural loads, but you're going to have to apply some engineering judgement to estimating maximum lift for performance purposes.
     
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  8. MikeJohns
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    MikeJohns Senior Member

    It's an interesting subject.

    Within a host of usable rudder sections and even planforms, the most significant variable in directional control of a ship is rudder area.
    Consider that this is an arbitrary empirical choice based on nothing more precise than lateral plane area. But even then there’s no guarantee of adequate maneuverability or coursekeeping. No matter what foil section is chosen.

    From empirical data there are a number of sections and profiles that work well, and when considering rudders operating behind a prop and ‘forces normal to the rudder’ I’d reiterate that free stream lift/drag data is not even applicable given the flow field the ships rudder experiences Which is why you won’t find much on refining sections for the best lift drag ratio in applied NA texts such as Molland for example.

    For the general practice NA, CFD fails to predict ship maneuverability with any reliability as ITTC point out after every review. That’s why model basins are still so essential and only validated CFD ship models have any useful output once the solutions have been refined. The complexity of a hull- prop-rudder interaction is so high. Even modelling just the rudder in the prop stream is no reliable model, just small changes in mesh can have large changes in output or even a complete failure of the solver to converge.
     
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