Increasing the Efficiency of a hydrofoil dagger board *HELP*

Discussion in 'Hydrodynamics and Aerodynamics' started by ross whitaker, Feb 16, 2018.

  1. ross whitaker
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    ross whitaker Junior Member

    Hi i am currently student at university given the task of increasing the efficiency of a hydro foiling dagger board.
    I will be designing a dagger boar for the Americas cup catamarans and was wondering if anyone would know a NACA profile that i could base my design on ?
    Also if anyone knew a good method of measuring the lift and drag in a fluid tunnel i would greatly appreciate it.
    Many thanks for any help given !
     
  2. tspeer
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    tspeer Senior Member

    NACA sections are not the best choices for a hydrofoil. A NACA 6-series section might be a good start, but you'll need to use a tool like XFOIL to design sections specifically for the purpose. The section design for an AC catamaran daggerboard is driven by cavitation and structural constraints. And the largest gains are to be had are the in sizing of the foil, not the section design.

    Here is a process that might be used to design the foil:
    1. Determine the requirements. You will need to know how much lift the board has to produce in both the vertical and horizontal directions, for upwind and downwind sailing. Define the maximum speed and the takeoff speed. Determine the upwind and downwind cant angles. Study the Design Rule limitations.

    2. Start with the rondure (shape in the transverse Y-Z plane). This needs to meet the constraints of the Design Rule - maximum beam, maximum depth, within the required distance from the hull when retracted, maximum size of the foil. The height, radius, and turning angle of the elbow are important. The dihedral angle of the wing portion is a compromise between the height of the elbow and the Design Rule constraints. (This is what led ETNZ to put the pronounced kink in their wings.)

    3. Design the planform shape to provide the desired area minimize the induced drag while constraining the heeling moment. This can be done with a lifting line analysis or a vortex lattice code. You only need to consider flat plate camber lines at this stage. These first two steps are the most important for determining the efficiency. Make the daggerboard wing as long as possible to minimize the induced drag. Reduce chord to minimize the wetted surface at high speed, subject to cavitation limits.

    4. Determine the required thickness distribution for structural stiffness. It is critical that you push the structural design as hard as possible to minimize the thickness.

    5. By now you have established the physical thickness and lift coefficient for each station along the foil. You need to design sections that match the operating conditions at each station. Say, at the top of the board, half way down the shaft, at the shaft/elbow junction, middle of the elbow, elbow/wing junction, a couple of stations in the middle of the wing, wing tip. The upper sections do not need to be designed for high speeds because they will be out of the water, then. The sections in the elbow need to have reversed camber in the middle of the sections in order to flatten the suction side to compensate for 3D interference. You can push the thickness to the bottom surface and make it more convex because of the pressure relief it gets from the 3D shape of the elbow.

    At the maximum speed, the pressures on the bottom and top of the sections will be essentially the same and equal to the incipient cavitation threshold. All the lift will come from the aft loading of the section. At low speed, you need to ensure that the leading edge suction peak does not cause cavitation at the leading edge.

    You need to analyze the sections with different assumptions with regard to boundary layer transition. It is not at all clear how much laminar flow is possible in water, and it may change with the water quality of the day at the particular venue. I suggest analyzing with Ncrit=1, 3, and with the boundary layer tripped to be fully turbulent.

    6. Put the sections into a panel code to analyze for cavitation with 3D effects. Align the zero lift lines of the sections with the flat plate chord lines from the vortex lattice analysis. This ensures the effective twist from camber changes matches the design twist. The flow will be accelerated on the inside of the elbow compared to the 2D predictions, so you will need to iteratively modify the 2D design so the local velocity stays under the cavitation threshold in the middle of the elbow.

    7. Analyze the final design candidates using free-surface Navier Stokes CFD, if available.
     
  3. ross whitaker
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    ross whitaker Junior Member

    Hi,
    Thank you so much for your response it’s greatly appreciated !
    My university uses the software “solidworks” for design do you know if you can transfer data from the XFOIL profile into solidworks ?
    Many thanks
     
  4. tspeer
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    tspeer Senior Member

    Yes. Probably the most universal way is to delete the first line from the XFOIL coordinates file, leaving only the numbers. Then import that as a points file. You'll need to spline a curve through the points after you have them in Solidworks.

    XFOIL also has an RSAVE command at the top level that will output a Rhino script to create the section. You might be able to adapt that file for Solidworks. Or, if you have access to Rhino, you could execute the script and then export the section from Rhino as an iges or stl file that you'd read into Solidworks.
     
  5. ross whitaker
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    ross whitaker Junior Member

    Hi again, In regards to changing the coordinates inside of the XFOIL software are there any tutorials you know that can show me how to do this as there are very few tutorials for this software?
     
  6. tspeer
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    tspeer Senior Member

    It depends on what kind of changes you want to make. In general, there are four places in XFOIL where you can change the shape.
    1) At the top level, the INTE command will blend between two different airfoil shapes. This can be used to breed a new shape that has some of the characteristics of the two parent sections, or it can be used to interpolate between the root and tip shapes. INTE is a good way to generate a starting shape for the other methods.

    2) GDES mode is the way to make geometric changes to the section. There are several commands that will do this. TSET and TFAC will scale the thickness and camber. CAMB is a sub-mode that can be used to reshape the camber line or create a whole new camber shape. The MODI sub-mode will let you interactively draw changes to the contour. Unless you are very skilled, this is likely to introduce peaks and wiggles in the pressure distribution, but it can be a good way to get the shape started in the direction you want, and then use the inverse methods to fix it up.

    3) MDES mode is an inverse design method that shapes the entire airfoil. It has a MODI submode that is used to interactively draw changes to the design pressure distribution. MDES is my favorite way of modifying an airfoil's shape, in conjunction with trips back to the GDES mode to reset the thickness, etc. I also use MDES for smoothing an airfoil that has wiggles in the pressure distribution from imperfect coordinates.

    4) QDES mode is an inverse design method that only affects part of the airfoil shape. You designate a segment that you want to change, and then use the MODI submode to change the design pressure distribution for that segment as in MDES. This mode is useful when you have part of the section constrained to match some existing shape and you want to improve the performance using the part of the shape that is free to be modified.

    In practice, you will find yourself using all these methods as you iterate the design.
     
  7. ross whitaker
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    ross whitaker Junior Member

    Hi thank you i will have to try that later, I have been trying to import coordinates from UIUC Airfoil Data Site http://m-selig.ae.illinois.edu/ads/coord_database.html The files are in .dat format but i have then converted to a .txt file format and have saved it. then i tried to input the file with the LOAD command but this isnt working, i was wondering if you knew why this is ? the foil i am using is the ( Eppler E874 hydrofoil and was trying to input this into the XFOIL software to do analysis on and compare this with other profiles.
     
  8. tspeer
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    tspeer Senior Member

    That file is in Eppler code format. Here's a version you should be able to load directly into XFOIL.

    Eppler files are a bit of a pain to convert. You need to delete the lines that give the number of points (leaving just the name and the coordinates), sort the top surface so the coordinates run from trailing edge to leading edge, delete duplicate points at the leading edge, and save as a text file. It's easiest to do this in a spreadsheet, but I've not found a way for Excel to create a space delimited file instead of a tab delimited file. XFOIL hates tab characters, so you need to do a final edit to replace the tabs with spaces.
     

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  9. ross whitaker
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    ross whitaker Junior Member

    Thank you I believe the upload has completed, what software did you manage to convert the coordinates into those files as i have previously tried to use excel as i need to compare the Eppler E874 to different profiles such as (Eppler E836) (Eppler E838) (NACA 62-212) (Eppler 818)
     

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  10. tspeer
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    tspeer Senior Member

    I use Excel, as I describe in my previous post.
     
  11. ross whitaker
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    ross whitaker Junior Member

    okay thank you i shall try and give it a go and will let you know how it goes
     
  12. tspeer
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    tspeer Senior Member

    Probably the hardest part of learning XFOIL is the work flow. If you don't know what you're trying to do, you don't know what commands to look for. The sessions.txt file in the source code folder has some examples of how to do some common tasks. It may seem like gibberish when you first look at it, but if you follow along, putting in the commands in your own session, I think it'll become clearer.

    FWIW, I looked through the source code to see what commands were there that weren't documented. I think the RSAV command is the only one that would be useful to most people.
    Top level:
    RSAV Writes out current airfoil as a Rhino Macro command file that interpolates curve through airfoil points
    USAV Writes out current airfoil in integer coordinates
    INTX Interpolates two airfoils into an intermediate shape. [I'm not sure what the difference is between INTE and INTX.]

    GDES:
    NDEC Decrements name version number
    SINT [I'm not sure what it does, other than turning on tic marks when plotting the section shape.]

    OPER:
    WAK Sets length of wake
    NDEC Decrements name version number [Does the opposite of NINC.]
    DAMP Toggles between modified and default amplification

    MDES:
    DUMP Writes Cn to a file
     
  13. markdrela
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    markdrela Senior Member

    INTE interpolates x(s),y(s) at the same fractional s/stot location on each airfoil side.
    INTX interpolates y(x) at the same x/c location on each airfoil side.

    Default spline uses secant arc length as the spline parameter. SINT uses the true arc length. Doesn't make much difference in practice.
     
  14. tspeer
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    tspeer Senior Member

    Thanks, Mark!

    I think INTX may be useful with just a single airfoil. There are times when it's good to have the coordinates lined up, and using INTX to "interpolate" between and airfoil and itself would do that.
     

  15. markdrela
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    markdrela Senior Member

    I'm not sure what you mean by "line up" here. Since INTE always interpolates to the s/smax values taken from airfoil 0, it can do the following useful trick which I use frequently:

    Inputs:
    airfoil0
    airfoil1
    fraction = 1.0

    Output:
    airfoil1new which has the shape of airfoil1, but with the point count and spacing of airfoil0

    So airfoil0 and airfoil1new can be loaded into Excel columns and interpolated point by point in any way you want, e.g. with some arbitrary f(x) fraction, which gives more possibilities than the constant fraction that INTE assumes.
     
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