Is Matte Faster?

Discussion in 'Hydrodynamics and Aerodynamics' started by sabsfeigler, Aug 30, 2012.

  1. DCockey
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    DCockey Senior Member

    There are two different ways that surface roughness can affect the aero/hydrodynamic forces and moments on a body.

    One is by altering if/where separation occurs which in turn can alter the overall mean flow field and pressure distribution. This can significantly affect all forces and moments, not only drag. Depending on Reynold's number and geometry of the body a decrease in drag and increase in lift may occur with increasing roughness. The effect of dimples on a golf ball are probably the best known example of this phenomena. The paper Mikko linked to in post #70 http://www.usna.edu/naoe/people/SCHULTZ PAPERS/Miklosovic, Schultz & Esquivel JoA 2004.pdf shows another example; a Laser board at two different Reynolds numbers and various angles of attack with significant changes in lift, moment and drag in some situations.

    The other effect surface roughness can have is to alter the average shear stress at the surface without fundamentally altering the mean flow. Generally average shear stress increases with increasing roughness. This will generally have a much larger effect on drag than on other force and moment components. Mikko provided a link to a study of this:
    My understanding is almost all "engineering" CFD codes use models for at least some aspects of the boundary layer, and surface roughness can be one input into these models. The ability of these codes to predict surface roughness effects would be directly tied to these boundary layer models. Several decades ago I heard of cases where tuning these models to improve separation prediction made average shear stress predictions worse and vice-versa. Perhaps the modeling has improved since then.
     
  2. tunnels

    tunnels Previous Member

    So all thats being said is just hypothetical !! figures and predictions on paper and pie in the sky dreams and is not actual results of boards being use it in the water in a boat sailing one on one ?? :confused:
     
  3. Mikko Brummer
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    Mikko Brummer Senior Member

    Very nice explanation in your post #76, thank you. Using surface roughness to "tune" separation prediction in a CFD code actuallysounds like a good idea: Most often, predicting separation better is much more important than skin friction, at least when dealing with sails & masts.
     
  4. Joakim
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    Joakim Senior Member

    Their measurements showed that the surface was quite smooth. The unsanded (sprayed paint) was the roughest at 10-12 um (RMS or average roughness height), which below the limit of "hydraulically smooth" for a flat plate at the Re used (~18 um at the higher Re). Thus it is not surprising that the drag did not change when the surface was made smoother. Also the average/RMS roughness is below the roughness needed for turbulent transition (about 40 um for the higher Re) thus for a flat plate there really shouldn't be any changes at all.

    At higher AoA the velocity near the boundary layer is far higher than the free stream velocity. I guess that must be the reason why the surface is no longer smooth enough. I don't see much gain from the better performance at 13-15 AoA for a keel.

    For me the most interesting was the measured roughness vs. grit. If you compare those the grit size of the papers used, there is a huge difference. Grit 60 paper has 260 um grit size, but still the sanded surface has the average/RMS roughness of only 4-6 um and even the maximum peak to peak is only 30-40 um. Sanding with grit 600 (European/25um or US/15um???) only dropped the roughness to 2-3 um with 20 um peak to peak. So going to 10 times finer grit size made the surface only 50% smoother.

    So it would appear that there is really no point of sanding with finer than grit 220, which already seems to give a smooth enough surface up to ~30 knots.

    http://www.cs.rochester.edu/u/roche/rec.wood.misc/grit.sizes.htm
     
  5. Mikko Brummer
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    Mikko Brummer Senior Member

    In the flat plate test in the 2nd quoted paper, the results appear to be more "normal", with increasing drag with increasing roughness. The laser centerboard used similar surface roughnesses as the flat plate test, I took my 2nd ref. from there: "The tested surface finishes were the same as tested by Schultz18 in flat-plate frictional resistance experiments." Re may have been different, though.
     
  6. Mikko Brummer
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    Mikko Brummer Senior Member

    Obviously, for a keel an AoA of 13-15 is of no consequence, but I would prefer a CLmax 17% higher for the rudder, for sure. And if a polished surface is not needed, that does save you quite a bit of sanding hours...
     
  7. tunnels

    tunnels Previous Member

    But what about the direction its sanded fore and aft or up and down ????:confused:

    Ok how about round and round !!!!Ah but what the dia of round and round ?? big ,small, oval ,eliptical ,What ??
     
  8. Joakim
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    Joakim Senior Member

    I don't think the Laser keel paper really showed accurate enough values for drag to really see is there an effect or not at low AoA. The other paper used much higher Re, but still not high enough for a hull. And they did see some change in the drag.

    I don't think you can really get both. If the roughness has a positive effect on Cl at high AoA, it will have a negative effect on Cd at low AoA at least at a bit higher speed.

    Here are some (very old) papers about the subject. They report only negative results from added roughness:
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930092760_1993092760.pdf
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930092798_1993092798.pdf
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930092756_1993092756.pdf
     
  9. Erwan
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    Erwan Senior Member

    DCockey,

    Sorry, nothing interesting to add on rugosity, Just want to answer quickly your question

    Regarding Induced Drag, I calculate with the very basic formula, both for the sail and foils, the goal is to have an idea of the relative magnitude of the different drags.

    Regards Everybody

    EK
     
  10. DCockey
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    DCockey Senior Member

    The change in drag of lifting surfaces such as sails and foils is due to both induced drag from trailing vorticity which is estimated by the simple formula using aspect ratio, and by changes in skin friction due to changes in the velocities.
     
  11. Erwan
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    Erwan Senior Member

    Thanks David,

    Yes I do not account for the change in skin friction due to change in velocities., I am not that sophisticated.

    The purpose is just to establish a proxy of the different drags for the average Joe fleet sailor, and some guidelines for the average Joe garage designer & builder.

    So of course it is not that accurate, but as I noticed for instance that most of sailors have no idea on how much is the sail induced drag compare to the foils induced drag, I guess it could be usefull at least for me.

    Regards

    EK
     
  12. DCockey
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    DCockey Senior Member

    Erwan,

    Depending on the aspect ratio and lift of a lifting surface, the increase of drag with angle of attack and lift due to viscous effects can be larger than the induced drag due to trailing vorticity. So the inviscid induced drag may be a very poor proxy for what actually happens.

    Look at "drag polar" curves (drag vs lift) for symmetric airfoil sections such as found in Theory of Wing Sections by Abbott and Doenhoff. Compare the increase in drag with lift shown for the 2D sections without any trailing vorticity with your calculated induced drag at the same lift coefficients.

    The term "induced drag" is sometimes used to include all drag increase due associated with lift production, not just the drag increase associated with trailing vorticity which is the "proper" definition. This causes confusion and can lead to errors in models because the simple formulas found for "induced drag" based on aspect ratio only include the drag increase associated with trailing vorticity.
     
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  13. Erwan
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    Erwan Senior Member

    David,

    Thanks, it s exactly what I used to do, for the total drag, I get the Cd from XFOIL and add the induced drag with the classic formula.

    I guess it's accurate enough for my purpose.

    Regards

    EK
     
  14. DCockey
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    DCockey Senior Member

    Sounds good as long as you are getting the Cd from XFOIL for the sectional lift coefficient, not Cd for zero lift.
     

  15. Erwan
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    Erwan Senior Member

    Thanks for coaching David
     
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