Atmospheric surface layer profile

Discussion in 'Hydrodynamics and Aerodynamics' started by fastwave, Jun 4, 2020.

  1. fastwave
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    fastwave Senior Member

    Hi guys,
    I am wondering if anyone has come across literature or measurements of the wind profile For the first meter above the ground.
    I am aware about the log and power laws that are somewhat useable for large yachts.
    However my view is that the lowest couple of meters cannot be approximated using those models. Turbulent mixing is probably dominant and any attempt to model it is probably useless since it depends on so many conditions.
    However humans are a strange species and they try anyway.
    Any experimental measurements would help at least confirm some of my thoughts.
    Thank you
     
  2. jehardiman
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    jehardiman Senior Member

    Are we looking for it over smooth water, waves, or terrain? I know some of that is in Applied Fluid Dynamics Handbook by Robert D. Blevins · 2003.
     
  3. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Many years ago I was building aerohydrofoil models with the wings arranged to cancel the heeling moment. As a part of that I did measurements of the wind-profile up to 3.5 m above the water surface, since this was quite critical for the heeling moment cancellation of that configuration.
    My test-rig was a floating buoy with a 4 m high mast equipped with 4 vertical M-4 threaded rods. On those rods I attached nuts with cups on sprockets, that moved downwards as they rotated in the wind. I calibrated the sensors against an integrating wind-meter at a few different wind-speeds. They were quite linear. During the measurements I let them integrate for a minute or so. I think they moved some 50 to 100 mm downwards during each measurement.
    I could not find my report now, but I think I have it somewhere. But I remember the velocity profiles were usually pretty much like a turbulent boundary layer at zero pressure-gradient, with the viscous sublayer below the lowest sensor at 0.5 m height. On some occasion after a major windshift I got a different velocity profile.
     
  4. fastwave
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    fastwave Senior Member

    Hi Alexander,
    Thank you for the reply. That’s the kind of data/input I am looking for.
    Some vehicles operate in that 1m of the layer, eg rc yachts or racecars. I was pretty certain that it would not be worth using classic laws in those cases but some data would help come up with the best guess. Still wrong but hopefully better compared to uniform flow.
    Thank you for your reply
     
  5. AlexanderSahlin
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    AlexanderSahlin Junior Member

    I found my protocol from 1982! Measurements were taken at Möja Söderfjärd 200 m or more from land. Excuse me for the swedish, but the velocity profiles are quite clear I think. windspeed in m/sec.
     

    Attached Files:

  6. fastwave
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    fastwave Senior Member

    Fantastic! That will help me for sure with some initial thoughts.
    I also love the measuring approach. I have never seen anything like it. It made me smile and wish I had a different job.

    Did you correct the height as the cups rotated themselves lower :p

    Thank you.
     
  7. AlexanderSahlin
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    AlexanderSahlin Junior Member

    The heights in my plots were the averages during each measurement. But you ask for the lowest meter of the boundary layer, and my lowest sensor was at average 0.4 m above the water and the next sensor 1.4 m above the water. So I suggest you make a similar series of measurements in the range where you are going to sail, and report your measurements here. As you see it can be done with quite simple equipment :) Good Luck!
     
  8. Doug Halsey
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    Doug Halsey Senior Member

    I would be very interested in any conclusions you might have reached about the best sail designs for operating so close to the surface!
     
  9. fastwave
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    fastwave Senior Member

    Hi Alexander,
    I have plotted on your data together non-dimensionalised by the wind speed at 3.4m height.
    • The red line represents your average measurements.
    • The Blue line is Davenports simple power law with a coeff of 0.15. This is in line with open water coefficients.
    A very simple analysis which still does not really give more insight but it was helpful.
    Choosing a design with a wide operating window is important as always.
    Thank you
    upload_2020-6-8_10-28-39.png
     

    Attached Files:

  10. fastwave
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    fastwave Senior Member

    Hi Doug,
    the difference I see has nothing to do with the size of the boat and the height of the mast. It has to do more with the realtive boat speed to the wind speed.
    The small yachts reach hull speed much earlier which means the twist in the AWA profile due to the boat speed is very little.
    This is similar to a high perfomance full scale yacht to a heavy tradition cruiser.
    This makes a difference to the entry angles of your sections and your twist distribution.
     
  11. AlexanderSahlin
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    AlexanderSahlin Junior Member

    I think I shall give a few comments on the conditions when I measured. During the first measurement I had a 180° windshift, the first 3 measurements were also taken some 200 m leeward from land. With wind-directions ONO, O and SO I had more than 1 nm undisturbed wind from Möja Söderfjärd. Yes my measurements agree quite well with the Davenport power-law. The measurements were taken on 6 different days, sometimes clear sky, sometimes cloudy. And I never got the kind of velocity distribution you may expect in a stable layered atmosphere, as reported in e.g. C.A. Marchaj, Sailing Theory and Practice, page 371 for light wind and overcast sky. See my measurements 9, 10, 11, that were taken under overcast sky.
     
  12. Dejay
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    Dejay Senior Newbie

    Not sure if helpful but I found this:


    Looks like it's turtles all the way down!
     
    AlexanderSahlin likes this.
  13. AlexanderSahlin
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    AlexanderSahlin Junior Member

    My measurements agree quite well with a turbulent boundary-layer, as described by the Davenport power-law, also this near the water-surface. So the windspeed near the hull is about 75 % of that at the masthead. See fastwave's and my previous comments.
    There are many constraints to think about when you choose the lift-distribution of a sail. You can e.g. design for a limited heeling moment, or as in the case for our Paravane-Speedsailer, when the heeling moment was canceled, you try to get the most power from the wind with a mast-height that is practical to handle. Such a distribution will be a little wider at the middle than at the bottom, quite close to an ellipse, where you cut off the lowest part.
    On the Paravane-Speedsailer however, I designed the wing rectangular up to 75 % and tapered in the top. The wing was twisted to get uniform angle of attack when sailing 3.5 times the true windspeed. I think this was not so very far from the optimum distribution.
    When I did the wind-measurements, I was thinking about an aerohydrofoil, that was somewhat sensitive for the wind-gradient. But my conclusion from the measurements were that I would be able to handle the wind-gradient by ailerons on that aerohydrofoil.
     
  14. Doug Halsey
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    Doug Halsey Senior Member

    But not all "small yachts" are slow. The foiling Mini40's of Ian Holt (& others) are registering speeds approaching 20 knots and, with masts of only 2 meters or less, they're operating almost entirely in the region with the steepest & most rapidly changing gradients. The effect on them must be considerably more important than on most larger boats. Don't you think?
     

  15. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Yes, you can build models, that sail very fast relative the true wind. I tested a model of our Paravane Speedsailer with a 3m high mast, that sailed at 20 knots, or twice the true windspeed. Also Sailrocket did lots of model testing at high speed before they built their world-record craft at full scale.
    But my measurements indicate that the wind-gradient relative the mast-height is approximately the same for a 3 m high model as for a full-scale boat.
     
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