Friction Coefficient

Discussion in 'Hydrodynamics and Aerodynamics' started by jesdreamer, Sep 30, 2015.

  1. jesdreamer
    Joined: Jan 2013
    Posts: 88
    Likes: 1, Points: 0, Legacy Rep: 10
    Location: Alabama

    jesdreamer Junior Member

    Sucking down reduces area????

    I just had to question Phil/sweet's statement above -- He is responding to my questionable observation of YouTube kayak racers seeming to exhibit some slight but noticeable increase in draft at speed vs when stationary -- his response is "yes" and then he goes on to discuss

    I really don't understand his following statement....." surface sucking down and reducing the apparent area of the hull"

    It seems to me that with some typical flare in freeboard, if there was to be any "sucking down" action, that it would increase wetted area. Am I wrong, or do I just not understand the statement (I usually do understand PhilSweet's other comments, but this one bothered me)
     
  2. jesdreamer
    Joined: Jan 2013
    Posts: 88
    Likes: 1, Points: 0, Legacy Rep: 10
    Location: Alabama

    jesdreamer Junior Member

    Bow rise due to paddle pressure at lever arm??

    I feel some disagreement is still warranted -- I see the main reason for the kneeling position and the support leg forward as to provide a reaction moment to stabilize the hull and specifically restrict the hull from reacting in the way Phil is describing. I don't disagree with his paddle moment effect but I do feel there is an intentional or non-intentional equal and opposite reaction being applied by the paddler himself.

    As another observation, per the "hull sinking into the trough behind bow wave" as shown in Joakim's sketches in post #34 -- I see no evidence of such action in the YouTube videos of either 2-blade kayak or single blade canoe races referenced. We all know these paddlers are exceeding hull speed but I see no trough, no trailing wave, and in fact, little or no bow wave -- as noted earlier the bow rise condition seems related to the single blade canoe racer and the double blade kayak racers with their obviously more evenly applied propelling force seem to not show this effect at all --
     
  3. philSweet
    Joined: May 2008
    Posts: 2,258
    Likes: 150, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Using Bernoulli, since the water at the surface always at atmospheric pressure, it can't speed up. So if you had a sub, the water close by the hull amidship would be moving faster than the free stream. But in a surface ship, this effect reduces to 0 as you move from the keel to the surface.

    <edit> The above takes the view that the surface is flat and gravity plays no roll. That isn't consistent with what I wrote about the surface sucking down. The reality is that the flow curves around the hull, but near the surface, it also curves to form waves. Changes in the surface height can change the flow velocity, even of the surface.
     
  4. daiquiri
    Joined: May 2004
    Posts: 5,372
    Likes: 239, Points: 73, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Phil, with no offence intended, I am really not getting the sense of the above post and I fail to relate it to this discussion.

    It is all included in the Bernoulli equation - the dynamic, the static and the hydrostatic pressures. One cannot assume a flat surface, except for very special cases and flow regimes.

    Looks like the first half of your post is lacking the context. Otherwise, the post makes no sense to me.
     
  5. philSweet
    Joined: May 2008
    Posts: 2,258
    Likes: 150, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    The Op was relating a hull to a venturi, which is fine. But a venturi is usually taken as bounded all round, no free surface. A constant mass flow through the venturi reveals an accelerated average velocity, and the curved streamline theory can provide a "transverse" profile. I put transverse in quotes because except at the neck, the profile will be along a curved cut.

    If you take a solid of revolution like a submarine, you can do the same thing. You can calculate the flow velocity at any point x along the length, and it will be the same all around the girth.

    On a surface ship, the velocity is not the same along a girth even if the shape is a solid of revolution. Solving for this accurately is difficult. But it still comes down to visualizing the streamlines. If the pressure changes along the girth, you must be cutting across curved stream lines. So the flow is not parallel to the sides.

    I'm subtracting out the static pressure and farstream velocity, just looking at the velocity perturbation caused by the ship.

    For shallow craft in particular, more of this pressure relaxing gets distributed onto the bottom of the hull, not the sides, so it is more important in shallow craft. I'm typing while I'm eating lunch, so not a very detailed explaination. Back to work in a few.
     
  6. jesdreamer
    Joined: Jan 2013
    Posts: 88
    Likes: 1, Points: 0, Legacy Rep: 10
    Location: Alabama

    jesdreamer Junior Member

    When I began this thread I was not trying to solve anything accurately (as if such a thing could be possible in hull design), I was trying to gain a broader understanding of what all might be taking place as a hull travels through water -- I still feel the need for understanding cause and effect of aft suck-down, laminar vs turbulent, separation, wake hollow, induced drag, etc, etc. I feel that I do now have a better feel for viscous friction at or in boundary.
     
  7. Barry
    Joined: Mar 2002
    Posts: 1,215
    Likes: 117, Points: 63, Legacy Rep: 158

    Barry Senior Member

    Jesdreamer
    Are you suggesting that your terms "aft suck down" or "squat" are a result of lower pressures than atmospheric pressure in the rear area of the boat?

    Are you considering that your comments apply to planing and displacement hulls equally?
     
  8. DCockey
    Joined: Oct 2009
    Posts: 4,488
    Likes: 236, Points: 63, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    "Aft suck-down" is not caused by laminar vs turbulent flow, boundary layer thickness or similar. Also it is not caused by induced drag (drag due to trailing vorticity).
     
  9. jesdreamer
    Joined: Jan 2013
    Posts: 88
    Likes: 1, Points: 0, Legacy Rep: 10
    Location: Alabama

    jesdreamer Junior Member

    I am of the belief that additional draft greater than hydrostatic can occur when underway. And increased trim might relate to lower pressure under aft area with increased pressure at bow. (I realize that hollow between waves can also cause trim as historically related to hull speed).

    And I would expect this increased draft action to prevail in displacement as well as semi or full planing hulls (though not with equal effect due to distribution of wetted area, etc) -- disregarding prop angle forces or any forces other than hull traveling through water --

    So my answer would be Yes and Yes -- (but not necessarily equally)
     
  10. jesdreamer
    Joined: Jan 2013
    Posts: 88
    Likes: 1, Points: 0, Legacy Rep: 10
    Location: Alabama

    jesdreamer Junior Member

    Aft Suck-Down

    I never differentiated between laminar & turbulent, boundary layer, or any drag relating to a trailing vorticity -- I am of the feeling that increased water velocity along hull might be causing a reduced bouyancy pressure resulting in increased draft (which perhaps might show up toward aft due to pressure increase against projected bow) -- I am not referring to any dig-down action under sudden throttle from stationary in a high HP outboard or speedboat --

    DCockey might have different explaination for the effect -- Interested to hear it
     
  11. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 52, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member

    Here is Cp of a NACA 0012 foil at 0 AoA. Note how the lowest pressure is near the leading edge and at the trailing edge the pressure is already higher than "zero". The kayak hull will have somewhat similar velocity distribution and thus also pressure distrubution due to "bernoulli effect". But waves will make the pressure distribution different and the velocities will be much closer to far field (zero) since kayak is a narrow 3D object not a infinite 2D as NACA 0012 in the picture.

    As you notice from this velocity and pressure distribution, it would not cause an aft down trim.

    A kayak travels at about Fn=0.8 during sprint event. That is 2x hull speed. Then the bow wave is half of the LWL and thus the stern will be in the deepest part of the bow wave. I'm not very familiar how such a narrow hulls trim at those speeds, but more conventional boat hulls have about the highest trim angles then.

    To me it is quite obvious that the trim change of C1 during each stroke is due to weight and moment changes caused by the canoeist and not at all due to changes in velocity during each stroke.
     

    Attached Files:

  12. jesdreamer
    Joined: Jan 2013
    Posts: 88
    Likes: 1, Points: 0, Legacy Rep: 10
    Location: Alabama

    jesdreamer Junior Member

    I felt the front leg and foot position was intended to balance out the moment of paddle force down below hull level -- but maybe not entirely -- So I had thought perhaps sudden surge in velocity and possible related loss of rear end buoyancy could be causing the increased trim at each stroke. Now whether or not that is true and whether or not moment inballance is the cause, I think we might agree that this increase in trim at each stroke is bad. Would more wetted area at stern and thus more buoyancy back there help reduce increase in trim at each stroke?? I see this trim increase to mean that with each stroke, paddler is causing his own (short term) hill to climb similar to explanation of the problem at hull speed?? (I realize these paddlers are well above hull speed -- are they really riding the bow wave??)
     
  13. jesdreamer
    Joined: Jan 2013
    Posts: 88
    Likes: 1, Points: 0, Legacy Rep: 10
    Location: Alabama

    jesdreamer Junior Member

    Velocity related pressure??

    It seems to me that the NASA graph of pressure distribution along flow path across (upper half of) an aerofoil at zero angle of attack might be proving my point. In the NASA aerofoil case an increase in air velocity along the convex surface of aerofoil is yielding a reduction in fluid pressure normal to wing surface -- and this is cause of lift. I visualize similar effect on a hull in water except all is inverted and the resulting reduction in pressure might cause more draft (or trim depending on CG location) -- I realize there is little curvature along keel but there is significant curvature along immersed sides which can end up causing similar effect. Now since C1 paddler is already well above hull speed (in YouTube videos) and to some extent has front area riding the bow wave, I think the NASA data is explaining why aft hull might sink under the sudden surge forward during each stroke --

    I agree with Jokim's first sentence -- "The kayak hull will have somewhat similar velocity distribution and thus also pressure distribution due to "bernoulli effect".....

    But this 2nd sentence sounds like mumbo jumbo to me -- "But waves will make the pressure distribution different and the velocities will be much closer to far field (zero) since kayak is a narrow 3D object not a infinite 2D as NACA 0012 in the picture...." -- And I can't see why 2D wing analysis should have any different significance than a 3D hull in our current area of discussion

    Why is this train of thought wrong??
     
  14. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 52, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member

    In the simulation I made the NACA 0012 is 2D that is it is infinitely long perpendicular to the picture. Velcity is high where fluid is "pushed away" by the profile. That is when its thickness is close to the maximum and still increasing. As soon as thickness is constant the velocity starts to decreace and pressure starts to rise towards zero.

    Since this NACA 0012 is a 2D object fluid is "pushed" only in one direction (on one half of the profile). On a 3D object like a kayak hull, submarine, airplane fuselage etc. there is much more directions for the fluid to deflect.

    Imagine that 12 cm thick NACA 0012 is in a 24 cm high wind tunnel. At maximum thickness the free area is only 12/24 thus 50 % of the area and thus the average velocity is 2*V.

    Now take the same NACA 0012 profile as a rotated profile looking like a torpedo. It is still 12 cm thick and you put it in a 24 cm diameter cylindrical wind tunnel. Now the free are at the thickest poist is (12^2-6^2)/12^2 = 75% of the area and thus the average velocity is 1.33*V.

    Then you should also compare the profile of a kayak to that of a torpedo or a wing profile. Note how slowly the cross sectional area increases in a kayak compared to those other ones. This is to minimize waves. At the same time it makes velocity increase much smaller.

    Now take look at the pressure coefficient of an long and narrow AC yacht. Note how badly the Cp correlates to the Cp of a NACA 0012. Cp shows mostly the effect of waves (except for appendages).
     

    Attached Files:


  15. jesdreamer
    Joined: Jan 2013
    Posts: 88
    Likes: 1, Points: 0, Legacy Rep: 10
    Location: Alabama

    jesdreamer Junior Member

    Pressure reduction with increased valocity

    As I read through the above, I agreed with every point of the discussion. Also as I read through the post I got the Idea Joakim was really agreeing with everything I had said per velocity effect on draft and aft sinkage depending on CB and CG relative locations. So I get the impression he is agreeing with aft sinkage with increased velocity --

    Now what would happen if we moved paddler a little distance forward, since bow wave is already near midships with velocity around 2x hull speed, might we get the the hull to surf down the bow wave during each stroke instead of trying to climb a self-induced slope??
     
Loading...
Similar Threads
  1. dustman
    Replies:
    53
    Views:
    3,755
  2. rwatson
    Replies:
    34
    Views:
    2,864
  3. Slip
    Replies:
    3
    Views:
    951
  4. designing
    Replies:
    5
    Views:
    1,397
  5. nine6
    Replies:
    5
    Views:
    1,338
  6. Leo Lazauskas
    Replies:
    34
    Views:
    8,324
  7. hprasmus
    Replies:
    11
    Views:
    5,373
  8. Shayne Young
    Replies:
    12
    Views:
    2,552
  9. Leo Lazauskas
    Replies:
    50
    Views:
    7,419
  10. nimblemotors
    Replies:
    47
    Views:
    13,760
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.