resistance scaling

Discussion in 'Hydrodynamics and Aerodynamics' started by previah, Jan 19, 2012.

  1. previah
    Joined: Mar 2004
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: van hasseltlaan

    previah Junior Member

    Thanks for the responses guys! Obviously there are a lot of different opinions on this subject. What I have in mind is to use the admiralty formula to estimate the resistance:

    R ~ disp^(2/3) x V^2

    Thus:

    R2 = R1 x (disp2/disp1)^(2/3) x (V2/V1)^2

    Assuming V2 = V1 (in reality V2 < V1) then:

    R2 = R1 x (disp2/disp1)^(2/3)

    Obviously the real answer is far more complicated than this. But can this be used as a first quick and dirty estimate?
     
  2. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Only if there is leeway/angle of attack, assuming the usual situation of lateral symmetry.
     
  3. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 155, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    Induced drag is not relevant to this discussion.

    Using a surface-piercing parabolic strut allows the wave resistance to be
    calculated analytically so that the effects of draft (and hence displacement) can be easily discerned. .

    There is a fairly detailed discussion and examples using "Tuck's Strut" in Chapter 4, Section 3 of:
    "Hydrodynamics of high-speed marine vehicles"
    Faltinsen, O.M.
     
  4. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 155, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    Just to be obnoxiously pedantic, that's only strictly true if there is no free-surface.
    With waves created by the hull (and/or ambient waves) there is a vertical
    component of velocity that can create vortices at the bottom of the strut, even at zero AoA.
     
  5. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    I assume you are talking about vortices due to separation along the tip. If so then a free surface is not necessary for a span-wise component of velocity. Such vortices can also occur at sharp chines, tight radius bilges, etc.
     
  6. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Also can be vortices at the root of the strut, etc due the boundary layer along the hull rolling up and separating. Again, no free surface necessary.
     
  7. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 155, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    No, I was talking about (potential) flow on the flat bottom of a strut.
    I take your (equally pedantic) point about separation and other viscous effects. :)
     
  8. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    No vortices are generated in purely inviscid (theoretical) flow. There needs to be at least vanishingly small viscosity for separation to occur and for vortices form. There are several types of separation in three dimensions. One is essentially the same as two dimensional separation with the velocity on/near the surface going to zero and a "bubble" forming. Another is when the streamlines on/near the surface lift off and form trailing vortices without a "bubble". Classic case is along the leading edge of a delta wing at higher angles of attack. It can also occur on a tip of a wing with finite thickness even when there isn't an angle of attack or "lift".
     
    Last edited: Jan 20, 2012
  9. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 155, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member


    To test your idea, see:
    Robards, Simon William, "The hydrodynamics of high-speed transom-stern vessels"
    http://unsworks.unsw.edu.au/vital/access/manager/Repository/unsworks:3426
    Appendix D contains the resistance graphs.

    Specific resistance (Rt/Weight) for a variety of drafts and displacements
    is shown as a function of Froude number for many hull series (e.g.
    NPL, NOVA-I, II, III and IV, D-Series, Sklad, Series 63 etc.)

    Good luck!
    Leo.
     
  10. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 155, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    I consider the flow with a vertical component on the bottom of the (cut-off) strut to be similar to the flow over a low-aspect ratio wing at AoA. The bottom of a parabolic strut is like a lenticular wing, i.e. with a pointy leading edge at midspan and rounded wingtips. Therefore, there is a small amount of lift, and hence there will be induced drag due to wingtip vortices.
    There's no need for viscosity here.

    I'm happy to be disabused of this opinion, though!
     

    Attached Files:

  11. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    To expand a little further about potential flow, vorticity, and viscosity.

    For any object in a potential flow field, there is a potential flow solution which has continuous velocity potential everywhere exterior of the object and no trailing or other vorticity. However if there are zero radius corners/edges on the surface of the object then the velocities of that solution will be infinite at the zero radius corners/edges except for the special cases such as the flow aligned with the edge or the edge being a stagnation location. I'll stress that this is a theoretical solution and may or may not have physical significance.

    Infinite velocities don't actually occur so the theoretcial solution needs to be reconciled with reality. One way to do this is to introduce a sheet of vorticity which originates at the surface where the velocity is infinite and goes downstream to infinity. Outside of the sheet the flow is still irrotational (no vorticity). By adjusting the strength and distribution of vorticity in the sheet the infinite velocity can be made finite, and in fact there will not be any flow across the edge. This is one way to describe the Kutta condition.

    So why don't infinite velocities at sharp corners/edges occur in reality. The answer is viscosity. Any amount of viscosity, even an a tiny, tiny bit, would cause infinite stresses which leads to separation. The vorticity sheet is the idealization of the separation as the visicosity becomes vanishingly small.
     
  12. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    If you are talking about "leading edge suction" and the like that will have to wait. We're headed out to a concert. But I will say that can also be related to vanishingly small viscosity.
     
  13. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 155, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    No, I wasn't talking about LE suction.
    I over-thought myself into a mistake by imagining the (flat) bottom of a strut
    as a lifting surface. Of course, there is no pressure difference as with a real
    wing.

    I understand your points about vanishing viscosity.
    Without viscosity there can be no starting vortex so planes could not take off, let alone fly. Verified by experiments in zero-viscosity, super-cooled helium, I believe.
     
  14. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    A with a flat bottom, even a prismatic constant cross-section strut, would have flow across the edges between the sides of the strut and the bottom. If the edges are sufficiently "sharp" then in the real world (as opposed to the ideal mathematical world) separation would occur along the edges and could result in trailing vorticity.
     

  15. previah
    Joined: Mar 2004
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: van hasseltlaan

    previah Junior Member

    Leo,
    Thanx for the link. Unfortunately the data are for high speed vessel with Fn > 0.2, while the range I'm interested is for Fn < 0.2 instead. Moreover the admiralty formula, I think, is only applicable for low speed.

    Best regards,
    -Arman-

     
Loading...
Similar Threads
  1. Furkan
    Replies:
    2
    Views:
    633
  2. Ousmane
    Replies:
    22
    Views:
    1,963
  3. zstine
    Replies:
    22
    Views:
    3,875
  4. zstine
    Replies:
    8
    Views:
    1,663
  5. Furkan
    Replies:
    7
    Views:
    1,881
  6. Leo Ambtman
    Replies:
    24
    Views:
    4,254
  7. Claudio Valerio Parboni
    Replies:
    3
    Views:
    1,369
  8. dustman
    Replies:
    78
    Views:
    7,449
  9. Surfer Naval Architect
    Replies:
    4
    Views:
    1,566
  10. anuprdk
    Replies:
    3
    Views:
    1,900
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.