Re: HydroDynamic Lifting=BS

Bill Atkinson <[EMAIL PROTECTED]> writes
> Carl Douglas <[EMAIL PROTECTED]> wrote
> > Jon Anderson
> > <[EMAIL PROTECTED]> writes
> > >marco wrote:
> > >> So nobody that actually think that HDL is just BS?
> > >
> > >If you think very carefully it becomes clear there *has* to be some.
> > >You can argue over how much use it is, but if I throw a plate through
> > >the air it generates lift. If I get an oar and push it through the
> > >water like this:
> > >                  __________
> > >-----------------/          |
> > >----------------\           |  =====>>>>>
> > >                 \__________|
> > >
> > >then you're going to get some lift too. You need to read this in the
> > >right type of font to see my drawing.
> > >
> >
> > But some prefer to assume that hydrodynamic lift is irrelevant to
> > rowing. Having devoted quite a lot of time to practical experimentation,
> > & having put the assumptions of others to test, I consider lift very
> > relevant to rowing propulsion.
> >
> > I'm sorry, Marco, that I'm too busy right now to engage myself, once
> > more & purely for the fun of so doing, in this knotty little topic.  But
> > I do not share the excellent Bill Atkinson's relative disregard for
> > hydrodynamic lift in rowing propulsion.  At the Reynolds numbers (ratio
> > of inertial to viscous forces) & angles of attack at which oar blades
> > work near catch & finish blades would certainly be expected to develop
> > hydrodynamic lift.  So it is not reasonable to discount it, to make
> > broad assumptions about lift coefficients, or to play down the adverse
> > consequences of sub-optimal lift/drag characteristics resulting from
> > inappropriate blade curvatures & alignments.  Such issues may very well
> > affect the blades we use right now - after all, they are still only
> > slightly more advanced than flattened sticks ;^).
> >
> > I will gently suggest, for now, that those who doubt the relevance of
> > hydrodynamic lift to oared propulsion should, just for a start,
> > seriously experiment with oarblades of different curvatures.  Flat
> > blades are not difficult to construct.  Nor is it hard to mount a blade
> > back-to-front on an oarshaft, or reverse an oarsleeve.  You may learn
> > quite a lot.  You could also play with different axial alignments of the
> > blades.
> >
> > Go give it a try, & report back here.  As I did when I went out to test
> > the Krew-Klips I had formerly criticised - a lot of swimming (& 2 weeks
> > of an ear infection to follow that).  And I reported to RSR that they
> > did, contrary to my earlier opinion, work OK.
> >
> > Cheers-
> > Carl
>
> Carl et al:
>  I'm happy to see the lift/drag discussion slowly shearing away from
> mysticism and conventional wisdom.
>  Carl is on the right course- that of actually getting out "there"
> with the experimental flattened sticks, measuring means, and lots of
> frustrating time and work.  The problem is that the measuring part- if
> done conscienciously- is extermely equipment intensive and time
> consuming and, hence, very expensive.  Unless one is supported by an
> entity like the Australian Institute of Sport, some well financed
> Olympic hopefuls, or a university department of Applied Physics with
> deep pockest this research will probably never be done.

I respect your point, Bill, but think you exaggerate the difficulties & 
cost of such studies.  If we will not do the necessary research, whether 
in physical reality or by mathematical modelling in virtual fluids 
(which may well be harder to do with any confidence for so complex a 
topic), then we remain in the land of hearsay & supposition, in which 
only the loudest, not necessarily the wisest, voices are heard.

>  By way of clarification of my views on lift (and drag) are as
> follows:
>  1. HDL, as Carl emphasizes, is certainly real.
>  2. Counterintuitively, though, lift has no direct effect on the
> propulsive force exerted on the water via the rower's effort on the
> oarhandle <http://www.atkinsopht.com/row/propforc.htm>.
>  3. What lift DOES effect is the SLIP (and consequent efficiency) of
> the blade during the stroke.
>  4. The reason that lift has little effect in the first moments after
> the catch is twofold.  First, owing to the small catch bow angle the
> attack velocity, while initially relatively high, is countered by a
> small attack angle (initially zero).  Second, the attack angle depends
> upon the slip (produced by the rower's effort) but, until that effort
> builds up to significance, the attack angle remains small

There I must fundamentally disagree (sorry Bill!).  A lifting foil 
automatically adjusts its angle of attack to the load applied, so that 
the lift generated always matches the load applied.

I would further note that the entry is not instantaneous (please recall 
the recent thread on backsplash).  The blade is immersed over a finite 
time, during which the load continuously develops & the AOA 
correspondingly adjusts itself.

Further:  The oarblade prescribes a complex 3-D path into the water at 
the catch.  It is moving rapidly downwards as well as tip-forwards & 
swinging outwards.  So it makes an entry both with its lower edge & with 
its tip (effectively diagonally).  The vertical motion peaks around 
3.5m/sec, as does the velocity approximately axial with the oarshaft. 
And the back of the oar is curved in both planes, which helps the 
development of lift as the net fluid flow over the back of the blade 
continuously adjusts itself.

Add to which - all of this is occurring in a transient, not 
fully-developed, flow regime, in which lift coefficients may be very 
high.

Mmmm.

> <http://www.atkinsopht.com/row/liftdrag.htm> Fig. 4-7.
>  5. No one would argue that, near mid-stroke, drag did not play a
> significant role.  High drag (large surface) means low slip.
>  Let's try to foster a climate of experimentation!

But there I could not agree with Bill more.  Why is it that we work so 
darned hard, yet make negligible effort to understand where all that 
effort goes, & whether we could improve the way we apply it & the tools 
with which we apply it?

Cheers -
Carl
--
Carl Douglas Racing Shells        -
    Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
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