Re: Bubble line and waves - should there be more?

KC wrote:
Carl Douglas wrote:

KC wrote:
<snip>

Yes surface tension would also drop significantly. Not sure how big a
factor that would be in boat speed though.


Errr... no. Surface tension, a feature of the air/water & boat/water
interfaces, would only drop if you added a suitable surfactant. I
didn't think we were contemplating a bubble bath here, although I 'spose
that might give us cleaner finishes ;)



Well, the actual surface tension may not change, but I would think some
sort of "effective" surface tension would change, similar to the fact
that the actual density of the water does not change, since the bubbles
mixed in with it are not water, they are air (nor are they even water
vapor as in the case of deep cavitation from submarines, etc.) With
normal calm water you can "float" a pin/needle on the water due to
surface tension. I would bet that with the bubbles, the pin would
sink. Thus, the "effective" surface tension is reduced.



The "floating" pin, spider, etc., or the waterproof fabric in the rain, or the rolling drop of water on dry ground, or the beads of mercury when you break a thermometer (we used to play with mercury when I was a kid, but not no more!) are all examples of where surface tension rules. However, it is all a matter of scale.

At the very small scale, surface tension can be very important, & surface tension still affects the ease with which a water surface breaks up, atomises or forms bubbles & foams (as you'd see if detergent were injected into the bubble curtain, or when oil is spread over a water surface), but at the normal scale in which we row its effects are dwarfed by those of viscosity, density & inertia. Thus when we scale to the situation of a moving boat we find surface tension has no detectable influence on the resistance to motion of the boat or, the other side of the picture, the water's movement around the boat.

Over a century ago, Osborne Reynolds demonstrated that you could scale many fluid flow phenomena purely on the basis of the ratio of dynamic to viscous forces - which is expressed in the dimensionless Reynolds number expressed as:
characteristic dimension x velocity x density / viscosity

Reynolds number similarity is still a fundamental basis for directly comparing fluid flow situations which at first seem entirely different in character (e.g. air flowing over a wing or sail vs water flowing around a rudder or oar-blade).

Nor is surface tension relevant to speed. The viscosity & density of
the water, yes, but not its surface tension.


Good to know, I suspected as much but wasn't sure.

-Kieran


But if you bubble air through the water you row on, then surface tension is significant as it strongly affects the formation & density of the resulting foam & its flow characteristics around the blade.

Water supports a shell because it is sufficiently dense for that crewed shell to be able to displace a mass of water equal to its own all-up mass without that level water surface coming so far up the sides that it flows in (Archimedes, around 2250 years ago). If you immerse a boat in a foam of ~ 1/3 water density, then you'll sink because the boat has insufficient volume to displace its own mass of the stuff. So a large ship can be sunk by passing through a region of water affected by a submarine gas vent (one of the Bermuda triangle hypotheses). You'll also encounter the joys of 2-phase flow.

That 2-phase (mixed air/water) situation completely alters the flow characteristics of the fluid as well as its bulk density. That means the blade nolonger generates anything like the lift or drag that it did in solid water. I think a new coaching technique & much different equipment may then be needed. It could be a whole new sport!

;)
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
Write: The Boathouse, Timsway, Chertsey Lane, Staines TW18 3JY, UK
Email: carl@xxxxxxxxxxxxxxxxx Tel: +44(0)1784-456344 Fax: -466550
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)
.

Relevant Pages

  • Re: The Value of Science: Explanation vs Prediction
    ... Together with surface tension, I suppose - consider a stream of falling ... water being "stretched" by gravity, like pulling taffy; ... and the flow becomes turbulent. ... As the jet speeds up it's surface area decreases, ...
    (talk.origins)
  • Re: Counter Current
    ... the physics of a boat moving through water, 'lumping' up water ahead of it, ... side of the boat called the 'Counter Current'. ... As you move that hole forward, of course the water has to flow around ... If you wanted to go faster in a narrow channel you might try cruising ...
    (uk.rec.waterways)
  • Re: catch confusion
    ... purpose isn't to flow through the water and generate lift, but to stick in the water and provide leverage for prying the boat forward. ... I'm no closer to understanding the "hydrodynamic blade lift" concept than I was before. ... Lift is generated when the flow of a fluid aver a surface results in a fall in that fluid's internal pressure. ...
    (rec.sport.rowing)
  • Re: catch confusion
    ... purpose isn't to flow through the water and generate lift, ... stick in the water and provide leverage for prying the boat forward. ... no closer to understanding the "hydrodynamic blade lift" concept than I ...
    (rec.sport.rowing)
  • Re: What is the maximum size for a drop of water?
    ... Surface tension holds it together, ... In free fall one can assemble some rather large water drop. ... there are no internal stresses at the limit, and the water needs to be ... be superseded by own gravity. ...
    (talk.origins)