Topic split from Sheck thread - water velocity

[aainslie]

The answer is horribly complicated, with long discussions about laminar or non-laminar flow, reynold's numbers, etc etc. It also makes a BIG difference if the pipe is "plugged" or not, the shape of the "plug", etc etc.

There's definitely no single answer to this one.

Back to the original question - the difference in resistance would obviously be huge, but it wouldn't be directly proportional to the densities of the two fluids. And it doesn't simply scale up as the speed goes up. Also the force changes hugely on the shape of the object, and the acceleration until the force is no longer felt is hugely different. Hence physiologically these are going to be two very different experiences.

One example - imagine if you change the shape, e.g. by putting your hand out. In the water flow situation, no major difference - a definite tug would be experienced but you'll survive. In the airflow situation with an "equivalent" airflow, i.e. one that gives the same sense of pressure, when you put your hand out it'll probably get ripped out of its socket! The difference is that it'll be accelerated to a much higher velocity.

So - MY conclusion is that there's no definite or simple answer to this one. You'd have to give the exact body shape to an aerodynamicist and let them program for weeks to get an answer. And I stick with my preferred location - whether in an open passage OR as a plug!

More importantly - where is this "tube"? Sounds like laying a reel there could be... challenging! Edit - ah, Weeki Watchee. I get it now...

Another edit - does the effort required to move aginst this flow go up linearly or not? This is of course the more interesting question. The answer is, no, and here's a nice little discussion: http://en.wikipedia.org/wiki/Drag_%28physics%29

[phreaticus]

I understand the horrible complications, mathematicaly...yet... I would really like to know an approximate "resistance" number of a diver progressing in said 6 ft. diameter tube (obviously, you would NOT be swimming!)...let's say a diver with a four foot X two foot cross section (this would be a pretty skinny cave diver...not me!).
Whats the answer?
BTW...I remember seeing a article regarding this same question, based upon the flow in Little River, (when it really had flow!) waaaaaay back in the early 70's in an NACD conference paper...I think it was Tom Mount as the author....but again...brain cells fail me.....
Forest....you were there....you may have recolection?

[Kelly Jessop]

I understand the horrible complications, mathematicaly...yet... I would really like to know an approximate "resistance" number of a diver progressing in said 6 ft. diameter tube (obviously, you would NOT be swimming!)...let's say a diver with a four foot X two foot cross section (this would be a pretty skinny cave diver...not me!).
Whats the answer?
BTW...I remember seeing a article regarding this same question, based upon the flow in Little River, (when it really had flow!) waaaaaay back in the early 70's in an NACD conference paper...I think it was Tom Mount as the author....but again...brain cells fail me.....
Forest....you were there....you may have recolection?

The article you want is "The Geometry of Phreatic Karst Conduits" by William Cate written in 1973

[FW]

BTW...I remember seeing a article regarding this same question, based upon the flow in Little River, (when it really had flow!) waaaaaay back in the early 70's in an NACD conference paper...I think it was Tom Mount as the author....but again...brain cells fail me.....
Forrest....you were there....you may have recolection?

I wasn't in the NACD back then, so I didn't see the article.

[Kelly Jessop]

There was a Wilson at that workshop in 1973 and 1974,but not a Forrest.

[OFG-1]

Generally, in one dimentional flow, there is a continuity relationship between Flow, Area, and Velocity such that
Q = AV
where Q - Flow, A - Cross Sectional Area and V = velocity

It actually is quite different in a cave because the cross sectional area is varying and the flow is not one dimensional, every time the cave changes direction, there are eddy currents set up that distort the shape of the flow. Also, the potential energy of the system changes (depth) in accordance with the gravity. And all those pretty rocks create friction.

If you scooter dive JB, at the C&L bypass and beyond you can see this effect on the floor as the mud gets shaped with hills and troughs that we can see in the bottom, but some run into anyhow and heave nasty scooter tracks.

There is a computer program, "Flowmaster" from Haestad Methods that can compute this for you, but you would spend most of your life entering the different shapes at each station.

Cheers

[mpoucher]

Just curious (I'm no math wiz.)....if you increase the flow in a tube that is, say, six feet in diameter, by 60 %...what does that do to water velocity?
I don't think it is linear...correct?

The answer is that the increase in velocity with increased flow is linear, so the velocity (ft/s) would increase by 60% with a 60% increase in flow (cubic feet/s). The formula is Flow=velocity x area.

The force of the flow goes up as the square of the velocity, so if you double the flow/velocity, the force of the water goes up four times. So if the flow increase by 60%, the force of the flow increases by a factor of 2.56 (1.6 squared).

[aainslie]

Just curious (I'm no math wiz.)....if you increase the flow in a tube that is, say, six feet in diameter, by 60 %...what does that do to water velocity?
I don't think it is linear...correct?

The answer is that the increase in velocity with increased flow is linear, so the velocity (ft/s) would increase by 60% with a 60% increase in flow (cubic feet/s). The formula is Flow=velocity x area.

The force of the flow goes up as the square of the velocity, so if you double the flow/velocity, the force of the water goes up four times. So if the flow increase by 60%, the force of the flow increases by a factor of 2.56 (1.6 squared).

...and, looking at the Wiki quote I put in above, the POWER required goes up as the CUBE, i.e. you need a scooter (or whatever you're using - more like a scooter version of a Cigarette boat that is 1.6^3 or roughly 4x as powerful.