Buoyancy Inverted

[dumpsterDiver]

I might suggest that TSandM's experience is due to a small pressure differential between her lungs and her mouth/reg (assuming a perfectly horizontal posture, her reg will be higher than the mean of her lungs) which forces a subtly larger volume from her lungs upon exhalation (and a smaller volume into her lungs with inhalation) when on her back as opposed to on her front. This might make her less buoyant on her back.

As for being less buoyant feet-up vs. feet-down, I cannot rationalize this unless the act of inverting ones self drives one's BC slightly deeper, increasing compression.

Have fun diving!

I was just going to post the same suggestion. I myself never noticed this to occur, but it makes sense.

 

[Herk_Man]

1. Gravity's pull does not change based on orientation.

2. The effects of drag on the descent rate of the diver are going to be almost nil at the speeds we are talking about.

3. Either buoyancy is changing due to breathing or the diver is creating some sort propulsion (either to stop a descent when upright or creating a descent when inverted.)

 

[The Kraken]

OK, let's make it simple.

Let's leave out the air resistance with respect to this problem, it's insignificant to the equation.

Given: the diver's lungs are equally filled on each "dive" or "jump" (therefore the equilibrium of forces, to be construed in this instance as "buoyancy", remain constant)

Given: the diver's weight has not changed

Given: the diver's specific gravity has not changed

Given: the diver's specific volume has not changed

Given: gravity is a constant

Given: water has a specific gravity of 1

et ceterus paribus (all things being equal)

You jump off a 10 meter dive platform.

You go in head first (read "dive"), you go down several feet.

You jump off that same 10 meter dive platform and do a "belly flop", you may go down a foot or so.

Nothing has changed EXCEPT the cross sectional surface area presented to the fluid in which the object is suspended.

Resistance.

Resistance is an intergral function of buoyancy.

the K

 

[TSandM]

But resistance is a function of velocity, too; if I'm completely at rest, not moving, and perfectly balanced, and then I roll over onto my back and sink, where does the change in resistance come in?

 

[LeadTurn_SD]

But resistance is a function of velocity, too; if I'm completely at rest, not moving, and perfectly balanced, and then I roll over onto my back and sink, where does the change in resistance come in?

That has me puzzled.

To my vast shame, I really should be able to answer this question since Naval Architecture was a required course (that I struggled with ).

I wonder if the shape of a partially-filled wing chages enough as you roll "on top" of it to change its surface area.... does a "taco-ed" wing in a prone (belly down) postion present more surface area than a wing that flattens itself against your back when you roll into a supine (belly up) position?

I have no idea, and I really should know this.....

Best wishes.

 

[seaducer]

But resistance is a function of velocity, too; if I'm completely at rest, not moving, and perfectly balanced, and then I roll over onto my back and sink, where does the change in resistance come in?

The shape of the tank(s) vs your body? It seems to me a rounded object of similar mass sinks faster than a flat one.

Could it be from some other factor like a change in breathing pattern or lung volumes?

I'm no physicist...

 

[seaducer]

As for being less buoyant feet-up vs. feet-down, I cannot rationalize this unless the act of inverting ones self drives one's BC slightly deeper, increasing compression.

We used to do this trick all the time at the aquarium I dive at. I never noticed a change in bouyancy, only continued movement from the force of the manuever that placed me head down. A quick full inhalation stopped the descent and then normal breathing held me neutral.

 

[dumpsterDiver]

OK, let's make it simple.

Let's leave out the air resistance with respect to this problem, it's insignificant to the equation.

Given: the diver's lungs are equally filled on each "dive" or "jump" (therefore the equilibrium of forces, to be construed in this instance as "buoyancy", remain constant)

Given: the diver's weight has not changed

Given: the diver's specific gravity has not changed

Given: the diver's specific volume has not changed

Given: gravity is a constant

Given: water has a specific gravity of 1

et ceterus paribus (all things being equal)

You jump off a 10 meter dive platform.

You go in head first (read "dive"), you go down several feet.

You jump off that same 10 meter dive platform and do a "belly flop", you may go down a foot or so.

Nothing has changed EXCEPT the cross sectional surface area presented to the fluid in which the object is suspended.

Resistance.

Resistance is an intergral function of buoyancy.the K

Is this a joke or what? The depth of submergence of a springboard diver has very little to do with bouyancy and everything to do with drag coefficient. Buoyancy and "resistance" or hydrodynamic drag are independent.


The short answer is that upside down the diver probably inhales less deeply due to the pressure differential between the lungs and the second stage delivery pressure.

 

[The Kraken]

But resistance is a function of velocity, too; if I'm completely at rest, not moving, and perfectly balanced, and then I roll over onto my back and sink, where does the change in resistance come in?

The surface area of the object (you) presented to the resisting fluid in the water column.

OK. Let's just say that in a spread eagle position you are neutrally buoyant.

In this spread eagle position you present 1000 square inches of area against the downward pull of gravity in the water column and you are neutrally buoyant.


Now you relocate your position with your feet up and your head down. For the sake of argument let us say that now you present only 100 square inches of area against the downward pull of gravity.

Nothing else has changed.

You will sink. Inertia at rest, if you would, but not quite.

A good analogy:

Imagine you are pressing your fist against a piece of dry wall at X pounds of force.

Your fist will not break through.

Now you press your finger tip against that piece of dry wall with the same amount of force. Your finger tip WILL break through.

Why? Square inches of resistance.

Let's try a block of ice. You take a piece of wood 1" x 1" and press against that block of ice with a force of 100 lbs.

Aint gonnna happen. You will not make a dent.

But you take an ice pick and apply that same 100 lbs. of force, there's quite a remarkable difference of penetration.

It's a matter of applied surface area and force.

But in our case it's a matter of surface area and resistance.

the K

 

[The Kraken]

Is this a joke or what? The depth of submergence of a springboard diver has very little to do with bouyancy and everything to do with drag coefficient. Buoyancy and "resistance" or hydrodynamic drag are independent.


The short answer is that upside down the diver probably inhales less deeply due to the pressure differential between the lungs and the second stage delivery pressure.

You a gamblin man???

the K