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Robert January 21, 2007 17:52

velocity of air
Hi everybody:

Please fogive me but I am not a scientist or a student and I am losing my brain very quickly. I am not interested in the mathematical process but would like your feedback in laymans terms.

I need to know the velocity (ft per sec) of one cubic meter of air in a 10' column of water.

For simplification purposes please consider the following details:

Water and air temp are the same: 70 degrees F Water is filtered clean (not salt) For purposes of figuring drag please assume the bubble keeps the shape of a sphere.

I know this a complex formula and am at my wits end.

I'd be eternally grateful if someone could help.


mettler January 22, 2007 12:17

Re: velocity of air
are you trying to measure the velocity of a bubble of known volume in a column of water?

Robert January 22, 2007 12:41

Re: velocity of air
Thanks for your interest. Yes one cubic meter of air in a column of water not deeper than 10 feet;

Since I know a lot of the bottom line deals with drag, I'd like to simpliy the equation by confining the meter of air in a perfect (imaginary) sphere

Can you help?

mettler January 22, 2007 14:25

Re: velocity of air
your pressure that the bubble sees at it rises will be a function of depth. It is not a difficult problem. Look into buoyancy.

Robert January 22, 2007 19:42

Re: velocity of air
Thanks for your interest..would you take a look at these calculations and let me know if you think they are valid and conclusive:

There are lot of factors include in the speed of air bubble in water in which it depends. As the volume of required bubble is too large so it is mobile surface of bubble. Which moves in oscillation shape and trajectory. Density of fluid takes main role in the movement of bubble higher the density the velocity will be more Viscosity also has some role in the air bubble velocity. Viscosity is a resisting force for the air bubble. That's why in more viscous fluid air bubble velocity is less, bubble move fast in the less viscous fluid. Due to temperature fluctuation it's very minor change in the velocity of bubble so velocity of bubble is less depend upon temperature. Velocity is also depends upon the force of gravity .The direction of air bubble towards the acceleration so it is vertical. In clean water air bubbles move faster than the contaminated water. The value of k for clean water is 0 and it is standard.

The velocity of one cubic foot bubble is as under. Density of clean water between 70 to 80 F is 0.9982 g / cm 2. Kinematics viscosity of water is 0.01 .

V = 2*g*density ( 1+k ) / 3*r2*viscosity (2+3k ) K=0 Volume of bubble = 4/3*pi*r3 Volume=28316 cm3 28316=4/3*3.14*r3 R=18.91cm Density=0.9982 Viscosity=0.01 V=2*980*0.9982/3*357.5*0.01*2 Velocity=91.21 cm/s = 2.992 ft/s

The velocity of one cubic meter bubble is as under. Volume=1m3=1000000 cm3 R3=1000000*3/4*3.14 R=62.03 cm V=2*980*0.998 / 3*3849*0.01*2 Velocity=8.47cm/s Velocity=0.277 ft/s

Hence the velocities of both bubbles are not equal. As the size of bubble increases the velocity decreases. We conclude that many forces are acting at the surface of bubble in which it moves upward.

I am not a scientist so that's why I'm asking you guys with brains to tell me if I'm on the right track.

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