CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Main CFD Forum

velocity of air

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   January 21, 2007, 17:52
Default velocity of air
  #1
Robert
Guest
 
Posts: n/a
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.

Thanks
  Reply With Quote

Old   January 22, 2007, 12:17
Default Re: velocity of air
  #2
mettler
Guest
 
Posts: n/a
are you trying to measure the velocity of a bubble of known volume in a column of water?
  Reply With Quote

Old   January 22, 2007, 12:41
Default Re: velocity of air
  #3
Robert
Guest
 
Posts: n/a
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?
  Reply With Quote

Old   January 22, 2007, 14:25
Default Re: velocity of air
  #4
mettler
Guest
 
Posts: n/a
your pressure that the bubble sees at it rises will be a function of depth. It is not a difficult problem. Look into buoyancy.
  Reply With Quote

Old   January 22, 2007, 19:42
Default Re: velocity of air
  #5
Robert
Guest
 
Posts: n/a
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.

  Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
problem with two phase flow (air injected in water) miles_davis OpenFOAM 14 October 11, 2011 18:53
UDF, air flow through a cell----get velocity in lig FLUENT 0 September 6, 2010 14:02
Inlet Velocity in CFX aeroman CFX 12 August 6, 2009 19:42
air bubble is disappear increasing time using vof xujjun CFX 9 June 9, 2009 08:59
the air velocity is not zero on the wall? Li Rong CFX 4 June 13, 2007 13:09


All times are GMT -4. The time now is 01:48.