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

Modelling free falling water into a air filled duct?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   August 23, 2010, 03:35
Default Modelling free falling water into a air filled duct?
  #1
New Member
 
Michael Sanders
Join Date: Aug 2010
Posts: 4
Rep Power: 6
sandmike_83 is on a distinguished road
Hello to everybody. I'am trying to represent using Ansys CFX a 10 m long duct initially filled with air. The duct has only one way in or out, a hole at the top through which water is introduced.

I was trying to simulate it using the surface model, with homogeneous model,transient, double precision,

I get two strange results:

1. When monitoring the water volume fraction in the duct I noticed that In the first time step (just when the water is invading the access hole) all the domain achieves a water volume fraction of 0.5. To me this sounds like a numerical problem, but I don't know if anyone has experienced something like this?

2. The velocity at which the water falls is more or less the same as the inlet velocity (2 m/s). Shouldn't it be close to the free falling speed of a waterfall (around 10 m/s)

Any advise is welcome
Thanks for your help

MS
sandmike_83 is offline   Reply With Quote

Old   August 23, 2010, 07:43
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,651
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Can you post some images?
ghorrocks is offline   Reply With Quote

Old   August 23, 2010, 08:19
Default
  #3
New Member
 
Michael Sanders
Join Date: Aug 2010
Posts: 4
Rep Power: 6
sandmike_83 is on a distinguished road
some pictures below, hope the quality is enough.
The velocity at the inlet is 2 m/s.
the duct diameter is around 2"
The duct lenght is around 10 m
The Mesh is not very refined, but enough for a first approximation

I am starting to believe that the first problem is related somehow with the given initial conditions. Seems to me that the solver is imposing a 0.5 volume fraction for the entire domain, ignoring the initial conditions file.



Now, for the second, no clue so far


Thanks for the possible help Mr Horrocks
Attached Images
File Type: jpg 0_01_s.jpg (15.3 KB, 33 views)
File Type: jpg 0_2_s.jpg (14.7 KB, 31 views)
File Type: jpg 0_2_s_(2).jpg (13.6 KB, 35 views)
sandmike_83 is offline   Reply With Quote

Old   August 23, 2010, 18:30
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,651
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Your first question is an initial conditions problem. Just define the IC as VF=0.

The second question is dodgy. Firstly, the flow will take some time to accelerate to terminal velocity. You will probably have to run it for longer. Secondly, do you know what terminal velocity is? It is a function of drop size, fluid density, drop shape and other parameters - and it is also a function of the surrounding air flow. Your flow is clearly different to your reference waterfall. Additionally, a waterfall will entrain an air gust with it so the water droplets see reduced drag and this will increase the terminal velocity.
ghorrocks is offline   Reply With Quote

Old   August 24, 2010, 03:27
Default
  #5
New Member
 
Michael Sanders
Join Date: Aug 2010
Posts: 4
Rep Power: 6
sandmike_83 is on a distinguished road
Ok, I will try changing the initial conditions to see what I get.

Yes, I did some calculations to determine terminal speed of one water dropplet , and I got that result, somehow similar to that of a waterfall. Terminal velocity is also achieved in a very small lenght, but maybe in this case due to the trapped air, it will require longer distances.

Thanks for your help

MS
sandmike_83 is offline   Reply With Quote

Reply

Tags
cfx, falling water

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
Setfields inoutlet and water and air patches erik023 OpenFOAM Pre-Processing 1 September 29, 2008 10:05
Modelling of bubble merging submerge on water Carlos Main CFD Forum 0 October 26, 2005 09:30
moving water from a chamber using air Matt Mason CFX 0 December 2, 2003 20:46
Terrible Mistake In Fluid Dynamics History Abhi Main CFD Forum 12 July 8, 2002 09:11
Asymmetric water free jet study Norberto Parreira Main CFD Forum 7 June 25, 2001 08:23


All times are GMT -4. The time now is 04:53.