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

Hydro Penstock Boudary conditions

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

Like Tree2Likes
  • 1 Post By ghorrocks
  • 1 Post By ghorrocks

Reply
 
LinkBack Thread Tools Display Modes
Old   November 29, 2011, 16:50
Default Hydro Penstock Boudary conditions
  #1
New Member
 
Claude Munger Poirier
Join Date: Nov 2011
Posts: 3
Rep Power: 5
STAC is on a distinguished road
Good Afternoon All,

I am doing a Steady State CFD analysis of a Hydro Penstock Y connexion both pipes have 11 ft in diam. Connexion is at 62 degrees. Two Inlets and One Outlet with ANSYS 13.

For boundary conditions I am giving the Velocities at inlet 1 and 2. At the outlet (0nly one) I am putting a zero Pressure.

Some of my collegues are arguing that I should also input the elevation of all inlets and the outlet to take into account the weight of the water body.

Moreover, they want me to give a non-zero pressure value at the outlet since I am not under atmospheric pressure.

Are they correct and am I wrong. (inlet velocities are 4 and 5 m/sec) differential in elevation is about 20 meter or 60 ft.

Thank you for your help - A Boundary lost friend
STAC is offline   Reply With Quote

Old   November 29, 2011, 17:23
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,245
Rep Power: 82
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
The question is whether the head makes any difference. Do you have buoyancy, variable fluid properties or some fluid path which is affected by the head pressure? If so then better include it. If not then you can ignore it and your pressure field will be pressure relative to the local hydrostatic pressure.
STAC likes this.
ghorrocks is offline   Reply With Quote

Old   November 29, 2011, 19:56
Default More Info
  #3
New Member
 
Claude Munger Poirier
Join Date: Nov 2011
Posts: 3
Rep Power: 5
STAC is on a distinguished road
The water head at the outlet is 73 meters static pressure. There are no buoyancy and water flow is almost laminar not turbelent.

My goal is only to check if there is a case where I may have cavitation nothing more.

But they are stubbern with that outlet pressure because the penstock continue for almost 100 m before reaching atmospheric pressure.

The outlet boundaries are the only concern for my results to be accepted.

so If it is required I will impose the pressure but I think it is a mistake.
STAC is offline   Reply With Quote

Old   November 30, 2011, 05:59
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,245
Rep Power: 82
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Why do you say the flow is "almost laminar"? For a 11ft diameter duct with water that would mean a flow of no faster than about 1 mm/s. Unless you are modelling these low flow velocities then your flow is turbulent.

I never said to apply a pressure boundary condition. You apply what ever boundary conditions describe the flow best. What I was talking about is whether the hydrostatic head would offset your results or not.
ghorrocks is offline   Reply With Quote

Old   November 30, 2011, 11:26
Default
  #5
Member
 
Join Date: Dec 2009
Posts: 57
Rep Power: 7
Graham81 is on a distinguished road
If you want to model cavitation, your colleagues are right I think. The static pressure levels may be relative to reference and to eachother, but vapor pressure is an absolute value. If the static head at the inlets is higher, it will have to drop more to induce cavitation.
Graham81 is offline   Reply With Quote

Old   November 30, 2011, 17:57
Default
  #6
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,245
Rep Power: 82
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
There is a big difference between checking whether cavitation is possible and modelling cavitation.

Checking if cavitation is possible can be done with a simple model with no hydrostatic pressure. You just check whether the pressure is below the vapour pressure, with an offset for the local hydrostatic pressure. You do not need to model the hydrostatic pressure to do this.

Modelling the cavitation is a different matter. Then you do need to include the hydrostatic pressure in the model.
STAC likes this.
ghorrocks is offline   Reply With Quote

Old   December 1, 2011, 08:38
Default
  #7
Member
 
Join Date: Dec 2009
Posts: 57
Rep Power: 7
Graham81 is on a distinguished road
Obviously.
Graham81 is offline   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
HELP,how to define the periodic boudary conditions using the icem mesh? dada1204 FLUENT 2 May 1, 2012 17:06
Y Penstock BOUNDARY Conditions STAC Main CFD Forum 0 November 29, 2011 16:16
Free surface boudary conditions with SOLA-VOF Fan Main CFD Forum 10 September 9, 2006 12:24
Far field boudary conditions in FDM James Main CFD Forum 3 July 20, 2005 02:44
boudary conditions for flow past a sphere srijit goswami Main CFD Forum 3 January 27, 2001 08:28


All times are GMT -4. The time now is 09:08.