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

gravity driven flow- boundary conditions?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   November 15, 2003, 23:47
Default gravity driven flow- boundary conditions?
  #1
Shankar
Guest
 
Posts: n/a
Hi,

I am studying flow through a U-tube like structure. The water level on the left hand side is higher than the water level on the right and this height difference alone causes flow. So for boundary conditions, I specify the left-side water level as "pressure-inlet" and the right-side one as "pressure-outlet".(can't have "outflow" if u have "pressure-inlet")And I turn the gravity on with a value of -9.8 m/s2 in the Y-direction.

For the pressure values to be specified FLUENT manual says that the hydrostatic pressure differences should not be included. So, I specified 101325 Pa for both inlet and outlet and got good convergence of the equations, but later I realised we can only input gauge pressure, which is Pabs-Patm which becomes zero in my case. When I change the pressure value to zero at both ends (with gravity on), I get divergence of the continuity equation.

Can anybody tell me what I am doing wrong?

Thanks, Shankar.
  Reply With Quote

Old   November 17, 2003, 02:54
Default Re: gravity driven flow- boundary conditions?
  #2
ashish
Guest
 
Posts: n/a
Hello Shankar,

Static pressure will not be zero. static pressure on the left limb is the static pressure of the water column i.e. rho x g x ht. ans this pressure is causing the flow. I think you have to sepcify inlet gauge pressure as rho x g x ht. If you specify zero pressure on either side there will be problem in continuity that is very correct. There is a difficulty in calculating driving force for the flow (which is zero in your case), and thus you are getting divergence.

Bye, ashish
  Reply With Quote

Old   November 18, 2003, 13:54
Default Re: gravity driven flow- boundary conditions?
  #3
Shankar
Guest
 
Posts: n/a
Hello Ashish,

I tried the flow with a simple vertical pipe for which we know the analytical solution.

Pressure at the bottom=0Pa Pressure at top=rho*g*H. gravity is off. reference pressure location is (0,0,0) located at the centre of the bottom surface and operating pressure=101325 Pa.

The solution converges rapidly to 10E-6.

But still the results are way off the analytical results.

Shankar.
  Reply With Quote

Old   November 26, 2003, 15:43
Default Re: gravity driven flow- boundary conditions?
  #4
co2
Guest
 
Posts: n/a
pressure at the bottom should be rho*g*H
  Reply With Quote

Old   November 26, 2003, 16:45
Default Re: gravity driven flow- boundary conditions?
  #5
Shankar
Guest
 
Posts: n/a
When the gravity is turned off in the case of a straight pipe flow, I guess it doesnt matter which end you give the rho*g*H pressure as long as it is with respect to the reference pressure location?

Shankar.
  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
Wind turbine simulation Saturn CFX 34 October 16, 2014 05:27
Implementation of boundary conditions for FVM Tom Main CFD Forum 7 August 26, 2014 05:58
mass flow in is not equal to mass flow out saii CFX 2 September 18, 2009 08:07
Internal flow simulation boundary conditions Kishore FLUENT 1 July 10, 2007 11:42
Boundary conditions? Tom Main CFD Forum 0 November 5, 2002 02:54


All times are GMT -4. The time now is 05:58.