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

PressureinletOutletVelocity&Total pressure

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

Like Tree3Likes
  • 2 Post By tomf
  • 1 Post By tomf

Reply
 
LinkBack Thread Tools Display Modes
Old   January 23, 2013, 12:55
Default PressureinletOutletVelocity&Total pressure
  #1
Senior Member
 
Sasan Ghomi
Join Date: Sep 2012
Location: Iran - Tehran
Posts: 185
Rep Power: 4
sasanghomi is on a distinguished road
Hi every body
I have a question about a boundry that boundry condition for velocity is

type pressureInletOutletVelocity;
phi phi;
value uniform (0 0 0);
(what is this?????)

and for pressure is :
type totalPressure;
U U;
phi phi;
rho none;
psi psi;
gamma 1.4;
p0 uniform 1e5;
value uniform 1e5;
(what is this?????)
I know that for pressure we have :
P0=P+(0.5) (rho )(U^2)
what is the value for velocity and pressure??????
Also I know pressureInletOutletVelocity = inletOutlet+ PressureInletVelocity
Can any one explain this boundry condition exactly??????
please help me....
sasanghomi is offline   Reply With Quote

Old   January 24, 2013, 05:31
Default
  #2
Senior Member
 
Tom Fahner
Join Date: Mar 2009
Location: Delft, Netherlands
Posts: 224
Rep Power: 10
tomf is on a distinguished road
Send a message via MSN to tomf Send a message via Skype™ to tomf
Dear Sasan,

The pressureInletOutletVelocity is indeed a combination of inletOutlet and pressureInletVelocity. It means that in case there is an outward flux, the boundary condition is zeroGradient, extrapolating from the internal field outwards, if the flux is inwards to the cell, the pressure is used to determine the magnitude of the patch-normal velocity, similar to the way pressureInletVelocity is used.

The entries in "value" are basically initial values (for the velocity vector, or the static pressure), that get updated during the first iteration/timestep, so they need to be appropriate. If you have an estimate to what either of them should be, then you can use those values, but the current setup seems fine to me.

Regards,
Tom
tomf is offline   Reply With Quote

Old   January 24, 2013, 11:27
Default
  #3
Senior Member
 
Sasan Ghomi
Join Date: Sep 2012
Location: Iran - Tehran
Posts: 185
Rep Power: 4
sasanghomi is on a distinguished road
Hi Tom,
thanks for your reply..
But I have two questions:
1)when we have outwards flux the boundry condition for velocity is zeroGradient and what is the role of extrapolation in this boundry???Do you mean that in this boundry the boundry condition is zeroGradient and boundry value evaluated by extrapolation from internal field?????
2)You said , when we have inwards flux the velocity evaluates from pressure but in this case the pressure is dependent on velocity (P0=P+(0.5) (rho )(U^2))...Can you explain more about it?
thanks,
Sasan.
sasanghomi is offline   Reply With Quote

Old   January 24, 2013, 12:44
Default
  #4
Senior Member
 
Tom Fahner
Join Date: Mar 2009
Location: Delft, Netherlands
Posts: 224
Rep Power: 10
tomf is on a distinguished road
Send a message via MSN to tomf Send a message via Skype™ to tomf
Hi Sasan,

Quote:
Originally Posted by sasanghomi View Post
1)when we have outwards flux the boundry condition for velocity is zeroGradient and what is the role of extrapolation in this boundry???Do you mean that in this boundry the boundry condition is zeroGradient and boundry value evaluated by extrapolation from internal field?????
I mean that when you start solving and the solution indicates that the patch that you have this boundary condition for is an outlet, the values for this patch are such that there is no gradient normal to the patch (Thus, the value on the patch is the same as the value of the first cell inside the domain as seen from the patch). So yes that is what you describe.

Quote:
Originally Posted by sasanghomi View Post
2)You said , when we have inwards flux the velocity evaluates from pressure but in this case the pressure is dependent on velocity (P0=P+(0.5) (rho )(U^2))...Can you explain more about it?
thanks,
Sasan.
Well you have an iterative method, with some pressure-velocity (Simple/Pimple/Piso) coupling, so in an iterative fashion, there is a solution for the pressure and the velocity that correspond to each other. The pressure is always dependent on the velocity and vice versa, the combination of these boundary conditions enforces this on the boundary.

I hope this answers your questions?

Regards,
Tom
sasanghomi likes this.
tomf is offline   Reply With Quote

Old   January 24, 2013, 14:33
Default
  #5
Senior Member
 
Sasan Ghomi
Join Date: Sep 2012
Location: Iran - Tehran
Posts: 185
Rep Power: 4
sasanghomi is on a distinguished road
Dear Tom,

I got it...But please answer my last question :

I think all of the iterative methods (PISO/SIMPLE/PIMPLE) need to have specified value(or specified condition) on boundaries....please correct me if I am wrong...????????
I want to conclude in this case that the boundry value is not fixed in all of the solution's time and the boundry value evaluates at each time step (by iterative procedure)....
Is my conclusion correct??

thanks,
Sasan.
sasanghomi is offline   Reply With Quote

Old   January 25, 2013, 04:07
Default
  #6
Senior Member
 
Tom Fahner
Join Date: Mar 2009
Location: Delft, Netherlands
Posts: 224
Rep Power: 10
tomf is on a distinguished road
Send a message via MSN to tomf Send a message via Skype™ to tomf
That is indeed the case. The only thing that is fixed is the total pressure, only static and dynamic pressure will change between time/iteration steps.
tomf is offline   Reply With Quote

Old   January 25, 2013, 06:09
Default
  #7
Senior Member
 
Sasan Ghomi
Join Date: Sep 2012
Location: Iran - Tehran
Posts: 185
Rep Power: 4
sasanghomi is on a distinguished road
Dear Tom,
thank you very much

Sasan.
sasanghomi 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
Calculation of the Governing Equations Mihail CFX 7 September 7, 2014 06:27
Unsteady pressure differential between inlet and outlet of the pipe for single phase joshi20h FLUENT 0 September 26, 2012 12:41
How to get the total pressure in the UDF? zgzhai Fluent UDF and Scheme Programming 2 September 12, 2012 06:15
Pressure instabilities with interDyMFoam for the floatingObject case nbadano OpenFOAM Running, Solving & CFD 14 March 8, 2011 09:19
Setting pressure and velocity in inlet Asghari FLUENT 5 September 22, 2006 13:23


All times are GMT -4. The time now is 03:53.