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-   -   PressureinletOutletVelocity&Total pressure (http://www.cfd-online.com/Forums/openfoam/112190-pressureinletoutletvelocity-total-pressure.html)

sasanghomi January 23, 2013 12:55

PressureinletOutletVelocity&Total pressure
 
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....

tomf January 24, 2013 05:31

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

sasanghomi January 24, 2013 11:27

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.

tomf January 24, 2013 12:44

Hi Sasan,

Quote:

Originally Posted by sasanghomi (Post 403807)
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 (Post 403807)
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 January 24, 2013 14:33

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.

tomf January 25, 2013 04:07

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.

sasanghomi January 25, 2013 06:09

Dear Tom,
thank you very much

Sasan.


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