pressure inlet and outlet boundary condition

[INTURU SRINIVAS 20PHD0548]

Dear Griffith,

I want to apply the inlet and outlet pressure boundary conditions as shown in the figure

Top boundary was open to the atmosphere and defined as a pressure-inlet. Thus, the boundary condition was defined as opening with a relative pressure set to 0 Mpa. And the right boundary of computational domain was applied pressure.

I have a confusion in setting the pressure boundary conditions. Could you please help me to set these boundary conditions?

Thanks and regards

Srinivas

[Amneet Bhalla]

Why do you want to have a pressure outlet at the end of the domain?

 Note that pressure would be very close to (not exactly) hydrostatic near the domain and you cannot impose a pressure condition (I think) near the end as it would to an inconsistency (imposed vs hydrostatic configuration the system is trying to achieve).

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[INTURU SRINIVAS 20PHD0548]

Dear Amneet,

Thanks for your reply.

Why do you want to have a pressure outlet at the end of the domain?

The boundary conditions in the image which I sent was taken from DOI: 10.1109/ACCESS.2018.2794064. I want to reproduce the result of Numerical Wave Tank given in the paper.

Note that pressure would be very close to (not exactly) hydrostatic near the domain and you cannot impose a pressure condition (I think) near the end as it would to an inconsistency (imposed vs hydrostatic configuration the system is trying to achieve).

Is this applicable only in the case of multi-phase flows (like wave tanks) or any application?

Thanks and regards

Srinivas

[INTURU SRINIVAS 20PHD0548]

In continuous to the previous mail, If I want to maintain a boundary at atmospheric pressure, what is the exact representation of the boundary condition? a = 0, b = 1, and g = 0 or a = 0, b = 1, and g = -101325Pa.

Thanks and regards

Srinivas

[Amneet Bhalla]

On Mon, Dec 4, 2023 at 8:35 PM 'INTURU SRINIVAS 20PHD0548' via IBAMR Users <ibamr...@googlegroups.com> wrote:

Dear Amneet,

Thanks for your reply.

Why do you want to have a pressure outlet at the end of the domain?

The boundary conditions in the image which I sent was taken from DOI: 10.1109/ACCESS.2018.2794064. I want to reproduce the result of Numerical Wave Tank given in the paper.

Well people mean different things when they say pressure boundary conditions. Moreover, this paper is using ANSYS Fluent for their simulations. Who knows what commercial codes implement for pressure boundary conditions. The commercial codes sometimes also silently change user specified "incompatible" solver options.   

Note that pressure would be very close to (not exactly) hydrostatic near the domain and you cannot impose a pressure condition (I think) near the end as it would to an inconsistency (imposed vs hydrostatic configuration the system is trying to achieve).

Is this applicable only in the case of multi-phase flows (like wave tanks) or any application?

I was specifically referring to the multiphase flow case. Close to the end of the wave damping zone, water is stationary. Pressure in the air phase would be close to atmospheric pressure (= 0 here as air has very low density), whereas in the water phase it would be hydrostatic p = rho g z. If you try to impose p(x_outlet) = p_specified, it will conflict with what the flow physics is trying to achieve. 

In IBAMR we can actually impose a pressure value. In many research codes, projection method is used to solve incompressible Navier Stokes,  in which pressure is an auxiliary variable. Therein incompatible boundary conditions may work. Not here!  

Thanks and regards

Srinivas

On Monday, December 4, 2023 at 7:26:27 PM UTC+5:30 Amneet Bhalla wrote:

Why do you want to have a pressure outlet at the end of the domain?

 Note that pressure would be very close to (not exactly) hydrostatic near the domain and you cannot impose a pressure condition (I think) near the end as it would to an inconsistency (imposed vs hydrostatic configuration the system is trying to achieve).

On Mon, Dec 4, 2023 at 5:02 AM 'INTURU SRINIVAS 20PHD0548' via IBAMR Users <ibamr...@googlegroups.com> wrote:

Dear Griffith,

I want to apply the inlet and outlet pressure boundary conditions as shown in the figure

Top boundary was open to the atmosphere and defined as a pressure-inlet. Thus, the boundary condition was defined as opening with a relative pressure set to 0 Mpa. And the right boundary of computational domain was applied pressure.

I have a confusion in setting the pressure boundary conditions. Could you please help me to set these boundary conditions?

Thanks and regards

Srinivas

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--Amneet 

[Amneet Bhalla]

The normal traction bc is 

-p + 2 * mu * du_n/dn = g.   (i)

u_n is the normal component of velocity and n is outward normal. If the  tangential velocity u_t is set to be a constant (i.e., du_t/dt = 0) then by incompressibility condition 

du_n/dn + du_t/dt = 0 (ii)

==> du_n/dn  = 0

This makes (i) to be 

-p = g 

Thus to impose a pressure value of 0 on the top boundary set g equal to 0. To set pressure equal to 1 atm on the top boundary, set g = -101325Pa. However, setting a specific value of pressure at the top boundary is arbitrary. Even if it is set to 0, who can alway add 101325 to pressure values obtained from the simulation.  

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--Amneet 

[INTURU SRINIVAS 20PHD0548]

Thank you so much for the explanation professor.