[hfsf]

Hello all,

I'm sorry for such a beginner's question, but what's the guideline to define velocity and pressure boundary conditions (BC) in simple pipe flows using OPENFOAM?

Afeter some research, I realized that the major papers describe such situation as follows:

At the inlet, the boundary conditions are 0 for pressure (for icoFoam solvers, at last) and a prescribed velocity. At the outlet, the BC are a zero gradient for velocity and pressure.

My question is: Defining zero gradient to velocity don't make the system to stay closed, I mean, if there's no flux at the outlet, the natural tendency is the increase of pressure. I would appreciate some guidance in such effort.

Thanks in advance.

[blacksquirrel]

For a simple case I would suggest:
velocity: INLET fixedValue; OUTLET zeroGradient
pressure: INLET zeroGradient; OUTLET fixedValue

[bscphil]

Hello Hanniel,

the first and frequently-used option for simulating pipe flow is to define:
-> mass flow (or volumetric flow rate or velocity) at your inlet
-> static pressure at outlet as ambient pressure
the boundary conditions must be self-stabilized, this is why you have to define:
-> U:
inlet: type fixedValue; value uniform (Ux Uy Uz);
outlet: type zeroGradient;
-> p:
inlet: type zeroGradient;
outlet: type fixedValue; value uniform 0;

A other option is to define pressure-driven boundary conditions, but i think this was not your question!? I hope this helps? Best greetz, Phillip

[hfsf]

Thak you guys for the quick answers.

I was wondering the numerical reasons behind such boundary conditions.
The BC for pressure at the outlet is fixedValue 0. The only way it would make sense is if he pressure solved in icoFoam was only the dynamic one, and given the fact that in the outlet the total pressure is the ambient, the dynamic is zero. But I can't figure out the reason behind the definition of the velocity outlet BC as zeroGradient.

Any recommendations of literature that might help understanding the boundary condition determination would be higly appreciated.

Thanks in advance!

[bscphil]

Originally Posted by hfsf

Any recommendations of literature that might help understanding the boundary condition determination would be higly appreciated.

Thanks in advance!

See e.g.: Computational Methods for Fluid Dynamics by J.H. Ferziger and M. Peric, @ chapter 7.7: Solution of the Navier-Stokes Equations, Boundary Conditions for the Navier-Stokes Equations

[aryan789]

Hi,
I have to draw a t shaped circular pipe in ansys workbench where the vertical pipe diameter is larger than the horizontal one.I tried to merge and connect them by selecting their faces..but proper joining doesn't occur..flat surface is present..suggest what to do