Hi all,

I've had a problem of specifying the boundary condition at the inlet of an opened-end tube. I use CFD to simulate flow inside the tube having an embedded momentum source. The momentum source is located at the center of the horizontal tube and both ends (inlet and outlet) of the tube is opened to the atmosphere.

I specify zero static pressure at the outlet (reference to ambient). The flow inside the tube is driven by the given momentum source. However, I don't know what the boundary condition at the inlet of the tube should be.

Could anybody please give me some suggestion?

Thank you very much

Jackie

You need to either specify the velocity or the pressure at the inlet.

Hello Mike,

I tried that, but fixing the velocity at the inlet is fixing the mass flow rate of the flow. So the momentum source inside the tube can not pump the flow in. So there's no effect on the flow like it has no momentum source.

My objective of this simulation is to see the flow near the inlet of a tube which open free for the flow in ,and is pumping by a momentum source. But it seem like we don't know the property such a thing at the inlet. I really don't have an idea to specify the BC at that position.

Have you had any idea again? Anybody who have an idea suggest me please.

Jackie

try to impose fully developed flow B.C. there, i.e. \vec{n} \cdot \nabla \vec{v}=0.

Sory Xueying ! again please, I don't understand the sentence i.e. \vec{n} \cdot \nabla \vec{v}=0. What is that sentense mean.

Thank you very much. Jackie

And one more question. Why do you think that inlet position will be a fully develloped.Could you please explain? I think that it's a position that the flow just come in. So, It should'n be a fully develloped.

Thanks you very much, Jackie

The boundary conditions may depend on the physics of your momentum source. However, assuming it is a small disc of fixed velocity at the centre-line of the tube (if you know the momentum and where it acts then you know the velocity) but surrounded by an annulus of unknown velocity then specifying an upstream total pressure condition and a downstream static pressure condition should allow the flow to evolve to an equilibrium condition.

Thank a lot Andy.

Jackie

My similar experience was :

Extend computational domain to far field(inflow region) where the flow is nearly staionary. Then, the total pressure of far field is nearly zero gauge pressure, nearly equal to the outflow static pressure because the velocity is very weak. So I could use two pressure boundary condition, for far field and pipe exit.

I found that, fluid was entrained from the wide area of far field with very weak velocity, to the pipe entrance. And I found that, as momentum source increased, the flow rate increased, too, which was, as you know, physically realistic.

Sincerely, Jinwook

Thank you very much Jin-Wook, your answer is very usefull and very clear for me.

Jackie

the latex expression means the normal vector dot-product with velocity gradient tensor is zero.

Sorry, I should specify the conditions when I say fully developed flow. Usually when we deal with open-flows, we are interested in some important region in the flow, so we just segment this inner region as a computational domain, thus it's fully developed.