Dear Nikesh,

As you know in Fluent you have to define in Boundary Condition inlet ( Pressure, Velocity etc.) , Outlet ( Pressure, Velocity etc.) , Centerline( in symmetric case) and Wall condition. Now, in your problem you know about your inlet and outlet Pressure. The most important thing now should be defined is your wall condition, not only the Momentum condition ( Stationary , Slip , Drieven ), also the thermal condiition at wall ( constant heat flux or constant Temperature or ..).  It backs to your problem to choose which type of these.

Regards

You can not impose in both boundaries the static pressure. The numerical solution is not stable. The physical reason is simple, you do not fix the mass flow rate with these conditions. The mathematical proof is not for the faint of heart... If it is a experimental setup, try to measure the total pressure with a pitot-tube. Et voilà, you have your problem solved.

Thanks Amin, and Henriques! 

@Amin: The wall condition is not a problem. Its a stationary wall and I am not concerned about the thermal condition too as for now. I have the pressure but only static at inlet and outlet. For simulations in Fluent, I require Total Pressure at the inlet. 

@Henriques: Yes that's exactly what I had in mind. And it might be possible to install a pitot tube too. However, the condition in real case is that I need to adjust the position of the poppet-valve such that the outlet pressure (static) is 1000 psi (with the inlet static pressure set at 2,800 psi). I guess it can only be done experimentally first and then proceed with the CFD for that particular position of the poppet valve. 

One more thing: Should I be concerned about this high pressure difference, 1,800 psi? Will it make the flow compressible? 

I think the flow is likely to be supersonic and highly compressible.

What is upstream of your inlet? Indeed, a compressed air tank at 2900psi just upstream of your simulation inlet might require different valve conditions to get down to the necessary outlet conditions, compared to inlet flow at Ma = 2 and 2900psi? Do you need to simulate from somewhere of known stagnation conditions, for example to include piping running to a large compressed air tank where fluid velocity is zero, so stagnation conditions at such an inlet are known?

Once you have enough boundary conditions, I guess the system condition you are searching for is "for these desired inlet/outlet conditions, where do the fluid forces forcing my valve open balance the spring force trying to close it?"

I guess you could look at this as a single variable optimisation problem. If the spring properties and inlet/outlet conditions are fixed, the free variable would be the valve position and the 'optimisation target' would be for sum of fluid forces acting on valve = spring force?

If the flow is supersonic as Anton says, you can only impose inlet boundary conditions since flow perturbations are not propagated upstream (all eigenvalues are positive, again the math is not easy). If imposed outlet BC in supersonic flow the solution is not stable and physically wrong.

I think that you should try "Pressure inlet" for the inlet boundary condition and "Pressure Outlet" for the outlet boundary conditions.

For compressible flow, you have to define it in " Material".

@ Anton:Upstream of the inlet is compressed air tank that maintains 2900 psi inlet pressure. Initially the poppet valve completely blocks the flow such that the pressure after the valve is at 1 atm (its an empty vessel too). The valve needs to be pressed slowly which releases pressure and I need to find that position of the valve where the pressure aft of the valve is 1000 psi. 

@ Henriques: I have still to check and try for compressible flow case. Will probably do it soon. 

@ Daniel: The figure I attached in my original question is just to give a tentative idea of my case. The inlet and outlet have same cross-sectional area. I am yet to work on compressible case. And yes I won't want to use k-epsilon model and I'm not concerned with temperature too at this point. 

@ Faten: Yes Pressure inlet and Pressure outlet would be the best bc's to be used here. But since fluent implies total pressure at inlet and static pressure at outlet, it won't be very straight forward. I only have static pressure at inlet and outlet. I understand that measuring experimentally the total pressure at inlet is the only way to go for my sims! 

@Bhattarai the upstream pressure is pressure in the air tank, stagnation pressure as measured by a pitot tube or static pressure measured at the wall entrance. If you know the pressure in the air tank and the static pressure or the stagnation pressure you have or problem solve! Tell us your exact conditions.

what are the compatible boundary conditions at inlet with outflow boundary condition in Fluent?

Assuming incompressible 3d flow we have:

Inlet -Velocity components or stagnation pressure and two velocity angles

Outlet - static pressure

The maths beyond this are not for the "faint of heart".

You could think inversely, applying the total pressure BC for stability reason and modifying the value until you get the required value of static pressure.

Hi, Thank you for your Question. I have also a similar question. I have a vertical pipe. The inside and outside of the pipe are full of static water and no flow. Some height above from the bottom of the pipe air will be injected inside the pipe. This air will be mixed with water and makes the water less denser than the water below the air injection point and outside of the pipe. So the denser fluid push the lower denser fluid and flow will be happened and continue. The bottom of the pipe is pressure inlet. What will be the "Gauge Total pressure" and "Supersoninc/Initial Gauge pressure"? Say, the pressure at the bottom of the pipe due to static water level is 3 bar. Please help me, if someone knows.

Joao C. C. Henriques May you please provide references for your answers wherein you have mentioned that eigen values are positive or the first one wherein you mentioned the numerical solution is not stable.

Thanks

(PS: Sorry for asking on this old post but I just tumbled upon this looking for answer to my query https://www.researchgate.net/post/Reflecting_boundary_condition_at_outlet .

Kindly do comment if you can provide some insights on this also. Thanks)