I have received a project on flow distribution through a 180 degree bend in a pipe. The mass flow rate, pressure, temperature, etc. are known for the inlet, so I have tried using a Mass-Flow-Inlet as my inlet boundary condition. The problem is that I need to examine the flow leaving the 180-degree bend (at the outlet)and I cannot figure out which boundary condition to apply. Essentially, nothing is known about the pipe outlet other than the diameter, and the goal is to distribute the flow as much as possible while mainaining a similar pressure. If anyone has any suggestions on which boundary conditions will yield a solution, I would certainly like to hear them. I have tried using an outflow and got immediate divergence. I can't use pressure outlet because the outlet conditions are unknown. I welcome all suggestions.

(1). A 180 degree bend pipe is likely to give you flow separation around the bend. (it depends on the pipe diameter and the radius of the bend). (2). So, you would like to move the exit of the pipe further down stream until the flow reattached and further developed. So, you can add a long pipe after the 180 degree bend. Flow separation will give you trouble in flow divergence. (3). The inlet condition can be uniform flow, fully developed flow or anything in between. All can have the same flow rate. So, you must specify your inlet flow condition. The code will not know which is your actual inlet condition. (4). So, use your mass flow rate to convert into the velocity profile at the inlet first, and then specify the velocity at the inlet. Uniform flow at the inlet and the fully developed flow at the inlet will give you different flow field around the bend. (5). With this ,my suggestion is: select the type of velocity profile at the inlet and calculate the velocity there as the input to the inlet condition. As for the exit condition, add a long pipe and specify the downstream condition there. By the way, flow field with internal flow separation will have convergence problem. You will have to deal with this part on your own.

Looks like John Chien has already given you good advice on how to handle the situation here. I just want to add two more comments.

1) As for the 'flow field with internal flow separation will have convergence problem', I agree to some extent. However, as long as you give good initial conditions to start with, it should work fine.

2) Don't worry about the mass flow inlet too much. John was right there. However, I don't think it is necessary to convert into velocity profile at the inlet, unless you know the spacial distritution. I would just keep the mass flow inlet.

Thanks for the advice (John and Xiao). I will be running several simulations again tomorrow, and over the weekend and I will probably post the outcome early next week. In the mean time, I welcome any additional information you would like to pass on. Thanks ~Justin