Fixing inlet boundary conditions
 

I'm trying to simulate a flow through a channel with a pressure-far-field inlet (the flow is compressible so I can't use a velocity inlet) and a pressure-far-field outlet. At the inlet boundary condition, my gauge pressure is 0, my Mach number is 2, and my temperature is 288.15 K. At the outlet boundary condition, based on my calculated values, my gauge pressure is 606174 Pa, my Mach number is 0.3306, and my temperature is 507.53 K. However, I am running into some trouble when I set my initial values. If they're too close to the inlet values (or somewhere in the middle), then my residuals explode, but if they're too close to the outlet values, then the residuals eventually converge, but my boundary conditions are basically ignored by Fluent when I look at the contours. For example, the pressure is way too high and the velocity is way too low. Is there any way to fix the boundary conditions so that they can't be changed when Fluent is running the calculations? This is so frustrating, and any help would be greatly appreciated.

You cannot use pressure farfield at outlet , out of question. To use pressure farfiled on inlet for internal flows, you must understand what pressure farfield boundary stands for.

Okay, thank you, that makes sense. If the conditions at the inlet are freestream, then I should be able to use a pressure farfield at the inlet, right?

What are some alternatives that you would suggest trying for the outlet? Maybe the pressure outlet or the mass flow outlet?

Just think what makes, pressure far field as pressure far field. Because it is far away from your object.

Secondly you can use pressure inlet condition at inlet. But it should be placed in a way so that static and total pressure should be almost equal at this boundary. This is like I am saying, think, how physical setup is created.

This is one reference for using pressure inlet and pressure outlet for pressure farfield conditions

https://www.researchgate.net/publica...ll_Simulations

Thank you. Since my flow is supersonic at the inlet (the Mach number is 2), I don't see how I can have the static pressure be equal to the total pressure, so I'll probably need to try something else.

I've changed my outlet to a pressure outlet and at the inlet I've tried a mass flow inlet and a pressure inlet, but I still keep running into the same issue where the pressure at the inlet is way to high when I view the contour and it ignores the boundary conditions that I set at the inlet. Do you know what could be causing this?

If the inlet flow is supersonic then you specify both the total and static pressure if you are using a pressure inlet, or the mass flow and static pressure if you are using a mass flow inlet.

Thank you. I'm currently deciding between a mass flow inlet and a pressure inlet.

To follow up, I'm now trying a pressure inlet and a pressure outlet.

For the pressure inlet, the Reference Frame is Absolute, the Gauge Total Pressure is 691485 Pa, the Supersonic/Initial Gauge Pressure is 0 Pa (simulating a Mach number of 2), the Direction Specification Method is Normal to Boundary, Prevent Reverse Flow is checked, and the Total Temperature is 518.67 K.

For the pressure outlet, the Backflow Reference Frame is Absolute, the Gauge Pressure is 606174 Pa, the Pressure Profile Multiplier is 1, the Backflow Direction Specification Method is Normal to Boundary, the Backflow Pressure Specification is Static Pressure, the Average Pressure Specification Averaging Method is Weak, the Target Mass Flow Rate is 669.09 kg/s (the upper and lower limits of absolute pressure are 5000000 and 1 Pa respectively), and 518.67 K.

My problem is that when I try running the calculations, when I check the contours, the inlet boundary conditions seem to be completely ignored by FLUENT. For example, the pressure is way too high and the velocity is way too low. I believe this is due to the fact that my initial values are much closer to the outlet values, but this is because if they're too close to the inlet values (or even somewhere in the middle), then my residuals explode really quickly. They still explode even if the initial values are closer to the outlet values, but not until much later. How would you suggest I initialize this? Should I try patching different initial values at the inlet, outlet, and in the middle?

Once again, any help would be greatly appreciated.

For a given set of pressure inlet conditions, there is both a supersonic and subsonic solution which are allowed.

The targeted mass flow rate outlet BC uses a Bernoulli-based approach and tends to not give a supersonic solution because when it senses that the flow rate is too low, it lowers the outlet pressure to increase the driving pressure difference. There are ways to still obtain a supersonic solution (expertz only)...

Try using a regular pressure outlet... You should have said from the beginning that you were using the mass flow rate option on the outlet.

Okay, thank you. I'll try that.

I've tried the pressure outlet without the mass flow rate option, but then my residuals explode. If I have the option checked, my residuals converge, but then my results are inaccurate.