CFX Mass Flow Outlet BC

  • Last Post 22 September 2018
awernick posted this 20 September 2018

I am running a compressor blade passage simulation in Ansys CFX and wish to use the mass flow outlet boundary condition. I have received a successful run however, when analyzing the solution, I noticed that my velocity profile at the exit appears to be normal with the boundary which should not be the case. I have already looked up in the CFX-Pre User Guide regarding the Mass Flow Exit and it states that "A positive value represents mass flow through the boundary in the specified flow direction. For details, see Flow Direction." The section I am quoting from is section Mass Flow Rate (Bulk Mass Flow Rate for Muliphase). The section it suggested to reference for details sent me to information on the flow direction specified in the inlet.

Is the flow direction for a mass flow exit tied to the flow direction of the inlet, or is there a way to specify the flow direction for an exit mass flow rate boundary condition? It may also be possible that I am looking in the wrong section of the CFX-Pre User Guide. Any guidance will help.


raul.raghav posted this 22 September 2018

The section you quoted talks about “inlet” boundary conditions. Check the description of mass flow rate for the “outlet”. For the outlet BC setup, there is no option to set the flow direction.

I believe that by prescribing an outlet mass flow rate BC, you’re enforcing the flow to be normal to the boundary. Alternatively, changing the inlet BC to mass flow rate and outlet to a zero static pressure would be a better option. If you know for sure that the flow through the existing outlet would not be normal to the boundary, you should either (I) move the outlet further downstream and prescribe a zero static pressure condition or (ii) change the outlet to opening.