# Reflection Free Boundary Condition

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April 14, 2021, 04:02
Reflection Free Boundary Condition
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Sanda
Join Date: Apr 2021
Posts: 4
Rep Power: 2
I am conducting pressure based transient incompressible fluid flow in viscoelastic tube in ANSYS fluent and I would like to create an absorbing boundary condition in the outlet of the tube.
ANSYS Fluent already include non-reflecting outlet boundary conditions (NRBC) but only applies for ideal gas (compressible) flow.
With Pressure Far Field BC, it is available only when density is calculated using ideal gas law.
With Outflow BC, it cannot be used for unsteady flow with variable density.
So, the last option is using UDF in the outlet of the tube and I have difficulty applying it.

I found a thesis that similar to my research approach. (https://folk.ntnu.no/leifh/alumni/20...aag_Hansen.pdf)
And I would like to create such boundary condition in ANSYS fluent.
In page 27 of the thesis, under Section 3.2 Boundary Conditions, in Subsection 3.2.1, the author applied reflection free boundary condition at the outlet of the vessel. I have short knowledge of how to apply the boundary condition at the outlet of a vessel.

I am learning UDF in ANSYS Fluent, but I am not sure how to apply boundary equation (Eq 106, page 28) at the outlet. I assume it will require to use UDM but the equation require to use the volume flow rate value from previous space step.
I manage to calculate in numerical analysis but Is there a tutorial to put an equation in UDF of ANSYS fluent?

Equation 106 requires to use data from the previous space exported from ANSYS so How to input a variable that need to read from the analysis?
Using UDM is one of the idea but not sure how to load the data.
Attached Images
 Reflection free Boundary Condition-page-001.jpg (99.4 KB, 5 views) Tube with fluid flow.png (55.4 KB, 3 views)

 October 19, 2021, 13:36 #2 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,575 Rep Power: 53 I don't get it. You are incompressible. You don't have waves and you don't have wave reflections. Why are you looking into BC's for removing unwanted waves?