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Different behavior of boundary conditions with dynamic mesh in Fluent

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Old   October 28, 2020, 04:00
Wink Different behavior of boundary conditions with dynamic mesh in Fluent
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Hi all,

I experience a problem with my pressure-outlet boundary conditions when executing FSI with Ansys Fluent and Ansys Mechanical APDL. I'm modeling an abdominal aortic aneurysm including the aorto-illiac bifurcation. At both outlets (called LCI and RCI), I'm applying a pressure-outlet boundary condition. After each time step, the boundary pressure for the next time step is calculated using a Windkessel model and a 'define_execute_at_end' UDF. At each iteration, I'm using a 'define_profile' UDF to couple the calculated pressure to the right boundary. One pressure is calculated for the entire outlet (since the pressure is based on the flow through the surface).

When I use these UDFs and monitor the pressure in a Fluent-only (rigid mesh) model, the pressures behave as expected. The pressure for all iterations within one timestep is the same and corresponds to the results visualized in CFDpost (figure 'fluent-only'). The pressure is constant along the entire outlet, with only small differences at the borders.

However, when I use the same method in my FSI simulation, some discrepancies occur (figure 'FSI-pressure'). Firstly, the pressure is not constant within one timestep (left). The CFDpost visualization (middle) shows that the pressure is not constant along the outlet. Furthermore, the pressure does not correspond to the prescribed pressure (right, top). This prescribed pressure should be the same in each iteration within a timestep, which is confirmed in the output file (right, at the top, the pressure for a few iterations in the first timestep are shown. At the bottom, the pressure for a few iterations in the second timestep are shown).

I haven't been able to find the cause of these unexpected results. It looks like the movement of the mesh invalidates the prescribed boundary conditions. However, I couldn't find any documentation about difference in boundary condition behavior for deforming meshes. The outlets are able to move in radial direction and are smoothed and remeshed (if needed).

Does anyone has any suggestions on how to solve this?

Thanks in advance!

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File Type: jpg fluent-only.jpg (110.5 KB, 5 views)
File Type: jpg FSI-pressure.jpg (174.8 KB, 5 views)
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boundary conditions, fluent, fsi, pressure

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