Dear Peter,
thank you for your answer. I really appreciate your help.
The idea of using the option "GradP discretization" in the NS solver helped me a lot. In fact I was using these pressure boundary conditions first on a pipe with rigid walls, but now I'm trying to run a testcase with elastic walls. The final goal being a validation of the FSI-Elmer solver in a blood flow testcase, I would like to compare the Elmer result with a "straight flexible tube"-case presented in the article of C.J.Greenshield [1]. In this paper, the initial condition is a uniform pressure and velocity field: p = 0, v = 0. At the inlet, a pressure step of 5kPa is prescribed, generating a pulse of pressure which propagates along the pipe. The outlet bc is constat pressure p = 0. On the outer wall of the artery, a zero traction condition is prescribed. I'm imposing similar boundary conditions in Elmer (at least I think), but unfortunately the wave propagates much faster than predicted by the Moens-Korteweg formula and it seems to get reflected from the outlet. I'm using gmres solver for NS equations and direct solver for elastic equations and mesh update. My time step = 0.1 ms
I also have the paper of E.Järvinen et al. [2], but I did not manage to solve my problem anyway.
The boundary conditions I'm imposing are listed below. I attached a picture with boundary numbering.
Code: Select all
!!!! INLET Blood !!!!
Boundary Condition 1
Target Boundaries = 1
Pressure = 5000.0
Velocity 1 = 0.0
Mesh Update 2 = 0.0
End
!!! Bottom artery skin
Boundary Condition 2
Target Boundaries = 2
Displacement 2 = 0
End
!!!! OUTER WALL
Boundary Condition 3
Target Boundaries = 3
Mesh Update 2 = 0.0
End
!!! Top artery skin
Boundary Condition 4
Target Boundaries = 4
Displacement 2 = 0
End
!!!! OUTLET Blood !!!!
Boundary Condition 5
Target Boundaries = 5
Velocity 1 = 0
Pressure = 0 !I know this might be a strange condition, but it is used in the first article
Mesh Update 2 = Real 0
End
!!!! SYMMETRY axis !!!!
Boundary Condition 6
Target Boundaries = 6
Velocity 1 = 0
Mesh Update 1 = Real 0
Mesh Update 2 = Real 0
End
!!!! FSI interface boundary INNER Wall
Boundary Condition 7
Target Boundaries = 7
Velocity 1 = 0
Velocity 2 = 0
FSI BC = True
!Velocity 1 = Equals Mesh Velocity 1
!Velocity 2 = Equals Mesh Velocity 2
Mesh Update 1 = Equals Displacement 1
Mesh Update 2 = Equals Displacement 2
Force BC = Logical True
End
I would like to ask you if you could be so kind and look at my BCs and tell me if you see an obvious mistake. I tried to play with many parameters and I re-checked the material properties, but the pulse wave velocity is anyway several times higher than it should be and I am stuck. Desperately stuck. If you want, I can send you the whole sif file.
I did not use any 1D model so far (request of my supervisor
)
I also tried to use a simple fortran file to impose zero inlet pressure for several iteratinos, and then make a jump to the value 5000 Pa. I am wondering if this could be a problem for Elmer, since I impose the change in pressure as a sudden step (as described in article [1]). I noticed that in [2], the change in pressure was linear.
Thank you very much for your help.
Emilie
The articles I refer to are:
[1] C. J. Greenshields, H. G. Weller: A unified formulation for continuum mechanics applied to fluid-structure interaction in flexible tubes, Int. Journal for Num. Methods in Engineering, 2005
[2] E. Järvinen, M. Lyly, J. Ruokolainen, P. Raback: Three-dimensional fluid-structure interaction modelling of blood flow in elastic arteries, ECCOMAS Conference, 2001