Two way FSI of a deformable pipe

omkar5990omkar5990 Member Posts: 8

Hello all,

I am new to FSI and have been working on a problem that involves a deformable pipe. The length of the pipe is 20 mm and the internal diameter is 3 mm. The thickness of the pipe is 1 mm. I am trying to simulate flow of water in the pipe for the case when the rear portion of the pipe (last 5 mm ) is made up of a more compliant material and has a pulsating external pressure being applied to it. Water flows into the pipe at 0.5 m/s for which I have used a velocity-inlet condition. What i am hoping to achieve is that due to deformation of the pipe at the rear end, there is reversal of water flow and it flows back towards the inlet. When I run the system coupling, it throws an error which says 'negative volume detected.' I have tried decreasing the time step and that has not helped. For a different run, I got an error which said that excessive distortion of some elements in the transient structural module is detected. I have tried refining the mesh and ramped the load over a much larger time. Could someone please help me troubleshoot the problem? I have attached the workbench file below.



  • SteveSteve Posts: 153Forum Coordinator
    edited May 2019


    A general recommendation for FSI cases is to perform a simplified analysis with each solver independently before moving to a full 2 way coupled simulation. This will allow you to debug any issues in one solver that are independent of the other solver. In Mechanical, this would mean estimating fluid forces and applying them as boundary conditions. In Fluent, this would mean estimating deformation and applying a moving boundary with a profile, or using a 1way coupled simulation to get the deformations from Mechanical.

    For your case, are you running a 2 way FSI simulation? ie fluid forces are sent from Fluent to Mechanical, and displacements are sent from Mechanical to Fluent? If so, you should consider whether or not this is necessary. If the fluid forces don't significantly impact the deformation of the tube, then you can use a 1 way analysis where you only send displacement data from Mechanical to Fluent. 

    The negative volume error means the mesh has deformed and folded onto itself. First, you'll need to determine if the negative volume occurs in the fluid or solid mesh by examining the output log file for each solver. If the negative volume is in the fluid mesh (and you're using Fluent), you'll need to use Smoothing and possibly Remeshing which are in the Dynamic Mesh Settings. You can read more about Smoothing and Remeshing here:

    If the negative volume occurs in the Mechanical mesh, you can try applying a ramping on the System Coupling data transfer of fluid forces. 

    As a reference I recommend doing the following System Coupling tutorial:



  • omkar5990omkar5990 Posts: 11Member
    edited May 2019

    Hi Steve,

    Thanks for your suggestions. I was initially running a 2 way simulation, and then switched to a 1 was FSI since I was only interested in the fluid behavior due to the displacement of the structural boundary. I got the simulation to work by coarsening my mesh and halving the maximum pressure applied on the pipe. However, when I increase the pressure upto the maximum pressure I want to apply, the solution fails. 

    I am using the remeshing method to set up the dynamic mesh since the ANSYS guide recommends using this if the boundary displacements are larger compared to the cell size. 

    Below is a figure of my setup and the table showing how I have implemented the varying pressure on the pipe.

    I am using a time step of 0.001 sec. The Fluent boundary condition is a pressure outlet condition as I was looking to apply a boundary condition that is equivalent to 'opening' in CFX. The simulation fails at about 0.35 sec. This is how the deformed pipe looks at that point.

    The negative cell volume is detected in the Fluent mesh. Do you think reducing the time step further would help? Do you have any other suggestions?

  • audreyaudrey Posts: 21Member
    edited December 2019

    Hello Omkar,

    I'm trying to simulating almost the same thing, a 10 cm collapsible tube. It is a latex tube so it properties are quite differents then the artery's because it is more elastic. I lead a flow in the tube and see what's happening to the pressure. When i want to grow the flow or dicrease the thickness, an error about an element which is to distorded happend.I guess you met the same problem, did you find a solution?


    Thank you.

  • audreyaudrey Posts: 21Member
    edited April 2020


    I don't know if you already find your answer but if not, i solved this problem by changing the mesh. It is written in ANSYS guide that remeshing option works only with a tetraed mesh.

  • sureshlankasureshlanka Posts: 2Member

    can anyone help me how to do 2 way fsi on deformable pipe?

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