Fluids

Fluids

Problem with partial overlap of solid and fluid domains in FSI simulation

    • jfonken
      Subscriber

      Hi all,

      For my PhD project on abdominal aortic aneurysms, I'm using Ansys System Coupling, Ansys APDL and Fluent to perform FSI simulations of patient-specific geometries. At the moment, we're working on obtaining patient-specific inlet velocity profiles to use in the CFD part of the FSI simulation. We want to prescribe this velocity at the inlet of the measured geometry. However, in the solid domain, we need to fix the displacements of the inlet (and outlets) in longitudinal and circumferential direction, which causes some numerical inaccuracies close to the inlet (and outlets). Therefore, we normally extend the patient-specific geometry by a few cm, to reduce the effects of the BCs of the solid domain in the region of interest. 

      This leads to a mismatch of solid and fluid inlets. To resolve this, I came up with the following idea (see picture):

      The red part is the patient-specific geometry, whereas the blue part is the extension at the side of the inlet. In my CFD (Fluent) simulation, I'm already able to create separate mesh zones for the red and blue parts. I want to prescribe the (patient-specific) velocity at the inlet of the red part and prescribe the current inlet pressure to the complete extension (blue). The extension can be exclude from the CFD simulation, only the pressure needs to be updated each step (according to the changes in pressure at the inlet). 

      However, I don't know how to achieve this. Does anyone have any recommendations on how to handle this problem? Or maybe a different approach to handle the mismatch between the solid and fluid domain?

      I also tried another approach (see picture):

      The real patient-specific geometry is used in the fluid domain and the velocity is prescribed at the inlet. The extended patient-specific geometry is used in the solid domain and is divided into the patient-specific (right) part and the extension (left). At the patient-specific part, a 2-way FSI interaction is prescribed, in which the pressures calculated in the fluid domain are transferred to the solid domain and the calculated displacements in the solid domain are transferred to the fluid domain. The extension is coupled to the inlet of the fluid domain using a 1-way FSI interface, so only the pressure of the fluid domain is transferred to the solid domain, no displacements are transferred. 

      However, in this case, no mapping can be performed between the extension and the inlet of the fluid domain, because the meshes aren't conformal. 

      Hopefully this post is clear to you, I understand it could be quite confusing 😅

      Please let me know if you have a suggestion!😊

       

      Best,

      Judith

    • Danica
      Ansys Employee

       

      Hi Judith, 

      So looking at the problem, you need to define either the inlet pressure or the inlet velocity profile, but not both. A velocity boundary varies the pressure until the velocity condition is met: a pressure boundary adjusts the mass flow until the pressure is met.  I would suggest a transient velocity profile defined at a point that is a suitable length upstream of the region of interest to allow for the flow to fully develop. Would suggest velocity over pressure as this will provide more stable solutions. 

      Have a look at the following for how to define this: 7.6. Transient Cell Zone and Boundary Conditions (ansys.com)

      What model are you using for blood viscosity?

      Danica

       

    • jfonken
      Subscriber

      Hi Danica,

      Thanks for your reply! I actually don't want the flow to develop any further, because I used Ultrasound Doppler to measure the velocity at the inlet, so I want to prescribe this measured velocity at the correct point (right at the beginning of the region of interest). I understand that I can't prescribe both inlet pressure and inlet velocity, that's why I was thinking about excluding the extension in the simulation, but only using it to transfer the inlet pressure to the solid domain. However, I then need a way to prescribe the inlet pressure to the extension, but I don't know how.

      I'm using the Carreau model for the viscosity.

       

      Best,

      Judith

    • Danica
      Ansys Employee

      Hi Judith, 

      I think that you can use an area-weighted average pressure report at the velocity inlet to get the pressure for each time-step. Then import these values into APDL using system coupling and use this value to define the pressure on the solid section of the model. 

    • jfonken
      Subscriber

      Hi Danica,

      That's an excellent idea indeed. I didn't know it was possible to import pressure values into APDL for each timestep using system coupling. Do you maybe have some documentation or examples that I can look into?

       

    • Rob
      Ansys Employee

      The only documentation we can share via the Community is accessable through Help in the Fluent solver, if you navigate up the tree you'll eventually get to the (nearly) full list of Ansys products. 

    • jfonken
      Subscriber

      Hi Rob,

      I have access to the Ansys Documentation, but I haven't found a way to import pressures into Ansys APDL using system coupling (yet). Do you have recommendations for certain commands or chapters of the documentation?

    • Rob
      Ansys Employee

      Only the System Coupling section - I'll ask this side. For most of the work I do it if bends it's going to fail so I don't tend to do any FSI. 

      • jfonken
        Subscriber

        Hi Rob,

        Have you been in touch with the System Coupling section to resolve this problem? 

        Best,

        Judith

    • jfonken
      Subscriber

      Using the *TREAD command, I'm able to import the Fluent pressure values into Mechanical APDL. However, this is only done before the first time step, but then no Fluent pressures are available yet. I would really appreciate any help to update the pressure table during the system coupling simulation. Which command(s) should I use to issue the *TREAD command (updating the pressure table) at every coupling step?

    • Rob
      Ansys Employee

      We did, I'll remind them. 

      • jfonken
        Subscriber

        Okay thanks, hopefully they can help me!

      • jfonken
        Subscriber

        Hi Rob,

        Any news from the system coupling section?

        • Rob
          Ansys Employee

          It would appear not, I'll try kicking again. My apologies. 

        • Rahul Mule
          Ansys Employee

          Hi Judith, 

          Sorry for the delayed response.

          Please refer External Data (ansys.com) to import data from text files into a Mechanical system or a System Coupling component system.

           

        • jfonken
          Subscriber

          Dear Rahul,

          Thanks for your input. I had a look at the referred documentation. However, it states that the external data is static and therefore only read in the analysis set-up. I was able to do this with the *TREAD command as well, but my problem is that it doesn't re-read the file at each coupling (time) step. Do you have any suggestions to solve this problem? Which command(s) should I use to issue the *TREAD command (updating the pressure table) at every coupling step? For Ansys APDL, it works well to use the *TREAD command at each load step, but Ansys APDL sees the system coupling process as a single load step.

           

          Best,

          Judith

    • Rahul Mule
      Ansys Employee

      Hi Judith,

      Both external data and *TREAD command will work for static data import and even if loaded data at each load step in APDL, SC will process it as single step.

      • jfonken
        Subscriber

        Hi Rahul,

        I indeed had this experience. Isn't it possible to alter something, such that SC will consist of multiple steps, such that new data can be loaded in each step?

    • Rahul Mule
      Ansys Employee

      Hi Judith,

      When a co-simulation coupling participant, such as a Fluent system, is connected to a Mechanical
      system via System Coupling, Mechanical is given access to data directly from the other participant. So, I would suggest you to create conformal mesh and specify the velocity at the extended part and transfer data directly from Fluent to Mechanical through system coupling.

      System Coupling (ansys.com)

      Regards,

      • jfonken
        Subscriber

        Hi Rahul,

        As explained in my initial question, when I prescribe the velocity at the extended part, the profile developes and therefore deviates from the measured profile at the measurement site. I was unable to prescribe a pressure at the extension part in Fluent, as schematically represented in the figure below. Do you have any suggestions on how to achieve this?

        • Rahul Mule
          Ansys Employee

          Hi Judith,

          As you wants to import the external data in MAPDL. External data won't get updated due to coupling it with MAPDL. In that case you can try one way coupling between the external data and MAPDL. You can add multiple data sets with external data in that case. Please refer "Importing Multiple Data Sets" section in External Data (ansys.com)for more information. 

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