Learning Forum

[Ahmed_Aissa]

Dear all,

As a sanity check for my FSI simulation, I prepared the fluid subsystem and selected the desired boundary conditions, including a gap model. Running a Fluent CFD standalone simulation, I obtained the expected fluid behavior with an infinite pressure wave as seen in the figure below, with the continuity residuals dropping below 1e-6 for a time-step size = 1e-5sec

[Karthik R]

Hello:
I'm not sure if I completely follow your explanation here. How is turning on Dynamic Meshing and running an incompressible flow simulation equivalent to a compressible flow run? Can you elaborate how these two cases are comparable?
Karthik

[Ahmed_Aissa]

Hi Kremella What I am trying to say here is that the dynamic mesh options are affecting somehow the governing equations, namely the mass balance. This observation comes from a comparison between 3 models: incompressible fluid (No dynamic mesh options), incompressible fluid with a dynamic mesh option ON, and compressible fluid.
For an incompressible fluid, I expect the pressure distribution to be as shown in the first figure (during the simulation start-up). This implies that at each time step, the pressure distribution does not change as long as contact is ON. Nonetheless, If I tick the dynamic mesh options and specify the moving domains, I get a different result (Fig 2), which is comparable to what I get when I change the fluid material to compressible.
My question is: Do the dynamic mesh algorithms within Fluent make changes to the continuity equation?
Thanks

[Karthik R]

Hello:
Dynamic mesh does not make any changes to the continuity equation. The change in residuals with the dynamic mesh you are facing is because the computational domain has changed and this is expected behavior. You will not be able to obtain the same residuals as the static mesh case.
What is your original mesh quality and what happens are the mesh changes? Can you track the max skewness of the mesh with time? My suspicion is that the mesh may be degrading.
Karthik

[Ahmed_Aissa]

Thank you for your reply.
I did not mention that the behavior occurs before I run the coupled simulation. When I run a Fluent standalone simulation the mesh is static (Even though the dynamic mesh is ticked on).
This is why I am a bit confused.
Thank you,

[Karthik R]

Hello Without coupling your simulation, you may need either a UDF or an expression to tell Fluent how you wish to move the mesh. This is perhaps why your mesh is static. What are you doing to move the mesh?
Karthik

[Ahmed_Aissa]

Hello,
To move the mesh, I assign system coupling surfaces, with the deforming interiors adjacent to the FSI interfaces following the Linearly Elastic Solid moving approach.
Best,
Ahmed