[FIA11]

Hello everyone,

I am simulating flow in an elastic tube with the following conditions:

• Tube diameter = 8mm.
• Tube Length = 60mm.
• Tube wall thickness = 1mm.
• Tube Young's Modulus = 0.4MPa.
• Fluid viscosity = 0.0035 Pa.s.
• Fluid density = 1050 Kg/m^3.

First of all i set up constant boundary conditions from literature as follows:

• inlet axial-velocity = 0.085858553 m/s.
• outlet pressure = 9662.149847 Pa.
• For the initial pressure at the inlet i used Hagen-Poiseuille equation to get an initial guess.

Initially i want to simulate the flow with constant boundary conditions, as i am currently not interested in a time dependent profile of the inlet and outlet conditions. So i segmented the profile of velocity and pressure into 0.1 intervals and calculated the average values over the segment using the trapezoidal method.
However, i still need to run the simulation in transient as i am coupling it with transient structural and after that i want to perform cyclic loading on transient structural.

I ran the simulation with the following boundary conditions as steady state.
After which, i imported the node data as initial conditions and adjusted the boundary conditions to the new results i calculated in fluent through area weighted average calculation of inlet and outlet total pressure and inlet axial velocity.
Then i proceeded to run the simulation coupled with the structural domain.

My solution analysis setup was as follows:

• Simulation time = 1s.
• Time step size = 2e-2s.
• Iterations of the coupled solution = 15 iterations.
• Iterations of the fluid domain = 50.

I am aware that my timestep size was too big. however, i have run this model numerous times before for smaller time step sizes and yet the general outcome is more or less the same, which is, the continuity equation struggles to reach my convergence criteria (standard 1e-3 as per fluent) in the initial time steps and takes much time to finally hit the desirable convergence criteria.

My question is, what am i doing wrong in this whole setup and workflow? is there any adjustment i must consider for better results?

I have attached below the residual plot for the continuity, xyz-momentum, and additionally i set up a monitor at the center of the tube for axial velocity rplot. The inlet velocity and outlet pressure profiles from literature are also attached.

Thank you in advance.


Inlet velocity profile.png

Outlet pressure profile.png

mid section velocity residuals.jpg

residual plot.jpg