[Ames]

Hi all,

I am trying to run a transient simulation in Ansys Fluent. I have a narrowed pipe where I prescribe a certain flow at the inlet and zero pressure at the outlet. The walls are rigid and a no slip condition was applied.

I was having some convergence issues when using the steady solver, so I want to switch to transient to see if these problem resolve. I want to prescribe the same flow for all time steps.

I have written a UDF that prescribes a paraboloid velocity profile at the inlet. I will initialize the velocity and pressure with 0.

I am trying to build up slowly the flow at the inlet instead of forcing it all at once in one time step. Could someone help me with how I can best prescribe the flow at the inlet? I am mostly struggling with deciding how fast I will have to ramp the velocity at the inlet. I understand how I can manage it using the UDF, but on what time scale can I best increase the flow from 0 to 5.9 mL/s?

I have a mesh of about 400,000 elements (mesh size 0.15mm) and in steady simulations I have seen max. velocity of 4.5 m/s. According to the courant number I would need a time step of 1e-5.

Thanks in advance!

[AlexanderZ]

from my point of view, criteria is convergence,
in best case if you can get converged simulation with 5.9 mL/s -> you don't need to decrease speed

for instance start with 0.59, check residuals, increase velocity and so on