On prediction of FIP in closed side branches by CFD

In single phase internal flow past closed side branches, the shear layer instability in the branch opening provides a tonal acoustic source propagating into the side branch. In the event
that one of the dominant frequencies of the shear layer oscillation coincides with an acoustic mode of the fluid in the side branch, the fluid dynamics and acoustics lock-in and high
amplitude resonant conditions can occur. If, in turn, the lock-in mode coincides with a structural mode of the piping system, the vibrations caused by the vibro-acoustic coupling can lead to rapid fatigue failure. The acoustic pressure amplitude of the lock-in mode at the
end of the closed side branch can be estimated via different means, such as energy balance methods or plane wave acoustic models. The required input source strength can be estimated
empirically or by use of Panel Methods. Another method, previously discussed in Macchion et al (2016), involves the use of computational fluid dynamics (CFD) which provide a more accurate representation of the three-dimensional geometry. This paper investigates and benchmarks the use of CFD to predict the occurrence and strength of the lock-in amplification in single and coaxial closed side branches by comparing experimental data with results from improved delayed detached eddy simulations (IDDES) and unsteady Reynolds-averaged Navier-Stokes simulations (URANS). The URANS results show good correlation with the experimental data, both in terms of
onset of resonance and in terms of the associated strength of the amplified pressure wave. The IDDES results show similar correlation for some experimental points. However, for other, the
strength of the amplified pressure wave is severely over predicted. This is possibly due to unphysical excitation of the lock-in mode through broadband turbulent noise caused by
uncertain inlet boundary conditions. For a single side branch configuration, URANS predicts the peak pulsation amplitude within a few percent. For a coaxial side branch configuration, the peak pulsation amplitude is conservatively predicted within approximately 60 %. This is
likely due to an overprediction of the acoustic source strength at the junction between the branches.