The effect of smoothing a simulated PT history by averaging over
100 points is shown in Fig. 10-right. It can be seen that the highfrequency
oscillations are still present in smoothed trace, and some
low-magnitude oscillations with a short period also remain. The
latter have period of 0.9 ms and are related to natural oscillations of
the bottle, as explained in the experimental part. On the other
hand, the origin of the high-frequency oscillations is not that clear.
It was previously believed that these frequencies were of numerical
nature caused by interactions on the solid-fluid interface (Greenshields
et al. [16]). However, detailed analysis of the pressure traces
and pressure distribution in the fluid domain shows that they have
a physical meaning, and are related to the presence of unconfined
waves in the fluid, as explained below.