## 3 Aeroacoustic Source Visualization
### Flow Quantities
Again, we analyzed four time instances and found that dominant pressure changes occur within the whole domain. If the glottal gap is narrow, then the pressure drop is high and vice versa. Both the pressure gradient change and the velocity change inside the glottal gap are large over one full VF cycle. As a consequence, the time derivative of the incompressible pressure is large inside the upstream and downstream region (based on the origin of the \(x\) coordinate). Based on these four time instances, we determine the phases, the flow undergoes during one VF oscillation cycle. Firstly, the resting vortical structures are blown out. Secondly, a new stable glottal jet is formed. Thirdly, this jet is decelerated. Fourthly, deceleration curls up the flow structures into vortices. Figure 7 displays the mean velocity magnitude and the TKE, respectively. The mean velocity is an important quantity inside the PCWE source term. It features in the convective source terms. As indicated, the shear layer zones are also subject to the dominant pressure structures. Furthermore, inside the glottal jet, the convective source terms will be large. The TKE is a measure of the mean kinetic energy that can be associated with vortices in the turbulent flow. As it is an averaged quantity, it does not change drastically during a full cycle and therefore only one time instance is visualized in Figure 7. As observed previously, it shows that most of the turbulent structures are located inside the glottal jet region overlapping with the regions of high mean velocity. Taking a closer look at the flow velocity, Figure 8 gives illustration of the velocity magnitude in the frequency domain. Just like for the flow pressure, at the VF fundamental frequency \(f_{0}\), the velocity changes globally and propagates downstream. The velocity profile for higher harmonics does not differ significantly from the ones at a non-harmonic frequency. The contour planes become small separated regions scattered around the whole computational domain, visualizing the turbulent structures inside the flow. Figure 7: Visualizations of the glottal jet inside the glottal region. Figure 9 shows the incompressible flow pressure gradient magnitude in the time domain. As the flow pressure was discussed previously, many of the following observations are connected to the pressure distribution. While the VFs are opening and turbulent structures are evolving, the gradient magnitude is rather low in all areas, except the highly turbulent regions close to the VFs. Thereafter, the glottal jet flow is visible with a high gradient magnitude inside the jet region caused by turbulent structures that are moving downstream. This process continues, while the VFs are closing. The area in which a large gradient magnitude is present enlarges because of the fluctuations caused by the decelerating flow. Once the VFs are fully closed, the remaining vortical structures generate the visible pressure gradient, in the otherwise resting fluid. In the frequency domain, Figure 10 depicts the oscillation mode of the incompressible flow pressure gradient magnitude \(|\
abla p^{\mathrm{ic}}|\). For the VF oscillation frequency, the time-harmonically visualized flow pressure gradient seems to not change sign throughout the plotted region, indicating an in-phase oscillation in the whole domain. This behavior is most likely due to the global pressure variations that were discussed previously. In contrast, the oscillation mode at higher harmonic frequencies is of similar magnitude as the oscillation mode at non-harmonic frequencies. The main observation is, however, the dominance of the mode at the VF oscillation frequency. Figure 8: Fluid velocity vizualization for two frequencies. Figure 9: Pressure gradient field magnitude \(|\
abla p^{\mathrm{ic}}|\) visualized by contours at \(|\
abla p^{\mathrm{ic}}|=2\cdot 10^{5},4\cdot 10^{5},6\cdot 10^{5}\mathrm{Pa} /m\).