Oscillations, Waves, and Interactions - GWDG

82 D. Ronneberger et al.
Figure 8. Pressure amplitude (a) and sound-induced static pressure decrease (b) within
a resonator section consisting of 32 cavities with the first radial resonance at 2.94 kHz.
The static pressure decrease is normalized to the pressure drop without sound irradiation
(24 mbar). From Ref. [29].
shows an example). In such cases the acoustic transmission coefficient decreases
when the length of the lined duct section is further increased, and since the spatial
development of the instability wave depends on the frequency, the maximum of the
transmission coefficient is reached at different frequencies for different lengths of the
lined duct section (the frequency decreased when the length is increased).
(iv) The amplitude of the instability wave saturates at high amplitudes of the incident
sound wave (see Fig. 7). So also the transmission coefficient decreases if a certain
sound amplitude is exceeded.
Besides the oscillating part of the pressure also the sound-induced decrease of the
static pressure was measured in several cases, and a close relationship between both
the components of the pressures has been found:
(i) While the pressure amplitude increases as a function of the axial coordinate, the
static pressure decreases, and vice versa (Fig. 8).
(ii) It may even happen at high sound frequencies that the static pressure increases
and that the flow resistance of the resonator section decreases with sound irradiation.
(iii) While the sound amplitude increases, the pressure drop becomes noticeable only
when the transmission coefficient starts to decrease.