Oscillations, Waves, and Interactions - GWDG

76 D. Ronneberger et al.
(flow velocity) / (speed of sound) power spectral density
40
db
20
10
0
0.3
0.2
0.3
0.2
0.1
a
b
A 1
A 1
0.1
0 10 20 30 dB 40
c
1 A
0 1 2
frequency [kHz]
3 4
Figure 2. Power spectral density (psd) of the pressure downstream and upstream of the
resonator section: (a) U/c = 0.27, downstream; (b) difference to the median of the distribution
of the psd, as a function of the flow velocity, downstream; (c) same as (b), but
upstream.
dependence on the incident sound amplitude [25–28]. These phenomena have meanwhile
turned out to be the effect of a strong seemingly convective instability of the
turbulent flow in the lined duct section. Various experimental observations and some
qualitative explanations of the phenomena will be presented in Sects. 2 and 3 while
the attempts and the difficulties to understand the instability will be discussed in
Sect. 4.
2 Experimental observations
2.1 Axisymmetric mode
2.1.1 Turbulent pressure fluctuations, sound transmission and static pressure drop
The sound amplification and the acoustic influence on the static pressure occur at
frequencies slightly above the first radial resonance frequency of the cavities. In
contrast to the original study with frequencies well below the resonance frequency,
most of the experiments to be reviewed in the following have therefore been performed
A 2
A 2
2 A
D
B1
A 3