This research introduces a new application of the three microphones method, which was originally developed to analyse standing waves, to measure the aeroacoustic power of a duct housing a shallow cavity coupled with a longitudinal acoustic mode. In addition, this work provides, for the first time, the spatial distribution of the aeroacoustic sound sources over the cavity region for this type of flow-sound-structure interaction pattern. Furthermore, this research includes a comprehensive study of the effect of cavity geometrical parameters on the characteristics of the cavity aeroacoustic source.
An experimental investigation of the aeroacoustic source of an axisymetric cavity in a pipeline is presented. This aeroacoustic source is generated due to the interaction of the cavity shear layer oscillation with the resonant acoustic field in the pipe. The source is determined under high Reynolds number, fully developed turbulent pipe flow. The experimental technique (Sound Wave Method, SWM) employs six microphones distributed upstream and downstream of the cavity to evaluate the fluctuating pressure difference generated by the oscillating cavity shear layer in the presence of externally imposed sound waves. The results of the dimensionless aeroacoustic sources are in good agreement with the concepts of free shear layer instability and the fluid resonant oscillation behavior.
A validation study is performed in order to validate the measurement technique and the measured source term from the SWM. The validation methodology consisted of comparing the self-excited resonance response obtained from self-excitation measurements with that estimated from an acoustic model supplemented with the measured source term using the SWM. The comparison depicts a very good agreement for the resonance frequency, lock-in ranges, and the resonance amplitude.
Extensive PIV flow measurements are performed to clarify the non-linear behavior of the aeroacoustic source at high levels of the acoustic particle velocity, and to understand the dependence of the flow-sound interaction patterns on the main system parameters such as the Strouhal number and excitation level. The results of a finite element analysis of the resonant sound field are combined with those of the PIV flow measurements into Howe’s aeroacoustic integrand to compute the spatial and temporal distributions of the aeroacoustic sources. The results are also compared with the measured aeroacoustic source strength obtained by means of the SWM. This comparison highlights the superior efficiency of the SWM technique. Identification of the aeroacoustic source distributions as function of the acoustic excitation level showed that the non-linear behaviour of the source strength, which occurs at moderate sound levels, is caused by a gradual transition in the vorticity field oscillation pattern; from a distributed vorticity cloud over the whole cavity length at small excitation amplitudes to a pattern involving rapid formation of (discrete) vortices at the leading edge which becomes dominant at large excitation levels.
The spatial distribution of the acoustic power over the cavity length at resonance condition shows sources of sound generation at the first and last thirds of the cavity mouth and an absorption sink in the middle third. This distribution is different from that observed for deep cavities and trapped modes of shallow cavities. Due to these differences in the aeroacoustic source distributions, the effects of cavity geometrical parameters for the present shallow cavity are not necessarily similar to those reported in the literature for deep cavities and trapped mode resonance cases.
A comprehensive study of the effect of cavity geometrical parameters (including rounding-off the cavity edges) on the aeroacoustic sound sources is also included. Nine cavity sizes are studied in three different groups of length to depth ratios (L/H) with three different cavity volumes for each group of L/H. The aeroacoustic source strength and the Strouhal number corresponding to its maximum value are found to increase in a systematic manner as the cavity volume is increased for the same L/H ratio. These results indicate that the aeroacoustic sources of shallow cavities are affected not only by the ratio L/H, but also by the cavity volume.
The effect of cavity edge curvatures on the resonance response is experimentally investigated by testing different sizes of curvatures at different locations (upstream, downstream or both edges). The results show that rounding-off the cavity edges causes a reduction in the vertical component of the acoustic particle velocity but also an increase in the cavity length. These two consequences have opposite effects on acoustic power generation and therefore, rounding-off the edges has no significant effect on the resonance amplitude in the present case, except for relatively large radius. / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/18288 |
| Date | 11 1900 |
| Creators | Mohamed, Saber Ragab Taha |
| Contributors | Ziada, Samir, Mechanical Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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