G Günes Nakiboglu

On the whistling of corrugated pipes: effect of pipe length and flow profile

Whistling behaviour of two geometrically periodic systems, namely corrugated pipes and multiple side branch systems, is investigated both experimentally and numerically. Tests are performed on corrugated pipes with various lengths and cavity geometries. Experiments show that the peak-whistling Strouhal number, where the maximum amplitude in pressure fluctuations is registered, is independent of the pipe length. Experimentally, a decrease of the peak-whistling Strouhal number by a factor of two is observed with increasing confinement ratio, i.e. the ratio of pipe diameter to cavity width. A numerical methodology that combines incompressible flow simulations with vortex sound theory is proposed to estimate the acoustic source power in periodic systems. The methodology successfully predicts the Strouhal number ranges of acoustic energy production/absorption and the nonlinear saturation mechanism responsible for the stabilization of the limit cycle oscillation. The methodology predicts peak-whistling Strouhal numbers in agreement with experiments and explains the dependence of the peak-whistling Strouhal number on the confinement ratio. Combined with an energy balance, the proposed methodology is used to estimate the acoustic fluctuation amplitudes.

Aeroacoustic power generated by multiple compact axisymmetric cavities : effect of hydrodynamic interference on the sound production

Aeroacoustic sound generation due to self-sustained oscillations by a series of compact axisymmetric cavities exposed to a grazing flow is studied both experimentally and numerically. The driving feedback is produced by the velocity fluctuations resulting from a coupling of vortex sheddings at the upstream cavity edges with acoustic standing waves in the coaxial pipe. When the cavities are separated sufficiently from each other, the whistling behavior of the complete system can be determined from the individual contribution of each cavity. When the cavities are placed close to each other there is a strong hydrodynamic interference between the cavities which affects both the peak amplitude attained during whistling and the corresponding Strouhal number. This hydrodynamic interference is captured successfully by the proposed numerical method.

Self-sustained Aeroacoustic Oscillations in Multiple Side-Branch Pipe Systems

ow along closed side branches of the pipe system. The investigation of this phenomenon is carried out on a scale model. Since the scale model shows an acoustic behavior similar to the compressor station it is used in order to characterize some of the design parameters that are inuencing the aeroacoustic behavior of the pipe network. These parameters are the shape of the edges at the junction between the side branches and the main pipe, the depth of the side branches and the geometrical symmetry of the system. The resonance modes of the pipe network are predicted by means of a plane wave acoustic model. A model for the evaluation of the global maximum pulsation amplitude from the knowledge of a local maximum pulsation amplitude is then presented.

A parametric study on the whistling of multiple side branch system as a model for corrugated pipes

Corrugated pipes are widely used in industry due to their inherent character of being globally flexible and locally rigid. Under certain conditions flow through the corrugated pipes causes severe noise and vibration problems. Thus, to understand the phenomenon and parameters that play role is a real asset for industry. This study is a continuation of a research based on multiple side branch system and presented together with results of an investigation performed on corrugated pipes. Many similarities between the corrugated pipes and multiple side branch system have been observed. A Strouhal number which uses as characteristic length the cavity width plus the upstream edge radius yields the best collapse of the data for both corrugated pipes and multiple side branch system. For both systems the upstream edge radius of the cavity has significant effect on pressure fluctuation amplitudes. It can increase the amplitude of the pressure fluctuation by an order of magnitude compared to sharp edges. The radius of the downstream edge has a less pronounced effect on the sound production. Strouhal numbers display two hydrodynamic modes the first with a Strouhal number around 0.1 and the second one varying in the range between 0.4 and 0.6. The variation in critical Strouhal number for the second hydrodynamic mode correlates with the relative corrugation volume compared to the pipe volume. Experiments with corrugated pipes reveal that 1 st hydrodynamic mode is limited to configuration with small relative corrugation volume. The first hydrodynamic mode was not yet observed in the multiple side-branch systems. Copyright

Onset of Flow Induced Tonal Noise in Corrugated Pipe Segments

Corrugated pipes combine small-scale rigidity and large-scale flexibility, which make them very useful in industrial applications. The flow through such a pipe can induce strong undesirable tonal noise (whistling) and even drive integrity threatening structural vibrations. Placing a corrugated segment along a smooth pipe reduces the whistling, while this composite pipe still retains some global flexibility. The whistling is reduced by thermoviscous damping in the smooth pipe segment. For a given corrugated segment and flow velocity, one would like to predict the smooth pipe length just sufficient to avoid tonal noise: the onset of whistling. A linear model based on empirical data is proposed that predicts the conditions at the onset of whistling for a composite pipe at moderately high Reynolds numbers, Re: 3000 < Re < 100; 000. Experimental results for corrugated pipes of eight different corrugation geometries are presented revealing fair agreement with the theory. Based on these results, a universal qualitative prediction tool is obtained valid for corrugated pipe segments long compared to the acoustic wave-length.

Corrugated pipe segment with anechoic termination: critical Mach number for whistling

Corrugated pipes combine local stiffness with global flexibility, which makes them very useful. A drawback of corrugated pipes is the whistling, which results from coupling of vortex shedding with acoustic plane waves due to the flow through the pipe. We consider the possibility to reduce whistling by placing a short corrugated pipe segment along a long smooth pipe. A linear model is proposed in which the magnitude of the dipolar sound source is determined empirically. The model allows to estimate the critical Mach number of the flow through the pipe above which the whistling will always occur even for anechoic pipe terminations.

Hydrodynamic interaction between whistling axisymmetric cavities in the presence of a coupling longitudinal standing wave

An axisymmetric cavity along a pipe is a commonly encountered construction in industrial applications, which can also be considered as a unit element of a corrugated pipe. At critical conditions such a configuration causes severe noise problems, called whistling. Whistling is a self-sustained oscillation, driven by a flow-acoustic interaction. In the current study, the hydrodynamic interaction between two such cavities is addressed in the presence of a coupling standing wave along the pipe. The phenomenon is investigated both experimentally and numerically. The hydrodynamic interaction has a strong effect both for the amplitude and for the Strouhal number of the whistling, which depends critically on the separation distance between two adjacent cavities.