Devis Tonon

Aeroacoustics of pipe systems with closed branches

Flow induced pulsations in resonant pipe networks with closed branches are considered in this review paper. These pulsations, observed in many technical applications, have been identified as self-sustained aeroacoustic oscillations driven by the instability of the flow along the closed branches. The fundamental aspects of the flow induced pulsations are discussed, with particular attention to the description of the sound sources. A single mode model for the prediction of the self-sustained oscillations is presented, the “energy balance”. This model consists of the evaluation of the amplitude of each acoustic mode of the system by means of a balance between the acoustic source power and the acoustic power losses. The main components of this prediction method are discussed; these are the evaluation of the acoustic behavior of a pipe network and the modeling of the sound sources and the acoustic losses. Several field and scale model examples of pipe systems displaying self-sustained oscillations are presented, in order to discuss the parameters influencing the aeroacoustic behavior of pipe networks. Finally some counter-measures for the prevention of self-sustained oscillations are reviewed and perspectives for future work are considered.

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

Whistling of short corrugated pipes: Experimental investigation of the source locations

The goal of this study is to investigate the issue of the aeroacoustic source location within corrugated pipes. For this purpose, a configuration with a short pipe and well defined boundary conditions is chosen. These investigations have been carried out focusing on the first two whistling modes. This allows for a precise knowledge of the distribution of the acoustic velocity and pressure within the pipe. The methodological investigation of the source locations has been carried out by using straight pipe segments to replace the corrugated section near the acoustic velocity nodes and near the acoustic velocity antinodes. The sections of the corrugated pipe near the acoustic velocity antinodes has been identified as the main location of the sound sources.

Flow-induced pulsations in double closed branch systems

Flow-induced pulsations are frequently observed in pipe networks, when the main ow passes along a number of closed branches. Although there have been detailed studies on the pulsations generated by the separating ow along a closed side branch, the configurations with the ow entering or leaving the side branch have not been extensively analyzed. In this study, strong ow-induced pulsations have been measured in configurations with a mean ow diverging into a side branch and entering the main pipe from a side branch. Pulsations amplitudes almost as strong as observed in con??gurations with closed side branches have been measured.

Experimental investigation of the source locations for whistling short corrugated pipes

The goal of this study is to investigate the issue of the aeroacoustic source location within corrugated pipes. A configuration with a short pipe and well defined boundary conditions has been chosen, in order to have a precise knowledge of the distribution of the acoustic velocity and pressure within the pipe. The source locations have been investigated methodologically by using straight pipe segments to replace the corrugated section near the acoustic velocity nodes or near the acoustic velocity antinodes. The main locations of the sound sources have been identified to be the sections of the corrugated pipe near the acoustic velocity antinodes. Copyright ©2010 by ASME.

Aeroacoustics of a wall perforation in the pure grazing flow regime: effect of the perforation geometry

Acoustical dampers are used in order to avoid the noise propagation. Well known examples are the aero-engine liners, the IC-engine exhaust muffers, and the liners in combustion chambers. These devices comprise wall perforations, responsible for their sound absorbing features. Understanding the effect of the flow on the acoustic properties of a perforation is essential for the design of acoustic dampers. In the present work the effect of grazing flow on the impedance of slit shaped wall perforations is experimentally investigated by means of a multi-microphone impedance tube. Measurements are carried out for perforations with different geometries. The focus of the experiments is on the Strouhal number dependence of the acoustic impedance. An analytical model of the aeroacoustic behaviour of a two-dimensional wall perforation subject to grazing flow is proposed. These theoretical results are used to qualitatively explain the effect of the perforation geometry on its acoustic absorption properties.

A Semi-analytical Model for Perforated Pipe Mufflers

A quasi-steady semi-analytical model is proposed to predict the grazing-bias flow in a perforated-pipe muffler at low Mach numbers for high Reynolds numbers based on the diameter of the perforation ...

Numerical Prediction Method for the Acoustic Characterization of Network Elements in Presence of Mean Flow

In the present paper a numerical prediction method for the acoustic characterization of network elements is developed. The acoustic field inside the tested elements is evaluated solving the linearized Euler equations (LEE), or the acoustic perturbation equations (APE), with a discontinuous Galerkin method (DGM). The pressure field is post-processed with a plane wave decomposition method to obtained left- and right-running plane waves at each element port. To characterize the element either the scattering matrix or the impedance is evaluated. For the solution of the hydrodynamic mean flow field the open source OpenFOAM toolbox is used. The numerical method is applied to evaluate the scattering matrix of a sudden area discontinuity both with medium at rest and in presence of mean flow and to evaluate the impedance of an orifice plate as function of the frequency. For both cases the results obtained by the numerical simulations are compared with experimental data.