Experimental evidence of an instability over an impedance wall in a duct with flow

An experimental investigation of the acoustical behaviour of a liner in a rectangu- lar channel with grazing flow has been conducted. The liner consists of a ceramic structure of parallel square channels: 1mm by 1 mm in cross section, 65 mm in length, and a surface density of 400 channels/inch square. The channels are rigidly terminated, thus constituting a locally reacting structure. In the absence of flow the liner reacts classically: There is a significant decrease in transmission coefficient around the frequency of minimal impedance. When the wall is exposed to a grazing flow this behaviour is changed: an increase in transmission coefficient appears at this resonance frequency. The transmission coefficient can be even rise above 1 (up to 3 for a Mach number of 0.3). This behaviour is caused by the appearance of a hydrodynamic instability above the liner. Furthermore, the stationary pressure drop induced by this liner is deeply affected by its acoustic behaviour. When a sound wave is added, at the resonance frequency of the liner, the pressure drop can increase by a factor 3 when the Mach number is 0.3. This effect is attributed to a modification of the turbulent boundary layer induced by the acoustic wave

Hydrodynamic instability and sound amplification over a perforated plate backed by a cavity

International audienceThe long-wavelength hydrodynamic behaviour over a cavity-backed perforated plate, in a duct with a mean shear flow, is studied numerically using the multimodal method, where the acoustic and hydrodynamic disturbances are calculated from the linearized Euler equations. The flow-acoustic coupling near the perforated plate is first solved hole by hole, and results indicate a well-defined large-scale hydrodynamic wave over the plate, with a wavelength close to the plate length at the peak sound amplification frequency when a plane acoustic wave is introduced from the upstream duct. Since the hydrodynamic wavelength is one order larger than the period of the perforation, the effect of the perforated plate is then described by a homogeneous plate impedance. It is shown that the homogenized approach approximately represents the discrete approach in this problem

Transmission loss predictions for dissipative silencers of arbitrary cross section in the presence of mean flow

A numerical technique is developed for the analysis of dissipative silencers of arbitrary, but axially uniform, cross section. Mean gas flow is included in a central airway which is separated from a bulk reacting porous material by a concentric perforate screen. The analysis begins by employing the finite element method to extract the eigenvalues and associated eigenvectors for a silencer of infinite length. Point collocation is then used to match the expanded acoustic pressure and velocity fields in the silencer chamber to those in the inlet and outlet pipes. Transmission loss predictions are compared with experimental measurements taken for two automotive dissipative silencers with elliptical cross sections. Good agreement between prediction and experiment is observed both without mean flow and for a mean flow Mach number of 0.15. It is demonstrated also that the technique presented offers a considerable reduction in computational expenditure when compared to a three dimensional finite element analysis

Experimental observation of a hydrodynamic mode in a flow duct with a porous material

This paper experimentally investigates the acoustic behavior of a homogeneous porous material with a rigid frame (metallic foam) under grazing flow. The transmission coefficient shows an unusual oscillation over a particular range of frequencies which reports the presence of an unstable hydrodynamic wave that can exchange energy with the acoustic waves. This coupling of acoustic and hydrodynamic waves becomes larger when the Mach number increases. A rise of the static pressure drop in the lined region is induced by an acoustic excitation when the hydrodynamic wave is present

Orifice impedance under grazing flow: Modal expansion approach

Acoustic lining is used in jet engines to reduce noise radiation. The liners consist of up to several layers of perforated plate backed with honeycomb structure. A key aspect in their performance is the in uence of the grazing ow on the perforates' impedance. In our research we concentrate on the effect of ow on a single orifice. Comparison of previous experiments with existing theory was unsatisfactory. Especially we concluded that the boundary layer thickness of the grazing ow is an important factor. In this paper we present a theoretical model, which accounts for boundary layer characteristics. Two parallel 2D ducts with partially an interconnection, representing the orifice domain, are considered. The eigen modes in the ducts and orifice domain are solved numerically for any given ow profile. By subsequent matching of the modal expansions of the acoustical field between the domains, the acoustic impedance of the orifice is determined. Comparison with impedance tube measurements shows reasonably good agreement for both resistance and end correction. However, numerical convergence of the model is not yet established

Influence of mean flow profile and geometrical ratios on scattering of sound at a sudden area expansion in a duct

The scattering of sound at a sudden area expansion in a duct with subsonic mean flow has been modelled with a multimodal method. Technological applications are for instance internal combustion engine exhaust silencers and silencers in industrial duct systems. Both 2D rectangular and 2D cylindrical geometries are considered. The influence of the mean flow profile, and the-in this method-associated application of an acoustic Kutta condition at the edge of the area discontinuity, is investigated. The scattering coefficients for the plane waves are found to change smoothly as the flow profile is changed gradually from one, where the acoustic Kutta condition is applied to one where it is not applied. Furthermore, for high Strouhal numbers no difference is observed in the results for the scattering coefficients obtained for different flow profiles. Also, at low Strouhal numbers the magnitudes of the scattering coefficients are the same for different profiles. The influence of the ratio of the heights (in 2D rectangular geometry), respectively, radii (in 2D cylindrical geometry), of the ducts upstream and downstream of the area expansion on the scattering coefficients is examined. Around a certain Strouhal number, a specific feature in the scattering coefficients is observed when the ratio of the duct heights or radii is less than 0.5. This is found to be connected to a strong interaction between the first evanescent acoustic mode and the hydrodynamic instability mode. For non-uniform flow even an apparent jump between the first evanescent acoustic mode and the hydrodynamic unstable mode and a corresponding jump in scattering coefficients is observed, when employing causality analysis according to the Briggs-Bers or Crighton-Leppington procedure. This implies that in fact an absolute instability occurs. © 2009 Elsevier Ltd. All rights reserved