Application of Viscothermal Wave Propagation Theory for Reduction of Boundary Layer Induced Noise

Boundary layer induced noise, i.e. noise inside the aircraft resulting from the turbulent boundary layer enclosing the fuselage, is known to dominate air-cabin noise at cruise conditions. In this paper a method is described to design trim panels containing a large number of coupled tubes to effectively reduce this type of noise. The theory of viscothermal wave propagation in tubes, as presented by Tijdeman [3], is discussed. To illustrate the procedure the absorption coefficient for a panel containing a number of non-coupled tubes is calculated. Initial results optimising the tubes’ length and radii for a desired fictive absorption coefficient are presented and prove the applicability of the method

Optimised Sound Absorbing Trim Panels for the Reduction of Aircraft Cabin Noise

The EU project FACE (Friendly Aircraft Cabin Environment) aims to improve the environmental comfort in aircraft cabins. As part of this project, this paper focuses on the reduction of noise in aircraft cabins. For modern aircraft flying at cruise conditions, this cabin noise is known to be dominated by turbulent boundary layer noise. The purpose of this work is to reduce the resulting sound pressure levels in the cabin by means of optimised sound absorbing trim panels with quarter-wave resonators. Sound absorption with quarter-wave resonators is mainly realised by dissipation of sound energy as a result of viscous and thermal losses. The viscothermal wave propagation of the air inside the resonators is efficiently and accurately described by the so-called low reduced frequency model. By optimisation of the dimensions of the resonators, desired sound absorption characteristics can be obtained for different specified frequency ranges. This means that the panels can be tailored to different positions in the aircraft cabin with different prevailing sound pressure levels. Results of optimisations for various frequency ranges show that a very good agreement is obtained between the desired and the calculated absorption curves. With the same optimisation procedure, panels have also been tuned for the dominant frequency range of a sound spectrum measured in a modern aircraft. Experimental validation of the numerically predicted optimal configurations, by means of impedance tube measurements, shows that a fairly good agreement is obtained between the numerical and experimental results

Reconstruction of the Settlement History of Building

Although it is a well-known fact that buildings can settle, it is often not known how much settlement has occurred since the construction. Three case studies in the Netherlands are presented which deal with the following questions: has the settlement process stopped or is it continuing and if so, what settlements can still be expected in the future? All three cases show large settlements of up to a maximum of 0.8 m since construction. This paper shows how the magnitude of the settlement since the construction can be reconstructed by analysing settlement data, covering only a relatively short period of time

A finite element approach to the prediction of sound transmission through panels with acoustic resonators

Previous research by the authors has shown that sound radiated by a vibrating panel can be reduced considerably by using tuned acoustic resonators. The length of the tube resonators determines the frequency range in which sound is reduced. The shape of the spectrum is determined by the ratio of the cross-sectional areas of the resonators to the area of the panel. Maximum sound reduction is achieved if the volume velocities at the surface of the vibrating panel and those at the entrance of the resonators are equal in magnitude but opposite in phase. Up to now, the effect of the resonators on the radiated sound has been studied with a one-dimensional analytical model. In this paper, a three-dimensional acousto-elastic model is developed using the finite element method. The purpose of this model is to study the influence of the flexibility and the boundaries of the panel, as well as the presence of rooms behind and in front of the panel on the sound transmission. Modelling the complete structure, including the resonators and the interaction with the air inside the resonators, is computationally expensive. Therefore, an alternative approach is developed. Because of the repetitive pattern of resonators in the panel, the structural part of the panel is modelled with superelements. To enable coupling between the structural part of the model and the air behind and in front of the panel, a new interface element is derived. The formulation of this interface element also includes the acoustic behaviour of the resonators. Sound transmission loss calculations are made for one configuration and the results are compared with the results obtained with a one-dimensional analytical model

Application of acoustically tuned resonators for the improvement of sound insulation in aircraft

One of the aims of the EU project FACE (Friendly Aircraft Cabin Environment) is to reduce aircraft interior noise. For modern aircraft flying at cruise conditions, the turbulent boundary layer is the main source for cabin noise. Normally, the turbulent boundary layer causes the trim panels to vibrate, and hence to radiate sound into the aircraft cabin. The purpose of the present work is to reduce this kind of noise by means of sound insulating trim panels with tuned acoustic resonators1. The length and the radius of these resonators are tuned in such a way that the volume velocities at the vibrating panel surface and at the entrance of the resonators are equal in magnitude but opposite in phase. In this way, maximum reduction of the radiated sound can be achieved for a specified frequency range. Because of the repetitive pattern of the resonators in the panel, the influence of the resonators on the sound radiated in normal direction by the panel is studied with a one-dimensional model. The so-called low reduced frequency model is extended to describe the viscothermal wave propagation in the vibrating resonators. An advantage of the viscothermal effects is that, in the low frequency range, more sound reduction is obtained than if these effects are not present or very small. Calculations show that a large reduction of the radiated sound can be achieved. The model is also validated by experiments in an impedance tube. Good agreement is found between theory and measurements

Book of Buechner (Book Review)

Reviewed Title: Brown, Dale. The Book of Buechner. Louisville, KY: Westminster John Knox Press, 2006, 394pp. ISBN 978-0-664-23113-2

Locally Adaptive Frames in the Roto-Translation Group and their Applications in Medical Imaging

Locally adaptive differential frames (gauge frames) are a well-known effective tool in image analysis, used in differential invariants and PDE-flows. However, at complex structures such as crossings or junctions, these frames are not well-defined. Therefore, we generalize the notion of gauge frames on images to gauge frames on data representations $U:\mathbb{R}^{d} \rtimes S^{d-1} \to \mathbb{R}$ defined on the extended space of positions and orientations, which we relate to data on the roto-translation group $SE(d)$, $d=2,3$. This allows to define multiple frames per position, one per orientation. We compute these frames via exponential curve fits in the extended data representations in $SE(d)$. These curve fits minimize first or second order variational problems which are solved by spectral decomposition of, respectively, a structure tensor or Hessian of data on $SE(d)$. We include these gauge frames in differential invariants and crossing preserving PDE-flows acting on extended data representation $U$ and we show their advantage compared to the standard left-invariant frame on $SE(d)$. Applications include crossing-preserving filtering and improved segmentations of the vascular tree in retinal images, and new 3D extensions of coherence-enhancing diffusion via invertible orientation scores