Effect of Thermal Losses and Fluid-Structure Interaction on the Transfer Impedance of Microperforated Films

It has been shown previously that incompressible computational fluid dynamics (CFD) models can be solved in the time domain to calculate the transfer impedances of microperforated panels. However, these models require relatively lengthy run times, do not allow for thermal losses due to irreversible heat transfer to the panels, and rely on the assumption that the solid parts of the panels are rigid. In the present work, compressible, thermo-acoustic models, solved in the frequency domain, have been used to compute thermal losses in addition to viscous losses; these calculations enable the visualization and spatial localization of both loss mechanisms. Thermal losses prove to be relatively small compared to viscous losses in typical geometries, but they become progressively more important as the frequency increases. Additionally, the fully-coupled fluid-structure interaction (FSI) problem has been solved to determine the range of parameters within which the transfer impedance of a rigid microperforated panel can be added in parallel to the impedance of a limp panel ( ) to account for panel flexibility. In particular it will be shown under what conditions the relative motion between the fluid velocity through the perforations and the velocity of the panel, including its phase, must be explicitly considered

Computational investigation of microperforated materials: end corrections, thermal effects and fluid-structure interaction

The concept of microperforated noise control treatments was introduced by Maa 1975; in that theory, the transfer impedance of the microperforated layer was calculated based on oscillatory viscous flow within a small cylinder combined with resistive and reactive end corrections. Initially, microperforated materials were the subject of mostly academic study since practical implementations were rare owing to the cost of manufacturing the materials with acceptable accuracy. However, recently, new manufacturing procedures have dramatically lowered the cost of these materials, and perhaps as a result, there has been renewed interest in studying their properties. Since 1975, Maa’s original theory has been widely used to predict the performance of microperforated materials. However, in principal, that theory can only be used to describe cylindrical perforation, while in practice, perforations are rarely cylindrical. In addition, there have been questions about the dependence of end corrections on frequency, and on the effect of coupling between the motion of the fluid in the perforations and the solid sheet in which they are formed. Additionally, in his original paper, Maa drew a distinction between the dissipative properties of thermally conducting and adiabatic materials. The latter topic, in particular, has not been considered by any investigators since the idea was introduced. The purpose of our presentation is to introduce the numerical tools that can be used to address the open questions mentioned above, and to highlight important results obtained by using those tools

Design of Lightweight Fibrous Vibration Damping Treatments to Achieve Optimal Performance in Realistic Applications

I n recent work, it has been shown that conventional sound absorbing materials (e.g., lightweight fibrous media) can provide structural damping when placed adjacent to vibrating structures, including infinite panels, partiallyconstrained panels and periodically-supported panels typical of aircraft structures. Thus, a fibrous layer may serve two functions at once: absorption of airborne sound and the reduction of structure-borne vibration. It has also been found that the damping is primarily effective below the critical frequency of the structure, and that the damping results from viscous interaction between the fibrous layer and the evanescent near-field of the panel, in the region where incompressible flow caused by the panel vibration oscillates primarily parallel with the panel surface. By using a near-field damping (NFD) model based on the Biot model for acoustical porous media, it has been shown that a properly-optimized fibrous layer can provide levels of damping comparable with those provided by conventional, constrained-layer, visco-elastic, damping treatments. Based on the idea that vibrating structures exhibit a certain wavenumber/frequency response spectrum, the focus of the current study has been on evaluating the power dissipated by a fibrous treatment as a function of wavenumber and frequency, and on identifying the material microstructure (i.e., fiber size) required to maximize the power dissipation, and hence damping, in a specific wavenumber/frequency range. To demonstrate the wavenumber/frequency-matching procedure, an example involving a simplified model of a vehicle component will be considered here, and it will be shown how a fibrous layer can be designed to maximize its damping effectiveness when applied to a realistic base structure, such as an automotive floor pan

Examining the dynamics of social cohesion: A call for a different perspective on scaling impacts of real-world laboratories

Social cohesion is an important impact category for scaling real-world laboratory experiments. This idea has been largely overlooked in the transformative research debate. Based on observations within real-world laboratories that focused on iterative, co-creative, and practice-based climate change adaptation, we identify social cohesion, first, as a prerequisite for real-world laboratory impacts. Second, social cohesion can itself be an impact, enhancing the scaling potential of real-world laboratories. Cooperation can pave the way for amplifying real-world laboratories’ activities temporally and spatially

Numerical Modelling of the Acoustics of Low Density Fibrous Media having a Distribution of Fiber Sizes

The focus of the present work was on a low density, polymeric, fibrous medium, comprising a mix of blown micro-fibers, having a relatively broad fiber size distribution, and a second, so-called staple fiber component having a very narrow fiber size distribution. The airflow resistivity of such a material is usually considered to be its most important macroscopic property when it comes to defining the material’s acoustical properties (since the tortuosity and the porosity are very close to unity, for example). Thus, in the first instance, it is of interest to be able to calculate the flow resistivity of such a material on the basis of the distributions of the various fiber sizes, the densities of the fibers and the bulk density of the material. A recent survey of methods for predicting the flow resistivity of fibrous media has revealed a wide variety of approaches, largely based on a knowledge of a material’s solidity (1 minus the porosity), and mean fiber spacing, but in all cases it is assumed that the fiber radius is uniform. An example of such an approach is the work of Tarnow who has developed a model based on the viscous drag experienced by uniform-sized fibers positioned within randomly-spaced Voronoi cells. Recent measurements have shown that Tarnow’s “perpendicular random” model allows accurate predictions of flow resistivity for fibrous media comprising a single fiber component having a very narrow fiber size distribution. It has also been shown that Tarnow’s theory can be modified to account for multiple fiber components having different fiber size distributions. It is then assumed that the flow resistivity calculated in that way can be used to predict the acoustic properties of the medium, although the latter approach takes no specific account of the range of fiber sizes existing within the fibrous medium. Thus, in the present work, two specific issues are addressed: first, how accurate is the new method of calculating the flow resistivity, and secondly, can the effect of fiber size distributions be neglected when predicting sound propagation through such media. The execution of this work has primarily involved the use of numerical tools, GeoDict and Comsol. In particular, Fiber Geo was used to generate a variety of fiber arrays having different orientations and fiber size distributions, and FlowDict was used to compute the pressure drop resulting from low speed viscous flow through a cell of fibers, from which the flow resistivity can be calculated, for example. In the first stage of the work, fibers were modeled as occupying a fluid volume. It is possible to specify fiber sizes and orientations in order to represent materials consisting of fibers having a specified distribution of fiber radii, based for example of micro CT scans of real materials. Flow resistivity results for sets of fibrous media having differing solidities were computed in this way. These results are for practical purposes “exact”, and so provide benchmarks against which parameterized predictions such as presented in reference can be compared. Close agreement with the latter results has been found. Further, the effect of fiber orientation has been studied, which has allowed the prediction of direction dependent flow resistivity in non-isotropic fiber arrays. In the second phase of the work, the fiber geometries considered above were imported into Comsol. By using that software, all of the JCA parameters can be calculated, hence making it possible to calculate the acoustical properties of the fibrous media: e.g., complex densities and sound speeds. At the same time, finite element models of the fiber arrays can be generated, and then linearized visco-thermal models may be solved to yield the “exact” wave propagation characteristics of the fiber arrays. Initially, rigid models were studied: i.e., no motion of the fibers was allowed. Subsequently, full fluid-structure interaction models were implemented to allow for fiber motion in response to oscillatory acoustic flows. In this way, the propagation properties of limp porous materials may be predicted, as can the properties of elastic fiber networks when the fibers are connected. By using the models described here, wave propagation in fiber arrays having realistic bi-modal fiber size distributions has been predicted and compared with corresponding predictions made using conventional Biot-based models. As a result, it has been possible to draw conclusions regarding the ability of those models to accurately represent wave propagation in fibrous media having relatively broad fiber size distributions

Predictors for prehospital first-pass intubation success in Germany

(1) Background: Endotracheal intubation in the prehospital setting is an important skill for emergency physicians, paramedics, and other members of the EMS providing airway management. Its success determines complications and patient mortality. The aim of this study was to find predictors for first-pass intubation success in the prehospital emergency setting. (2) The study was based on a retrospective analysis of a population-based registry of prehospital advanced airway management in Germany. Cases of endotracheal intubation by the emergency medical services in the cities of Tübingen and Jena between 2016 and 2019 were included. The outcome of interest was first-pass intubation success. Univariate and multivariable regression analysis were used to analyse the influence of predefined predictors, including the characteristics of patients, the intubating staff, and the clinical situation. (3) Results: A total of 308 patients were analysed. After adjustment for multiple confounders, the direct vocal cord view, a less favourable Cormack–Lehane classification, the general practitioner as medical specialty, and location and type of EMS were independent predictors for first-pass intubation success. (4) Conclusions: In physician-led emergency medical services, the laryngoscopic view, medical specialty, type of EMS, and career level are associated with FPS. The latter points towards the importance of experience and regular training in endotracheal intubation

A Comparison between Glass Fibers and Polymeric Fibers when Serving as a Structural Damping Medium For Fuselage-like Structures

In previous studies, theories have been developed and simulations performed to show that properly designed fibrous media can be very effective at reducing structural vibration as well as absorbing airborne sound. Therefore, the potential range of noise control applications for fibrous media such as glass and polymeric fiber has been broadened due to this multi-functionality. Since the acoustical, sound absorption properties of these two kinds of fibers are well-known, the current study focused on a comparison of their damping performance when they are used as layered dampers for a fuselage-like structure: i.e., a panel with frames. A layer of each kind of fiber was modeled as resting on the stiffened panel, which was then driven by a convective pressure. The solution for the panel motion was first found in the wavenumber/frequency domain, and an inverse transform was then applied to obtain the spatial results. Through model calculations, the structural damping characteristics of these two fibrous dampers were analyzed and quantified in terms of an equivalent loss factor. In addition, microstructures were designed for each fiber type that results in optimal damping performance over a frequency range of interest. Guidelines were also established for the effective use of these media in structural damping applications

Prediction of Airflow Resistivity of Fibrous Acoustical Media having Double Fiber Components and a Distribution of Fiber Radii

Presented here is a new airflow resistivity (AFR) prediction model that accounts for situations in which a fibrous medium comprises more than one fiber component, and when the radius of each fiber component varies within a certain range. The study started with the evaluation of existing AFR models, which were mostly developed for single-component fibrous media with uniform radius (SCUR). After comparing the SCUR predictions results with AFR measurements of different single-component fiber samples, a model of Tarnow’s was shown to yield reasonable prediction accuracy and was chosen as the starting point for further development. The Tarnow model was first modified to make it capable of predicting the AFR for double-component fibrous media with uniform radii (DCUR). It was then further modified by adding the effect of fiber radius distributions to the DCUR models and making it capable of predicting the AFR for double-component fibrous media with various radii (DCVR). After adjusting the distribution parameters of both components, the DCVR model prediction results were verified by comparison with AFR measurements on different double-component fiber samples. It was found, for example, that the DCVR model prediction results were affected by the implicit differences in their starting points: i.e., the assumption as to whether the material consisted of pores (e.g., Doutres and Horoshenkov) or fibers (e.g., Tarnow)

Structural Damping by Layers of Fibrous Media Applied to a Periodically-Constrained Vibrating Panel

It has recently been demonstrated that layers of fibrous, acoustical material can effectively damp structural vibration in the sub-critical frequency range. In that frequency range, the acoustical near-field of a panel consists of oscillatory flow oriented primarily parallel with the panel surface. When a fibrous layer occupies that region, energy is dissipated by the viscous interaction of the near-field and the fibrous medium, and the result is a damping of the panel motion. Previously, the damping effect has been demonstrated to occur for line-driven, infinite panels and panels with isolated constraints. In this article, the focus is instead on periodically-constrained panels driven into motion by a convective pressure distribution. The constraints are allowed to have translational and rotational inertias and stiffnesses. This arrangement is intended to represent a very simple model of an aircraft fuselage structure. By considering the power flows in this system, it is possible to compute an equivalent loss factor, and then to identify the fibrous layer macroscopic parameters that result in optimal damping at a given mass per unit area. Finally, given that information, it is possible to identify the microstructural details, e.g., fiber size, that would be required to achieve that damping in practice