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Solving Galbrun's Equation with a Discontinuous galerkin Finite Element Method

Over many years, scientists and engineers have developed a broad variety of mathematical formulations to investigate the propagation and interactions with flow of flow-induced noise in early-stage of product design and development. Beside established theories such as the linearized Euler equations (LEE), the linearized Navier–Stokes equations (LNSE) and the acoustic perturbation equations (APE) which are described in an Eulerian framework, Galbrun utilized a mixed Lagrange–Eulerian framework to reduce the number of unknowns by representing perturbations by means of particle displacement only. Despite the advantages of fewer degrees of freedom and the reduced effort to solve the system equations, a computational approach using standard continuous finite element methods (FEM) suff ers from instabilities called spurious modes that pollute the solution. In this work, the authors employ a discontinuous Galerkin approach to overcome the difficulties related to spurious modes while solving Galbrun's equation in a mixed and pure displacement based formulation. The results achieved with the proposed approach are compared with results from previous attempts to solve Galbrun's equation. The numerical determination of acoustic modes and the identification of vortical modes is discussed. Furthermore, case studies for a lined-duct and an annulus supporting a rotating shear-flow are investigated.

Open Access
Rating Level as a Method to Assess the Impact of Speech Noise on Cognitive Performance and Annoyance in Offices

In Germany, the rating level is an important parameter to assess noise immissions in occupied offices. The rating level denotes the energy-equivalent sound pressure level during a measurement period with speech sounds and considers penalties for tonal, informational and impulsive constituents. There is little evidence that the rating level correlates with the performance and perceived annoyance of office workers. This study evaluates 89 different sound conditions under which subjects have to complete a short-term memory task and a questionnaire in laboratory conditions with respect to their relationships with the rating level. The relationships of the penalty for impulsiveness and the penalty for tonality or informational constituents with the rating level are analyzed separately. In addition, the penalty for tonality or informational constituents is substituted by percentile level statistics, namely the difference between the 10th and 90th percentile levels. In contrast to the penalty for tonality or informational constituents, this metric is objectively measurable. Using the rating level to assess the noise at office workplaces could be improved by using percentile level statistics to account for informational constituents. To improve the predictive validity, it is suggested to report the penalties separately.

Anthropogenic Noise Mapping of a Periurban Natural Park on the Coastal Area in the SW of Spain: Case Study of Bird's Ecosystem Protection

This article presents a methodological approach to try to respond to some of the protection and management needs against the noise of a peri-urban natural park. The methodology presented is based on the generation of "ad hoc" noise maps. To analyze its possibilities and the limits of use, a coastal park surrounded by a densely populated area in the southwest of Spain is used as a case study. In this study, birds in their diverse ecosystems are the main target noise receiver of the study. The source of noise pollution considered is the traffic noise of the highways and the urbanized areas surrounding the park. However, the methodology can be extrapolated to any source of noise and other protection figures. An adequate diagnosis of the environmental noise would help to overcome the supposed incompatibility between the preservation of nature and the tourist exploitation of natural spaces. With this in mind, it has also been proposed as target noise receivers, the ornithologists and visitors who wish to become bird-watchers and bird-listeners. To this end, it has been proposed to produce noise maps with certain methodological guidelines that fit on a case-by-case basis. Several heights are used in this paper, adapting the map to noise receivers. With the same purpose, noise level maps in octave bands were developed. The tonal frequencies of interest are those that the birds use in their songs (according to the species, normally between 2 and 8 kHz). The maps have been contrasted with noise measurements carried out throughout the park. The study shows that in the areas most exposed to the noise of the Rio San Pedro and the university campus, noise levels at 2 kHz can reach 74 dB during the peak traffic hours. In addition, a large percentage of the area of both areas is affected by noise levels that exceed 50 dB (100% and 44% respectively). We are also concerned that a small population of birds has been counted in these areas based on preliminary observations at peak traffic times. The results can help the decision-makers to evaluate how traffic noise invades different ecosystems and where it can mask the sound of birds.