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Acoustics, Volume 2, Issue 3 (September 2020) – 4 articles

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Open AccessArticle
Numerical Investigation of Influence of Entropy Wave on the Acoustic and Wall Heat Transfer Characteristics of a High-Pressure Turbine Guide Vane
Acoustics 2020, 2(3), 524-538; https://doi.org/10.3390/acoustics2030028 - 09 Jul 2020
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Abstract
As an indirect noise source generated in the combustion chamber, entropy waves are widely prevalent in modern gas turbines and aero-engines. In the present work, the influence of entropy waves on the downstream flow field of a turbine guide vane is investigated. The [...] Read more.
As an indirect noise source generated in the combustion chamber, entropy waves are widely prevalent in modern gas turbines and aero-engines. In the present work, the influence of entropy waves on the downstream flow field of a turbine guide vane is investigated. The work is mainly based on a well-known experimental configuration called LS89. Two different turbulence models are used in the simulations which are the standard k-ω model and the scale-adaptive simulation (SAS) model. In order to handle the potential transition issue, Menter’s ð-Reθ transition model is coupled with both models. The baseline cases are first simulated with the two different turbulence models without any incoming perturbation. Then one forced case with an entropy wave train set at the turbine inlet at a given frequency and amplitude is simulated. Results show that the downstream maximum Mach number is rising from 0.98 to 1.16, because the entropy waves increase the local temperature of the flow field; also, the torque of the vane varies as the entropy waves go through, the magnitude of the oscillation is 7% of the unforced case. For the wall (both suction and pressure side of the vane) heat transfer, the entropy waves make the maximum heat transfer coefficient nearly twice as the large at the leading edge, while the minimum heat transfer coefficient stays at a low level. As for the averaged normalized heat transfer coefficient, a maximum difference of 30% appears between the baseline case and the forced case. Besides, during the transmission process of entropy waves, the local pressure fluctuates with the wake vortex shedding. The oscillation magnitude of the pressure wave at the throat is found to be enhanced due to the inlet entropy wave by applying the dynamic mode decomposition (DMD) method. Moreover, the transmission coefficient of the entropy waves, and the reflection and transmission coefficients of acoustic waves are calculated. Full article
(This article belongs to the Special Issue Aeroacoustics of Turbomachines)
Open AccessArticle
The Effect on Room Acoustical Parameters Using a Combination of Absorbers and Diffusers—An Experimental Study in a Classroom
Acoustics 2020, 2(3), 505-523; https://doi.org/10.3390/acoustics2030027 - 04 Jul 2020
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Abstract
Several room acoustic parameters have to be considered in ordinary public rooms, such as offices and classrooms, in order to present the actual conditions, thus increasing demands on the acoustic treatment. The most common acoustical treatment in ordinary rooms is a suspended absorbent [...] Read more.
Several room acoustic parameters have to be considered in ordinary public rooms, such as offices and classrooms, in order to present the actual conditions, thus increasing demands on the acoustic treatment. The most common acoustical treatment in ordinary rooms is a suspended absorbent ceiling. Due to the non-uniform distribution of the absorbent material, the classical diffuse field assumption is not fulfilled in such cases. Further, the sound scattering effect of non-absorbing objects such as furniture are considerable in these types of rooms. Even the directional characteristic of the sound scattering objects are of importance. The sound decay curve in rooms with absorbent ceilings often demonstrate a double slope. Thus, it is not possible to use reverberation time as room parameter as a representative standalone acoustic measure. An evaluation that captures the true room acoustical conditions therefore needs supplementary parameters. The aim of this experimental study is to show how various acoustical treatments affect reverberation time T20, speech clarity C50 and sound strength G. The experiment was performed in a mock-up of a classroom. The results demonstrated how absorbers, diffusers and scattering objects influence room acoustical parameters. It is shown that to some extent the parameters can be adjusted individually by using different treatments or combination of treatments. This allows for the fine-tuning of the acoustical conditions, in order to fulfill the requirements for achieving a high-quality sound environment. Full article
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Open AccessArticle
Climate Change, Security, Sensors
Acoustics 2020, 2(3), 474-504; https://doi.org/10.3390/acoustics2030026 - 03 Jul 2020
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Abstract
A concise threefold illustration is given: (i) of climate change on the gigayear (Ga) time scale through the nanosecond (nsec) time scale, (ii) of the role of the performance of solid materials, concerning both manmade and natural structures with [...] Read more.
A concise threefold illustration is given: (i) of climate change on the gigayear (Ga) time scale through the nanosecond (nsec) time scale, (ii) of the role of the performance of solid materials, concerning both manmade and natural structures with reference to security, and (iii) of the exploitation of the electrostatic energy of the atmospheric electrical circuit—which is an enormous reservoir of natural “clean” energy. Several unfortunate misunderstandings are highlighted that bias the present generally agreed beliefs. The typical natural pace of the Earth’s “electrocardiogram”, ~27.4 Ma, is such that, at present, for the first time humankind must challenge an Earth’s “heartbeat”. A correct use of sensors is needed to get an efficient monitoring of the ongoing climate change. Both anthropic and natural drivers are to be considered. A brief reminder is given about sensors that ought to monitor solid materials—with application (i) to every kind of machinery, building, viaduct or bridge, vehicle, aircraft, rocket, etc. and (ii) for a correct (and unprecedented) monitoring of the electric field at ground, which is the prerequisite for the exploitation of the electrostatic energy of the atmosphere. In every case, a systemic approach is always needed. Every specialized investigation often misses the true physics of phenomena. The resulting great complication can be tackled by means of suitable approximate and “simple” models, which always have to be correctly tested. The impact on the biosphere is manifested as a steady regeneration of microorganisms at the deep ocean floors, supplied by endogenous CH4. Microorganisms are thus the beginning of an ever rejuvenating food chain. The natural climate change implies a permanent evolution of living forms. On the longer time-scale, a permanent cycle occurs of species extinction and/or generation. In addition, owing to such a process, some living forms are likely to also exist underground on other planetary objects. That is, life ought to be a ubiquitous intrinsic endogenic feature of matter in the universe, while life’s survival, evolution and/or extinction, ought to depend on the available hosting environment. Full article
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Open AccessArticle
Improved Room Acoustics Quality in Meeting Rooms: Investigation on the Optimal Configurations of Sound-Absorptive and Sound-Diffusive Panels
Acoustics 2020, 2(3), 451-473; https://doi.org/10.3390/acoustics2030025 - 27 Jun 2020
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Abstract
This work deals with the improvement of the room acoustic quality of two medium sized meeting rooms through the investigation of the optimal placement of absorption and diffusive panels on the walls and ceiling. Acoustic measurements have been carried out in the existing [...] Read more.
This work deals with the improvement of the room acoustic quality of two medium sized meeting rooms through the investigation of the optimal placement of absorption and diffusive panels on the walls and ceiling. Acoustic measurements have been carried out in the existing untreated rooms with ODEON 13 room acoustics measurement and prediction software, and the Adobe Audition plugins Aurora. Simulations of different combinations of sound absorption and diffusion treatments have been carried out with the updated version of the software, ODEON 15. The panels were positioned in the meeting rooms following the guidelines of the DIN 18041 standard and the scientific literature. The results advise the application of absorptive materials on the ceiling or around the borders, creating a reflective middle area, and on the upper part of one the lateral walls, including the rear wall. Configurations with diffusers do not generally bring significant improvements. The Speech Transmission Index (STI) is a less sensitive parameter for the different acoustic scenarios, compared to Reverberation Time (T) and Clarity (C50). The research also outlined a design workflow, useful to successfully design meeting rooms and rooms for speech in general, which allows to determine the optimal number and location of acoustic panels and to minimize the costs. Full article
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