Reduction of fan noise bymeans of (circular) side-resonators; theory and experiment

One of the main noise sources in computers are the cooling fans. An important aspect of the noise they generate is tonal noise produced at the rotational frequency of the fan, the blade passing frequency (BPF), and its higher harmonics. Previous research pointed out that so-called side resonators can be applied successfully to reduce this tonal noise. A variety of side resonator geometries is available. A model describing viscothermal wave propagation in a circular side-resonator and the connected duct and the preliminary results of a parameter study of that model where presented previously. The experimental verification of the model is presented here

Thin broadband noise absorption through acoustic reactance control by electro-mechanical coupling without sensor

published_or_final_versio

A study of aerodynamic noise in air duct systems with turning vanes

Buka je jedan od problema u mnogim industrijama. U ovom radu prikazana je analiza aerodinamički nastale buke koja se javlja pri strujanjima vazduha preko usmerivača vazduha oblika aeroprofila u vazdušnim kanalima. Na osnovu Proudmanove jednačine i Lajthilove akustične analogije širokopojasni nivo buke u vazdušnom kanalu može biti izračunat. Za modeliranje turbulentnog modela, k - e turbulentni model je korišćen a potrebne konstante su eksperimentalno određene merenjem u aerotunelu za podzvučne brzine, nakon kojih su vršena merenja buke za u neposrednoj blizini usmerivača vazduha oblika aeroprofila. Nekoliko rešenja je analizirano, kanal sa unutrašnjim aeroprofilom, kanal sa centralnim aeroprofilom, kanal sa spoljašnjim aeroprofilom kao i kombinacija prethodnih slučajeva. Analizom dobijenih rezultata, a u cilju ispunjavanja svih zakonskih propisa koji se tiču industrijske buke, koji postaju sve striktniji, akustična analiza i projektovanje moraju biti primenjeni u većini industrijskih sistema jer buka predstavlja opasnost po zdravlje radnika u radnom okruženju.Noise is becoming one of the problems in many industrial applications. In this paper the aerodynamic noise in air duct system that arises from air flow passing over a surface of turning vanes is investigated. Based on Proudman's formula using Lighthill's acoustic analogy broadband acoustic noise model can be predicted. To model the turbulent flow in an air duct k - e turbulent model is used and required constants are obtained experimentally in a low speed wind tunnel followed by noise measurement in the vicinity of deflected airfoil. Several designs are investigated: Inner airfoil in the duct elbow, center positioned air foil, outer positioned and the overall combination of all previous cases. It was found that in order to satisfy all noise requirements and regulation, which are becoming more strict nowadays, the acoustic analysis and design must be performed in most industrial systems since the noise levels arising from the operating industrial equipment may represent occupational and health hazard

A study of aerodynamic noise in air duct systems with turning vanes

Buka je jedan od problema u mnogim industrijama. U ovom radu prikazana je analiza aerodinamički nastale buke koja se javlja pri strujanjima vazduha preko usmerivača vazduha oblika aeroprofila u vazdušnim kanalima. Na osnovu Proudmanove jednačine i Lajthilove akustične analogije širokopojasni nivo buke u vazdušnom kanalu može biti izračunat. Za modeliranje turbulentnog modela, k - e turbulentni model je korišćen a potrebne konstante su eksperimentalno određene merenjem u aerotunelu za podzvučne brzine, nakon kojih su vršena merenja buke za u neposrednoj blizini usmerivača vazduha oblika aeroprofila. Nekoliko rešenja je analizirano, kanal sa unutrašnjim aeroprofilom, kanal sa centralnim aeroprofilom, kanal sa spoljašnjim aeroprofilom kao i kombinacija prethodnih slučajeva. Analizom dobijenih rezultata, a u cilju ispunjavanja svih zakonskih propisa koji se tiču industrijske buke, koji postaju sve striktniji, akustična analiza i projektovanje moraju biti primenjeni u većini industrijskih sistema jer buka predstavlja opasnost po zdravlje radnika u radnom okruženju.Noise is becoming one of the problems in many industrial applications. In this paper the aerodynamic noise in air duct system that arises from air flow passing over a surface of turning vanes is investigated. Based on Proudman's formula using Lighthill's acoustic analogy broadband acoustic noise model can be predicted. To model the turbulent flow in an air duct k - e turbulent model is used and required constants are obtained experimentally in a low speed wind tunnel followed by noise measurement in the vicinity of deflected airfoil. Several designs are investigated: Inner airfoil in the duct elbow, center positioned air foil, outer positioned and the overall combination of all previous cases. It was found that in order to satisfy all noise requirements and regulation, which are becoming more strict nowadays, the acoustic analysis and design must be performed in most industrial systems since the noise levels arising from the operating industrial equipment may represent occupational and health hazard

Aeronautical engineering: A special bibliography with indexes, supplement 82, April 1977

This bibliography lists 311 reports, articles, and other documents introduced into the NASA scientific and technical information system in March 1977

STUDIES TO IMPROVE EXHAUST SYSTEM ACOUSTIC PERFORMANCE BY DETERMINATION AND ASSESSMENT OF THE SOURCE CHARACTERISTICS AND IMPEDANCE OPTIMIZATION

It is shown that the relationship between an impedance change and the dynamic response of a linear system is in the form of the Moebius transformation. The Moebius transformation is a conformal complex transformation that maps straight lines and circles in one complex plane into straight lines and circles in another complex plane. The center and radius of the mapped circle can be predicted provided that all the complex coefficients are known. This feature enables rapid determination of the optimal impedance change to achieve desired performance. This dissertation is primarily focused on the application of the Moebius transformation to enhance vibro-acoustic performance of exhaust systems and expedite the assessment due to modifications. It is shown that an optimal acoustic impedance change can be made to improve both structural and acoustic performance, without increasing the overall dimension and mass of the exhaust system. Application examples include mufflers and enclosures. In addition, it is demonstrated that the approach can be used to assess vibration isolators. In many instances, the source properties (source strength and source impedance) will also greatly influence exhaust system performance through sound reflections and resonances. Thus it is of interest to acoustically characterize the sources and assess the sensitivity of performance towards source impedance. In this dissertation, the experimental characterization of source properties is demonstrated for a diesel engine. Moreover, the same approach can be utilized to characterize other sources like refrigeration systems. It is also shown that the range of variation of performance can be effectively determined given the range of source impedance using the Moebius transformation. This optimization approach is first applied on conventional single-inlet single-outlet exhaust systems and is later applied to multi-inlet multi-outlet (MIMO) systems as well, with proper adjustment. The analytic model for MIMO systems is explained in details and validated experimentally. The sensitivity of MIMO system performance due to source properties is also investigated using the Moebius transformation