Acoustic scattering in a small centrifugal compressor based on the use of linearized equations in a rotating frame

Numerical solutions of acoustic wave scattering are often used to describe sound propagation through complex geometries. For cases with flow, various forms of the convected equation have been used. A better alternative that includes vortex-sound interaction is instead to use the linearized and harmonic forms of the unsteady fluid flow governing equations. In this paper, a formulation of the linearized equations that include rotational effects, in an acoustic computation using a rotating frame of reference in a stationary geometry, is presented. We demonstrate that rotational effects can be important, e.g., when computing the transmission loss through high-speed compressors. The implementation of the proposed addition to the existing schemes is both simple and numerically inexpensive. The results are expected to have an impact on the research and development related to noise control of high-performance turbo-machinery, e.g., used in automotive or aviation applications at operating conditions that can be represented by steady background flows

Reducing low frequency tonal noise in large ducts using a hybrid reactive-dissipative silencer

© 2017 Elsevier Ltd Noise generated by fans or turbines normally consists of a combination of narrow and broadband noise. To lower transmitted noise levels, it is attractive to use a combination of reactive and dissipative elements. However, this approach presents a number of challenges for larger systems. This is because reactive elements are commonly placed around the duct circumference where they are normally only effective up to the frequency at which the first higher order mode cuts on in the duct. For larger systems, this means that reactive elements work only in the low, and often very low, frequency range, whereas dissipative elements, which are distributed across the duct cross-section, generally work well in the medium to high frequency range. This can cause noise problems in the low to medium frequency range in larger systems. This article presents an alternative approach for delivering noise attenuation over the low to medium frequency range that is suitable for application in larger duct systems. This approach takes advantage of those splitter silencer designs commonly used in larger systems to integrate a reactive element into the splitter design. This delivers a hybrid splitter that uses a combination of dissipative and reactive elements so that the reactive element partitions the main airway. This has the advantage of introducing a quasi-planar transverse sound pressure field for each resonator in the low to medium frequency range, including frequencies above the first cut-on. It is demonstrated using predictions and measurements taken for a number of example silencers, that this approach enables reactive elements to work over an extended low to medium frequency range, including at frequencies above the first cut-on mode in the main duct. Accordingly, it is shown that a hybrid dissipative-reactive splitter design is capable of delivering improved levels of attenuation in the crucial low to medium frequency range

Private Equity Firms as Gatekeepers

Notwithstanding the considerable attention private equity receives, there continues to be substantial confusion about what private equity does and whether this creates value. Calls for more aggressive regulation of the industry reflect a skeptical view of private equity as—at best—a zero-sum game, in which profits are generated only at the expense of other constituencies. The standard defense of private equity points to its corporate governance advantages as a source of value. This Article identifies an overlooked and increasingly important way in which private equity creates value: private equity firms act as gatekeepers in the debt markets. As repeat players, private equity firms use their reputations with creditors to mitigate the problems of borrower adverse selection and moral hazard in the companies that they manage, thereby reducing creditors’ costs of lending to these companies. Private equity-owned companies are thus able to borrow money on more favorable terms than standalone companies, all else being equal. By acting as gatekeepers, private equity firms render the debt markets more efficient and provide their portfolio companies with an increasingly valuable borrowing advantage. Ironically, then, debt may well be private equity’s greatest asset

Transmission loss predictions for dissipative silencers of arbitrary cross section in the presence of mean flow

A numerical technique is developed for the analysis of dissipative silencers of arbitrary, but axially uniform, cross section. Mean gas flow is included in a central airway which is separated from a bulk reacting porous material by a concentric perforate screen. The analysis begins by employing the finite element method to extract the eigenvalues and associated eigenvectors for a silencer of infinite length. Point collocation is then used to match the expanded acoustic pressure and velocity fields in the silencer chamber to those in the inlet and outlet pipes. Transmission loss predictions are compared with experimental measurements taken for two automotive dissipative silencers with elliptical cross sections. Good agreement between prediction and experiment is observed both without mean flow and for a mean flow Mach number of 0.15. It is demonstrated also that the technique presented offers a considerable reduction in computational expenditure when compared to a three dimensional finite element analysis

Experimental evidence of an instability over an impedance wall in a duct with flow

An experimental investigation of the acoustical behaviour of a liner in a rectangu- lar channel with grazing flow has been conducted. The liner consists of a ceramic structure of parallel square channels: 1mm by 1 mm in cross section, 65 mm in length, and a surface density of 400 channels/inch square. The channels are rigidly terminated, thus constituting a locally reacting structure. In the absence of flow the liner reacts classically: There is a significant decrease in transmission coefficient around the frequency of minimal impedance. When the wall is exposed to a grazing flow this behaviour is changed: an increase in transmission coefficient appears at this resonance frequency. The transmission coefficient can be even rise above 1 (up to 3 for a Mach number of 0.3). This behaviour is caused by the appearance of a hydrodynamic instability above the liner. Furthermore, the stationary pressure drop induced by this liner is deeply affected by its acoustic behaviour. When a sound wave is added, at the resonance frequency of the liner, the pressure drop can increase by a factor 3 when the Mach number is 0.3. This effect is attributed to a modification of the turbulent boundary layer induced by the acoustic wave

Cost-Benefit Analysis, Static Efficiency and the Goals of Environmental Law

Measurement of plane wave acoustic transmission properties, so called two-port data, of flow duct components is important in many applications. It is an important tool for instance in the development of mufflers for IC-engines. Measurement of two-port data is difficult when the flow velocity in the measurement duct is high because of the flow noise contamination of the measured pressure signals. The plane wave acoustic two-port is a 2x2 matrix containing 4 complex quantities at each frequency. To experimentally determine these unknowns the acoustic state variables on the inlet and outlet side must be measured for two independent test cases. The two independent test cases can be created by: changing the acoustic load on the outlet side leading to the so-called two-load technique or by using one acoustic source on the inlet side and one acoustic source on the outlet side leading to the so-called two-source technique. In the latter case the independent test cases are created by first using the source on the inlet side and then the source on the outlet side. As pointed out by Åbom it is also possible to run both sources simultaneously to create more than two independent test cases. This over-determination could be used to improve the measurement results for instance if the data is contaminated by flow-noise. In this paper over-determination is tested by applying up to 5 different test cases. This procedure has been applied to a single orifice test object.QC 20120531</p

Soviet-Polish relations, 1919-1921

The Polish-Soviet War of 1919-1921 was a direct consequence of the ideological objectives pursued by the belligerents. Ideology shaped the political agenda and the diametrically opposed war aims of both states, and was implemented through the foreign policy, diplomatic negotiation and military engagements pursued. This proved to be the principal obstacle to the establishment of cordial relations. As western democracy and Russian Marxism battled it out, war was inevitable. Externally, the Paris Peace Conference provided the necessary conditions for the resumption of traditional Russian-Polish hostilities, whilst the Allied States consistently demonstrated their absolute inability to directly influence either the development, or outcome, of the conflict. Redressing the balance of historiography, this thesis includes a greater examination of the conflict from the perspective of the Soviet regime. This firmly controlled the Russian decision-making process. By charting the war, it becomes clear that both states deliberately pursued a dual offensive: traditional diplomatic negotiation and military campaign as conditions dictated. However, in addition, Soviet Russia developed a unique and innovative, revolutionary, agit-prop, diplomatic medium. This enabled adept Soviet diplomats to win the majority of diplomatic battles during the conflict, although often negotiating from a militarily weak position. Nevertheless, the regime ultimately failed in its objective: to ignite socialist revolution in western Europe. The mistaken Soviet decision in July 1920 to cross the ethnographic border to forcefully sovietise Poland, in opposition to Marxist doctrine, irreversibly altered the complexion of the war and proved its pivotal turning point. This culminated politically with the short-lived establishment of the Provisional Revolutionary Committee in Białystok, and militarily, with the decisive defeat of the Red Army at the Battle of Warsaw. It is now certain that the Red Army offensive into Poland in July 1920 aimed not only at the sovietisation of Poland, but at spreading the socialist revolution to Western Europe and overthrowing the Versailles settlement. The European revolutionary upsurge had largely extinguished during the previous year and in August 1920, Communist ideology ultimately failed to inspire the vast majority of the Polish population. Thus, by utilising the Soviet military to secure its war aims, Lenin and the Politburo inadvertently signed the death-warrant of socialist revolution in Poland at the beginning of the twentieth century.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Analysis of acoustic networks including cavities by means of a linear finite volume method

[EN] A procedure allowing for the analysis of complex acoustic networks, including three-dimensional cavities described in terms of zero-dimensional equivalent elements, is presented and validated. The procedure is based on the linearization of the finite volume method often used in gas-dynamics, which is translated into an acoustic network comprising multi-ports accounting for mass exchanges between the finite volumes, and equivalent 2-ports describing momentum exchange across the volume surfaces. The application of the concept to a one-dimensional case shows that it actually converges to the exact analytical solution when a sufficiently large number of volumes are considered. This has allowed the formulation of an objective criterion for the choice of a mesh providing results with a prefixed error up to a certain Helmholtz number, which has been generalized to three-dimensional cases. The procedure is then applied to simple but relevant three-dimensional geometries in the absence of a mean flow, showing good agreement with experimental and other computational results.This work has been partially supported by Ricardo Software, and by Ministerio de Ciencia e Innovacion through Grant DPI2009-14290. The authors thank Dr. F.D. Denia for his kind computational assistance.Torregrosa, AJ.; Broatch, A.; Gil, A.; Moreno Martínez, D. (2012). Analysis of acoustic networks including cavities by means of a linear finite volume method. Journal of Sound and Vibration. 331(20):4575-4586. https://doi.org/10.1016/j.jsv.2012.05.023S457545863312

Methodology for experimental validation of a CFD model for predicting noise generation in centrifugal compressors

Centrifugal compressors working in the surge side of the map generate a broadband noise in the range of 1-3 kHz, named as whoosh noise. This noise is perceived at strongly downsized engines operating at particular conditions (full load, tip-in and tip-out maneuvers). A 3-dimensional CFD model of a centrifugal compressor is built to analyze fluid phenomena related to whoosh noise. A detached eddy simulation is performed with the compressor operating at the peak pressure point of 160 krpm. A steady flow rig mounted on an anechoic chamber is used to obtain experimental measurements as a means of validation for the numerical model. In-duct pressure signals are obtained in addition to standard averaged global variables. The numerical simulation provides global variables showing excellent agreement with experimental measurements. Pressure spectra comparison is performed to assess noise prediction capability of numerical model. The influence of the type and position of the virtual pressure probes is evaluated. Pressure decomposition is required by the simulations to obtain meaningful spectra. Different techniques for obtaining pressure components are analyzed. At the simulated conditions, a broadband noise in 1-3 kHz frequency band is detected in the experimental measurements. This whoosh noise is also captured by the numerical model. (C) 2014 Elsevier Inc. All rights reserved.This work has been partially supported by the Spanish Ministerio de Economia y Competitividad through grant No. TRA2012-36954. Part of the computational resources used in this work have been provided by Supercomputing Center of Universitat Politecnica de Valencia and are thus gratefully acknowledged.Broatch Jacobi, JA.; Galindo Lucas, J.; Navarro García, R.; García Tíscar, J. (2014). Methodology for experimental validation of a CFD model for predicting noise generation in centrifugal compressors. International Journal of Heat and Fluid Flow. 50:134-144. https://doi.org/10.1016/j.ijheatfluidflow.2014.06.006S1341445