Broadband sound power determination in flow ducts

This paper presents a new experimental technique, which enables not only the calculation of in-duct transmitted sound power for dominant tones, but also for broadband noise over a wide frequency range. The new method also distinguishes between sound power transmitted in and against the direction of flow. It can be performed using wall-flush installed pressure sensors as well as radial sensor rakes in the flow. The method is mathematically derived based on the analytical solution of the wave equation describing sound propagation in ducts with superimposed static mean flow. The analysis technique requires cross-correlation measurements, e.g. between azimuthally traversable sensor arrays and at least one fixed reference sensor. It assumes an invariant mean broadband noise sound field. The broadband sound power determination method was experimentally verified in a low speed DLR laboratory fan experiment against the ISO 5136 standard for in-duct sound power determination. The experimental setup consists of a single-stage compressor in a flow duct with a diameter of 500 mm. An azimuthally traversable duct section located 2m downstream of the rotor can be equipped alternatively with a sampling tube for measurements after ISO 5136 or microphone rakes for in-duct mode detection. For mode detection, 8 microphones were installed in the moving duct section, which was traversed over 36 azimuthal positions. A wall-flush mounted microphone was used as a reference sensor. Experimental results are presented for three operating conditions and two different compressor tip clearances

Conceptual estimation of the noise reduction potential of electrified aircraft engines

The advent of electrified propulsion systems for civil aircraft promises not only notable reductions of CO2 and NOx emissions, but also of perceived noise. In an attempt to estimate the noise reduction potential of fully electric aircraft engines, the current study compares the noise generated by classical turboprop and turbofan engines with noise spectra calculated for electrified engines. The calculation is based on published far-field sound pressure level spectra at different noise certification points, which are then modified to account for the absence of combustion-related noise sources. In addition to the overall sound pressure level, changes to the effective perceived noise level are also taken into account. The results clearly show that the electrification of the engine alone will not lead to the notable noise reductions that are required in order to achieve the goals for future aviation set by the European Commission. Instead, continued research is necessary to further reduce noise sources that will continue to be present in novel electrified aircraft systems, such as fan noise and airframe noise

Latest research on the reduction of aircraft noise at the source

Der Vortrag gibt einen Überblick zum Stand der Quelllärmforschung und der Minderungs von Schallquellen bei Verkehrsflugzeugen. Es werden zunächst die Hauptschallquellen an modernen Verkehrsflugzeugen Triebwerk, Flugwerk und Installationschallquellen diskutiert. Im zweiten Schritte werden entwickelte Lärmminderungstechnologien besprochen und schließlich ein kurzer Ausblick auf hochinnovative Lärmminderungsansätze in Richtung neuer lärmarmer Flugzeugkonfigurationen gegebe

Dissipationseigenschaften periodisch durchströmter Liner bei streifendem Schalleinfall

Perforierte Wandauskleidungen mit Kavität, auch Liner genannt, stellen eine wichtige Schallreduktionstechnologie in durchströmten Kanälen wie beispielsweise Flugzeugtriebwerken dar. Bei modernen, emissionsarmen Triebwerken sind die zu dämpfenden Frequenzbestandteile eher tieffrequent und breitbandig auf einen größeren Frequenzbereich verteilt. Hierfür benötigen konventionelle, auf dem Resonatorprinzip basierende Linerkonzepte einen deutlich größeren Bauraum, der in Triebwerken zumeist nicht verfügbar ist. Daher besteht Bedarf an neuartigen Systemen mit breitbandiger Dämpfung. Vor diesem Hintergrund wird in dieser Arbeit das Wirkungsprinzip des sogenannten Zero-MassFlow-Liners (ZML) genauer untersucht. Das Dissipationsverhalten wird durch eine periodische Durchströmung der perforierten Wand erweitert. Für die zusätzliche Dissipation werden Wirbelbildung und -ablösung an den Löchern der perforierten Wand, ähnlich wie bei konstant durchströmten, sogenannten Bias-Flow-Linern (BFL), verantwortlich gemacht. Der Vorteil dieses Konzeptes ist, dass gegenüber dem BFL keine zusätzliche Luftversorgung benötigt wird. Im Labor kann eine periodische Durchströmung beispielsweise mit einem Lautsprecher hergestellt werden. In dieser Studie wird das Dissipationsverhalten periodisch durchströmter Liner in Abhängigkeit von der Schallschnelle in den Löchern und der Anregungsfrequenz untersucht. Die Ergebnisse zeigen für verschiedene Wandauskleidungen, dass das Dissipationsverhalten mit steigender Schallschnelle einen breitbandigeren Verlauf annimmt und die Anregungsfrequenz einen vergleichsweise geringen Einfluss hat

Comparative Study of Impedance Eduction Methods

The absorption efficiency of acoustic liners used in aircraft engines is characterized by the acoustic impedance. World wide, many grazing ow test rigs and eduction methods are available that provide values for that impedance. However, a direct comparison and assessment of the data of the di erent rigs and methods is often not possible because test objects and test conditions are quite di erent. Only a few papers provide a direct comparison. Therefore, this paper together with a companion paper, present data measured with a reference test object under similar conditions in the DLR and NASA grazing ow test rigs. Additionally, by applying the in-house methods Liner Impedance Non-Uniform ow Solving algorithm (LINUS, DLR) and Convected Helmhholtz Equation approach (CHE, NASA) on the data sets, similarities and differences due to underlying theory are identi ed and discussed

Reduction of inertial end correction of perforated plates due to secondary high amplitude stimuli

The decrease in reactance of perforated plates at high sound pressure amplitudes is of interest for the design of Helmholtz resonator liners. It is associated with the loss of end correction due to flow separation at the orifices. In practical applications, complex acoustic signals impinge on perforations. The loss of end correction due to multiple stimuli of unrelated frequency and phase has not been considered yet. This study assesses and presents an empirical approximation for the reduction of end correction of perforated plates at primary frequencies when flow separation is induced by an additional secondary unrelated high amplitude stimulus

Design and Evaluation of a Zero Mass Flow Liner

In this study, the concept of a Zero Mass Flow Liner is evaluated. The concept enables impedance control by the induction of periodic bias flow through the perforated facing sheet of the liner. The periodic bias flow is generated by a secondary high amplitude acoustic actuation. By means of the periodic bias flow, the liner can be tuned to different operating points in a given range of grazing flow velocities. The equivalent fluid impedance model for perforated plates is modified to account for the effects of periodic bias flow and grazing flow. An optimization routine, based on a genetic algorithm, is implemented. The method is applicable to any liner concept and uses the impedance of the lined surface as boundary condition in a numerical simulation. Thereby, a set of liner parameters is derived in order to obtain the desired damping characteristics. Based on the results of the optimization, a Zero Mass Flow Liner is manufactured and consequently evaluated experimentally. The damping characteristics are evaluated in form of the dissipated energy along the lined surface. Prediction and measurements show agreement. The Zero Mass Flow Liner delivers broad band dissipation of high peak value over a range of grazing flow Mach numbers. Under grazing flow conditions, the effect of periodic bias flow is reduced. This poses high energy requirements in high Mach number flow regimes which might restrict the applicability of the Zero Mass Flow concept to grazing flows of low Mach numbers

Messtechnische Analyse von Schallmoden in Strömungen mit Umfangsinhomogenitäten

Im Kontext neuer Flugzeugkonzepte werden alternative Positionen der Triebwerke, etwa eingelassen in den Rumpf, diskutiert. Ein in Hinsicht auf die aerodynamische Effizienz vielversprechendes Konzept sieht das Einsaugen der Rumpfgrenzschicht (Boundary Layer Ingestion - BLI) durch das Triebwerk vor, wodurch es zu einer über den Triebwerksquerschnitt, insbesondere nicht umfangssysmmetrischen, ungleichmäßigen Strömung kommt. Bisherige Studien zeigen, dass die durch die Rotor-Stator-Interaktion angeregten Schallmoden unter diesen Zuströmungsbedingungen Schallenergie in andere Modenordnungen streuen. Diese Streuung zieht die Notwendigkeit der kompakten mathematischen Beschreibung durch eine Modenbasis nach sich, deren Umfangsmoden bedingt durch die gestörte Symmetrie der Hintergrundströmung nicht mehr rein harmonisch sind und deren radiale Formfunktionen sich mathematisch nicht nur aus Besselfunktionen zusammensetzen lassen. Solch ein alternatives Modenmodell, das auf der regulären Störungsrechnung basiert, wird vorgestellt und auf die praktische Anwendbarkeit bei Schallfeldmessungen mit Mikrofonringen untersucht

Preliminary Safety Assessment of PEM Fuel Cell Systems for Electrified Propulsion Systems in Commercial Aviation

This paper analyses polymer electrolyte membrane fuel cell systems (PEMFCSs) as main energy provider for electrified aircraft propulsion, identifies potential weaknesses as well as safety challenges and presents potential solutions. The general design, operating principles and main characteristics of hydrogen-fuelled low temperature PEMFCSs are described. The safety assessment process in aviation according to Aerospace Recommended Practices ARP4754A and selected methods according to ARP4761 are introduced. The functions of fuel cell systems in electrified aircraft powertrains are analysed and visualised in functional structure trees on aircraft, powertrain and fuel cell system level. By means of a Functional Hazard Analysis (FHA), potential malfunctions and their effects are investigated and their severity is evaluated. Critical failure modes are identified and requirements for acceptable failure probabilities are stipulated. Within the scope of a Fault Tree Analysis (FTA), the components of a fuel cell system are assigned to the identified functional structure trees and potential causes of critical failure modes are examined. The results of the mentioned analyses reveal design challenges associated with the application of fuel cell systems in electrified aircraft propulsion, for instance concerning functional independence as well as solutions for cold start conditions, heat transfer and lightweight design

Modelling of Acoustic Liners Consisting of Helmholtz Resonators Coupled with a Second Cavity by Flexible Walls

Acoustic liners are an effective way to dampen aircraft noise. Conventional single-degree-of-freedom liners consist of a perforated facesheet backed with a honeycomb structure and a rigid end plate. Their damping excels near their resonance frequency which is anti-proportional to the cavity depth (lambda/4-resonator) or the cavity volume (Helmholtz resonator). However, this is a challenge for low-frequency noise with long wavelengths due to the limited installation space. We therefore propose a resonator in which the back cavity is divided into two cavities by a flexible plate. The aim is to combine the damping mechanisms of the Helmholtz resonator with the material damping of the flexible plate. With carefully chosen parameters, this flexible plate resonates well below the Helmholtz frequency. We derived an analytic model based on waveguide theory to predict the impedance of the resonator concept. The Helmholtz equation was solved to (numerically) determine the scattering coefficients of a channel section in which one wall is lined with the predicted resonator impedance. The predicted dissipation agreed well with experimental data from measurements at the aero-acoustic wind tunnel DUCT-R