A robust extension to the triple plane pressure mode matching method by filtering convective perturbations

Time-periodic CFD simulations are widely used to investigate turbomachinery components. The triple-plane pressure mode matching method (TPP) developed by Ovenden and Rienstra extracts the acoustic part in such simulations. Experience shows that this method is subject to significant errors when the amplitude of pseudo-sound is high compared to sound. Pseudo-sound are unsteady pressure fluctuations with a convective character. The presented extension to the TPP improves the splitting between acoustics and the rest of the unsteady flow field. The method is simple: i) the acoustic eigenmodes are analytically determined for a uniform mean flow as in the original TPP; ii) the suggested model for convective pressure perturbations uses the convective wavenumber as axial wavenumber and the same orthogonal radial shape functions as for the acoustic modes. The reliability is demonstrated on the simulation data of a low-pressure fan. As acoustic and convective perturbations are separated, the accuracy of the results increases close to sources, allowing a reduction of the computational costs by shortening the simulation domain. The extended method is as robust as the original one--giving the same results for the acoustic modes in absence of convective perturbations.Comment: Accepted 15-05-11 by International Journal of Aeroacoustics to be published in the special issue focusing on turbomachinery aeroacoustic

Hybrid methods for the prediction of the tonal noise emission of turbo engines

Die effiziente Berechnung der Schallemission moderner Flugzeugtriebwerke unter realistischen Betriebsbedingungen ist erst durch die Kopplung verschiedener numerischer Verfahren möglich. Die physikalisch/mathematisch korrekte, algorithmisch effiziente und für den Anwender bequeme Umsetzung der Kopplung der verschiedenen Berechnungsmethoden ist letztendlich maßgebend für deren Einsatz im industriellen Auslegungsprozess eines Flugtriebwerks. In dieser Arbeit werden unterschiedliche Ansätze zur Kopplung der verschiedenen Berechnungsverfahren dargestellt, gegen experimentelle Ergebnisse validiert und es werden Empfehlungen zu deren Anwendung gegeben. Gegenstand des anwendungsorientierten Teils der Untersuchung sind zwei skalierte Turbofantestaufbauten und ein generischer offener gegenläufiger Rotor. Die konkrete Kopplungsstrategie orientiert sich an den strömungsmechanischen Eigenheiten der verschiedenen Anwendungsfälle. In dieser Arbeit werden drei Ansätze zur Kopplung untersucht: Die direkte Interpolation der Schwankungsgrößen aus dem CFD- in das CAA-Gebiet für die zuströmseitige Schallausbreitung beim Turbofan. Die Rekonstruktion der akustischen Schwankungen im CAA-Gebiet anhand der Ergebnisse der CFD-Simulation unter Verwendung eines analytischen Modells für die abströmseitige Schallausbreitung beim Turbofan. Die Projektion der Schwankungsgrößen aus dem CFD-Gebiet zu beliebigen Beobachterpositionen durch Anwendung eines FW-H-Ansatzes für den Fall eines offenen gegenläufigen Rotors. Es wird gezeigt, dass bei Berücksichtigung der Eigenheiten der verschiedenen Verfahren und dementsprechend sorgfältiger Auslegung der Rechengitter die absoluten Schallpegel mit einer Genauigkeit von 1 - 2~dB wiedergegeben werden können. Die benötigten Ressourcen, in erster Linie Bearbeitungszeit und Rechenzeit auf einem Computercluster, sind gegenüber dem Ressourcenverbrauch eines experimentellen Vorgehens erheblich reduziert. Die vorgestellten Ansätze zur Kopplung der verschiedenen numerischen Verfahren wurden in dem Programmpaket Connect3D umgesetzt. Dieses ist in der Programmiersprache C++ implementiert, wobei ein stark modularer Ansatz verfolgt wird. Auf diese Weise ist es möglich die Ansprüche an Robustheit, Wartbarkeit und Erweiterbarkeit, welche eine praxisnahe Anwendung mit sich bringt, langfristig zu erfüllen.The efficient prediction of the noise emission of modern aircraft engines is enabled only by the coupling of different numerical approaches. For a hybrid method to be useful in the industrial design process it is important, that first the implementation is valid from a physical and mathematical point of view, second it is efficient from a algorithmic point of view and last but not least it's application should be comfortable for the user. The scope of this thesis is to describe approaches to couple the different numerical tools. The final hybrid process chain is validated against experimental data and some advise will be given for the correct application of the hybrid method. The focus of the application oriented part of the thesis are two scaled turbo fan test rigs and one generic contra rotating open rotor. The particular approach to couple the different numerical tools is oriented at the flow features of the specific application. Three different coupling approaches are given in further detail: First, to compute the sound field upstream of a ducted turbo fan the perturbation variables are interpolated from the CFD-domain to the CAA-domain. Second, to compute the sound field downstream of a ducted turbo fan the acoustic perturbation variables will be reconstructed in the CAA-domain based on an analytical model which is calibrated with the source information from the CFD-domain. Third, to compute the far field pressure of a contra rotating open rotor a dedicated FW-H approach is used. It benefits from the tonal and axisymmetric nature of the problem. It is shown that the measured sound pressure levels can be reproduced with an accuracy of 1 - 2 dB if the characteristic features of each method are considered and a careful design of each computational mesh is done. The different coupling approaches are implemented in the program package Connect3D, which is implemented in C++ with extensive use of methods from the field of modern software engineering. Thus it is possible to cope with the different demands that come along with an application oriented software development on a long term perspective. These are robustness, maintainability and extensibility of the different applications

Advanced Numerical Methods for the Prediction of Tonal Noise in Turbomachinery, Part II: Time-Linearized Methods

This is the second part of a series of two papers on unsteady computational fluid dynamics (CFD) methods for the numerical simulation of aerodynamic noise generation and propagation. It focuses on the application of linearized RANS methods to turbomachinery noise problems. The convective and viscous fluxes of an existing URANS solver are linearized and the resulting unsteady linear equations are transferred into the frequency domain, thereby simplifying the solution problem from unsteady time-integration to a complex linear system. The linear system is solved using a parallel, preconditioned general minimized residual (GMRES) method with restarts. In order to prescribe disturbances due to rotor stator interaction, a so-called gust boundary condition is implemented. Using this inhomogeneous boundary condition, one can compute the generation of the acoustic modes and their near field propagation. The application of the time-linearized methods to a modern high-bypass ratio fan is investigated. The tonal fan noise predicted by the time-linearized solver is compared to numerical results presented in the first part and to measurements.</jats:p

Natural Flocculant from a Combination of Moringa oleifera Seeds and Cactus Cladodes (Opuntia ficus-indica) to Optimize Flocculation Properties

The lack of access to clean water worldwide and organic, inorganic as well as biological contamination of existing freshwater sources are a major problem for around 2 billion people, especially in the countries of the global south. One sign of polluted water is turbidity. It is generally caused by colloidal and particulate suspended solids. Chemical flocculants are often used to reduce turbidity and thus eliminate the mostly harmful substances that cause it. However, these have some disadvantages, such as cost and availability, so increasingly natural plant-based flocculants are coming into focus and are considered as an alternative option. In this study, Moringa seeds (Moringa oleifera) and cactus cladodes (Opuntia ficus-indica) were investigated as innovative and environmentally friendly flocculants for water treatment. The parameters investigated included absolute turbidity reduction and flocculation activity, as well as shear strength of the resulting flocs. The flocculation experiments were conducted as simultaneous tests in beakers. Experiments were conducted using both a laboratory-prepared model suspension with an initial turbidity of approximately 139 NTU and natural surface water with an initial turbidity of approximately 136 NTU. The flocculant dosages used ranged from 100 to 300 mg/L. The results show that although Moringa seeds had the highest flocculation activity (up to 93%), the flocs were very fragile and were destroyed again even at low induced shear forces. Flocculants from cactus yielded stable flocs, but the flocculation activity (maximum at 54%) was not as high as that of Moringa. The combination of the two materials resulted in a flocculant with sufficiently high flocculation activity (76%) and stable flocs, which could withstand higher shear forces potentially induced in further treatment steps

Versuchsdaten Flockung

Datensatz von Flockungsversuchen mit verschiedenen natürlichen Flockungsmitteln zur Flockungsaktivität und Schwermetalladsorption Data set of flocculation experiments with different natural flocculants for flocculation activity and heavy metal adsorption