This work proposes to address the computation of flow-generated noise, including the high frequency components in an acceptable computational time, relative to industrial applications. In this framework, hybrid methods based on aeroacoustic analogies are relevant to predict the corresponding sound, in term of computational time and easy implementation, and are separating the sources of sound computation from the sound propagation itself. Both parts of the hybrid method are then investigated.<p><p>Firstly, the sources of sound should be computed accurately, reproducing most of the energy content of the turbulent flow. The present study is mainly based on LES computations, that are a good compromise between the accuracy and the time required to obtain the sources, especially for low Mach number application, where incompressible flow computations can be used. The influence of the computational parameters is then studied through the investigation of the influence of the boundary conditions, flow solver, mesh refinement, LES subgrid-scale model and computational domain definition. <p><p>Secondly, the sound sources propagation is computed using classical analogies based most of the time on free field Green's functions. They are limited to the low frequency range of the emitted sound spectrum, where the source is compact, if incompressible acoustic sources are used. They are then proposed to be combined with Amiet's theory for airfoil noise to cover the remaining high frequency part of the sound spectrum. The limitations of Amiet's theory are analysed through the geometrical and acoustical far-field assumptions. Furthermore, an inverse strip method is proposed to extend Amiet's theory to spanwise varying flow conditions and taking correctly into account the spanwise wave number effects. <p><p>The leading-edge noise mechanism is studied through the interaction of the turbulent region of a jet with a NACA0012 airfoil. The hybrid method, consisting in an incompressible LES computation combined with Curle's analogy, showed its limitation to the low frequency range of the sound spectrum when low-order CFD are used. The Amiet's theory, based on the modelisation of the upstream velocity spectrum, provides a good sound prediction at high frequency, as far as the airfoil thickness is accounted for. <p><p>The Trailing-edge noise mechanism is studied through the flow around a mid-span cut of a blade (CD airfoil) of an automotive cooling fan. <p>Three different acoustic methods are used :Curle's analogy is using wall-pressure fluctuations over the airfoil surface, Ffowcs-Williams and Hall's analogy is using volumetric velocity informations around the trailing-edge and Amiet's theory for trailing-edge noise is using the wall-pressure spectrum around the trailing-edge and the corresponding spanwise correlation. The methods using wall-pressure informations are shown to directly transfer the informations of the wall-pressure spectrum to the sound spectrum. Differences are appearing in the higher frequency range where the airfoil starts to be non compact, invalidating the use of Curle's analogy at such frequencies compared to the other methods, taking scattering effects into account implicitly in their formulations. <p><p>Finally, in order to reduce the computational cost, Amiet's theory for trailing-edge noise is proposed to be driven from steady RANS computations. Two methods to compute the wall-pressure spectrum from boundary-layer informations are studied. Their respective robustness and reliability are analysed in an uncertainty quantification framework, in case of varying velocity profiles upstream the airfoil. Both models showed similar results and tendency compared to the wall-pressure LES spectrum, as far as the flow topology remains similar. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

This thesis considers linear perturbations to the steady flow of a compressible inviscid perfect gas along a cylindrical or annular duct. Particular consideration is given to the model of the duct boundary, and to the effect of curvature of the duct centreline. For a duct with a straight centreline and a locally-reacting boundary, the acoustic duct modes can be segregated into ordinary duct modes and surface modes. Previously-known asymptotics for the surface modes are generalized, and the generalization is shown to provide a distinctly better approximation in aeroacoustically relevant situations. The stability of the surface modes is considered, and previous stability analyses are shown to be incorrect, as their boundary model is illposed. By considering a metal thin-shell boundary, this illposedness is explained, and stability analysed using the Briggs-Bers criterion. The stability of a cylindrical thin shell containing compressible fluid is shown to differ significantly from the stability for an incompressible fluid, even for parameters for which the fluid would otherwise be expected to behave incompressibly. The scattering of sound by a sudden hard-wall to thin-shell boundary change is considered, using the Wiener-Hopf technique. The causal acoustic field is derived analytically, without the need to apply a Kutta-like condition or to include an instability wave, as had previously been necessary. Attention is then turned to a cylindrical duct with a curved centreline and either hard or locally-reacting walls. The centreline curvature (which is not assumed small) and wall radii vary slowly along the duct, enabling an asymptotic multiple scales analysis. The duct modes are found numerically at each axial location, and interesting characteristics are explained using ray theory. This analysis is applied to a hard-walled RAE 2129 duct, and frequency-domain solutions are convolved to give a time-domain example of a pulse propagating along this duct. Finally, some numerical work on the nonlinear propagation of a large-amplitude pulse along a curved duct is presented. This is aimed at modelling a surge event in an aeroengine with a convoluted intake.

The noise (unwanted sound) from fans of all sizes, operating in close proximity to people, can be a design constraint due to annoyance or, in the worse cases, health damage. Of the total noise, aeroacoustic noise - produced by unsteadiness in the air - often represents a significant source and is intrinsically linked to the aerodynamic features of the flow field. In this work, the aeroacoustics of low-speed fans are investigated using a compact mixed-flow fan as a test case. The low-speed regime is less developed compared to large-scale, high-speed machines and is increasingly relevant to applications such as micro air vehicles, small wind turbines, and other environmental comfort technologies found in buildings or vehicles. The test case fan Reynolds number is of the order of 104 which is a couple of orders lower than those generally found in gas turbines. Its main sources are therefore best identified experimentally in the absence of proven alternative methods. In order to do this, a way of quantifying fan noise is developed in tandem with control of the aerodynamic operating point. Following a study of sources of the significant broadband and tonal noise, a low-order noise prediction scheme is developed and applied to predict tonal noise with reference to Reynolds number effects. The new, duct-based rig and method has several advantages over the existing sound power measurement rig built to the ISO 5136 standard at Dyson. The approach, which makes no assumptions about the relative power of different modes, has resulted in a rig that is much shorter. Unlike the ISO rig, it is capable of accurate narrow-band tone measurements with sources which excite strong non-plane-wave duct modes (as the modal structure of the sound is determined) for the frequencies of interest. Tests have been carried out at different operating points with a range of geometry modifications produced with 3D printing techniques. In terms of tonal sources which particularly impact sound quality, the mixed-flow impeller alone produces tones due to very high sensitivity to inflow distortion of the mean flow (giving unsteady blade loading). This means that the product inlet must be designed very carefully to optimally condition the flow. Periodicity in the impeller outlet flow produces rotor-stator interaction tones even with a number of guide vanes chosen to satisfy the Tyler-Sofrin theory cut-off criteria. This is thought to be due to abrupt radius change after the guide vanes in the rig (while the theory assumes constant radius). In the product, abrupt radius change also occurs. The sensitivity of the broadband level to inflow turbulence was confirmed to be low in the rig, although the in-product inflow appears much less ideal. The main broadband noise source in rig tests is suggested to be impeller self-noise as only small reductions in rotor-stator interaction noise are achieved with far fewer vanes. The low-order modelling scheme to understand the fundamental unsteady loading noise mechanism compares well to experiments for sample rotor-stator interaction tones. The velocity fluctuations which induce this noise, measured experimentally with a 2D hotwire, are shown to increase in intensity as Reynolds number is reduced towards 104. This is due to a higher importance of viscosity which can give boundary layers that are thicker and liable to laminar separation. Surface treatments such as boundary layer trips could be used to prevent such separation and potentially reduce noise. Based on the thesis findings, further tests, simulations and possible design modifications are suggested to understand and reduce the important noise sources.

Parciální diferenciální rovnice hrají důležitou v inženýrských aplikacích. Často je možné tyto rovnice řešit pouze přibližně, tj. numericky. Z toho důvodu vzniklo množství diskretizačních metod pro řešení těchto rovnic. Uvedená nespojitá Galerkinova metoda se zdá jako velmi obecná metoda pro řešení těchto rovnic, především pak pro hyperbolické systémy. Naším cílem je řešit úlohy aeroakustiky, přičemž šíření akustických vln je popsáno pomocí linearizovaných Eulerových rovnic. A jelikož se jedná o hyperbolický systém, byla vybrána právě nespojitá Galerkinova metoda. Mezi nejdůležitější aspekty této metody patří schopnost pracovat s geometricky složitými oblastmi, možnost dosáhnout metody vysokého řádu a dále lokální charakter toho schématu umožnuje efektivní paralelizaci výpočtu. Nejprve uvedeme nespojitou Galerkinovu metodu v obecném pojetí pro jedno- a dvoudimenzionalní úlohy. Algoritmus následně otestujeme pro řešení rovnice advekce, která byla zvolena jako modelový případ hyperbolické rovnice. Metoda nakonec bude testována na řadě verifikačních úloh, které byly formulovány pro testování metod pro výpočetní aeroakustiku, včetně oveření okrajových podmínek, které, stejně jako v případě teorie proudění tekutin, jsou nedílnou součástí výpočetní aeroakustiky.

Recent advances in manufacturing and growing concerns on the sustainability of aviation environment have led to a remarkable interest in electrical unmanned aerial systems (UASs) in the past decade. Among various UAS types, the newly designed quadrotor biplane tailsitter class is capable of delivering a wide range of civilian and military tasks, relying on its Vertical Take-Off and Landing (VTOL) capability as well as great maneuverability. Nevertheless, as such UASs employ rotors to generate thrust, and wings to generate lift, and operate at less-understood low to mid-Reynolds flow regime, they experience complicated flight aerodynamics with a noise generation mechanism which is different from common aircrafts. The present work aims at addressing this knowledge gap by studying the aerodynamics and aeroacoustics of a UAS of this type designed by the Army Research Lab. High-fidelity computational fluid dynamics (CFD) simulations are carried out for a wide range of operating conditions to understand the physics involved in the UAS aerodynamics and characterize its performance. Relying on the CFD results, a physics-informed reduced order model (ROM) is developed based on machine learning algorithms, to predict the propellers effects on the wings and calculate the dominant loads. The results of this study indicate that the UAS aerodynamics is significantly influenced by the propeller-wing interaction, which makes it challenging to estimate the loads by classic methods. The proposed physics-informed ROM shows a promising performance based on its computational cost and accuracy. Additionally, it is found that the aeroacoustics of the UAS is ruled by a two-way mechanism through which the propellers and the structure impose unsteadiness on each other.

Turbochargers play an important role in increasing the energetic efficiency andreducing emissions of modern power-train systems based on downsized recipro-cating internal combustion engines (ICE). The centrifugal compressor in tur-bochargers is limited at off-design operating conditions by the inception of flowinstabilities causing rotating stall and surge. They occur at reduced enginespeeds (low mass flow rates), i.e. typical operating conditions for a betterengine fuel economy, harming ICEs efficiency. Moreover, unwanted unsteadypressure loads within the compressor are induced; thereby lowering the com-pressors operating life-time. Amplified noise and vibration are also generated,resulting in a notable discomfort. The thesis aims for a physics-based understanding of flow instabilities andthe surge inception phenomena using numerical methods. Such knowledge maypermit developing viable surge control technologies that will allow turbocharg-ers to operate safer and more silent over a broader operating range. Therefore,broadband turbulent enabled compressible Large Eddy Simulation (LES) cal-culations have been performed and several flow-driven instabilities have beencaptured under unstable conditions. LES produces large amounts of 3D datawhich has been post-processed using Fourier spectra and Dynamic Mode De-composition (DMD). These techniques are able to quantify modes in the flowfield by extracting large coherent flow structures and characterize their relativecontribution to the total fluctuation energy at associated. Among the mainfindings, a dominant mode was found which describes the filling and emptyingprocess during surge. A narrowband feature at half of the rotating order wasidentified to correspond to co-rotating vortices upstream of the impeller faceas well as elevated velocity magnitude regions propagating tangentially in thediffuser and the volute. Dominant mode shapes were also found at the rotatingorder frequency and its harmonics, which manifest as a spinning mode shapelocalized at the diffuser inlet. From the compressible LES flow solution one can extract the acoustic infor-mation and the noise affiliated with the compressor. This enable through datacorrelation quantifying the flow-acoustics coupling phenomena in the compres-sor. Detailed comparison of flow (pressure, velocity) and aeroacoustics (soundpressure levels) predictions in terms of time-averaged, fluctuating quantities,and spectra is carried out against experimental measurements. / <p>QC 20160406</p>

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