Acoustic and Large Eddy Simulation studies of azimuthal modes in annular combustion chambers

The objectives of this paper are the description of azimuthal instability modes found in annular combus- tion chambers using two numerical tools: (1) Large Eddy Simulation (LES) methods and (2) acoustic solv- ers. These strong combustion instabilities are difficult to study experimentally and the present study is based on a LES of a full aeronautical combustion chamber. The LES exhibits a self-excited oscillation at the frequency of the first azimuthal eigenmode. The mesh independence of the LES is verified before ana- lysing the nature of this mode using various indicators over more than 100 cycles: the mode is mostly a pure standing mode but it transitions from time to time to a turning mode because of turbulent fluctu- ations, confirming experimental observations and theoretical results. The correlation between pressure and heat release fluctuations (Rayleigh criterion) is not verified locally but it is satisfied when pressure and heat release are averaged over sectors. LES is also used to check modes predicted by an acoustic Helmholtz solver where the flow is frozen and flames are modelled using a Flame Transfer Function (FTF) as done in most present tools. The results in terms of mode structure compare well confirming that the mode appearing in the LES is the first azimuthal mode of the chamber. Moreover, the acoustic solver provides stability maps suggesting that a reduction of the time delay of the FTF would be enough to sta- bilise the mode. This is confirmed with LES by increasing the flame speed and verifying that this modi- fication leads to a damped mode in a few cycles

Adjoint-Based Aerodynamic Optimisation of Wing Shape Using Non-uniform Rational B-Splines

Abstract Numerical shape optimisation with adjoint CFD is applied using the NURBS based Parametrisation method with Continuity Constraints (NSPCC) for aerodynamically optimising three dimensional surfaces. The ONERA M6 wing is re-parametrised with NURBS surfaces including weight adjustments to repre- sent the three dimensional wing accurately, resulting in fewer control points and smoother variation of curvature. The NSPCC CAD kernel is coupled with the in- house flow and adjoint solver STAMPS and a gradient-based optimiser to minimise the drag of the ONERA M6 wing in transonic Euler flow conditions. Optimisation results are presented for the B-Spline and NURBS parametrisations

Solving an inverse coupled conjugate heat transfer problem by an adjoint approach

A framework for obtaining adjoint gradients for coupled conjugate heat transfer problems is presented. The framework is tailored to partitioned approaches in which separate solvers are used for the fluid and solid domains. The exchange of sensitiv- ities between adjoint fluid and solid solvers is necessary in order to obtain gradients and how this is achieved is described. The effectiveness of the procedure is demonstrated by solving a conjugate heat transfer problem using a gradient based approach. The presented method can be extended to sensitivity analysis of multidisciplinary problems where both solvers offer adjoint derivatives

Algorithmic differentiation of code with multiple context-specific activities

International audienceAlgorithmic differentiation (AD) by source-transformation is an established method for computing derivatives of computational algorithms. Static data-flow analysis is commonly used by AD tools to determine the set of active variables, that is, variables that are influenced by the program input in a differentiable way and have a differentiable influence on the program output. In this work, a context-sensitive static analysis combined with procedure cloning is used to generate specialised versions of differentiated procedures for each call site. This enables better detection and elimination of unused computations and memory storage, resulting in performance improvements of the generated code, in both forward and reverse mode AD. The implications of this multi-activity AD approach on the static analysis of an AD tool is shown using data flow equations. The worst-case cost of multi-activity AD on the differentiation process is analysed and practical remedies to avoid running into this worst-case are presented. The method was implemented in the AD tool Tapenade, and we present its application to a 3D unstructured compressible flow solver, for which we generate an adjoint solver that performs significantly faster when multi-activity AD is used

LES Study of Transverse Acoustic Instabilities in a Swirled Kerosene/Air Combustion Chamber

LES is used to study self-excited transverse modes in an atmospheric, square combustor (p = 1 bar). Simulations over a range of different mass flow rates show that transverse modes are present for all cases and exhibit varying RMS pressure amplitudes up to 0.4 bar. Analysis of LES results shows that transverse modes are due to a lock-in mechanism between an hydrodynamic unstable mode typical of swirling flows (the PVC mode or Processing Vortex Core) and the cavity modes. A control method using damping compliant walls (named DCWs) is applied to control the acoustic mode in the LES and to characterize the PVC in the absence of acoustic forcing. This method shows that the highest pressure oscillations appear when the PVC frequency is close to the frequency of the first transverse acoustic mode. A 3D Helmholtz solver is then used to predict the stability limits obtained by 3D LES. To capture transverse modes, a new flame transfer function (FTF) formulation is derived where local heat release perturbations are controlled by the orthoradial acoustic velocity fluctuations. The FTF is measured in the LES and when it is included in the Helmholtz solver, it allows to recover the stability zones observed in the LES

Overview on low temperature co-fired ceramic sensors

Low Temperature Co-fired Ceramics (LTCC) is one of the microelectronic techniques. This technology was initially developed as an alternative to Printed Circuit Boards (PCB) and classical thick-film technology, and it has found application in the fabrication of multilayer ceramic boards for electronic devices. Fast and wide development of this technology permitted the fabrication of 3D mechanical structures and integration with various different processes. Thanks to this, LTCC has found application in the manufacturing of various microelectronic devices. This paper presents an overview on LTCC technology and gives a detailed summary on physical quantity sensors fabricated using LTCC technique

First Study of Combined Blazar Light Curves with FACT and HAWC

For studying variable sources like blazars, it is crucial to achieve unbiased monitoring, either with dedicated telescopes in pointing mode or survey instruments. At TeV energies, the High Altitude Water Cherenkov (HAWC) observatory monitors approximately two thirds of the sky every day. It uses the water Cherenkov technique, which provides an excellent duty cycle independent of weather and season. The First G-APD Cherenkov Telescope (FACT) monitors a small sample of sources with better sensitivity, using the imaging air Cherenkov technique. Thanks to its camera with silicon-based photosensors, FACT features an excellent detector performance and stability and extends its observations to times with strong moonlight, increasing the duty cycle compared to other imaging air Cherenkov telescopes. As FACT and HAWC have overlapping energy ranges, a joint study can exploit the longer daily coverage given that the observatories' locations are offset by 5.3 hours. Furthermore, the better sensitivity of FACT adds a finer resolution of features on hour-long time scales, while the continuous duty cycle of HAWC ensures evenly sampled long-term coverage. Thus, the two instruments complement each other to provide a more complete picture of blazar variability. In this presentation, the first joint study of light curves from the two instruments will be shown, correlating long-term measurements with daily sampling between air and water Cherenkov telescopes. The presented results focus on the study of the variability of the bright blazars Mrk 421 and Mrk 501 during the last two years featuring various flaring activities.Comment: 6 pages, 2 figures. Contribution to the 6th International Symposium on High Energy Gamma-Ray Astronomy (Gamma2016), Heidelberg, Germany. To be published in the AIP Conference Proceeding

Glass powder doping of nanocrystal-doped fibres: challenges and results

Incorporating new optical materials as nanocrystals into glass fibres for new functionalities has recently become a hot research topic. Our team (funded by the European FET Open project NCLAS) investigates the introduction of nanoscale laser crystallites into the core of optical fibres using the glass powder doping method. Active Y2O3:Pr3+ nanocrystals (NCs) were prepared via different synthesis methods, and structurally and spectroscopically characterized. After modification of technological parameters, the optimised NCs have been proposed as a luminescence centres to embed into germanate and silicate glass hosts. Glasses were analysed in terms of optical (transmission, refractive index matching to NCs) and thermal (thermal stability, viscosity, thermal expansion coefficient) parameters. Crystallisation issues during fibre drawing were particularly investigated. In a first step, glass powder-NCs mixing techniques and fibre preform preparation were developed. It was shown that temperature cycle profiles including dwell time and heating/cooling ramp rates influenced the glass-NCs properties and can lead to glass crystallisation or NCs dissolution. The sintering investigations pointed out the melting temperature limits to preserve active NCs in the glasses. In germanate glasses, Y2O3:Pr3+ dissolution was noticed at 800°C. In the case of the silicate glass compositions these regions vary from 700°C to 1050°C. The results allowed to select optical fibre drawing conditions performed by the powder-in-tube method. Their distribution uniformity is not yet sufficient, requiring further optimisation of the drawing kinetics.The research project funded by the European FET Open project NCLas: NanoCrystals in Fibre Lasers, Grant agreement number: 82916

Contribución al conocimiento de Porosagrotis gypaetina (Guen.) (Lep.:Noctuidae)

p.15-22Este trabajo tiene por finalidad brindar una descripcion detallada de los diferentes estados de desarrollo, asi como de los estadios larvales, de Porosagrotis gypaetina (Guen.) y estimar sus principales parametros biologicos. Se trata de una oruga conocida vulgarmente como gusano pardo que frecuenta cultivos de alfalfa, trebol bianco, maiz y girasol y determinadas malezas. Los caracteres considerados para su identificacion fueron, en el huevo: numero y distribucion de costas; en la larva: pigmentacion, distribucion de manchas y cerdas corporales; en la pupa: tamaño, forma y color y caracteristicas del cremaster; y en el adulto: ubicacion y coloracion de maculas y nervaduras alares. La emergencia de imagos alcanzo su maximo en abril y mayo. El periodo embrionario se completo en 22 a 26 dias. Aproximadamente la mitad de las larvas cumplieron su ciclo en 6 estadios y las restantes en 7; la duracion total del periodo larval fue de 134 a 141 dias, sin considerar la forma prepupal e independientemente del numero de estadios. Las orugas permanecieron como prepupas durante la temporada estival (aproximadamente 161 dias). El estado pupal duro 40 a 57 dias. Las observaciones realizadas permiten expresar que, inediante los caracteres descriptos, es factible reconocer la especie a traves no solo de los adultos, sino de sus estados inmaduros. Posee una sola generacion anual; transcurre el inviemo como larva; el daño tipico de corte lo produce a partir del cuarto estadio larval