4 research outputs found
Delayed-time domain impedance boundary conditions (D-TDIBC)
Defining acoustically well-posed boundary conditions is one of the major numerical and theoretical challenges in compressible NavierāStokes simulations. We present the novel Delayed-Time Domain Impedance Boundary Condition (D-TDIBC) technique developed to impose a time delay to acoustic wave reflection. Unlike previous similar TDIBC derivations (Fung and Ju, 2001ā2004 [1], [2], Scalo et al., 2015 [3] and Lin et al., 2016 [4]), D-TDIBC relies on the modeling of the reflection coefficient. An iterative fit is used to determine the model constants along with a low-pass filtering strategy to limit the model to the frequency range of interest. D-TDIBC can be used to truncate portions of the domain by introducing a time delay in the acoustic response of the boundary accounting for the travel time of inviscid planar acoustic waves in the truncated sections: it gives the opportunity to save computational resources and to study several geometries without the need to regenerate computational grids. The D-TDIBC method is applied here to time-delayed fully reflective conditions. D-TDIBC simulations of inviscid planar acoustic-wave propagating in truncated ducts demonstrate that the time delay is correctly reproduced, preserving wave amplitude and phase. A 2D thermoacoustically unstable combustion setup is used as a final test case: Direct Numerical Simulation (DNS) of an unstable laminar flame is performed using a reduced domain along with D-TDIBC to model the truncated portion. Results are in excellent agreement with the same calculation performed over the full domain. The unstable modes frequencies, amplitudes and shapes are accurately predicted. The results demonstrate that D-TDIBC offers a flexible and cost-effective approach for numerical investigations of problems in aeroacoustics and thermoacoustics
A novel modal expansion method for low-order modeling of thermoacoustic instabilities in complex geometries
This work proposes an improvement to existing methods based on modal expansions used for the prediction of thermoacoustic instabilities in zero Mach number flow conditions. Whereas the orthogonal basis made of the acoustic eigenmodes of the domain bounded by rigid walls is classically used, an alternative method based on a modal expansion onto an over-complete set of acoustic eigenmodes is proposed. This allows avoiding the misrepresentation of the acoustic velocity field often observed near non rigid-wall boundaries. A Low Order Model network utilizing a state-space framework is then built upon this novel type of modal expansion. Several test cases, going from non reacting ducts to a complex geometry with combustion, are studied to assess the potential of the approach. The methodology not only successfully mitigates the misrepresentation in the acoustic field in the presence of non-rigid-wall boundaries, but it also drastically improves the convergence speed. The modularity of the method and its ability to handle complex geometries are illustrated by considering a configuration featuring an annular chamber, an annular plenum, as well as multiple burners. This novel technique is expected to bring worthy improvements to existing Low Order Models using modal expansions for the prediction of combustion instabilities
Energy analysis and discretization of nonlinear impedance boundary conditions for the time-domain linearized Euler equations
Time-domain impedance boundary conditions (TDIBCs) can be enforced using the impeda-nce, the admittance, or the scattering operator. This article demonstrates the computational advantage of the last, even for nonlinear TDIBCs, with the linearized Euler equations. This is achieved by a systematic semi-discrete energy analysis of the weak enforcement of a generic nonlinear TDIBC in a discontinuous Galerkin finite element method. In particular, the analysis highlights that the sole definition of a discrete model is not enough to fully define a TDIBC. To support the analysis, an elementary physical nonlinear scattering operator is derived and its computational properties are investigated in an impedance tube. Then, the derivation of time-delayed broadband TDIBCs from physical reflection coefficient models is carried out for single degree of freedom acoustical liners. A high-order discretization of the derived time-local formulation, which consists in composing a set of ordinary differential equations with a transport equation, is applied to two flow duct
ModƩlisation Thermoacoustique de Bas Ordre et Simulation de la Fonction de Transfert d'une Flamme Diphasique
Les instabiliteĢs thermoacoustiques continuent dāeĢtre un obstacle majeur dans le deĢveloppement des systeĢmes de combustion des turbines aĢ gaz. Ces instabiliteĢs sont caracteĢriseĢes par des oscillations de pression de grande amplitude dans la chambre de combustion. Elles sont indĆ©sirables car elles entraiĢnent de fortes vibrations augmentant le bruit et les Ć©missions de polluants, provoquant des contraintes thermiques et meĢcaniques excessives sur les composants de la chambre de combustion, voire menacĢ§ant lāinteĢgriteĢ structurelle du systeĢme complet. La simulation aux grandes eĢchelles (LES) sāest aveĢreĢe eĢtre un outil puissant capable de preĢdire de nombreux pheĢnomeĢnes de combustion instationnaire, y compris les instabiliteĢs. Cependant, les couĢts de calcul eĢleveĢs associeĢs empeĢchent cette approche dāeĢtre utiliseĢe en phase de conception pour analyser toutes les conceptions possibles et les conditions de fonctionnement auxquelles les instabiliteĢs restent extreĢmement sensibles. Cāest pourquoi les modeĢles de bas ordre (LOM) sont preĢcieux et compleĢtent bien les LES, en particulier pendant les eĢtapes de preĢconception de la chambre de combustion. Bien que la plupart des outils LOM disponibles effectuent des simplifications physiques importantes (par exemple, lineĢarisation de lāacoustique, reĢponse aĢ la flamme), ils utilisent eĢgalement geĢneĢralement des geĢomeĢtries trop simplifieĢes. Lāun des principaux objectifs de ce travail est de remeĢdier aĢ cette dernieĢre limitation et dāameĢliorer les techniques LOM existantes pour pouvoir geĢrer des geĢomeĢtries reĢalistes complexes.
Une grande partie du travail sāarticule autour du deĢveloppement et de la validation dāun nouvel outil de modeĢlisation de reĢseaux acoustiques baseĢ sur des ex- pansions modales (Galerkin Series) et des meĢthodes dāespace dāeĢtats (viz. STORM) pour preĢdire et analyser les instabiliteĢs. Dans STORM, un systeĢme complexe aĢ analyser est deĢcomposeĢ et repreĢsenteĢ comme un reĢseau dāeĢleĢments geĢomeĢtriques plus simples (sous-domaines), de connexion (couplage), de flamme et dāeĢleĢments dāimpeĢdance. Les caracteĢristiques uniques de STORM sont la technique dāexpansion modale sur des Frame reĢcemment introduite pour modeĢliser lāacoustique dans les sous-domaines du reĢseau et la meĢthodologie dite des connexions spectrales de surface qui a eĢteĢ deĢveloppeĢe reĢcemment au CERFACS. Ensemble, ils permettent des inter- connexions transparentes entre les sous-domaines avec une acoustique 1D/2D/3D et construisent des reĢseaux repreĢsentant des configurations complexes pertinentes pour lāindustrie. Les meĢthodes dāapproximation rationnelle sont discuteĢes pour incorporer des modeĢles reĢalistes dāinteraction flamme/acoustique (cāest-aĢ-dire, les fonctions de transfert de flamme (FTF) dans les reĢseaux STORM. Lāimportance de quelques contraintes physiques, en particulier la causaliteĢ, dans les algorithmes deĢri- vant ces modeĢles de reĢponse de flamme dāordre infeĢrieur, dans le domaine temporel, dans lāespace dāeĢtats et baseĢs sur les donneĢes aĢ partir de donneĢes de simulation expeĢrimentales ou dāordre eĢleveĢ, est mise en eĢvidence. Un type speĢcial dāeĢleĢment dāimpeĢdance de reĢseau, DECBC (Delayed Entropy Coupled Boundary Condition), est eĢgalement deĢveloppeĢ pour faciliter la preĢdiction des instabiliteĢs mixtes entropie- acoustique. Dans lāensemble, STORM preĢsente un outil efficace, modulaire et flexi- ble pour preĢdire les instabiliteĢs thermoacoustiques et devrait aider aĢ deĢterminer les reĢgimes de stabiliteĢ et les strateĢgies de controĢle passif optimales.
Dans la deuxieĢme partie mineure de la theĢse, le forcĢ§age acoustique de la flamme de pulveĢrisation tourbillonnante turbulente est simuleĢ en utilisant lāapproche Euler- Lagrange (EL) LES. Lāobjectif eĢtait de calculer le FTF et dāeĢvaluer la pertinence du cadre de modeĢlisation de la combustion diphasique EL-LES existant pour un tel probleĢme dāidentification de systeĢme. Des travaux reĢcents ont deĢmontreĢ le potentiel de EL-LES pour preĢdire avec preĢcision lāinstabiliteĢ auto-entretenue. Cependant, les simulations forceĢes preĢsentent certaines difficulteĢs et la FTF obtenue numeĢriquement sāeĢcarte des valeurs de reĢfeĢrence expeĢrimentales dāenviron 20 aĢ 30%. Les reĢsultats restent sensibles, en geĢneĢral, aux parameĢtres de modeĢlisation, si bien que dāautres investigations seront neĢcessaires pour ameĢliorer les modeĢles et la fideĢliteĢ des preĢvisions