A generalized non-reflecting inlet boundary condition for steady and forced compressible flows with injection of vortical and acoustic waves

This paper describes a new boundary condition for subsonic inlets in compressible flow solvers. The method uses characteristic analysis based on wave decomposition and the paper discusses how to specify the amplitude of incoming waves to inject simultaneously three-dimensional turbulence and one-dimensional acoustic waves while still being non-reflecting for outgoing acoustic waves. The non-reflecting property is ensured by using developments proposed by Polifke et al. [1, 2]. They are combined with a novel formulation to inject turbulence and acoustic waves simultaneously at an inlet. The paper discusses the compromise which must be sought by the boundary condition formulation between conflicting objectives: respecting target unsteady inlet velocities (for turbulence and acoustics), avoiding a drift of the mean inlet velocities and ensuring non-reflecting performances for waves reaching the inlet from the computational domain. This well-known limit of classical formulations is improved by the new approach which ensures that the mean inlet velocities do not drift, that the unsteady components of velocity (turbulence and acoustics) are correctly introduced into the domain and that the inlet remains non-reflecting. These properties are crucial for forced unsteady flows but the same formulation is also useful for unforced cases where it allows to reach convergence faster. The method is presented by focusing on the expression of the ingoing waves and comparing it with the classical NSCBC approach [3]. Four tests are then described: (1) the injection of acoustic waves through a non reflecting inlet, (2) the compressible flow establishment in a nozzle, (3) the simultaneous injection of turbulence and ingoing acoustic waves into a duct terminated by a reflecting outlet and (4) a turbulent, acoustically forced Bunsen-type premixed flame

Hydrodynamic - acoustic filtering of a supersonic-underexpanded jet

The main noise that is perceived inside the cabin of an airplane originates in the turbofan engines and their associated exhaust jets. Due to flight conditions, a pressure difference appears at the exit of the secondary flow which engenders a series of expansion and compression waves known as shock-cells

Shock-cell noise of supersonic underexpanded jets

International audienceShock-cell noise is a particular noise that appears in imperfectly expanded jets. Under these expansion conditions a series of expansions and compressions appear following a shock-cell type structure. The interaction between the vortices developed at the lip of the nozzle and the shock-cells generates what is known as shock-cell noise. This noise has the particularity to be propagated upstream with a higher intensity. This publication will focus on the shock-cell noise generated by an axisymmetric under-expanded 10 to the power of 6 Reynolds single jet. The LES computations are carried out using the elsA code developed by ONERA and extended by CERFACS with high-order compact schemes. They are validated against experimental results. The LES simulation is initialized with a RANS solution where the nozzle exit conditions are imposed. Even though no inflow forcing is applied, good agreement is obtained in terms of flow structures and broadband shock-cell noise that is propagated to the farfield by means of the Ffowcs-Williams & Hawkings analogy

Shock-cell noise of supersonic underexpanded jets

Shock-cell noise is a particular noise that appears in imperfectly expanded jets. Under these expansion conditions a series of expansions and compressions appear following a shock-cell type structure. The interaction between the vortices developed at the lip of the nozzle and the shock-cells generates what is known as shock-cell noise. This noise has the particularity to be propagated upstream with a higher intensity. This publication will focus on the shock-cell noise generated by an axisymmetric under-expanded 10 to the power of 6 Reynolds single jet. The LES computations are carried out using the elsA code developed by ONERA and extended by CERFACS with high-order compact schemes. They are validated against experimental results. The LES simulation is initialized with a RANS solution where the nozzle exit conditions are imposed. Even though no inflow forcing is applied, good agreement is obtained in terms of flow structures and broadband shock-cell noise that is propagated to the farfield by means of the Ffowcs-Williams & Hawkings analogy

Modal structure of a supersonic under-expanded jet

Le bruit de choc est un bruit particulier qui intervient lorsque qu'un jet n'est pas parfaitement détendu. On observe alors des cellules de choc en aval de la tuyère, composées d'onde de compression et de détente. Les interactions entre la turbulence et ces cellules de choc sont responsable de la génération du bruit de choc. Ce bruit se caractérise par une directivité marqué vers l'amont de l'écoulement ainsi qu'une forte intensité. Dans cette étude, nous nous intéressons à l'analyse modale de la structure d'un jet supersonique sous-détendu caractérisé par un nombre de Reynolds Re=UjDj/vj = 10(6), calculé par simulation aux grandes échelles (SGE) à l'aide du code elsA développée par l'ONERA avec l'intégration de schémas d'ordre élevé du CERFACS

Cluster-based reduced-order modelling of a mixing layer

We propose a novel cluster-based reduced-order modelling (CROM) strategy of unsteady flows. CROM combines the cluster analysis pioneered in Gunzburger's group (Burkardt et al. 2006) and and transition matrix models introduced in fluid dynamics in Eckhardt's group (Schneider et al. 2007). CROM constitutes a potential alternative to POD models and generalises the Ulam-Galerkin method classically used in dynamical systems to determine a finite-rank approximation of the Perron-Frobenius operator. The proposed strategy processes a time-resolved sequence of flow snapshots in two steps. First, the snapshot data are clustered into a small number of representative states, called centroids, in the state space. These centroids partition the state space in complementary non-overlapping regions (centroidal Voronoi cells). Departing from the standard algorithm, the probabilities of the clusters are determined, and the states are sorted by analysis of the transition matrix. Secondly, the transitions between the states are dynamically modelled using a Markov process. Physical mechanisms are then distilled by a refined analysis of the Markov process, e.g. using finite-time Lyapunov exponent and entropic methods. This CROM framework is applied to the Lorenz attractor (as illustrative example), to velocity fields of the spatially evolving incompressible mixing layer and the three-dimensional turbulent wake of a bluff body. For these examples, CROM is shown to identify non-trivial quasi-attractors and transition processes in an unsupervised manner. CROM has numerous potential applications for the systematic identification of physical mechanisms of complex dynamics, for comparison of flow evolution models, for the identification of precursors to desirable and undesirable events, and for flow control applications exploiting nonlinear actuation dynamics.Comment: 48 pages, 30 figures. Revised version with additional material. Accepted for publication in Journal of Fluid Mechanic

Identification of temporal and spatial signatures of broadband shock-associated noise

Broadband shock-associated noise (BBSAN) is a particular high-frequency noise that is generated in imperfectly expanded jets. BBSAN results from the interaction of turbulent structures and the series of expansion and compression waves which appears downstream of the convergent nozzle exit of moderately under-expanded jets. This paper focuses on the impact of the pressure waves generated by BBSAN from a large eddy simulation of a non-screeching supersonic round jet in the near-field. The flow is under-expanded and is characterized by a high Reynolds number Rej=1.25×106 and a transonic Mach number Mj=1.15. It is shown that BBSAN propagates upstream outside the jet and enters the supersonic region leaving a characteristic pattern in the physical plane. This pattern, also called signature, travels upstream through the shock-cell system with a group velocity between the acoustic speed Uc−a∞ and the sound speed a∞ in the frequency–wavenumber domain (Uc is the convective jet velocity). To investigate these characteristic patterns, the pressure signals in the jet and the near-field are decomposed into waves traveling downstream (p+) and waves traveling upstream (p−). A novel study based on a wavelet technique is finally applied on such signals in order to extract the BBSAN signatures generated by the most energetic events of the supersonic jet

Modélisation de paroi pour la simulation des grandes échelles avec une approche numérique d’ordre élevé

Cette étude porte sur le couplage d’un modèle de paroi avec une approche numérique d’ordre élevé pour la réalisation de calculs aéroacoustiques d’écoulements pariétaux à hauts nombres de Reynolds par la Simulation des Grandes Echelles (SGE). La modélisation de paroi est basée sur l’utilisation de lois analytiques et est combinée à des schémas numériques implicites d’ordre élevé développés en Volumes Finis (VF). Les performances de ce couplage sont évaluées en réalisant la simulation d’un écoulement de canal plan bi-périodique turbulent à un nombre de Reynolds Re = 2000, puis la SGE d’un jet subsonique isotherme à un nombre de Reynolds ReD = 5,7 x 10(5)

A Mesh Adaptation Strategy to Predict Pressure Losses in LES of Swirled Flows

The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Program (FP/2007-2013) / ERC Grant Agreement ERC-AdG 319067-INTECOCIS