SUMMARY Taking place within more extensive work that focuses on hybrid methods in aeroacoustics, the present study is devoted to the data transfer operations that are to be performed between two stages of a hybrid calculation. More precisely, the article focuses on two typical operations that usually accompany such data transfer, which are (i) the sampling rate reduction and (ii) the interpolation of the unsteady perturbations to be transmitted from one stage to the other. First part of the paper analyzes the two main issues of such operations, which are the spuriousing and the aliasing phenomena. For doing so, the usual notions of the interpolation theory are revisited before they are synthesized within an original approach. The here proposed formalism allows to understand better both the spuriousing and the aliasing phenomena, as well as to accurately predict the impact of the latter on the data to be transmitted in terms of signal degradation. Second part of the paper provides an illustration and a validation of these theoretical developments via a direct application to a typical aerodynamic noise problem (aeroacoustic emission by a 2D cylinder cross flow). There, it is further shown how the here proposed formalism can help in improving aeroacoustics hybrid calculations by predicting (and thus possibly minimizing) the bias to be induced on the acoustic extrapolation stage because of the aliasing and/or spuriousing effects inherited from the sampling rate reduction and/or interpolation of CFD data—which is likely to occur in any hybrid scenario. Copyright © 2012 John Wiley & Sons, Ltd.

The present work is relevant from the numerical prediction of aircraft noise via aeroacoustics hybrid methods. It is part of a more extensive effort, which final objective is the development of a robust and accurate CFD-CAA weak coupling methodology. Within this framework, we focus here on the so-called surface coupling approach, a technique that allows conducting weakly coupled CFD-CAA calculations. Such approach (which had been previously developed and validated on simpler cases) is here applied to a ralistic problem of aircraft noise, given by the acoustic emission of a nose landing gear in approach flight (a configuration that was addressed in the Airbus LAGooN program). For doing so, several preliminary tasks are first addressed, which are carefully handled and thoroughly detailed. Two CFD-CAA coupled calculations are then conducted, both being based on i) a same CFD dataset coming from an unsteady aerodynamic calculation (zonal DES), and ii) two distinct CAA calculations of different characteristics ; first, a CFD-CAA calculation is conducted for the so-called 'baseline' configuration (i.e. isolated gear within a free-field flow), so as to validate the coupling procedure, as well as to generate a reference solution for subsequent assessment of the mean flow effects induced by the experimental set-up. The validation of the coupling procedure is conducted via a direct comparison of the CFD-CAA results with either experimental or numerical (CFD, CFD-FWH) outputs obtained in the near-, mid, and/or far-field. With the view of assessing the mean flow effects, an alternative CFD-CAA calculation is then performed, which incorporates the realistic sheared jet flow characterizing the anechoic facility. This allows assessing the (partial) convection and refraction effects induced by such jet mean flow, which helps underscoring better the relevance of the present CFD-CAA hybrid approach when it comes to handle real-life noise problems.

SummaryThe present paper addresses the optimization of finite‐difference schemes when these are to be used for numerically approximating spatial derivatives in aeroacoustics evolution problems. With that view in mind, finite‐difference operators are firstly detailed from a theoretical point of view. Secondly, time, the way such operators can be optimized in a spectral‐like sense is recalled, before the main limitations of such an optimization are highlighted. This leads us to propose an alternative optimization approach of innovative character. Such a novel optimization technique consists of enhancing the scheme's formal accuracy through a minimization of its leading‐order truncation error. This so‐called intrinsic optimization procedure is first detailed, before it is thoroughly analyzed, from both a theoretical and a practical point of view. The second part of the paper focuses on two particular intrinsically optimized schemes, which are carefully assessed via a direct comparison against their standard and/or spectral‐like optimized counterparts, such a comparative exercise being conducted utilizing several academic test cases of increasing complexity. There, it is shown how intrinsically optimized schemes indeed constitute an advantageous alternative to either the standard or the spectral‐like optimized ones, being allotted with both (i) the better scalability of the former scheme with respect to grid convergence effects when the grid density increases and (ii) the higher accuracy of the latter scheme when the discretization level becomes marginal. Thanks to that, such intrinsically optimized schemes offer very good trade‐offs in terms of (i) accuracy; (ii) robustness; and (iii) numerical efficiency (CPU cost). Copyright © 2015 John Wiley & Sons, Ltd.

In the present work, an optimization methodology to compute the best control parameters, χ and Δ, for the selective frequency damping method is presented. The optimization does not suppose any a priori knowledge of the flow physics, neither of the underlying numerical methods, and is especially suited for simulations requiring large quantity of grid elements and processors. It allows for obtaining an optimal convergence rate to a steady state of the damped Navier-Stokes system. This is achieved using the Dynamic Mode Decomposition, which is a snapshot-based method, to estimate the eigenvalues associated with global unstable dynamics. Validations test cases are presented for the numerical configurations of a laminar flow past a 2D cylinder, a separated boundary-layer over a shallow bump, and a 3D turbulent stratified-Poiseuille flow.

The present work constitutes a fraction of a more extensive study that is devoted to numerical methods in acoustics. More precisely, we address here the interpolation process, which is more and more frequently used in Computational Acoustics-whatever it is for enabling multi-stage hybrid calculations, or for easing the proper handling of complex configurations via advanced techniques such as Chimera grids or Immersed Boundary Conditions. In that regard, we focus here on high-order interpolation schemes, so as to analyze their intrinsic features and to assess their effective accuracy. Taking advantage of specific advances that had been previously achieved by the present authors regarding centered and standard high-order interpolation schemes, we here focus on spectral-like optimized and/or noncentered ones. The latter are analyzed thoroughly thanks to dedicated theoretical developments, which allow highlighting better what their strengths and weaknesses are. Among others, the various ways such interpolation schemes can degrade acoustic signals they are applied to are carefully investigated from a theoretical point-of-view, before they are numerically assessed thanks to several academic test cases. Besides that, specific criteria that could help in optimizing interpolation schemes better are provided, along with generic rules about how to minimize the signal degradation induced by existing interpolation schemes, in practice. Finally, conclusions are drawn about the effective accuracy one can expect from those noncentred and/or spectral-like optimized interpolations.