The paper reviews research conducted at ONERA over the last thirty years on the transonic buffet. We first present the transonic buffet phenomenon and we explain its importance for aeronautical applications. Then, a distinction is made between the 2D buffet produced by an airfoil and the 3D buffet that characterizes swept wings of finite span. The 2D buffet amounts to a pure oscillation of the shock phase-locked with the detachment and reattachment of the boundary layer downstream, whereas the 3D buffet takes the form of a pocket of broadband perturbations located in a limited portion of the wing. We recall that these mechanisms were first studied in the 1980s through a series of tests conducted in the transonic wind tunnel ONERA T2 at Toulouse and in the large transonic wind tunnel ONERA S2Ma at Modane. Since this pioneering work, progress in the measurement techniques has led to the constitution of a comprehensive database of the 2D buffet that we describe. This database, obtained in the wind tunnel ONERA S3Ch at Meudon, has been extensively used to validate various CFD tools, with the latter being used in turn to investigate the buffet physics. We illustrate this collaboration between simulation and physics by recalling that a linear stability analysis of accurate Reynolds-Averaged-Navier-Stokes (RANS) solutions made it possible to prove that the buffet on a 2D airfoil stems from a global instability mechanism. We also review more recent tests done in the case of a laminar airfoil, which reveal very distinct behaviors of the buffet flow. This illustrates how sensitive the buffet is to the nature of the boundary layer. The last section of the paper gives a short overview of advanced simulations for these different test cases. In the conclusion, we list research perspectives, which include some more general topics such as data assimilation.

The objective of this paper is to present an overview of the work performed at Onera over the last decade on the characterization and control of the buffet phenomenon. This aerodynamic instability induces strong wall pressure fluctuations and as such limits aircraft envelope, consequently it is interesting to understand the origin of this instability and to try to delay its onset, in order to improve aircraft performance, but also to provide more flexibility during the design phase. First, results from wind tunnel tests on 2D airfoils are presented to explain the 2D buffet phenomenon and since it is used as validation test case for numerical simulations. Then, results from several wind tunnel tests on a 3D configuration are presented. The 3D buffet phenomenon is characterized using steady and unsteady wall pressure measurements and LDV. Then, several types of flow control have been investigated, either passive (mechanical vortex generators) or active (fluidic VGs, fluidic trailing-edge device (TED)). It is shown than mechanical and fluidic VGs are able to delay buffet onset in the angle-of-attack domain by suppressing the separation downstream of the shock. The effect of the fluidic TED is different, the separation is not suppressed but the rear wing loading is increased and consequently the buffet onset is not delayed in the angle-of-attack domain, but only in the lift domain. Closed-loop control of the fluidic VGs is also investigated, to adapt the mass flow rate to the aerodynamic conditions.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Buffet Characterization and Control for Turbulent Wings J. Dandois, P. Molton, A. Lepage, A. Geeraert, V. Brunet, J.B. Dor, E. Coustols

We demonstrate the feasibility of detection of the nature (laminar/turbulent/transitional) of the aerodynamic boundary layer of a profile of a wing aircraft model, using a Distributed FeedBack (DFB) Fiber Laser as optical fiber sensor. Signals to be measured are pressure variations : Delta P similar to 1Pa at few 100Hz in the laminar region and Delta P similar to 10Pa at few kHz in the turbulent region. Intermittent regime occurring in-between these two regions (transition) is characterized by turbulent bursts in laminar flow. Relevant pressure variations have been obtained in a low-speed research-type wind tunnel of ONERA Centre of Toulouse. In order to validate the measurements, a "classical" hot film sensor, the application and use of which have been formerly developed and validated by ONERA, has been placed at the neighborhood of the fiber sensor. The hot film allows measurement of the boundary layer wall shear stress whose characteristics are a well known signature of the boundary layer nature (laminar, intermittent or turbulent) [1]. In the three regimes, signals from the fiber sensor and the hot film sensor are strongly correlated, which allows us to conclude that a DFB fiber laser sensor is a good candidate for detecting the boundary layer nature, and thus for future integration in an aircraft wing. The work presented here has been realized within the framework of "Clean Sky", a Joint Technology Initiative of the European Union.

This paper is a review on the dynamics of vortices in fluids which get involved in aircraft wakes. Basic notions useful to appraise their dynamics are: inertial waves, 3D instabilities due to vortex interaction, vortex merging, vortex breakdown and turbulence. Each one of these topics is illustrated by means of experimental or numerical results.

At the beginning of 2000, ONERA launched an internal project (PRF: Projet de Recherche Fédérateur) aimed at investigating a few aerodynamic problems linked to the future supersonic aircraft. Five topics are studied: - Laminar-turbulent transition and laminar flow control - Development of a "quiet" supersonic wind tunnel - Turbulent flows at high values of Reynolds numbers - Shock wave/boundary layer interaction - Jet noise The paper provides an overview of the current work performed in this project, which will be completed by the end of 2004.

At the beginning of 2000, ONERA launched an internal project (PRF: Projet de Recherche Federateur) aimed at investigating a few aerodynamic problems linked to the future supersonic aircraft. Five topics are studied: - Laminar-turbulent transition and laminar flow control - Development of a quiet supersonic wind tunnel - Turbulent flows at high values of Reynolds numbers - Shock wave/boundary layer interaction - Jet noise The paper provides an overview of the current work performed in this project, which will be completed by the end of 2004.

Depuis plusieurs années, l'ONERA a consacré un effort important aux recherches pour tenter de réduire la traı̂née de frottement, sachant que cette dernière composante représente environ 40 à 50 % de la traı̂née totale d'un avion de transport transsonique moderne. Parmi les méthodes passives généralement étudiées, la modification de la géométrie de la paroi par l'utilisation de rainures longitudinales ou «riblets», semble être la plus attrayante car elle est relativement facile à mettre en œuvre et très prometteuse pour une application à l'avion. Ainsi, dans cet article, une synthèse des études expérimentales menées à l'ONERA et dans le monde, pour évaluer les performances de telles parois rainurées, est proposée. Des résultats typiques de modifications de l'écoulement turbulent, mesuré ou calculé, seront également analysés, afin de documenter les effets significatifs attribués à la présence de ces parois rainurées dans l'écoulement.

The aim of this experimental study is to scrutinize the flow in the vicinity of a non zero thickness trailing edge, i.e. just upstream and downstream of the base, using Laser Doppler Anemometry. Indeed, boundary layer surveys as well as wake surveys have been performed for different thicknesses and shapes of trailing edges, Experiments have been conducted with two-dimensional trailing edges in both two- and three-dimensional flows. Much of the discussion presented in this paper is relevant to flow field modification induced by thick cambered trailing edges.

A detailed experimental study has been launched at ONERA/DMAE, aiming at the description of the flow pattern in the vicinity of thick cambered trailing edges. LDV and pressure measurements have provided a very detailed database, which has been considered, for validation of computational tools. Thus, several one- and two-equation turbulence models, implemented in the Reynolds-averaged Navier–Stokes (RANS) solver developed at ONERA, have been applied to the experimental configurations. Typical features are carefully discussed with emphasis set on both pressure distributions (prediction of correct rear loading) and near-wake surveys (capture of the topological vortex-type structure).

The study presents results for experiments conducted in the transonic pressurized T2 wind tunnel of Centre d'Etudes et de Recherches de Toulouse. The 1/80th scale model of a modern subsonic transport aircraft was mounted in the test section using a fin-sting. It consisted of a fuselage, a detachable horizontal rear stabilizer, a belly fairing, and tip-truncated wings. The tip-truncated wings were designed by Aerospatiale to reproduce the correct downwash in the rear fuselage region. Tests were conducted at a freestream Mach number of 0.82 and a Reynolds number evaluated with the model chord length of approximately 2.5 x 10(6). Various measurements were performed, mainly in the vicinity of the fuselage downstream rear part: oil-flow visualizations, pressure distributions, boundary-layer surveys along the fuselage symmetry lines and some lateral lines using a laser Doppler anemometry system, and near-wake surveys with both pressure and velocity measurements, These experiments were designed to allow a fine description of the flows developing along the rear part of a fuselage in a configuration similar to actual flight applications.

An experimental program was set up in the CERT/ONERA's T2 wind tunnel, which is transonic, pressurized, and has self-adaptive walls. A 1/80th-scale model of a modern transonic transport aircraft was tested in two configurations: 1) fuselage without horizontal stabilizer and 2) fuselage with horizontal stabilizer, Various measurements were performed: oil-now visualizations, pressure distributions, boundary-layer surveys along the fuselage symmetry lines using a three-dimensional laser Doppler anemometry system, and wake surveys with both pressure and velocity measurements in a plane downstream of the fuselage base. Moreover, both inviscid and viscous computations were carried out separately under experimental conditions. This article reports a detailed analysis of the computations and experiments conducted on the model in both configurations; the objective of this study was to reach a better understanding of the flows developing along the rear part of a fuselage.

: The purpose of this paper is to provide a current overview of turbulent skin friction drag reduction concepts which have potential for reducing aircraft fuel consumption. Then, this review lectures will be organized around four main topics. First, after briefly reviewing what is known about the structure of the turbulent boundary layer, possible mechanisms for both active and passive devices will be discussed ; concentrating on techniques offering nett drag benefits, either through inner or outer layer manipulation. Indeed, both experimental and numerical results will be presented for these boundary layer manipulators. Available data and computer model predictions for low and high subsonic speeds, transonic conditions, and supersonic flow (including flight tests) will be reviewed.

: The use of in-flow manipulator devices, such as LEBU's originated from research into the optimum use of screens and honeycombs for controlling (reducing) free-stream turbulence in wind tunnels 197. Such 'flow management' techniques were first applied to boundary layers by researchers at NAL Bangalore about 15 years ago 198, and a rapid succession of further studies (conducted first at IIT 199, NASA 2OO in the USA, then at the FFA in Sweden 201 and the Cavendish Laboratory of the University of Cambridge, England 202) led to the identification of single or preferably tandem (in-line) thin element devices as the best means of achieving any nett drag reduction.

The present paper summarizes one of the latest experimental studies, undertaken at ONERA/CERT in subsonic flows. It deals with the efficiency of internal manipulators, commonly named by many researchers riblets, in three-dimensional boundary layer flows. The flow developing on both sides of an ONERA D section aerofoil, set at 22.5° angle of sweep, has been manipulated using different riblet vinyl adhesive films. These latter were covering almost 85% of the chord length. Total drag variations were estimated from Pitot tube surveys, performed at about one chord length behind the aerofoil trailing edge. Two chord Reynolds numbers have been considered: 2.65 105 and 4.25 105. On both sides of the aerofoil, grooves have been aligned with the free-stream flow direction. Skin-friction drag decreases of up to 5–6% have been recorded when the dimensionless rib height, scaled with wall units, is less than 15–20.