Analysis and Comparison of Transonic Buffet Phenomenon over Several Three-Dimensional Wings

International audienceThe transonic buffet is a complex aerodynamic instability that appears on wings and airfoils at a high subsonic Mach number and/or angle of attack. It consists of a shock oscillation that induces pressure and notably lift fluctuations, thus limiting the flight envelope of civil aircraft. The aim of the present Paper is to improve the understanding of the flow physics of the three-dimensional transonic buffet over swept wings through the analysis and comparison of four different experimental databases. In particular, the objective is to identify characteristic values of the phenomenon such as Strouhal numbers, convection velocities, buffet onset, etc. It is shown that some dimensionless numbers are kept constant among the different databases and consequently can be considered as characteristics, whereas others change. The key factors in the understanding of the three-dimensional transonic buffet phenomenon lie in explaining common features but also the variability of transonic buffet characteristics in different configurations. In particular, it is shown that three-dimensional buffet is characterized by a Strouhal number in the range 0.2–0.3 and a spanwise convection velocity of 0.245 0.015 U∞, where U∞ denotes the freestream velocity. These characteristic ranges of frequencies are larger than those of the two-dimensional buffet phenomenon, which suggests different physical mechanisms

La recherche expérimentale en aérodynamique à l’ONERA – L’exemple du buffet transsonique

International audienceThe 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 limitedportion 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.L'article passe en revue les recherches menées à l'ONERA au cours des trente dernières années sur le buffet transsonique. Nous présentons d'abord le phénomène du buffet transsonique et nous expliquons son importance pour les applications aéronautiques. On distingue ensuite le buffet 2D produit par une aile bidimensionnelle et le buffet 3D qui caractérise les ailes en flèches d’envergure finie. Le buffet 2D se présente sous la forme d’une oscillation d’ensemble de tout l’écoulement couplant un déplacement de l’onde de choc et le décollement de la couche limite en aval de ce choc. Le buffet 3D prend quant à lui la forme d'une poche de perturbations localisées dans une portion limitée de l'aile. Nous rappelons que ces mécanismes ont d'abord été étudiés à l’ONERA dans les années 80 à travers une série de tests réalisés dans la soufflerie transsonique T2 à Toulouse et dans la grande soufflerie transsonique S2 de Modane. Ces travaux pionniers ont ensuite été approfondis dans la soufflerie S3Ch de Meudon de manière à élaborer une base de données complète du buffet 2D sur une aile 2D en régime turbulent, que nous décrivons. Cette base de données a été utilisée de façon extensive, à l’ONERA et dans d’autres institutions pour valider différents outils de simulation, ces derniers étant alors utilisés à leur tour pour étudier la physique du buffet. Nous illustrons cette collaboration entre la simulation et la physique en rappelant qu'une analyse de stabilité linéaire de solutions précises des équations de Navier-Stokes moyennées au sens de Reynolds (RANS) a permis de prouver que le buffet 2D provient d'un mécanisme d'instabilité globale. Nous passons également en revue des essais plus récents réalisés dans la soufflerie S3Ch sur le cas d'une aile 2D laminaire qui révèlent des comportements très distincts par rapport au cas turbulent. Cela illustre la sensibilité du buffet à la nature de la couche limite. Le dernier paragraphe du document donne un bref aperçu des simulations avancées de ces différents cas tests. Dans la conclusion, nous énumérons les perspectives de recherche sur le sujet, qui incluent aussi des thématiques méthodologiques plus générales telles que l'assimilation de données

Closed-loop separation control over a sharp edge ramp using Genetic Programming

We experimentally perform open and closed-loop control of a separating turbulent boundary layer downstream from a sharp edge ramp. The turbulent boundary layer just above the separation point has a Reynolds number $Re_{\theta}\approx 3\,500$ based on momentum thickness. The goal of the control is to mitigate separation and early re-attachment. The forcing employs a spanwise array of active vortex generators. The flow state is monitored with skin-friction sensors downstream of the actuators. The feedback control law is obtained using model-free genetic programming control (GPC) (Gautier et al. 2015). The resulting flow is assessed using the momentum coefficient, pressure distribution and skin friction over the ramp and stereo PIV. The PIV yields vector field statistics, e.g. shear layer growth, the backflow area and vortex region. GPC is benchmarked against the best periodic forcing. While open-loop control achieves separation reduction by locking-on the shedding mode, GPC gives rise to similar benefits by accelerating the shear layer growth. Moreover, GPC uses less actuation energy.Comment: 24 pages, 24 figures, submitted to Experiments in Fluid

Comparison of synthetic jet actuators based on sharp-edged and round-edged nozzles

Axisymmetric synthetic jet actuators based on a loudspeaker and on two types of flanged nozzles were tested and compared experimentally. The first type of the nozzle was a sharp-edged circular hole. The second one had a special design with fillets at inner and outer nozzle exit and with a small step in the middle of the nozzle. The function of the step was to prevent the flow reattachment during the extrusion stroke. The actuators with the two types of nozzles were operated at resonance and were compared first qualitatively using a simple phase locked flow visualization. Then the hot-wire anemometer was used to measure velocity distributions along nozzle axis and velocity profiles at the nozzle exit. Comparison of the nozzles was based on evaluation of the characteristic velocity and integral quantities (volumetric, momentum, and kinetic energy fluxes). It was found out that these quantities, which were evaluated at the nozzle exit, differ substantially for both nozzles. On the other hand the velocity flow field in farther distances from the nozzle exit area did not exhibit such prominent differences

Structure from motion photogrammetry in forestry : a review

AbstractPurpose of ReviewThe adoption of Structure from Motion photogrammetry (SfM) is transforming the acquisition of three-dimensional (3D) remote sensing (RS) data in forestry. SfM photogrammetry enables surveys with little cost and technical expertise. We present the theoretical principles and practical considerations of this technology and show opportunities that SfM photogrammetry offers for forest practitioners and researchers.Recent FindingsOur examples of key research indicate the successful application of SfM photogrammetry in forestry, in an operational context and in research, delivering results that are comparable to LiDAR surveys. Reviewed studies have identified possibilities for the extraction of biophysical forest parameters from airborne and terrestrial SfM point clouds and derived 2D data in area-based approaches (ABA) and individual tree approaches. Additionally, increases in the spatial and spectral resolution of sensors available for SfM photogrammetry enable forest health assessment and monitoring. The presented research reveals that coherent 3D data and spectral information, as provided by the SfM workflow, promote opportunities to derive both structural and physiological attributes at the individual tree crown (ITC) as well as stand levels.SummaryWe highlight the potential of using unmanned aerial vehicles (UAVs) and consumer-grade cameras for terrestrial SfM-based surveys in forestry. Offering several spatial products from a single sensor, the SfM workflow enables foresters to collect their own fit-for-purpose RS data. With the broad availability of non-expert SfM software, we provide important practical considerations for the collection of quality input image data to enable successful photogrammetric surveys

NARX Modeling and Extremum-Seeking Control of a Separation

International audienceA numerical study concerning a SISO active closed-loop separation control on a rounded step is presented. A first study of the synthetic jet frequency effect on the separation shows that the mean separation bubble surface is minimized if the mean pressure of a single wall pressure sensor is maximized. With the aim of designing a closed-loop strategy for the control of the recirculation bubble, a NARX black-box model of the pressure signal is identified using a single unsteady RANS simulation. The basic extremum- seeking algorithm is improved with an adaptive gain, to guarantee algorithm performance and this modification is validated against the nonlinear blackbox model of the forced flow. Then, the robust adaptive closed-loop is applied in realtime with an unsteady RANS simulation. Closed-loop results show the ability of the extremum-seeking control with adaptive gain to automatically control the separation, by minimizing the recirculation bubble surface using an unsteady RANS simulation

NACA Research Memorandums

Report presenting an investigation of the variables pertinent to the control of gas temperatures of gas-turbine engines during starting conducted by obtaining time records of the variables involved during actual starting conditions. Poor control of gas temperatures during starting was found to be caused by an accumulation of fuel in the engine before ignition and excessive fuel-flow rates at the time of ignition