Search

This paper deals with the computation of heat exchange in transitional boundary layers for the prediction of the laminar–turbulent transition detection using infrared thermography. An aerothermal coupling between a heat equation solver and a boundary-layer solver is presented here. This coupling is first used to provide guidelines in the academic framework of a flat plate, to achieve an enhanced temperature gradient at the transition onset in order to improve infrared imaging, and thus the transition position detection. The method is then successfully applied on a real case, by comparing the numerical predictions to measurements obtained during transonic wind-tunnel tests on a two-dimensional wing model.

Two separate experiments are conducted on a three bladed NACA0012 rotor operating at low Reynolds numbers using phase-locked infrared thermography coupled with simultaneous force and torque measurements. The first, focuses on the effects of freestream turbulence on boundary layer transition over the suction side of the aerofoil of the rotor in an advancing configuration. Freestream turbulence (FST) was generated in an open section wind tunnel using grids and was characterized using Hot-Wire anemometry. In general, when the rotor was subjected to FST, an increase in thrust and efficiency was observed, which could be due to the FST suppressing flow separation or by inducing early transition. The second experiment, consisted of a parametric study on the impact of forcing boundary layer transition using roughness placed on the suction side of the aerofoil, in a hover configuration. The height of the roughness was varied from 52-220μm and was placed at all at at 10% chord, over the entire span of the blade. Force and torque measurements revealed that there could be a optimal roughness height that could lead to a performance increase.

International audience; Reducing aircraft fuel consumption by maximising the extent of laminar flow on wings assumes that the initial flow, along the wing's attachment line, is laminar. However, if the wing is attached to a solid wall, the wing's attachment line can be contaminated by the turbulent boundary layer developing over the solid wall for flow conditions summarised in a critical Reynolds number (R) greater than 250. Since typical R values encountered in flight can be well above 400, techniques, such as wall suction along the wing's leading edge were developed to further delay the threshold R at which contamination occurs. The present paper presents the results from an experimental investigation performed on the ONERA DTP-A model fitted with leading edge suction capabilities. The experiment was performed in the ONERA F2 wind tunnel in the framework of the EU-funded Clean Sky 2 HLFC-WIN project (LPA-IADP platform), while the suction panels were manufactured by Aernnova, an aero-component manufacturing company. Hot film measurements and infra-red thermography showed that attachment line contamination could effectively be delayed up to threshold R values of 1000 for large suction flow rates. Although panels from different manufacturing processes and with different geometric characteristics were tested, no significant difference from these parameters were observed.

International audience; An experimental investigation on the flow topology and performance of a rotor operating at low Reynolds numbers is presented. The feasibility of laminar to turbulent transition experiments over small rotors is demonstrated. Phase-locked infrared thermography coupled with simultaneous force and torque measurements were used to study a three bladed NACA0012 rotor with a radius of 0.125 m and an angle of attack of 10 degrees. Boundary layer transition was fostered using two-dimensional (2D) and threedimensional (3D) isolated roughness elements, placed at approximately 5% and 28% of the rotor blades chord. In the smooth rotor configuration, a 3D flow topology is observed, consisting of a clear laminar region closer to the blade root and a turbulent region at the blade tip. It was found that the state of the boundary layer can significantly affect the rotor’s performance, with the forcing of laminar to turbulent transition generally resulting in a loss of performance when compared to the smooth reference rotor case.

International audience; The transition from laminar to turbulent boundary layer has been largely investigated in the literature on the experimental side as well as on the numerical side but the processes specifically involved in both transitional and unsteady flows (unsteady aerodynamic or aeroelasticity) have to be studied more in details. This paper describes an experimental investigation achieved through a wind tunnel test of an oscillating swept wing in low speed range. In order to force the apparition of Cross Flow instabilities, the model was installed with a 60° sweep angle under favorable pressure gradient ensuring the absence of Tollmien Schlichting instabilities. The model was equipped with hot-film sensors to estimate the laminar or turbulent state of the boundary layer through the measurement of wall shear stress. The synchronous acquisition of dynamic motion and hot-film signals allowed to analyse the unsteady effects on the Cross Flow induced transition. The wind tunnel test program was achieved following a parametric approach through the variation of significant parameters: the amplitude and frequency of the dynamic actuation, the flow speed velocity and the mean value of the airfoil angle of attack. The paper describes the main insights of the test campaign and the associated database and presents how the test setup provided a well-adapted mean to study dynamic motions of the transition location. In comparison to steady configuration, the transition was not located at a fixed chordwise position but covered an area with an intermediary state of the boundary layer between the laminar and turbulent states. Several tools and post processing methods were specifically tuned to calculate quantities of the transition position (time resolved signal, mean properties through a phase averaging process, intermittency ratio) in addition to classical time and frequency analyses. One of the main outcomes pointed out the mean location of the laminar-turbulent transition was weakly influenced by unsteady motion. The general trend indicated a IFASD-2019-120 2 more upstream location of few percent in chordwise in comparison to the steady case without any motion. The experimental results constitute a first step in the understanding of the interaction of Cross Flow instabilities and dynamic motion and in the assessment of ONERA modelling and numerical simulation capabilities.

L’influence de la rugosité de surface sur la transition laminaire/turbulente d’une couche limite 2D incompressible est examinée de manière expérimentale en soufflerie. En particulier, l’effet de la rugosité sur la réceptivité, et surtout l’amplification des ondes de Tollmien-Schlichting (ondes T-S) est analysée. Une expérience dont la rugosité de surface peut être partiellement changée est conçue est usinant une plaque plane pour que trois inserts rectangulaires amovibles y soient placés. Ces inserts sont soit lisses (usinés en métal) soit rugueux, fabriqués par une imprimante 3D à stéréolithographie. La plupart des résultats sont obtenus par l’anémométrie à fil chaud.Si les premiers résultats ont permis de mettre en lumière un potentiel effet de la rugosité de surface sur la réceptivité des ondes de Tollmien-Schlichting, l’essentiel de ce manuscrit est consacré à l’étude de la suramplification de ces instabilités en présence de rugosité de surface. L’accent est dans un premier temps mis sur l’étude des phénomènes par lesquels la rugosité de surface suramplifie les ondes T-S. Aucune déformation du profil moyen n’est constatée mis à part une légère inflexion à proximité immédiate de la rugosité, mais les études effectuées montrent la limite de l’anémométrie à fil chaud pour les études dans les régions aussi proches de la paroi. La possibilité que la rugosité de surface génère des stries est étudiée et confirmée, mais dans certains cas seulement. Cette génération de stries semble en effet n'avoir lieu que lorsque la rugosité de surface possède les longueurs d’ondes adaptées, au sens de la théorie des perturbations optimales, pour déclencher ce type d’instabilités.L’avancée de la position de transition est ensuite étudiée de manière quantitative en fonction des paramètres de hauteur moyenne, de position et de longueur de la zone rugueuse. Des positions de transition obtenues dans les cas avec rugosité sont déduits des $Delta N$, représentant un surcroît d'amplification des ondes T-S. Pour une longueur de rugosité donnée, la hauteur moyenne adimensionnée par l’épaisseur de déplacement ($R_a/delta_1)$ semble être un paramètre majeur et l'évolution $Delta N$ induit par la rugosité en fonction de ce paramètre est proche d'une loi linéaire. Aucune influence de la position le long de la plaque plane n’est observée tant que la réceptivité n’est pas modifiée. La longueur de la zone rugueuse se révèle également être un paramètre fondamental, et donner lieu là encore à une évolution proche d'une loi linéaire de $Delta N$.Basées sur ces résultats expérimentaux, deux modélisations, dites par $Delta N$ et $Delta N$ réparti inspirées des modèles en $Delta N$ utilisés dans le cadre des rugosités 2D localisées ont été mises au point pour calculer le Delta N induit par une surface rugueuse. La première est une approche globale quand la seconde propose une formulation locale. The influence of surface roughness on the laminar/turbulent transition of an incompressible 2D boundary layer is examined experimentally in a wind tunnel. In particular, the effect of roughness on the receptivity and especially the amplification of Tollmien-Schlichting waves (T-S waves) is analyzed. An experiment in which the surface roughness can be partially changed is designed by machining a flat plate so that three removable rectangular inserts are placed on it. These inserts are either smooth (machined from metal) or rough, fabricated by a stereolithography 3D printer. Most of the results are obtained by hot wire anemometry.If the first results have highlighted a potential effect of surface roughness on the receptivity of Tollmien-Schlichting waves, the main part of this manuscript is devoted to the study of the overamplification of these instabilities in the presence of surface roughness. The emphasis is first put on the study of the phenomena by which the surface roughness overdrives the T-S waves amplification. No distortion of the mean profile is observed except for a slight inflection in the immediate vicinity of the roughness, but the studies carried out show the limitation of hot-wire anemometry for studies in regions so close to the wall. The possibility that surface roughness generates streaks is studied and confirmed, but only in some cases. This generation of streaks seems to take place only when the surface roughness has the appropriate spanwise wavelengths, according to the theory of optimal perturbations, to trigger this type of instability.The advance of the transition position is then studied quantitatively as a function of the parameters of average height, position and length of the rough zone. From the transition positions obtained in the cases with roughness are deduced $Delta N$, representing an overamplification of the T-S waves. For a given roughness length, the average height scaled by the displacement thickness ($R_a/delta_1)$ seems to be a major parameter and the $Delta N$ evolution induced by the roughness as a function of this parameter is close to a linear law. No influence of the position along the flat plate is observed as long as the receptivity is not modified. The length of the rough area is also a fundamental parameter, and shows an evolution close to a linear law of $Delta N$ too.Based on these experimental results, two models, called by $Delta N$ and $Delta N$ distributed inspired by the $Delta N$ models used in the framework of localized 2D roughness, have been developed to calculate the $Delta N$ induced by a rough surface. The first one is a global approach while the second one proposes a local formulation.

The boundary layer transition from a laminar to a turbulent state increases the wall friction drag. Particularly on future aircrafts, one way of reducing fuel consumption, and answering both an environmental and economic issue, consists in delaying the transition farther downstream. In this context, the aim of this work is to characterize the impact of Dielectric Barrier discharge (DBD) plasma actuators on the boundary layer transition. When powered with an alternative high voltage, these active actuators produce a pulsed body force which is tangential to the wall and, under some conditions, enables to modify the boundary layer mean velocity profiles to delay the transition. Under other conditions, the unsteady body force amplifies modal instabilities (Tollmien-Schlichting waves) may destabilize the boundary layers, leading to a promoted transition. A first experiment enabled to highlight this competition between the stabilizing mean effect and the destabilizing unsteady effect by measuring respectively a transition delay and a transition promotion. A numerical study based on local stability analyses wass conducted in parallel and showed that a wide frequency range of TS waves is damped by the mean body force, which explains the transition delay. A second experiment, focusing on the mean effect, enabled to show the influence of the actuator position and the cumulative effect of several actuators on the transition delay; La transition laminaire-turbulent au sein de la couche limite qui se développe sur les parois des aéronefs augmente fortement la traînée de frottement. Ainsi, afin de répondre à une problématique à la fois environnementale et économique, une piste envisagée pour réduire la consommation en carburant des aéronefs du futur est de diminuer la trainée en reculant cette transition le plus en aval possible. Dans ce cadre, l’objectif de cette thèse est de caractériser expérimentalement et numériquement l’effet d’actionneurs à plasma de type Décharge à Barrière Diélectrique sur la transition. Alimentés par une haute tension alternative, ces actionneurs actifs produisent une force volumique pulsée qui permet, sous certaines conditions, de modifier les profils de vitesse moyenne dans la couche limite et de reculer la transition. Sous d’autres conditions, le caractère instationnaire de cette force volumique peut entrainer une amplification des instabilités modales naturellement présentes dans la couche limite (ondes de Tollmien-Schlichting) et ainsi conduire à une transition prématurée. Une première expérience a permis de mettre en évidence cette compétition entre l’effet moyen stabilisant et l’effet instationnaire déstabilisant en mesurant respectivement un recul et une avancée de la transition. Parallèlement à ces activités expérimentales, une étude numérique, basée sur des analyses de stabilité linéaire, a montré que l’effet moyen de la force volumique permettait d’atténuer une large gamme de fréquences d’ondes TS dans la couche limite et d’expliquer le recul de transition observé expérimentalement. En se concentrant sur l’effet moyen, une seconde expérience a permis d’étudier l’influence de la position de l’actionneur ainsi que l’effet cumulatif de plusieurs actionneurs sur le recul de transition.