Voilure supercritique | EPFL Graph Search

Une voilure supercritique est en aéronautique un profil d'aile dont la vitesse critique est plus grande que pour une voilure conventionnelle. La vitesse critique de l'avion est celle qui provoque localement un écoulement supersonique de l'air sur la partie supérieure de l'aile et induit une trainée d'onde. Le Mach critique est celui où, l'avion étant à une vitesse inférieure à celle du son, l'écoulement commence à devenir supersonique sur une partie du profil. Le régime d'écoulement est alors transsonique. Histoire Grâce à la mise au point des réacteurs dans les années 1950, les avions purent atteindre de grandes vitesses. Ils furent confrontés aux phénomènes liés à la compressibilité de l'air. Celle-ci engendre la formation d'ondes de choc, induit une variation de la portance et augmente fortement la traînée : c'est le mur du son. La vitesse des avions de ligne est relativement proche de la vitesse du son (Mach 1). Les filets d'air subissent une accélération autour

Predicting unsteady flow separation in response to a flow disturbance

Julien Dominique Claude Deparday, Guosheng He, Sabrina Henne, Karen Ann J Mulleners

We present here a summary of the activities and results of dynamically stalling airfoils from the UNFoLD lab at EPFL that have been presented at the meetings of the NATO AVT-282 discussion group on Unsteady aerodynamic response of rigid wings in gust encounters during the past three years. The results are based on experimental data for a sinusoidally pitching OA209 airfoil in a wind tunnel at

\Rey=\num{9.2e5}

, a sinusoidally pitching NACA0015 airfoil profile with a trailing edge flap in a wind tunnel at

\Rey= \num{5.5e5}

, and a sharp-edged flat plate undergoing a ramp-up motion in a recirculating water channel at

\Rey= \num{77 500}

. The first two data sets are used to study the onset of dynamic stall and the third one is used to analyse post-stall load fluctuations. Based on the experimental data, we derived a new model of the leading edge suction parameter that accurately predicts the value and the timing of the maximum leading edge suction and dynamic stall onset on a pitching airfoil. The model is based on thin airfoil theory and links the evolution of the leading edge suction and the shear layer during stall development. By including an oscillating trailing edge flap, the lift polars can be significantly altered but the dynamic stall time delay is only marginally affected by the kinematics of the flap. The maximum lift coefficient is strongly affected by both the main wing and the trailing edge flap kinematics. For a flat plate without flap, the maximum lift coefficient and the subsequent post stall peak values increase with increasing pitch rate up to a critical pitch rate beyond which the entire lift response become independent of the pitch rate. The convective time delay to reach the primary lift peak decreases with increasing pitch rate up to the critical pitch rate and the time delay between subsequent peaks slightly decreases until the limit cycle oscillation period is reached.

American Institute of Aeronautics and Astronautics, Inc2020

Interaction between a surface dielectric barrier discharge and transonic airflows

Samantha Pavon

The use of plasmas in aerodynamics has become a recent topic of interest. The potentialities of different types of plasmas are being investigated for low velocity and high velocity flow control, as well as for plasma-assisted combustion. Dielectric barrier discharges (DBDs) are good candidates since the transition of the glow or filamentary discharge to an arc is prevented by the dielectric barrier. Moreover, surface DBDs allow to ionize the gas very close to the dielectric surface and can be used to ionize the boundary layer around an object immersed in a flow. The research in flow control has basically followed two main paths: the study of DBDs in low-speed airflows and the study of volume glow or corona discharges in supersonic airflows. Until today, there has been an important technological barrier in experimental investigations with surface DBDs. Atmospheric pressure surface DBDs in air have been difficult to maintain for long operation times, typically several hours, because reactive species created in the plasma (for example atomic oxygen) generate intensive etching of the electrode and dielectric materials. Oxidation of the electrodes or reduction of the dielectric thickness will eventually lead to plasma extinction or arcing respectively. This important issue has prevented detailed studies of DBDs in extreme environment, namely in high-speed airflows. In the present work, a solution to this technological problem has been found and is presented. Low temperature co-fireable ceramic (LTCC) technology allows, for the first time, to fully encapsulate the electrodes in a ceramic matrix and maximize the lifetime of the DBD system. Encapsulation improves the reproducibility of the experiments. Moreover, the plate can be manufactured in a curved shape. This technological advance permits, in the frame of the research presented here, to carry out a detailed experimental investigation of DBDs in high-speed flows. The goal of this experimental research is to improve the physical understanding of the interaction between a local atmospheric discharge, causing a localized weak ionization of the surrounding airflow, and the shock wave structure in transonic and supersonic flows typical for aeronautic applications. The fundamental nature of the research makes it relevant in a large domain of applications such as sonic boom alleviation, the reduction of aerodynamic losses (drag reduction) or combustion improvement. The surface dielectric barrier discharge is first characterized without airflow in order to understand the influence of the applied electrical conditions and the structure of the DBD plate on the discharge regime its spatial distribution. Current curves and photomultiplier measurements show that the DBD comprises a filamentary and a continuous (glow- or corona-like) component. Increasing the applied voltage ramp (dU2/dt) results in an increase in the filament generation rate and current peak amplitude. The geometry of the electrodes has little effect on the burning voltage but plays a role in the filament generation rate. Encapsulation reduces the rate of filament production and the expansion of the plasma around the upper electrode but it generates a more uniform distribution of the plasma on the surface and as a function of time. The behavior of the surface DBD in a high-speed air flow is first studied in a simple aerodynamic configuration: a flat DBD plate mounted on one wall of the nozzle. It is demonstrated that the DBD generated by this system can be sustained in supersonic airflows up to free stream Mach numbers of M∞=1.1. Current and time-resolved light emission measurements (photomultiplier) show that there are modifications in the discharge regime at high airflow velocity. For overall discharge, the filamentary to continuous component ratio is increased with increasing flow velocities, the plasma becomes relatively more filamentary. For individual microdischarges, the light pulse emission duration is reduced by one order of magnitude. These measurements indicate that there is a change in the breakdown mechanism and it is proposed that a transition from Townsend breakdown to streamer breakdown occurs when the airflow velocity is increased. The inverse problem is then addressed, the effects of the DBD on the airflow, in a case where the plasma is generated on the surface of an airfoil and interacts with the shock structure in the transonic flow field around that airfoil. Although no significant effects of the surface DBD on the normal shock generated in the transonic flow have been observed with the Schlieren visualizations and far-field pressure measurements, the flow modifies the plasma characteristics in a very significant way. In addition to the effects of the flow velocity (as observed in the flat plate experiment), the significant variation of pressure on the surface of the airfoil plays an important role. Decreasing pressure increases the number of filaments and favors high current peak generation. It shows that the discharge characteristics cannot be completely controlled and that they depend on the flow field.

EPFL2008

Local Heat Transfer Distributions of Small Impingement Configurations: A Detailed Experimental Procedure Using Transient Heat Transfer Methods

Peter Ott, Alexandros Terzis, Bernhard Weigand

Impingement cooling is widely used in the cooling of turbine airfoils at critical regions where high heat transfer rates are required due to the large local convection coefficients that can be achieved. An accurate determination of local heat transfer distributions, is therefore, essential for reliable calculations of blade metal temperatures. In modern cast-in turbine airfoils, e.g. Lutum et al. [1], narrow impingement cooling channels can be formed, where the coolant is practically injected within the wall rather than the hollow of the airfoil, as shown in Figure 1. Narrow impingement channels consist of single or double rows of several cooling jets generating a relatively low amount of crossflow while the small wall thickness of a typical airfoil (~2mm) dictates significant heat transfer in all the interior surfaces of the cavity. A turbine airfoil can include a plurality of such small impingement configurations in the spanwise direction ensuring homogenous distribution of the material temperature. Contrary to the large amount of literature sources for multi-array impingement cooling systems, e.g. a review can be found in Weigand and Spring [2], little amount of research focused on narrow impingement channels with limited information for the sidewalls and impingement plate. A few studies can be found in Gillespie et al. [3], Chambers et al. [4], Ricklick et al. [5], Lamont et al. [6] and Terzis et al. [7]. This paper describes the overall experimental procedure and the heat transfer distributions for all the interior walls of a narrow impingement channel consisting of a single row of five inline jets. Heat transfer coefficients are evaluated with the transient liquid crystal technique considering also thermocouple thermal inertia effects.

2014