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International audience; An experimental investigation of the transonic flow past the laminar OALT25 airfoil has been conducted to analyze the impact of laminar flow upon the shock wave dynamics and the existence of a laminar buffet like phenomenon. Tests have been carried out at freestream Mach numbers varying in the range 0.7 to 0.8, angle of attack from 0.5 to 4 degrees and with two tripping configurations at the upper surface of the wing. The (airfoil) chord based Reynolds number is about 3 millions. Results obtained from pressure taps and sensors measurements, as well as Schlieren visualizations of the flow reveal the presence of a laminar buffet phenomenon in sharp contrast with the turbulent phenomenon, as it features a (freestream and chord) based normalized frequency of about unity while turbulent buffet occurs for a frequency close to 0.07 (Jacquin et al., AIAA Journal, vol. 47, number 9, 2009). A low frequency mode, at a frequency about 0.05 is also present in the laminar situation, notably lower than the high frequency component. The latter exhibits strong oscillations of the shock foot and vertical wavelike deformations of the shock wave and the former moves the shock back and forth over a small portion of chord, quite similarly to the turbulent phenomenon. The mean flow past the laminar wing is characterized by a laminar separation bubble under the shock foot which likely contributes much to the novel dynamics revealed by the present experiments. Two control strategies of the unsteady shock wave are implemented, one consisting of three-dimensional bumps and one consisting of steady jets blowing transversely to the freestream. It is found that bumps provide a significant reduction of the buffet intensity in the laminar situation. The jets are able to completely remove the flow unsteadiness in both laminar and turbulent conditions.

The buffet onset boundary is associated with a change in stability of the flow solution. The buffet onset boundary has been computed for a NACA 0012 aerofoil using a modified bifurcation tracking method. The algorithm combines the projection aspect of the Recursive Projection Method with two different direct bifurcation tracking methods. This considerably reduces the cost of the bifurcation tracking methods by solving for the bifurcation point on a small subspace of the system containing only the least stable dynamics. The method has been extended to allow the boundary to be computed as geometrical parameters, camber and thickness, are changed. This enables rapid evaluation of the effect of different aerofoil designs on the boundary. Results are presented comparing the full bifurcation tracking methods to their projected equivalent and show that although the projected methods lack the numerical accuracy of the full counterpart, the trends of the boundaries agree well.

International audience; Unsteady Pressure Sensitive Paint is applied to measure and investigate the shock dynamics on a transonic laminar airfoil which exhibits a rapid shock oscillation at a normalized frequency, based on chord and freestream velocity, close to one. The main motivation is to assess the capability of the PSP technique to capture the pressure fluctuations imposed at the upper surface of the wing by the oscillating shock wave, in the light of the recent developments performed at ONERA of the unsteady Anodized-Aluminum PSP method. Beyond frequency resolution, it is found that three main difficulties arise when applying PSP for the present unsteady laminar flow. The first concerns the roughness of the PSP layer, which must remain low to prevent unwanted transition of the laminar boundary layer. The second deals with the correction of illumination as compressibility and wing deformations may affect it. The third is about the correction of the temperature sensitivity of the PSP, which is mandatory in the present high speed flow situation. These three questions are carefully addressed. In addition, measurements for the case when the boundary layer at the upper surface of the airfoil is made fully turbulent by forced tripping are carried out for reference. In this case the dynamics is much slower, with a normalized frequency close to 0.05. The PSP pressure fields offer a rich database to analyze the flow features. In particular the spatial sampling provided by the PSP helps to qualify the two-dimensional nature of the shock movements and to question the wave propagation process implied by these unsteadiness.; La PSP insationnaire est utilisée pour la mesure et l'étude de la dynamique de choc sur un profil laminaire en transsonique, configuration qui présente une oscillation rapide du choc pour une fréquence normalisée proche de un. L'objectif est d'évaluer la capacité de la méthode par PSP à capturer les fluctuations de pression générées à la surface de la voilure au regard des développements récents réalisés à l'Onera sur la PSP constituée d'un revêtement d'aluminium anodisé. Au-delà du temps de réponse de la PSP et donc de la résolution en fréquence, trois autres difficultés sont apparues pour ce type d'application en écoulement laminaire. La première concerne la rugosite de la peinture qui doit être suffisamment faible pour ne pas déclencher la transition de la couche limite laminaire. La deuxième concerne les variations du champ d'illumination liées à la déformation de la maquette en cours d'essai, ainsi qu'à la l'observation en milieu compressible. La troisième est liée à la sensibilité à la température de la PSP. L'analyse des résultats prend en compte chacun de ces trois aspects. Parallèlement, le cas d'un écoulement turbulent obtenu par le déclenchement forcé de la couche limite est considéré comme référence. Dans ce cas, la dynamique du choc est notablement plus lente, à une fréquence normalisée de 0,05. Les mesures PSP fournissent une base de données riches pour analyser la structure de l'écoulement. En particulier, la résolution spatiale des cartes PSP permet de qualilfier la nature bi-dimensionnelle du battement de choc et de s'intéresser au processus de propagation d'onde associé au mode instationnaire.

Numerical simulations are performed to study the aeroelastic responses of an elastically suspended airfoil in transonic buffet flow, by coupling the unsteady Reynolds-averaged Navier-Stokes (RANS) equations and structural motion equation. The current work focuses on the characteristic analysis of the lock-in phenomenon. Great attentions are paid to studying the frequency range of lock-in and the effects of the three parameters, namely the structural natural frequency, mass ratio and structural damping, on lock-in characteristic of the elastic system in detail. It is found that when the structural natural frequency is close to the buffet frequency, the coupling frequency of the elastic system is no longer equal to the buffet frequency, but keeps the same value as the structural natural frequency. The frequency lock-in occurs and stays present until the structural nature frequency is near the double buffet frequency. It means that the lock-in presents within a broad range, of which the lower threshold is near the buffet frequency, while the upper threshold is near the double buffet frequency. Moreover, the frequency range of lock-in is affected by mass ratio and structural damping. The lower the mass ratio and structural damping are, the wider the range of lock-in will be. The upper threshold of lock-in grows with the mass ratio and structural damping decreasing, but the lower threshold always keeps the same.

Purpose The development of reusable space launchers requires a comprehensive knowledge of transonic flow effects on the launcher structure, such as buffet. Indeed, the mechanical integrity of the launcher can be compromised by shock wave/boundary layer interactions, that induce lateral forces responsible for plunging and pitching moments. Design/methodology/approach This paper aims to report numerical and experimental investigations on the aerodynamic and aeroelastic behavior of a diamond airfoil, designed for microsatellite-dedicated launchers, with a particular interest for the fluid/structure interaction during buffeting. Experimental investigations based on Schlieren visualizations are conducted in a transonic wind tunnel and are then compared with numerical predictions based on unsteady Reynolds averaged Navier–Stokes and large eddy simulation (LES) approaches. The effect of buffeting on the structure is finally studied by solving the equation of the dynamics. Findings Buffeting is both experimentally and numerically revealed. Experiments highlight 3D oscillations of the shock wave in the manner of a wind-flapping flag. LES computations identify a lambda-shaped shock wave foot width oscillations, which noticeably impact aerodynamic loads. At last, the experiments highlight the chaotic behavior of the shock wave as it shifts from an oscillatory periodic to an erratic 3D flapping state. Fluid structure computations show that the aerodynamic response of the airfoil tends to damp the structural vibrations and to mitigate the effect of buffeting. Originality/value While buffeting has been extensively studied for classical supercritical profiles, this study focuses on diamond airfoils. Moreover, a fluid structure computation has been conducted to point out the effect of buffeting.

Transonic flow over an airfoil involves shock induced oscillation at certain free steam Mach number and angle of attack due to the interaction of shock wave with airfoil boundary layer which consequences fluctuating lift and drag coefficient, aero acoustic noise and vibration, high cycle fatigue failure (HCF), buffeting and so on. In the present study, Reynolds Averaged Navier-Stokes (RANS) equations have been used to predict the transonic buffer and corresponding aerodynamics behaviour over NASA SC(2) 0714 supercritical airfoil. RANS computations have been performed at free stream Mach number of 0.77 while the angle of attack was varied from 2° to 7°. The results obtained from the numerical computation have been validated with the experimental results. Mach contour, lift and drag coefficient, and pressure history over the airfoil surface have been analyzed and confirmed the transonic buffet phenomena.

International audience; The prediction of shock-induced oscillations over transonic rigid airfoils is important for a better understanding of the buffeting phenomenon. The unsteady resolution of the Navier-Stokes equations is performed with various transport-equation turbulence models in which corrections are added for nonequilibrium flows. The lack of numerical efficiency due to the CFL stability condition is circumvented by the use of a wall law approach and a dual time stepping method. Moreover, various numerical schemes are used to try and be independent of the numerical discretization. Comparisons are made with the experimental results obtained for the supercritical RA16SC1 airfoil. They show the interest in using the SST correction or realizability conditions to get correct predictions of the frequency, amplitude and pressure fluctuations over the airfoil.