The interaction of a line vortex with a collinearly aligned jet is a prototypical configuration for various important applications in aeronautics. The purpose of this study is to analyze the impact of the jet flow on the kinematics and dynamics of a trailing vortex. A particular emphasis is on the effect of a variable relative jet–vortex spacing. To this end, we realized four different jet–vortex configurations in a wind tunnel experiment at a chord-based Reynolds number of 1.7×105 using high-speed stereo particle image velocimetry measurements in five transversal planes located between 2 and 26 chords behind the wing. Stochastic analyses reveal that the jet generally contributes an external excitation to the vortex as a function of the mutual spacing. Compared with the configuration without jets, the vortex amplification increases upon reducing the jet–vortex spacing. Most notably, for all but the closest spacing, the vortex response is qualitatively the same, changing mainly in magnitude. For the closest spacing, however, the dynamics is considerably different, which we suspect to be a consequence of jet entrainment. Proper orthogonal decomposition reveals, for the first time, that the collinearly aligned jet tends to excite a progressively broader range of vortex modes as the jet–vortex spacing is reduced. A close examination of the vortex mean flow seems to preclude linear vortex instabilities, while the vortex characteristics hint toward some form of receptivity mechanism to disturbances being located in the free stream. Our analyses are useful to validate simulation tools on configurations combining simultaneous lift and thrust effects.

This paper provides an overview of scientific activities performed at ONERA on Boundary Layer Ingestion (BLI) for civil transport aircraft. Four scientific challenges are treated: quantification of aircraft aerodynamic performance with BLI engine installation, prediction of engine fan surge with BLI flow distortion, prediction of flow distortion in an engine intake, prediction of aeroelastic behaviour of engine fan submitted to flow distortion. These activities include numerical and experimental work, with RANS, URANS CFD and aeroelastic computations as well as tests in research and industrial facilities. Some experimental databases are already delivered and compared to computations and preliminary conclusions are presented.

PurposeThe transonic buffet is a complex aerodynamics phenomenon that imposes severe constraints on the design of high-speed vehicles, including for aircraft and space launchers. The origin of buffet is still debated in the literature, and the control of this phenomenon remains difficult. This paper aims to propose an original scenario to explain the origin of buffet, which in turn opens promising perspectives for its alleviation and attenuation.Design/methodology/approachThis work relies on the use of numerical simulations, with the idea to reproduce the buffet phenomenon in a transonic aileron designed for small space launchers. Two numerical approaches are tested: unsteady Reynolds averaged Navier–Stokes (URANS) and large-eddy simulation (LES). The numerical predictions are first validated against available experimental data, before to be analysed in detail to identify the origin of buffet on the studied configuration. A complementary numerical study is then conducted to assess the possibility to delay the onset of buffet.FindingsThe buffet control strategy is based on wall cooling. By adequately choosing the wall temperature, this work shows that it is feasible to delay the emergence of buffet. More precisely, this paper highlights the crucial role of the subsonic flow inside the boundary layer, showing the existence of upstream travelling pressure waves that are responsible for the flow coupling between both sides of the airfoil, at the origin of the buffet phenomenon.Originality/valueThis paper proposes a new scenario to explain the origin of buffet, based on the use of a Fanno and Rayleigh flow analogies. This approach is used to design a control solution based on a modification of the wall temperature, showing very promising results.

This paper gives the main results of an experimental test campaign aimed at quantifying the effects of very low roughness levels on the flat plate turbulent drag in transonic conditions. This work, in a field not completely understood yet, the one of the transitionally rough regime, has revealed its importance by showing that some surfaces considered as hydrodynamically smooth in past studies might not have been. This issue can be strongly critical in the delicate exercise of experimental/numerical comparisons, and more specifically, when it comes to absolute drag predictions, if the fluid dynamics computations use only, as they almost always do, infinitely smooth surfaces. The experiments on the S8Ch wind tunnel have involved high-level measurement techniques, such as micro-drag evaluation with a three-component balance and near-wall laser Doppler velocimetry, for Mach numbers from 0.55 to 0.8 and maximum Reynolds numbers based on the sample length and the boundary layer thickness of about 2.6 × 106 and 0.13 × 106, respectively, the highest friction Reynolds number being close to 5000. A dozen surface samples were tested, with average roughness values from less than 0.25 μm (mirror-polished aluminum) to more than 10 μm (commercial sandpapers), and in between standard-machined or painted samples (including pressure sensitive paintings). A pragmatic computational fluid dynamics study reproducing this test campaign was completed. It is based on the well-known equivalent sand grain roughness height approach and also on the Musker correlation, which was adapted. The results and validity of such Reynolds-averaged Navier–Stokes simulations in that particular regime are discussed. Anyway, both experimental and numerical outcomes of this work are in agreement to indicate that drag can be significantly impacted even for roughness Reynolds numbers potentially below the usual threshold values often considered in the engineering world (i.e., about 3.5–5). And the cross-analysis of surface drag production and roughness characteristics has allowed the decisive role played by the rms (or average) roughness height, the skewness and kurtosis coefficients, and especially the slope parameter to be confirmed.

This paper presents and discusses the results obtained with a MEMS (Micro-Electro-Mechanical System) high temperature gradient sensor for time-averaged and fluctuating skin-friction measurements in highly turbulent flows. Designed as a robust wall-mounted suspended hot-wire structure, the micro-sensor was made using conventional microfabrication techniques, compatible with microelectronics for designing integrated smart systems. Successfully implemented into two air wind tunnels, the sensor was tested in a large range of turbulent flows, with mainstream velocities going up to 270 m/s (Mach number of 0.79), which corresponds to the mean velocity of airliner cruise flights. The experiments demonstrated the wide dynamic range of the micro-sensor without reaching its limits. The micro-sensor thereby demonstrated its value for measuring turbulence in aerodynamic applications, being particularly suitable for aeronautics.

The performance of two different PSP used at DLR and ONERA is compared by carrying out bench mark testing using the oft-studied configuration of an oscillating shock wave (up to 100 Hz) on a bump in the Mach 1.4 flow of the ONERA S8Ch wind tunnel.The shock is made to oscillate by rotating a cam with elliptical cross section, placed at a further downstream position, at frequencies of 15, 30 and 50 Hz.Both paint types are well known and much has been published on their use: the ONERA PSP is a Ruthenium complex Ru(dpp) 3 Cl 2 placed as a thin layer on an anodized aluminum substrate, while the DLR PSP is a Platinum complex PtTFPP on a base coating containing TiO 2 particles.S8 run conditions were held constant, and separate test run series were carried out with each paint.Instationary calibration was also carried out using a special test rig.Fourier analyses of the PSP results in S8 enabled a semi-quantitative comparison of the time response of both paints.This is the first published attempt (to the authors' knowledge) of carrying out a side-by-side comparison of the characteristics of these two paints on such a flow configuration in a wind tunnel.Particular emphasis is placed on their handling properties and, above all, their time responses.

The presented paper gives an overview of several projects addressing the experimental characterization and control of the buffet phenomenon on 3D turbulent wings in transonic flow conditions. This aerodynamic instability induces strong wall pressure fluctuations and therefore limits flight domain. Consequently, to enlarge the latter but also to provide more flexibility during the design phase, it is interesting to try to delay the buffet onset. This paper summarizes the main investigations leading to the achievement of open and closed-loop buffet control and its experimental demonstration. Several wind tunnel tests campaigns, performed on a 3D half wing/fuselage body, enabled to characterize the buffet aerodynamic instability and to study the efficiency of innovative fluidic control devices designed and manufactured by ONERA. The analysis of the open-loop databases demonstrated the effects on the usual buffet characteristics, especially on the shock location and the separation areas on the wing suction side. Using these results, a closed-loop control methodology based on a quasi-steady approach was defined and several architectures were tested for various parameters such as the input signal, the objective function, the tuning of the feedback gain. All closed-loop methods were implemented on a dSPACE device able to estimate in real time the fluidic actuators command calculated mainly from the unsteady pressure sensors data. The efficiency of delaying the buffet onset or limiting its effects was demonstrated using the quasi-steady closed loop approach and tested in both research and industrial wind tunnel environments.

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.

Aim: Some years ago, in the framework of the Motar Cooperation Agreement between ONERA and DLR, it had been agreed to carry out comparison testing of the two instationary PSP methods on a common test configuration with an unsteady flow produced in a suitable wind tunnel. The results from this comparison would form the basis for a benchmarking of the two different approaches, leading to an aid in decision-making as to which method is better suited for a particular application. / Test configuration: For many years there has been interest at ONERA on the (quasi) two-dimensional unsteady transonic flow in a channel with strong interactions between an oscillating shock and a separated boundary layer: various diagnostic methods were used in measurements on their S8Ch transonic wind tunnel at ONERA Meudon (Schlieren, pressure sensors, PIV, LDV), including also PSP with Ru/AA paint (uPSP). Flow separation of the boundary layer is produced over a convex contour profile (a bump) on the floor of the tunnel, with the subsequent flow acceleration leading to formation of a downstream shock wave. The sectional area of a second throat, situated downstream of the bump, is varied periodically (ca. 15 Hz or higher) by rotation of an elliptical rod (cam) placed downstream of the throat position, leading to forced upstream pressure fluctuations and hence an oscillation (movement in flow direction) in the shock position. The resulting pressure fluctuations on the bump can then be measured with the help of pressure sensors (Kulite) fixed in the floor, and also with PSP coated on the floor. / Procedure/Approach: The floor can also be coated with the DLR iPSP paint. As with ONERA, there exists sufficient optical access for placement of the DLR excitation LEDs and cameras adjacent to or above the test section. PSP measurements were then be carried out with a new set of measurements using both ONERA and DLR paints, consecutively. / Results: Pressure measurements with Kulite and iPSP/uPSP are compared. Phase correlation is good, but there remain some differences in amplitude for both paints. Static and instationary calibration were carried out on samples of both paints; the latter showed no phase loss up to 120 Hz, although the amplitude with iPSP dropped due to surface heating by the LED over time. Fourier analyses of Kulite and PSP showed good agreement. A table was compiled with a summary of characteristics for both paints: there is no “better” paint, since the choice is governed mainly by desired application.

This paper presents the results of a coupled experimental and numerical study aimed at evaluating the influence of typical aircraft surface imperfections on the flat-plate drag production in fully turbulent conditions. A test campaign involving high- level measurement techniques, such as microdrag evaluation, near- wall laser Doppler velocimetry, and oil- film interferometry, has been carried out at several Mach numbers from 0.5 to 1.3 to quantify the impact of a large range of flat- plate imperfections. Forward- facing and backward- facing plain and chamfered steps of different heights have been studied. A whole numerical study, based on Reynolds- averaged Navier- Stokes computations, has been completed and used for validation purposes. Given the very small order of magnitude of the forces to be measured and calculated, the relative comparison between experimental and numerical outcomes is satisfactory. Even if some local discrepancies exist, results show an overall good agreement in the positioning of the different imperfection drag productions. Such investigations are of prime interest to determine industrialization tolerances or excrescence geometries offering the best compromises between manufacturing costs and aerodynamic performances.

This paper presents an overview of the work performed recently at ONERA on the control of the buffet phenomenon. This aerodynamic instability induces strong wall pressure fluctuations and as such limits aircraft envelope; consequently, it is interesting to try to delay its onset, in order to enlarge aircraft flight envelop, but also to provide more flexibility during the design phase. 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 to higher angles of attack, but only to higher lift coefficient. Then, a closed loop control methodology based on a quasi-static approach is defined and several architectures are tested for various parameters such as the input signal, the objective function or, the tuning of the feedback gain. All closed loop methods are implemented on a dSPACE device calculating in real time the fluidic actuators command from the unsteady pressure sensors data.

This paper presents an overview of the work performed at Onera over the last decade on the control of the buffet phenomenon. This aerodynamic instability induces strong wall pressure fluctuations and as such limits aircraft envelope, consequently it is interesting to try to delay its onset, in order to improve aircraft performance, but also to provide more flexibility during the design phase. 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 to higher angles of attack, but only to higher lift coefficient. The objective of these wind tunnel tests was to prepare the closed-loop control of the buffet phenomenon, to adapt the mass flow rate to the aerodynamic conditions.

The presented paper aims at giving an overview of the work performed in the ONERA's internal research project BUFET'N Co addressing the experimental characterization and the control of the buffet phenomenon on 3D turbulent wings in transonic flow conditions. This paper summarizes the main investigations leading to the definition of a closed loop methodology of buffet control and its experimental demonstration. Several wind tunnel tests campaigns enabled to characterize the buffet aerodynamic instability and to study the efficiency of fluidic control devices designed and manufactured by ONERA. The analyses of the open loop databases (wall pressure fluctuations) demonstrated the effects on the usual buffet characteristics, especially on the shock location and the separation areas on the wing suction side. Using these results, a closed loop control methodology based on a quasi-static approach was defined and several architectures were tested for various parameters such as the input signal, the objective function, the tuning of the feedback gain... All closed loop methods were implemented on a dSPACE device calculating in real time the fluidic actuators command from the unsteady pressure sensors data. The efficiency of delaying the buffet onset or limiting its effects was demonstrated using the quasi-static closed loop approach and tested in both research and industrial wind tunnel environment.

This paper presents the results of a coupled experimental and numerical study aimed at evaluating the influence of usual surface imperfections on the flat plate drag production. A test campaign involving high level measurement techniques such as micro drag evaluation, near wall laser Doppler velocimetry and oil film interferometry has been carried out at several Mach numbers corresponding to compressible flow regimes, in order to quantify the impact of a large range of flat plate imperfections. Forward facing and backward facing plain and chamfered steps of different heights have been studied. A whole numerical study has also been completed to be used as a validation/comparison reference for the wind tunnel measurements. Being given the really small order of magnitude of the forces to be measured or computed, the relative experimental / numerical comparison is satisfactory. Results show an overall good agreement in the positioning of the different imperfection drag productions.

Experiments studying the wing tip vortex, generated by a rectangular wing with a symmetric airfoil section and a straight tip, manipulated by surface plasma actuators are presented in this paper. Objectives are focused on modifying vortex development to alter streamwise vorticity downstream of the wing trailing edge. Actual limitation of the ionic wind magnitude obtained with DBD actuators leads to investigate vortex control by action on the transverse component of the freestream flow velocity. Different DBD arrangements installed near the wing tip were tested to examine effects of momentum addition, firstly on the pressure and suction side surfaces, and secondly, simultaneously located more precisely on the main vortex separation and attachment lines. Stereoscopic PIV measurements were performed in different planes orthogonal to the vortex development axis to visualize and characterize wing tip vortices. According to DBD arrangements, results of the actuation show a slight displacement of the wing tip vortex and a reduction of the mean streamwise vorticity level in its core.

This paper presents an application of the Pressure Sensitive Paint (PSP) technique to investigate two-dimensional unsteady flow in a tran sonic channel. This work takes place in the study of the transonic interaction between an o scillating shock-wave and a separated boundary layer in a channel flow. Oscillation of th e shock-wave is forced thanks to a periodic variation of the downstream throat section given by a rotating elliptical shaft located near this throat inducing pressure perturba tions moving upstream. The channel’s lower wall is equipped with a contour profile ‐ or a bump ‐ allowing for flow separation. In order to achieve reduced response time, we use Anodized-Aluminum coating as PSP (AAPSP) instead of usual paint. Thus, an aluminum insert including the bump has been manufactured in the contour profile and coated with AA-PSP. Images are acquired by using a fast frame rate camera and PSP results are compared with pressure tap and Kulite sensor measurements implemented in the insert. Spectral analysis have been carried out to assess the ability of AA-PSP for understanding unsteady aspects of the flow against unsteady pressure sensors.

This paper describes the results obtained during the JEDI project carried out in cooperation between ONERA and Airbus. The aim of these studies was first to acquire a very complete database of a modern-type engine jet installation under a swept wing in various transonic flow conditions. Interactions between the engine jet, the pylon and the wing were studied thanks to many “advanced” measurement techniques. In parallel, accurate RANS simulations were carried out from simple ones with the Spalart-Allmaras model to more complex ones like the DRSM-SSG turbulence model. In the end, Zonal-DES simulations were performed too to compare different simulation techniques. All numerical results are accurately validated thanks to the experimental database acquired in parallel. This complete and complex study of modern civil aircraft engine installation allowed many upgrades in our understanding and simulation methods to be obtained. Furthermore, a setup for engine jet installation studies has been validated for possible future works in the S3Ch transonic research wind-tunnel. The main conclusions are summed up in this paper.

In the framework of the ONERA's internal research project BUFET'N Co, a joint experimental and numerical work has been performed on the buffet control by both passive (mechanical VGs) and active (fluidic VGs) actuators. First, numerical simulations have been performed to help to the actuator parameter choice (actuator position, orifice diameter, pitch and skew angles…) and to the design of the wing model's inserts which have been tested in the ONERA S3Ch wind tunnel. Three-dimensional RANS computations have been realized to study both passive (mechanical VGs) and active (fluidic VGs) actuators. Concerning the mechanical VGs, three parameters have been studied: the skew angle, the spanwise spacing and the chordwise position. The effect of those three parameters on the lift, the wall pressure distribution has been investigated to determine the optimal parameters. Concerning the fluidic VGs, two parameters have been studied: the pitch and skew angles. Then, wind tunnel tests have been performed on a half-model in transonic conditions in the ONERA S3Ch wind tunnel. Experimental results have shown that both mechanical and fluidic VGs were effective to suppress the flow separation which occurs without control between the shock foot and the trailing edge and which is at the origin of the buffet phenomenon. Numerical simulations done on this half-model with mechanical VGs have exhibited a good quantitative agreement with the experimental data.