Hammerhead launcher configurations, characterized by a larger diameter in the payload fairing than the rest of the launch vehicle, face substantial challenges during transonic operations due to their susceptibility to flow separation and intense pressure fluctuations. This experimental study investigates the influence of the nose and boat-tail geometry on the flow around hammerhead configurations in the transonic regime (Ma=0.7-0.8) and for various angles of attack (α=0-4°). To gain a general understanding of the main flow features, such as shockwave formation, separated flow in the boat tail region, and flow reattachment, oil flow and schlieren visualizations were employed. Schlieren visualizations were also utilized to characterize the level of unsteadiness in these regions. Additionally, particle image velocimetry was employed to quantify variations in the velocity field. The study's findings reveal an optimization of flow performance in the presence of a bi-conic nose, attributed to the creation of two-shockwave structures with relatively low intensity. This is in contrast to the ogive and conic noses, which exhibit a single, more detrimental shockwave structure (with the conic nose being the least favorable configuration). The investigation into different boat tail angles indicates that adopting low-angle boat tails (5° and 15° compared to 34°) leads to a noticeable reduction in the separated area, albeit associated with an increase in the range of oscillation of the shockwave structures.

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Modern launcher configurations, often characterized by a larger diameter in the payload fairing than the rest of the launch vehicle (hammerhead configuration), face significant challenges during transonic operations, given their susceptibility to flow separation and intense pressure fluctuations. This experimental study aims to reconstruct pressure from PIV under transonic conditions using Taylor's hypothesis. The focus is on the ESA VEGA-E hammerhead launcher model, investigated in the transonic regime at Ma=0.8 and at α=0°. Initially, the methodology of the pressure reconstruction algorithm is described and the validity of Taylor's hypothesis is established through validation with numerical data. Subsequently, the experimental data are first used to provide a general characterization of the flow phenomenon, highlighting key features such as an oscillating shockwave, separated and reattached flows. Later, the reconstruction of pressure from velocity data is compared to unsteady pressure transducer data, for instantaneous, average and standard deviation values, obtaining a very good agreement (ΔCpAvg~0.01-0.02 and ΔCpStd~0.02). The experimental set-up is also designed to showcase the impact of neglecting the out-of-plane velocity component on pressure reconstruction, showing comparable data for CpAvg, while larger discrepancies in terms of CpStd in particular in the separated area region.@en

The flight envelope of an aircraft operating at high subsonic velocities is bounded by several limitations, one of these consists in thewing experiencing oscillations of a shockwave on its suction side for a certain range ofMach number (Ma), angle of attack (®) and Reynolds number (Re). This phenomenon is referred to as transonic buffet and it may ultimately result in violent structural oscillations of the wing (the so-called buffeting), in addition to the oscillations of the aerodynamics loads. Notwithstanding the relevance of this topic, there is not yet an only explanation regarding its mechanism, therefore, the first aim of this experimental project is to obtain further insight on the physics of transonic buffet (Part I). As a second objective, in Part II different strategies for the control of buffet have been investigated. The experiments of this study have been carried out in the transonic-supersonic wind tunnel of TUDelft on supercritical airfoil and wings based on theOAT15A airfoil. The behavior of this phenomenon has been scrutinized using optical experimental techniques, such as particle image velocimetry (PIV), schlieren, and, background oriented schlieren (BOS)... @en

This experimental study investigates the use of shock control bumps (SCBs) for controlling transonic buffet. Threedimensional SCBs have been applied on the suction side of an OAT15A supercritical airfoil with the experiments conducted in the transonic–supersonic wind tunnel of Delft University of Technology at fully developed buffet conditions (Ma 0.7, α 3.5 deg and Re 2.6 × 10⁶). The effectiveness of the SCBs for different spanwise array spacings (ranging from 20 to 30%c) was verified using two optical techniques: schlieren visualization and particle image velocimetry. Both techniques confirmed the potential of controlling buffet using such devices, resulting in a reduction of the flow unsteadiness in terms of both shock oscillation and pulsation of the separated area. A dedicated particle image velocimetry investigation in a spanwise–chordwise measurement plane was conducted in order to characterize the effect of the spatial distribution of the bumps, focusing on the interaction of the shock-wave structures along the span. The configuration with a spacing of Δy_SCB 25%c was demonstrated to be the most efficient in reducing the transonic buffet oscillations and was able to reduce the reverse flow region size as compared to the clean configuration.

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In this experimental study, panel flutter induced by an impinging oblique shockwave is investigated at a freestream Mach number of 2, using the combination of planar particle image velocimetry (PIV) and stereographic digital image correlation (DIC) to obtain simultaneous full-field structural displacement and flow velocity measurements. High-speed cameras are employed to obtain a time-resolved description of the panel motion and the shockwave-boundary layer interaction (SWBLI). In order to prevent interference between the PIV and DIC systems, an optical isolation is implemented using fluorescent paint, dedicated light sources, and camera lens filters. The effect of the panel motion on the SWBLI behavior is assessed, by comparing it with the SWBLI on a rigid wall. The results show that panel oscillations occur with a maximum amplitude of ten times the panel thickness. The dominant frequencies observed in the panel oscillation (424 Hz and 1354 Hz) match the main spectral content of the reflected shockwave position. A further POD analysis of the panel displacement spatial distribution shows that these two frequency contributions are well captured by the first two POD modes, which correspond, respectively, to a first and a third bending mode shape and account for 92% of the total oscillation energy. The fluid-structure coupling is studied by identifying, in the flow, the regions of maximum correlation between the panel displacement and the flow velocity fluctuations. The results obtained prove that the inviscid flow region upstream of the SWBLI is perfectly in phase with the panel oscillation, while the downstream region has a delay of one quarter of the flutter cycle.

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This experimental study investigates the possibility of controlling transonic buffet by means of a trailing edge flap with an upward deflection (referred to as “upper trailing edge flap”, or: UTEF). Different geometries (straight and serrated) and dimensions of UTEFs (with heights ranging between 1 and 2% of the chord) have been studied with respect to their impact on the buffet behavior. The effectiveness of the UTEFs has been investigated with schlieren and particle image velocimetry (PIV) in the transonic-supersonic wind tunnel of TU Delft at Ma = 0.70, α = 3.5°. The schlieren results demonstrated the efficacy of the use of UTEFs for reducing the range of the buffet oscillations when the height of the UTEF was equal to at least 1.5%c. This result was corroborated by a flow characterization with PIV data and which highlighted that, in presence of a control system, not only the shock oscillation range is reduced but also the intensity of the separated area pulsation. The use of serrated UTEFs, despite having an effect on the local flow field, was found to be ineffective in alleviating buffet oscillations. The adoption of the best behaving UTEF configuration (straight 2%c UTEF) proved to only slightly alter the circulation value compared to the clean configuration, while it also proved to be effective in an off-buffet condition (Ma = 0.74 and α = 2.5°).

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This study experimentally investigates the effects of the sweep angle and finite wing on transonic buffet, studying two-dimensional (2-D) and three-dimensional wing configurations. Background-oriented schlieren and stereographic particle image velocimetry (PIV) have been used as measurement techniques, performing experiments on an OAT15A airfoil (clamped to both the side windows of the wind tunnel), an unswept wing, and two swept wings with sweep angles of 15 and 30 deg, respectively. The three wings are also based on the OAT15A airfoil and are clamped at the wind tunnel only at their root (free wingtip). All wings have been tested at a constant normal Mach number (Ma∞n=0.7) with respect to the leading edge. The results show that the buffet oscillations are much stronger for the airfoil than for the three finite-span wings. A large difference in the buffet behavior can be noticed between the airfoil and the unswept wing, as is also seen in oil flow visualizations. This difference is particularly evident in correspondence of the more outboard spanwise locations, suggesting that for the unswept wing, an important role could be played by finite-wing effects: notably, the tip vortex. A spectral analysis has shown that for the swept wings, the classical 2-D buffet peak (occurring at f=160  Hz for the present conditions) is substantially attenuated, whereas additional contributions in the range of 450–850 Hz appear. The PIV results showed, for the 30 deg sweep angle wing, a periodical occurrence of a secondary supersonic area downstream of the main shock wave structure, which is absent for the other wing models. The stereographic PIV configuration allowed the reconstruction of the spanwise-oriented velocity component, obtaining, in proximity of the trailing-edge, values of the spanwise velocity component (80–100  m/s) which are in agreement with the spanwise convection of buffet cells observed in the literature in this region.@en

In this study the effect of wing sweep on transonic buffet is studied experimentally to reveal the differences between two-dimensional (2D) and three-dimensional (3D) wing configurations. Background oriented schlieren (BOS) and stereographic particle image velocimetry (PIV) have been used as measurement techniques, performing experiments on: an airfoil, an unswept wing and two swept wings with a sweep angle of 15° and 30° respectively (all wings are based on the OAT15A airfoil). All wings have been tested at a constant normal Mach number (Ma∞n=0.7) with respect to the leading edge. The results show that the buffet oscillations are much stronger for the airfoil than for the three finite-span wings. A large difference in the buffet behavior can be noticed between the airfoil and the unswept wing, particularly in correspondence of the more outboard spanwise locations, suggesting that in the latter an important role could be played by finite-wing effects, notably the tip vortex. A spectral analysis has shown that for the swept wings the classical 2D buffet peak (occurring at f=160 Hz for the present conditions) is substantially attenuated, while additional contributions in the range of 450-850 Hz appear. The PIV results showed, for the 30° sweep angle wing, a periodical occurrence of a secondary supersonic area downstream of the main shockwave structure, which is absent for the other wing models. The stereographic PIV configuration allowed the reconstruction of the spanwise oriented velocity component, obtaining in the trailing edge area, spanwise outboard velocities (80-100 m/s) which are in agreement with the spanwise convection of buffet cells observed in literature in this region.@en

Abstract: Transonic buffet behaviour of the supercritical airfoil OAT15A was investigated experimentally at flow conditions Ma= 0.7 and α= 3. 5 ^∘, using schlieren and particle image velocimetry (PIV). The general behaviour of the buffet cycle was characterised with short-exposure schlieren visualisation and phase-averaged PIV measurements. A spectral analysis showed that the shock oscillation occurs with a dominant contribution at 160 Hz (St = 0.07, in good agreement with the literature) and between 25 and 55 % of the chord of the airfoil. Proper Orthogonal Decomposition (POD) was applied to the PIV data to extract the main modes connected with buffet. It is found that the first three most energetic modes capture around 65 % of the total fluctuating kinetic energy. The first and the third modes have a main frequency peak at 160 Hz and are well representing the separated area and the shock oscillation. The second mode was, instead, associated with an asymmetrical behaviour of the separated area and of the shear layer and displays a main peak at 320 Hz, being double the main buffet cycle frequency. Finally, it was shown that by using the 11 most energetic POD modes, an accurate reduced-order model (ROM) is obtained, which when subtracted from the instantaneous velocity fields allows the visualisation of the small-scale structures present in the flow, such as the upstream travelling waves (UTWs) and the vortex shedding in the separated area near the trailing edge. The analysis allowed to estimate the velocity of the UTWs, obtaining values in good agreement with the literature. In contrast, the analysis of the vortex dynamics in the trailing edge area revealed that vortices shed at the shock foot, which convect downstream in an area detached from the airfoil surface, cannot be considered responsible for the creation of UTWs in view of the mismatch in frequency of the two phenomena. Graphic abstract: [Figure not available: see fulltext.]

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This experimental study has the objective of providing new insight into the role of upstream traveling waves (UTWs) in the transonic buffet phenomenon, using the background-oriented schlieren (BOS) technique and corroborating the results with particle image velocimetry. The experiments were carried out on the supercritical OAT15A airfoil under transonic conditions, at a Mach number of 0.7, an angle of attack of 3.5°, and a chord-based Reynolds number of x6. The specific scope of the investigation is the characterization of the spanwise organization of the buffet phenomenon; therefore, the measurements consider a streamwise-spanwise-oriented field of view on the suction side of the airfoil. A particular topic of interest is the propagation and orientation of upstream traveling pressure waves (UTWs) that occur in transonic buffet. The experimental setup used allowed to confirm the two-dimensionality of the velocity field and of the shockwave, but revealed that the UTWs propagate at a non-zero orientation. Processing of the BOS images with two different procedures (normal and differential), has furthermore allowed to extract the frequency and propagation velocity of the UTWs, which have been confirmed to behave as acoustic waves, traveling at the speed of sound relative to the flow. A further analysis has given hints that the strength of the UTWs is modulated during the buffet cycle and, therefore, in support of the feedback-mechanism description of transonic buffet.

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View Video Presentation: https://doi-org.tudelft.idm.oclc.org/10.2514/6.2021-2558.vid

This experimental study investigates the use of shock control bumps (SCBs) for controlling transonic buffet. Three-dimensional SCBs have been applied on the suction side of an OAT15A supercritical airfoil with the experiments conducted in the transonic-supersonic wind tunnel of TU Delft for fully developed buffet conditions (Ma=0.7, α=3.5° and Re=2.6·10^6). The effectiveness of the SCBs for different spanwise spacings (ranging from 20%c to 30%c) was verified using two optical techniques: schlieren visualization and particle image velocimetry (PIV). Both techniques showed the possibility of controlling buffet using such devices, resulting in a reduction of the unsteadiness present in the flow, both in terms of shock oscillation and pulsation of the separated area. A dedicated PIV investigation in a spanwise-chordwise measurement plane was then conducted in order to understand the effect of the spatial distribution of the bumps, focusing on the interaction of the shockwave structures along the span. The configuration with a spacing of ΔxSCB=25%c demonstrated to be the most efficient in reducing the transonic buffet oscillations.@en

This study investigates the possibility of controlling transonic buffet by means of a Gurney Flap with an upward deflection at the trailing edge of the airfoil (UGF). Different geometries and dimensions of UGFs have been studied for their impact on the buffet behaviour. The effectiveness of the UGFs has been tested experimentally with Schlieren and PIV in the transonic-supersonic wind tunnel of TU Delft at Ma=0.7, a=3.5. It is found that the best performing UGF is a straight UGF with a height of 1.5% or 2% of the chord. These UGFs allow the reduction of the energy associated with buffet. This result has been corroborated by a flow description of the phases of buffet, a spectral analysis and a POD approach. In addition, the straight UGFs resulted to be effective in a less developed buffet condition too. @en

This paper investigates the appearance, propagation and orientation of upstream travelling waves (UTWs) in transonic buffet. Their role has been studied experimentally on a supercritical airfoil (OAT15A) in the transonic-supersonic wind tunnel of TU Delft at Mach number of 0.7, and an angle of attack of 3.5. In the experiments Background Oriented Schlieren (BOS) and Particle Image Velocimetry (PIV) have been used, to observe the suction side of the airfoil in a streamwise-spanwise oriented plane. The experiments detected the frequency and propagation velocity of the UTWs, which have been confirmed to behave as acoustic waves that therefore travel at the speed of sound relative to the flow. The particular set-up configuration used confirmed the two-dimensionality of the velocity field and of the shock wave but showed that the UTWs propagate in the plane of measurement with a non-zero orientation. @en