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.

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.

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.

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°).

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.]

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.