Fig 3 - uploaded by Andrey Garbaruk
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Effect of extent of laminar flow (left) and angle of attack (right) on shock position from steady RANS solution for nonswept wing (Λ 0°).
Source publication
Global instability analysis is used to investigate the effects of extended regions of laminar flow on both unswept and swept infinite-span wings. The formulation is based on the Reynolds-averaged Navier–Stokes equations and differs from earlier studies on fully turbulent flows in the activation of the trip term in the Spalart–Allmaras eddy-viscosit...
Contexts in source publication
Context 1
... variation of the shock position with angle of attack and different values of the trip location are shown in Fig. 3. An extended region of laminar flow results in a downstream shift in the shock position, largely independent of the angle of attack. As the extent of laminar flow increases, the shock position becomes more sensitive to changes in x t (i.e., the slope of the X shock x t curve also increases). Note that over the considered range of the ...
Context 2
... flow increases, the shock position becomes more sensitive to changes in x t (i.e., the slope of the X shock x t curve also increases). Note that over the considered range of the angle of attack, the effect of increasing x t ∕c on the shock position is opposite to the effect of increasing angle of attack (comparing left and right frames in Fig. ...
Citations
... While computational turbulent buffet studies typically assume fully-turbulent conditions (i.e. a turbulent boundary layer from the leading edge), recently Garbaruk et al. [22] have examined the effect of forcing transition at different locations on the aerofoil upstream of the shock wave using steady RANS simulations and GLSA. They have shown that the essential characteristics of turbulent buffet remain the same irrespective of the location of transition, which is in agreement with the experimental findings of Dor et al. [23]. ...
... However, it was later demonstrated by Moise et al. [19] using LES and a modal reconstruction that laminar buffet has characteristics that are essentially similar to those reported in previous literature for turbulent buffet. The simulations in [19] were carried out only for laminar buffet, while the steady RANS simulations and GLSA in [22] were performed only for turbulent buffet and gaps remain in linking the two studies. ...
... The reduced buffet amplitude at moderate when transition is forced indicates that buffet is sensitive to the boundary layer characteristics upstream of the shock wave, particularly when switching from a laminar to a turbulent state. These findings align with the GLSA results in [22], where a buffet mode becomes unstable at lower when the boundary layer is tripped farther downstream. This suggests that for a constant , buffet intensity would increase with an increase in streamwise extent of the laminar boundary layer. ...
Transonic buffet is commonly associated with self-sustained flow unsteadiness involving shock-wave/boundary-layer interaction over airfoils and wings. The phenomenon has been classified as either laminar or turbulent based on the state of the boundary layer immediately upstream of the shock foot, and distinct mechanisms for the two types have been suggested. The turbulent case is known to be associated with a global linear instability. Herein, large-eddy simulations are used for the first time to make direct comparisons of the two types by examining free- and forced-transition conditions. Corresponding simulations based on the Reynolds-averaged Navier–Stokes equations for the forced-transition case are also performed for comparison with the scale-resolving approach and for linking the findings with existing literature. Coherent flow features are scrutinized using both data-based spectral proper orthogonal decomposition of the time-marched results and operator-based global linear stability and resolvent analyses within the Reynolds-averaged Navier–Stokes framework. It is demonstrated that the essential dynamic features remain the same for the two buffet types (and for the two levels of the aerodynamic modeling hierarchy), suggesting that both types arise due to the same fundamental mechanism.
... While computational turbulent buffet studies typically assume fully-turbulent conditions (i.e. a turbulent boundary layer from the leading edge), recently Garbaruk et al. [22] have examined the effect of forcing transition at different locations on the aerofoil upstream of the shock wave using steady RANS simulations and GLSA. They have shown that the essential characteristics of turbulent buffet remain the same irrespective of the location of transition, which is in agreement with the experimental findings of Dor et al. [23]. ...
... However, it was later demonstrated by Moise et al. [19] using LES and a modal reconstruction that laminar buffet has characteristics that are essentially similar to those reported in previous literature for turbulent buffet. The simulations in [19] were carried out only for laminar buffet, while the steady RANS simulations and GLSA in [22] were performed only for turbulent buffet and gaps remain in linking the two studies. ...
... The reduced buffet amplitude at moderate when transition is forced indicates that buffet is sensitive to the boundary layer characteristics upstream of the shock wave, particularly when switching from a laminar to a turbulent state. These findings align with the GLSA results in [22], where a buffet mode becomes unstable at lower when the boundary layer is tripped farther downstream. This suggests that for a constant , buffet intensity would increase with an increase in streamwise extent of the laminar boundary layer. ...
Transonic buffet is commonly associated with self-sustained flow unsteadiness involving shock-wave/boundary-layer interaction over aerofoils and wings. The phenomenon has been classified as either laminar or turbulent based on the state of the boundary layer immediately upstream of the shock foot and distinct mechanisms for the two types have been suggested. The turbulent case is known to be associated with a global linear instability. Herein, large-eddy simulations are used for the first time to make direct comparisons of the two types by examining free- and forced-transition conditions. Corresponding simulations based on the Reynolds-averaged Navier--Stokes equations for the forced-transition case are also performed for comparison with the scale-resolving approach and for linking the findings with existing literature. Coherent flow features are scrutinised using both data-based spectral proper orthogonal decomposition of the time-marched results and operator-based global linear stability and resolvent analyses within the Reynolds-averaged Navier--Stokes framework. It is demonstrated that the essential dynamic features remain the same for the two buffet types (and for the two levels of the aerodynamic modelling hierarchy), suggesting that both types arise due to the same fundamental mechanism.
... Similarly to [21,22], the higher frequencies are associated with spanwise travelling structures with a distinct convection velocity. In order to verify whether the 3-D buffet phenomenon can be connected to a globally unsteady mode, analogously to 2-D buffet ( [12,15]), a global stability analysis was conducted in [24][25][26][27]. Besides the 2-D like unsteady mode, spanwise modes with maxima of growth rate for wavelengths close to 1 and 10 chords were discovered. ...
... The global stability analysis proved to be a reliable tool to detect buffet onset also in the 3-D case, even though the aspects of the instability seem to differ from the 2-D buffet and to resemble stall-cells modes in low-speed flows [27]. Finally, in [26], a global stability analysis was performed together with a simulated transition to turbulence of the boundary layer. In this case, buffet onset occurs at slightly lower than in all the previous studies, which considered a fully turbulent flow. ...
Self-sustained shock wave oscillations on airfoils, commonly defined as shock buffet, can occur under certain combinations of transonic Mach number (M) and angle of attack (AOA) due to the interaction between the shock and the separated boundary layer. In order to
improve the understanding of this complex phenomenon, the flow over a supercritical profile (OAT15A) was experimentally investigated for a fixed Reynolds number of 3×10^6 and numerous aerodynamic conditions within the ranges 2.5°<AOA<6.5° and 0.71<M< 0.78. Deformation and force measurements were used to assess the actual rigidity of the model and its interaction with the flow. The tracking of the shock location by means of Background Oriented Schlieren allowed for studying the shock features and the frequency content of the buffet flows. Furthermore, the inversion of shock motion and buffet onset, which are referred to as buffet boundaries, were estimated. The inversion of the shock motion proved to be a necessary but not sufficient condition for buffet onset. The trends of the results showed a good agreement with the relevant literature cases. However, the buffet amplitude was smaller and buffet onset
occurred at considerably higher AOA. The comparison of literature results also revealed a general sensitivity of buffet features to both numerical and experimental parameters. For this reason, the influence was examined of the boundary layer suction at the vertical walls and the gap flow at the side windows on the buffet features. The buffet frequencies and amplitudes were slightly affected, but the buffet boundaries appeared to be virtually insensitive to these factors.
Given the large number of investigated aerodynamic conditions, these results are valuable for validation purposes of CFD simulations.
The present article assesses the capability of the partially averaged Navier-Stokes (PANS) method to reproduce accurately the self-sustained shock oscillations, also known as transonic buffet, occurring on airfoils and wings at transonic regime under certain conditions of Mach number and angle of attack. The test case under analysis is an OAT15A unswept wing at Mach number 0.73 and Reynolds number 3 million. The three-dimensional flow is studied by accounting for the wind tunnel walls adopted in the experiments of Jacquin et al. [1] in the simulations. The computations on a large-span, confined configuration reveal a strong three-dimensionality of the flow both before and after the buffet onset. Attention is paid to the comparison with unsteady Reynolds-averaged Navier Stokes (URANS) results, to show the benefits of PANS in resolving flow unsteadiness at different flow resolutions, especially on affordable CFD grids, at limited additional cost. In this context, the role of the mesh metrics and the local turbulence level in the formulation of the model is described, as well as the relation of this latter with the spatiotemporal discretization used for the numerical simulations. The aim is to extend the use of PANS and obtain accurate predictions of flow cases involving shock-wave boundary layer interactions without expensive approaches.
Self-sustained, low-frequency, coherent flow unsteadiness over rigid, stationary aerofoils in the transonic regime is referred to as transonic buffet. This study examines the role of shock waves in sustaining this transonic phenomenon and its relation to low-frequency oscillations (LFO) that occur in flow over aerofoils in the incompressible regime (Zaman et al. , J. Fluid Mech. , vol. 202, 1989, pp. 403–442). This is investigated by performing large-eddy simulations of the flow over a NACA0012 profile for a wide range of flow conditions under free-transition conditions. At low Reynolds numbers, zero incidence angle and sufficiently high free-stream Mach numbers, $M$ , transonic buffet occurs with shock waves present in the flow. However, when $M$ alone is lowered, self-sustained, periodic oscillations at a low frequency are observed even though shock waves are absent and the entire flow field remains subsonic at all times. At higher incidence angles, the oscillations are sustained at progressively lower $M$ and are present even at $M=0.3$ , where compressibility effects are low. A spectral proper orthogonal decomposition (SPOD) shows that the spatial structure of these oscillations is consistent for all cases. The SPOD modes are topologically similar, suggesting a connection between transonic buffet and LFO in the incompressible regime. Comparisons with other studies examining transonic buffet on various aerofoils, under forced-transition and fully turbulent conditions support this hypothesis. Future studies using tools of global linear stability analysis, especially at high free-stream Reynolds numbers are required to examine whether the underlying mechanisms of transonic buffet and incompressible LFO are the same.
