Similitude between 3D cellular patterns in transonic buffet and subsonic stall

Plante, Frédéric; Dandois, Julien; Laurendeau, Éric

doi:10.2514/6.2019-0300

Cited by 7 publications

(4 citation statements)

References 54 publications

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“…Considering the whole buffet cycle in figure 12 we see the forward and backward movement of the separation location and the strict two-dimensionality of the shock-train patterns. The only indication of potentially larger-scale structures is the spanwise waviness of the transitional flow structures seen in figure 12(e), but these are a weak feature and are only present for a small part of the buffet cycle.Similar to the present study, no significant 3D structures were observed in experiments by[65] on a two-dimensional NASA CRM airfoil, however stall and buffet cells have been reported by[66,67] at spanwise whavelengths λ z > 1c for different airfoil geometries. One would have expected to see significant larger scale features already in the present wide-domain simulation in the case that stall/buffet-cell behaviour was relevant for this airfoil at this flow condition, although it is not possible to say for sure that they would not form in even wider computational domains.C.…”

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confidence: 88%

Wide domain simulations of flow over an unswept laminar wing section undergoing transonic buffet

Zauner

Sandham

2020

Phys. Rev. Fluids

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Transonic buffet is an unsteady flow phenomenon that limits the safe flight envelope of modern aircraft. Scale-resolving simulations with span-periodic boundary conditions can provide detailed insight into the flow physics associated with buffet and can help to calibrate simplified models that are needed, for example, to develop more efficient wings based on laminar-flow supercritical sections. However, such simulations are often feasible only for severely restricted spanwise domains. In the current contribution, we analyse an unswept laminar-flow wing section (of Dassault Aviation's V2C profile) at a moderate Reynolds number of Re = 500,000 and a Mach number of M = 0.7 with spanwise domains equal to 5% and 100% of the airfoil chord. An implicit large-eddy simulation methodology, using a spectral error estimator to control the action of a high-order filter, is first validated against direct numerical simulations and then used for the domain width study. Quantitative differences, due to domain size, include an increase in amplitude and regularity of the buffet oscillations in the wider domain. Nevertheless, space-time analysis shows that key physical phenomena such as upstream-propagating shock waves are properly represented in the narrow domain and there is limited sensitivity to domain size of the aerodynamic coefficients. Even in the very wide domain, which is an order of magnitude wider than the largest turbulent structures measured at the trailing edge, certain features remain two-dimensional, including the shock and expansion waves that interact with the boundary layer upstream of transition. The transition mechanism is found to have subtle variations during a typical buffet cycle, with Kelvin-Helmholtz structures prominent during low-lift phases and oblique modes developing behind shock/boundarylayer interactions during high-lift phases. The availability of the wide-domain data is used for further study of the buffet mechanism, considering phase-averaged data and instantaneous flow fields to show the global structure of the buffet oscillation.

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confidence: 88%

Wide domain simulations of flow over an unswept laminar wing section undergoing transonic buffet

Zauner

Sandham

2020

Phys. Rev. Fluids

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“…In this case, although the overall lift coefficient is still increasing between these two incidences, a negative sectional lift-curve slope is expected at the outboard sections where the massive separation occurs. A link between buffet cells and stall cells has been reported by Plante, Dandois & Laurendeau (2019). In contrast at , separation can be observed over a wider spanwise area, with the S-shaped shock curvature occurring further inboard at around .…”

Section: Results and Analysissupporting

confidence: 53%

Analysis of a civil aircraft wing transonic shock buffet experiment

2019

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The physical mechanism governing the onset of transonic shock buffet on swept wings remains elusive, with no unequivocal description forthcoming despite over half a century of research. This paper elucidates the fundamental flow physics on a civil aircraft wing using an extensive experimental database from a transonic wind tunnel facility. The analysis covers a wide range of flow conditions at a Reynolds number of around 3.6 × 10 6 . Data at pre-buffet conditions and beyond onset are assessed for Mach numbers between 0.70 and 0.84. Critically, unsteady surface pressure data of high spatial and temporal resolution acquired by dynamic pressure-sensitive paint is analysed, in addition to conventional data from pressure transducers and a root strain gauge. We identify two distinct phenomena in shock buffet conditions. First, we highlight a low-frequency shock unsteadiness for Strouhal numbers between 0.05 and 0.15, based on mean aerodynamic chord and reference free stream velocity. This has a characteristic wavelength of approximately 0.8 semi-span lengths (equivalent to three mean aerodynamic chords). Such shock unsteadiness is already observed at low-incidence conditions, below the buffet onset defined by traditional indicators. This has the effect of propagating disturbances predominantly in the inboard direction, depending on localised separation, with a dimensionless convection speed of approximately 0.26 for a Strouhal number of 0.09. Second, we describe a broadband higher-frequency behaviour for Strouhal numbers between 0.2 and 0.5 with a wavelength of 0.2 to 0.3 semi-span lengths (0.6 to 1.2 mean aerodynamic chords). This outboard propagation is confined to the tip region, similar to previously reported buffet cells believed to constitute the shock buffet instability on conventional swept wings. Interestingly, a dimensionless outboard convection speed of approximately 0.26, coinciding with the low-frequency shock unsteadiness, is found to be nearly independent of frequency. We characterise these coexisting phenomena by use of signal processing tools and modal analysis of the dynamic pressure-sensitive paint data, specifically proper orthogonal and dynamic mode decomposition. The results are scrutinised within the context of a broader research effort, including numerical simulation, and viewed alongside other experiments. We anticipate our findings will help to clarify experimental and numerical observations in edge-of-the-envelope conditions and to ultimately inform buffet-control strategies.

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“…In the present calculation only about 8 periods were acquired due to the high CPU cost. The existence of buffet cells [46,32] [7] has been used. Indeed, this method is well adapted to study short data that are known to consist of sinusoids in white noise (e.g.…”

Section: Resultsmentioning

confidence: 99%

Towards an enhanced protection of attached boundary layers in hybrid RANS/LES methods

Deck

Renard

2020

Journal of Computational Physics

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A robust strategy for the RANS shielding of attached boundary layers in a hybrid RANS/LES context is presented, addressing two major issues. Firstly, the attached boundary layers must be detected to ensure their RANS treatment, but the original shielding function used by automatic methods such as DDES (2006) or ZDES mode 2 (2012) fails for fine meshes and/or with adverse pressure gradients, motivating the present study. Secondly, even with a stronger shielding, there should not be an excessive delay in the formation of instabilities and resolved LES content in free shear layers. Both objectives cannot be simultaneously reached by only tuning a constant in the original shielding function. The proposed new ZDES mode 2 consequently involves three main ingredients, namely a second shielding function detecting the outer part of the wake layer including with strong adverse pressure gradients, an inhibition function which switches off the second shielding when flow separation is detected, and a significant enhancement of the destruction of eddy viscosity in grey areas. The calibration of the method relies on a set of boundary layers at various Reynolds numbers and pressure gradients conditions and is confirmed by a priori tests in three-dimensional flows around curved geometries. The resulting case-independent model, the new ZDES mode 2, is assessed on four test cases, namely a flat-plate boundary layer, a mixing layer, a backward facing step and transonic buffet over a supercritical airfoil. Overall, it is shown that the new ZDES mode 2 is relevant with respect to four objectives: protection of attached boundary layers for any grid cell size (including infinite mesh refinement) and pressure gradient, RANS shielding at least as broad as the original shielding in any situation, minimum delay in the formation of resolved LES content, and full compatibility of the resulting subgrid scale model in the LES branch with the other modes of ZDES (modes 1 and 3) ensuring a continuous treatment of resolved turbulence across zones treated with different ZDES modes. The new robust ZDES mode 2 consequently is a case-independent answer to the demand for a general automatic and robust RANS/LES treatment of attached and massively separated flows.

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Similitude between 3D cellular patterns in transonic buffet and subsonic stall

Cited by 7 publications

References 54 publications

Wide domain simulations of flow over an unswept laminar wing section undergoing transonic buffet

Wide domain simulations of flow over an unswept laminar wing section undergoing transonic buffet

Analysis of a civil aircraft wing transonic shock buffet experiment

Towards an enhanced protection of attached boundary layers in hybrid RANS/LES methods

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