Analysis of a civil aircraft wing transonic shock buffet experiment

Abstract

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.

Link between subsonic stall and transonic buffet on swept and unswept wings: from global stability analysis to nonlinear dynamics

“…Most of the studies were carried out on half-wing body aircraft configuration. A broadband frequency content with a dominant frequency higher than the one of 2-D buffet was found experimentally (Roos 1985;Dandois 2016;Koike et al 2016;Paladini et al 2019b;Masini, Timme & Peace 2020). On such configurations, the convection of flow perturbations towards the wing tip was observed and buffet cells can be seen in the pressure-sensitive paint measurements of Sugioka et al (2018).…”

“…2016; Paladini et al. 2019 b ; Masini, Timme & Peace 2020). On such configurations, the convection of flow perturbations towards the wing tip was observed and buffet cells can be seen in the pressure-sensitive paint measurements of Sugioka et al.…”

Global instability of wing shock-buffet onset

“…A spanwise outboard propagation of buffet cells (a term coined by Iovnovich & Raveh (2015)), which is believed to constitute the instability, has been reported both in experimental and numerical studies (Lawson, Greenwell & Quinn 2016;Sartor & Timme 2017;Sugioka et al 2018). A spanwise inboard propagation, dominant along the shock front, has also been identified experimentally at lower frequencies (Dandois 2016;Masini et al 2017;Masini, Timme & Peace 2020). Timme & Thormann (2016) observed resonant flow due to forced wing vibration in the same lower frequency range, in addition to distinct flow responses around typical shock-buffet frequencies on wings.…”

Effect of Oxygen Mole Fraction on Static Properties of Pressure-Sensitive Paint

“…Aerodynamic measurements and flow visualization using PSP have been conducted for various measurement targets in various conditions [ 2 , 3 , 4 , 5 , 6 , 7 , 8 ]. Instantaneous measurements in particular have been actively conducted and developed in recent years [ 9 , 10 ].…”