Analysis of a civil aircraft wing transonic shock buffet experiment

L Masini,S Timme,A J Peace

doi:10.1017/jfm.2019.906

L Masini, S Timme + Show 1 more

Open Access

https://doi.org/10.1017/jfm.2019.906

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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 . 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.

Highlights

Transonic shock buffet on civil aircraft wings presents a key challenge for aerodynamicists, with no unequivocal explanation of the underlying flow mechanisms
An extensive experimental database has been analysed to elucidate the complex flow physics surrounding transonic swept-wing shock buffet. This phenomenon is normally characterised by unsteady shock dynamics with a broadband frequency signature mutually interacting with an intermittently separated boundary layer which results in the formation of three-dimensional cellular patterns and spanwise propagation of disturbances
The key insight is the identification of two distinct, possibly connected, phenomena dominating the flow physics around the onset of the shock buffet instability; (i) low-frequency shock unsteadiness, characterised by shock motion for Strouhal numbers between 0.05 and 0.15 that predominantly propagates pressure disturbances inboard, and (ii) broadband higherfrequency outboard-running pressure perturbations along the shock wave and in the shock-induced downstream separated region, at Strouhal numbers between 0.2 and 0.5, which agrees with the widely accepted definition of the aerodynamic instability

Summary

Introduction

Transonic shock buffet on civil aircraft wings presents a key challenge for aerodynamicists, with no unequivocal explanation of the underlying flow mechanisms. The term shock buffet typically refers to self-sustained shock oscillation and intermittent boundary-layer separation, induced by shock-wave/boundary-layer interaction (SWBLI) beyond critical parameter combinations such as Mach number and angle of attack This reportedly aerodynamic instability exhibits distinct characteristics on nominally two-dimensional aerofoils and proper three-dimensional wings, and despite over half a century of research the physical mechanisms remain elusive, especially in the case of swept wings (Giannelis, Vio & Levinski 2017). Transonic shock buffet leads to unsteady aerodynamic loads and a consequent structural response, referred to as buffeting, mutually interacting with the flow This inflicts a drag penalty with an associated increased environmental footprint, deteriorates the aircraft’s performance, handling qualities and structural fatigue life, and degrades passengers’ comfort. Shock buffet limits the flight envelope at high Mach number and load factor, motivating continuous scrutiny from both industry and academia

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