On the co-existence of transonic buffet and separation-bubble modes for the OALT25 laminar-flow wing section

Transonic buffet is an unsteady flow phenomenon that limits the safe flight envelope of modern aircraft. Scale-resolving simulations with span-periodic boundary conditions are capable of providing new insights into its flow physics. The present contribution shows the co-existence of multiple modes of flow unsteadiness over an unswept laminar-flow wing section, appearing in the following order of increasing frequency: (a) a low-frequency transonic buffet mode, (b) an intermediate-frequency separation bubble mode, and (c) high-frequency wake modes associated with vortex shedding. Simulations are run over a range of Reynolds and Mach numbers to connect the lower frequency modes from moderate to high Reynolds numbers and from pre-buffet to established buffet conditions. The intermediate frequency mode is found to be more sensitive to Reynolds-number effects compared to those of Mach number, which is the opposite trend to that observed for transonic buffet. Spectral proper orthogonal decomposition is used to extract the spatial structure of the modes. The buffet mode involves coherent oscillations of the suction-side shock structure, consistent with previous studies including global mode analysis. The laminar separation-bubble mode at intermediate frequency is fundamentally different, with a phase relationship between separation and reattachment that does not correspond to a simple `breathing' mode and is not at the same Strouhal number observed for shock-induced separation bubbles. Instead, a Strouhal number based on separation bubble length and reverse flow magnitude is found to be independent of Reynolds number within the range of cases studied.

Aerodynamics, CFD, Large eddy simulation, Transonic buffet

10.1007/s10494-023-00415-4

1023-1057

Zauner, Markus

297472d4-56c0-458a-a510-ae43ecd5bd51

Moise, Pradeep

477a368b-c418-46b3-a94a-4b53049b93ba

Sandham, Neil D.

0024d8cd-c788-4811-a470-57934fbdcf97

4 May 2023

Zauner, Markus

297472d4-56c0-458a-a510-ae43ecd5bd51

Moise, Pradeep

477a368b-c418-46b3-a94a-4b53049b93ba

Sandham, Neil D.

0024d8cd-c788-4811-a470-57934fbdcf97

Zauner, Markus, Moise, Pradeep and Sandham, Neil D. (2023) On the co-existence of transonic buffet and separation-bubble modes for the OALT25 laminar-flow wing section. pp. 1023-1057 . (doi:10.1007/s10494-023-00415-4).

Record type: Conference or Workshop Item (Paper)

Abstract

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FTaC_OALT25_submit-10 - Accepted Manuscript

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Accepted/In Press date: 31 March 2023

Published date: 4 May 2023

Additional Information: Funding Information: The authors would like to thank ONERA for providing the OALT25 airfoil geometry. The authors also appreciate insightful discussions with V. Brion and J. Dandois, as well as F. Alves Protela. PM was supported by an EPSRC grant entitled “Extending the buffet envelope: step change in data quantity and quality of analysis” (Grant ID: EP/R037027/1). The simulations were performed on the Iridis5 cluster at the University of Southampton and on the UK national supercomputer facility ARCHER2, using computer time provided via the UK Turbulence Consortium grant EP/R029326/1. Funding Information: The authors would like to thank ONERA for providing the OALT25 airfoil geometry. The authors also appreciate insightful discussions with V. Brion and J. Dandois, as well as F. Alves Protela. PM was supported by an EPSRC grant entitled “Extending the buffet envelope: step change in data quantity and quality of analysis” (Grant ID: EP/R037027/1). The simulations were performed on the Iridis5 cluster at the University of Southampton and on the UK national supercomputer facility ARCHER2, using computer time provided via the UK Turbulence Consortium grant EP/R029326/1. Publisher Copyright: © 2023, The Author(s).

Keywords: Aerodynamics, CFD, Large eddy simulation, Transonic buffet