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Direct numerical simulations of forced and unforced separation bubbles on an airfoil at incidence

Direct numerical simulations of forced and unforced separation bubbles on an airfoil at incidence
Direct numerical simulations of forced and unforced separation bubbles on an airfoil at incidence
Direct numerical simulations (DNS) of laminar separation bubbles on a NACA-0012 airfoil at Re-c = 5 x 10(4) and incidence 5 degrees are presented. Initially volume forcing is introduced in order to promote transition to turbulence. After obtaining sufficient data from this forced case, the explicitly added disturbances are removed and the simulation run further. With no forcing the turbulence is observed to self-sustain, with increased turbulence intensity in the reattachment region. A comparison of the forced and unforced cases shows that the forcing improves the aerodynamic performance whilst requiring little energy input. Classical linear stability analysis is performed upon the time-averaged flow field; however no absolute instability is observed that could explain the presence of self-sustaining turbulence. Finally, a series of simplified DNS are presented that illustrate a three-dimensional absolute instability of the two-dimensional vortex shedding that occurs naturally. Three-dimensional perturbations are amplified in the braid region of developing vortices, and subsequently convected upstream by local regions of reverse flow, within which the upstream velocity magnitude greatly exceeds that of the time-average. The perturbations are convected into the braid region of the next developing vortex, where they are amplified further, hence the cycle repeats with increasing amplitude. The fact that this transition process is independent of upstream disturbances has implications for modelling separation bubbles.
direct numerical simulation, airfoil, aerofoil, laminar separation bubble, transitional separation bubble
0022-1120
175-207
Jones, L.E.
0a801680-fecd-400f-a771-b0ac1fcd02c6
Sandberg, R.D.
41d03f60-5d12-4f2d-a40a-8ff89ef01cfa
Sandham, N.D.
0024d8cd-c788-4811-a470-57934fbdcf97
Jones, L.E.
0a801680-fecd-400f-a771-b0ac1fcd02c6
Sandberg, R.D.
41d03f60-5d12-4f2d-a40a-8ff89ef01cfa
Sandham, N.D.
0024d8cd-c788-4811-a470-57934fbdcf97

Jones, L.E., Sandberg, R.D. and Sandham, N.D. (2008) Direct numerical simulations of forced and unforced separation bubbles on an airfoil at incidence. Journal of Fluid Mechanics, 602, 175-207. (doi:10.1017/S0022112008000864).

Record type: Article

Abstract

Direct numerical simulations (DNS) of laminar separation bubbles on a NACA-0012 airfoil at Re-c = 5 x 10(4) and incidence 5 degrees are presented. Initially volume forcing is introduced in order to promote transition to turbulence. After obtaining sufficient data from this forced case, the explicitly added disturbances are removed and the simulation run further. With no forcing the turbulence is observed to self-sustain, with increased turbulence intensity in the reattachment region. A comparison of the forced and unforced cases shows that the forcing improves the aerodynamic performance whilst requiring little energy input. Classical linear stability analysis is performed upon the time-averaged flow field; however no absolute instability is observed that could explain the presence of self-sustaining turbulence. Finally, a series of simplified DNS are presented that illustrate a three-dimensional absolute instability of the two-dimensional vortex shedding that occurs naturally. Three-dimensional perturbations are amplified in the braid region of developing vortices, and subsequently convected upstream by local regions of reverse flow, within which the upstream velocity magnitude greatly exceeds that of the time-average. The perturbations are convected into the braid region of the next developing vortex, where they are amplified further, hence the cycle repeats with increasing amplitude. The fact that this transition process is independent of upstream disturbances has implications for modelling separation bubbles.

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JonesSandbergSandhamJFM2008.pdf - Accepted Manuscript
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Submitted date: 21 August 2007
Published date: 10 May 2008
Keywords: direct numerical simulation, airfoil, aerofoil, laminar separation bubble, transitional separation bubble
Organisations: Aerodynamics & Flight Mechanics

Identifiers

Local EPrints ID: 55698
URI: http://eprints.soton.ac.uk/id/eprint/55698
ISSN: 0022-1120
PURE UUID: 12aaa3fd-6501-4d46-a17a-63ebdcc92c34
ORCID for R.D. Sandberg: ORCID iD orcid.org/0000-0001-5199-3944
ORCID for N.D. Sandham: ORCID iD orcid.org/0000-0002-5107-0944

Catalogue record

Date deposited: 05 Aug 2008
Last modified: 16 Mar 2024 03:03

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Contributors

Author: L.E. Jones
Author: R.D. Sandberg ORCID iD
Author: N.D. Sandham ORCID iD

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