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Instability and low-frequency unsteadiness in a shock-induced laminar separation bubble

Instability and low-frequency unsteadiness in a shock-induced laminar separation bubble
Instability and low-frequency unsteadiness in a shock-induced laminar separation bubble
Three-dimensional direct numerical simulations (DNS) of a shock-induced laminar separation bubble are carried out to investigate the flow instability and origin of any low frequency unsteadiness. A laminar boundary-layer interacting with an oblique shock-wave at M = 1:5 is forced at the inlet with a pair of monochromatic oblique unstable modes, selected according to local linear stability theory (LST) performed within the separation bubble. Linear stability analysis is applied to cases with marginal and large separation, and compared to DNS. While the parabolized stability equations approach accurately reproduces the growth of unstable modes, LST performs less well for strong interactions. When the modes predicted by LST are used to force the separated boundary-layer, transition to deterministic turbulence occurs near the reattachment point via an oblique-mode breakdown. Despite the clean upstream condition, broadband low-frequency unsteadiness is found near the separation point with a peak at a Strouhal number of 0:04, based on the separation bubble length. The appearance of the low-frequency unsteadiness is found to be due to the breakdown of the deterministic turbulence, filling up the spectrum and leading to broadband disturbances that travel upstream in the subsonic region of the boundary-layer, with a strong response near the separation point. The existence of the unsteadiness is supported by sensitivity studies on grid resolution and domain size that also identify the region of deterministic breakdown as the source of white noise disturbances. The present contribution confirms the presence of low-frequency response for laminar flows, similarly to that found in fully turbulent interactions.
0022-1120
5-26
Sansica, Andrea
f9d20b04-ef9c-4f72-9e14-f9e6d2e84ab5
Sandham, Neil
0024d8cd-c788-4811-a470-57934fbdcf97
Hu, Zhiwei
dd985844-1e6b-44ba-9e1d-fa57c6c88d65
Sansica, Andrea
f9d20b04-ef9c-4f72-9e14-f9e6d2e84ab5
Sandham, Neil
0024d8cd-c788-4811-a470-57934fbdcf97
Hu, Zhiwei
dd985844-1e6b-44ba-9e1d-fa57c6c88d65

Sansica, Andrea, Sandham, Neil and Hu, Zhiwei (2016) Instability and low-frequency unsteadiness in a shock-induced laminar separation bubble. Journal of Fluid Mechanics, 798, 5-26. (doi:10.1017/jfm.2016.297).

Record type: Article

Abstract

Three-dimensional direct numerical simulations (DNS) of a shock-induced laminar separation bubble are carried out to investigate the flow instability and origin of any low frequency unsteadiness. A laminar boundary-layer interacting with an oblique shock-wave at M = 1:5 is forced at the inlet with a pair of monochromatic oblique unstable modes, selected according to local linear stability theory (LST) performed within the separation bubble. Linear stability analysis is applied to cases with marginal and large separation, and compared to DNS. While the parabolized stability equations approach accurately reproduces the growth of unstable modes, LST performs less well for strong interactions. When the modes predicted by LST are used to force the separated boundary-layer, transition to deterministic turbulence occurs near the reattachment point via an oblique-mode breakdown. Despite the clean upstream condition, broadband low-frequency unsteadiness is found near the separation point with a peak at a Strouhal number of 0:04, based on the separation bubble length. The appearance of the low-frequency unsteadiness is found to be due to the breakdown of the deterministic turbulence, filling up the spectrum and leading to broadband disturbances that travel upstream in the subsonic region of the boundary-layer, with a strong response near the separation point. The existence of the unsteadiness is supported by sensitivity studies on grid resolution and domain size that also identify the region of deterministic breakdown as the source of white noise disturbances. The present contribution confirms the presence of low-frequency response for laminar flows, similarly to that found in fully turbulent interactions.

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Submitted date: 20 April 2016
Accepted/In Press date: 21 April 2016
e-pub ahead of print date: 31 May 2016
Published date: July 2016
Organisations: Aerodynamics & Flight Mechanics Group

Identifiers

Local EPrints ID: 393134
URI: https://eprints.soton.ac.uk/id/eprint/393134
ISSN: 0022-1120
PURE UUID: ce77d00a-c1f8-4ecd-9530-d470e0a4faff
ORCID for Neil Sandham: ORCID iD orcid.org/0000-0002-5107-0944

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Date deposited: 21 Apr 2016 11:19
Last modified: 06 Jun 2018 12:55

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