Burtsev, Anton 
ORCID: 0000-0002-8268-9088
(2023)
Linear Instability Mechanisms over Three-Dimensional Wings at Low Reynolds Numbers.
PhD thesis, University of Liverpool.
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Abstract
Identification of flow instabilities that cause a laminar flow to transition to turbulence is of great importance. Most instability studies of flows over lifting surfaces have focused on sim- plified models of laminar separation, which fail to address the essential three-dimensionality of the flow. The limited knowledge of linear instability mechanisms associated with three- dimensional separation on the wing surface and a lack of a deep understanding of the complex vortex dynamics arising from the wake instabilities behind a finite wing have motivated this work. The present thesis documents the instability mechanisms and vortex dynamics of a range of finite wing planforms at low Reynolds numbers (100 ≤ Re ≤ 400) and high angles of attack. Global linear stability analysis is conducted on steady base flows to identify the leading unstable modes of the separated flow. A parametric study revealed the existence of three families of unstable global modes, denominated A, B and C, that manifest themselves in a range of geometrical configurations. The origin of wake unsteadiness is associated with unstable global mode A, which originates at the peak recirculation zone of the three- dimensional laminar separation bubble (LSB) formed on the wing. There is a mostly linear relationship between reversed flow and the leading mode amplification rate when the three-dimensionality of the LSB is low. Adjoint global modes and structural sensitivity were computed for informing future flow control. The wavemaker of the leading global eigenmode lies inside the LSB at the spanwise location of peak recirculation. Increasing Re leads to the structure of the wavemaker becoming more compact in the spanwise direction and more dependent on the top and bottom shear layers of the LSB. The wake dynamics of the unsteady flow are analysed once the growth of the leading three-dimensional global mode has led the flow to nonlinear saturation. Several mechanisms (namely, some aspects of the LSB’s wing surface topology and the tip vortex meandering phenomenon) typically observed in turbulent flows at higher Re have been identified. This suggests that the undelaying physical mechanisms observed here may extend beyond the laminar regime and play a role in turbulent flows at higher Reynolds numbers. Analysis of the flow using data-driven techniques revealed the structure of the Wake mode associated with the higher frequency shedding of predominantly spanwise vortices near midspan and the Interaction mode responsible for the shedding of spanwise and braid vortices on the outboard section of the wing. Differences in the frequency between these modes manifest in the appearance of vortex dislocations. Present results establish a basis for understanding flow dynamics and instabilities on finite three-dimensional wings at low Reynolds numbers as a first step towards understand- ing turbulent flow at higher Reynolds numbers.
| Item Type: | Thesis (PhD) |
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| Divisions: | Faculty of Science and Engineering > School of Engineering |
| Depositing User: | Symplectic Admin |
| Date Deposited: | 29 Aug 2023 13:22 |
| Last Modified: | 01 Feb 2024 02:30 |
| DOI: | 10.17638/03169471 |
| Supervisors: |
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| URI: | https://livrepository.liverpool.ac.uk/id/eprint/3169471 |
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