| 1. |
- Kreilos, T., et al.
(författare)
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Bypass transition in boundary layers as an activated process
- 2015
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Ingår i: Proceedings - 15th European Turbulence Conference, ETC 2015. - : TU Delft.
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Konferensbidrag (refereegranskat)abstract
- We consider the spatio-temporal aspects of the transition to turbulence in a boundary layer above a flat plate exposed to free-stream turbulence. Combining results from the receptivity to free-stream turbulence with the observation of a double threshold from transition studies in e.g. pipe flow we arrive at a physically motivated prediction for the spatial distribution of nucleation events in boundary layers. We use a cellular automaton to implement a complete model for the spatial and temporal evolution of turbulent patches and show that the model reproduces the statistical features of the boundary layer remarkably well. The success of the modeling shows that bypass transition occurs as a spatiotemporally activated process, where transition is triggered by critical fluctuations imported from the free-stream turbulence.
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| 2. |
- Khapko, Taras, et al.
(författare)
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Edge states as mediators of bypass transition in boundary-layer flows
- 2016
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 801
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Tidskriftsartikel (refereegranskat)abstract
- The concept of edge states is investigated in the asymptotic suction boundary layer in relation to the receptivity process to noisy perturbations and the nucleation of turbulent spots. Edge tracking is first performed numerically, without imposing any discrete symmetry, in a large computational domain allowing for full spatial localisation of the perturbation velocity. The edge state is a three-dimensional localised structure recurrently characterised by a single low-speed streak that experiences erratic bursts and planar shifts. This recurrent streaky structure is then compared with predecessors of individual spot nucleation events, triggered by non-localised initial noise. The present results suggest a nonlinear picture, rooted in dynamical systems theory, of the nucleation process of turbulent spots in boundary-layer flows, in which the localised edge state plays the role of state-space mediator.
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| 3. |
- Khapko, Taras, et al.
(författare)
-
Fully localised edge states in boundary layers
- 2015
-
Ingår i: Proceedings - 15th European Turbulence Conference, ETC 2015. - : TU Delft.
-
Konferensbidrag (refereegranskat)abstract
- Investigation of the laminar–turbulent boundary is performed in a boundary-layer flow. Constant homogeneous suction is applied at the wall in order to prevent the spatial growth of the layer, leading to the parallel Asymptotic Suction Boundary Layer (ASBL). Edge tracking is performed in a large computational domain allowing for full spatial localisation of the structures on the laminar–turbulent separatrix. The obtained dynamics of the state goes through calm and bursting phases. During the latter the structure grows in size, shedding vortices downstream of its core which viscously decay during the calm phases. Comparison with the computation in spatially growing boundary layer is made. The influence of the Reynolds number and the path leading from the edge state to turbulent flow are considered.
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