Title: Investigation into boundary layer transition using wall-resolved large-eddy simulations and modeled inflow turbulence
Authors: Lobo, Brandon Arthur
Schaffarczyk, Alois Peter
Breuer, Michael  
Language: eng
Keywords: Laminar;Wind turbine;Turbulent;Transition;Large eddy simulation (LES)
Issue Date: 10-May-2022
Publisher: Copernicus Publications
Document Type: Article
Source: Wind Energ. Sci., 7, 967–990, 2022
Journal / Series / Working Paper (HSU): Wind Energy Science
Volume: 7
Issue: 3
Page Start: 967
Page End: 990
Publisher Place: Göttingen
The objective of the present paper is to investigate the transition scenario of the flow around a typical section of a wind turbine blade exposed to different levels of inflow turbulence. A rather low Reynolds number of Rec = 10^5 is studied at a fixed angle of attack but under five different turbulence intensities (TIs) up to TI = 11.2 %. Using wall-resolved large-eddy simulations combined with an inflow procedure relying on synthetically generated turbulence and a source-term formulation for its injection within the computational domain, relevant flow features such as the separation bubble, inflectional instabilities and streaks can be investigated. The study shows that the transition scenario significantly changes with rising TI, where the influence of inflectional instabilities due to an adverse pressure gradient decreases, while the influence of streaks increases, resulting in a shift from the classical scenario of natural transition to bypass transition. The primary instability mechanism in the separation bubble is found to be inflectional, and its origin is traced back to the region upstream of the separation. Thus, the inviscid inflectional instability of the separated shear layer is an extension of the instability of the attached adverse pressure gradient boundary layer observed upstream. The boundary layer is evaluated to be receptive to external disturbances such that the initial energy within the boundary layer is proportional to the square of the turbulence intensity. Boundary layer streaks were found to influence the instantaneous separation location depending on their orientation. A varicose mode of instability is observed on the overlap of the leading edge of a high-speed streak with the trailing edge of a low-speed streak. The critical amplitude of this instability was analyzed to be about 32 % of the free-stream velocity.
Organization Units (connected with the publication): Strömungsmechanik 
Publisher DOI: 10.5194/wes-7-967-2022
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