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Talks and Poster Presentations (without Proceedings-Entry):

B. Scheichl:
"Retardation of break-away separation by enhanced boundary layer turbulence: from the triple-deck to a novel Rayleigh stage";
Talk: Fluid Dynamics Group Seminar, Imperial College London, UK (invited); 2012-10-26.



English abstract:
Recently, a theory of incompressible two-dimensional time-mean turbulent boundary layer flow past the smooth impermeable surface of a rigid bluff body for arbitrarily large Reynolds numbers was established. It describes the local separation process in a self-consistent way and, most important, without the need to resort to a specific turbulence closure. However, notwithstanding the benefit of this asymptotic theory for a deepened understanding of turbulent separation, its status quo is unsatisfactory inasmuch as it copes with any position of inviscid flow detachment prescribed sufficiently far downstream from the front stagnation point associated with laminar-turbulent short-scale transition. We elucidate this specific arbitrariness on the basis of the triple-deck structure and solution of the accompanying vortex-flow problem that governs the pressure induced in the predominantly inviscid core of the separating boundary layer. Since attempts of finding further restrictions on the location of separation have not proven successful so far, it is believed that the turbulence intensity level of the boundary layer can be increased further and separation accordingly delayed. In this spirit, the previous flow description applies to moderately large turbulence intensities and a correspondingly wide range of separation points, but break-away of a boundary layer exhibiting the theoretically maximal turbulence intensity is expected to occur at an asymptotically small distance from the rear stagnation point referring to fully attached external potential flow. Here the latter is singularly perturbed such that the free streamlines detach smoothly and a newly defined distinguished limit allows the small streamwise velocity deficit of the incident boundary layer to be gradually transformed into a large one. The arising flow structure is susceptible to both symmetric separation on either side of the stagnation point and the merge of the free shear layers into a wake flow. Particular emphasis is placed on the Rayleigh stage giving rise to the counterpart of the aforementioned vortex-flow problem. Finally, the impact of the analysis on related problems, as the flow past the trailing edge of a plate or an airfoil, is dealt with briefly.

Keywords:
boundary layer theory, break-away separation, stagnating flow, triple deck, turbulence


Electronic version of the publication:
http://publik.tuwien.ac.at/files/PubDat_210837.pdf


Created from the Publication Database of the Vienna University of Technology.