B. Scheichl:
"Turbulent Separation at the Trailing Edge of a Bluff Body: Bridging Controversial Asymptotic Theories";
Talk: British Applied Mathematics Colloquium (BAMC) 2012, University College London, UK; 2012-03-27 - 2012-03-29.
A recent asymptotic theory dealing with incompressible two-dimensional time-mean turbulent boundary layer flow past the smooth impervious surface of a rigid bluff body for large global Reynolds numbers Re is extended insofar as the turbulence intensity level is increased to its maximum and separation correspondingly delayed. It then occurs at a distance of O[(log Re)-1/2(log log Re)-1/4] from the rear stagnation point referring to a fully attached external potential flow, a finding in reasonable agreement with experimental data. The beginning of the separation process is marked by a rather rapid transformation of the small streamwise velocity deficit characteristic of the incident boundary layer into large one -- a situation not encountered previously in the theory of turbulent boundary layers. Hence, a nearly inviscid slender shear layer forms immediately upstream of separation. However, in contrast to preceding studies of separation at a blunt trailing edge, here a self-consistent flow description requires the external flow to be locally weakly perturbed such that the free streamlines detach smoothly, in turn allowing for symmetric separation on either side of the stagnation point and the merge of the free shear layers into a relatively thick wake layer. The central mechanism of boundary layer separation then is no longer governed by the internal triple-deck structure typical for lower turbulence intensities but an Euler stage that accounts for the regularisation of the singular pressure gradient accompanying detachment of the external flow. Most notably, the qualitative results of the combined analytical/numerical study do not rest upon any specific Reynolds stress closure.
bluff-body flows, interactive boundary layers, perturbation methods, separation, turbulence
http://publik.tuwien.ac.at/files/PubDat_206920.pdf