A bypass wake induced laminar/turbulent transition

The process of laminar to turbulent transition induced by a von Karman vort ex street wake, was studied for the case of a flat plate boundary layer. Th e boundary layer developed under zero pressure gradient conditions. The vor tex sneer was generated by a cylinder positioned in the free stream. An X-t ype hot-wire probe located in the boundary layer, measured the streamwise a nd normal to the wall velocity components. The measurements covered two are as; the region of transition onset and development and the region where the wake and the boundary layer merged producing a turbulent flow. The evoluti on of Reynolds stresses and rms-values of velocity fluctuations along the t ransition region are presented and discussed. From the profiles of the Reyn olds stress and the mean velocity profile, a 'negative' energy production r egion along the transition region, was identified. A quadrant splitting ana lysis was applied to the instantaneous Reynolds stress signals. The contrib utions of the elementary coherent structures to the total Reynolds stress w ere evaluated, for several x-positions of the near wall region. Distinct re gions in the streamwise and normal to the wall directions were identified d uring the transition. (C) 1999 Editions scientifiques et medicales Elsevier SAS.