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Dominik Obrist
Institute of Fluid Dynamics, ETH Zurich Sonneggstr. 3, 8092 Zurich, Switzerland

Rolf Henniger
Institute of Fluid Dynamics ETH Zurich Sonneggstrasse 3, 8092 Zurich, Switzerland

Leonhard Kleiser
Institute of Fluid Dynamics ETH Zurich ETH Zentrum, CH-8092 Zurich, Switzerland


It is known from experimental investigations that the leading-edge boundary layer exhibits transition to turbulence at subcritical Reynolds numbers, i.e. at Reynolds numbers which lie below the critical Reynolds number predicted by linear stability theory. In the present work, we investigate this subcritical transition process by direct numerical simulations of a swept Hiemenz flow in a spatial setting. The laminar base flow is perturbed upstream by a pair of stationary counter-rotating vortices. This perturbation generates high- and low-speed streaks by a non-modal growth mechanism. Further downstream, these streaky structures exhibit a strong instability to secondary perturbations which leads to a breakdown to turbulence.
The observed transition mechanism has strong similarities to bypass transition mechanisms found for two-dimensional boundary layers. It can be shown that the transition strongly depends on the amplitude of the primary perturbation as well as on the frequency of the secondary perturbation.