caa a22 4500 606197141 CHVBK 20210128100928.0 cr unu---uuuuu 210128e20150101xx s 000 0 eng 10.1007/s00348-014-1878-2 doi (NATIONALLICENCE)springer-10.1007/s00348-014-1878-2 Stability of the laminar boundary-layer flow behind a roughness element [Elektronische Daten] [Yong-su Shin, Ulrich Rist, Ewald Krämer] Roughness elements in laminar boundary layers generate both high shear layers and streaky structures. Because these phenomena interact, it is difficult to precisely ascertain the dominant instability mechanisms. With the goal of explicating such interactions, we study the stability of a laminar boundary layer subject to a single roughness element at a Reynolds number subcritical of bypass transition. Our work involves two parts: bi-global linear stability theory (LST) analysis and corroborating experimental measurements. Linear stability analysis of a flat-plate boundary layer perturbed by streamwise streaks reveals the presence of several unstable modes. Of the dominant two modes, one exhibits spanwise symmetry and the other is antisymmetric. These modes are termed ‘varicose' and ‘sinuous,' respectively. Corroborating experiments were conducted in the laminar water channel of the University of Stuttgart. By simultaneously traversing two hot-film probes, we are able to confirm the presence of both eigenmodes predicted by LST and to extract relevant data for each: eigenvalues, eigenfunctions, growth rates and phase distributions. The main part of the experiments has been performed under ‘natural' conditions, i.e., in the absence of external forcing. As the amplitude of the sinuous part of the results is much smaller than the varicose one and hence affected by measurement noise, a case with asymmetric external forcing is presented as well. Despite some deficiencies of the setup, it is possible to enhance the sinuous mode with respect to the unforced case and to confirm its existence as an eigenmode of the flow. Springer-Verlag Berlin Heidelberg, 2015 Shin Yong-su Institut für Aerodynamik und Gasdynamik, Universität Stuttgart, Pfaffenwaldring 21, 70569, Stuttgart, Germany aut Rist Ulrich Institut für Aerodynamik und Gasdynamik, Universität Stuttgart, Pfaffenwaldring 21, 70569, Stuttgart, Germany aut Krämer Ewald Institut für Aerodynamik und Gasdynamik, Universität Stuttgart, Pfaffenwaldring 21, 70569, Stuttgart, Germany aut Experiments in Fluids Springer Berlin Heidelberg 56/1(2015-01-01), 1-18 0723-4864 56:1<1 2015 56 348 https://doi.org/10.1007/s00348-014-1878-2 text/html Onlinezugriff via DOI BK010053 XK010053 XK010000 Metadata rights reserved Springer special CC-BY-NC licence nationallicence 1 research-article jats NATIONALLICENCE NATIONALLICENCE NL-springer NATIONALLICENCE 856 40 https://doi.org/10.1007/s00348-014-1878-2 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Shin Yong-su Institut für Aerodynamik und Gasdynamik, Universität Stuttgart, Pfaffenwaldring 21, 70569, Stuttgart, Germany aut NATIONALLICENCE 700 1- Rist Ulrich Institut für Aerodynamik und Gasdynamik, Universität Stuttgart, Pfaffenwaldring 21, 70569, Stuttgart, Germany aut NATIONALLICENCE 700 1- Krämer Ewald Institut für Aerodynamik und Gasdynamik, Universität Stuttgart, Pfaffenwaldring 21, 70569, Stuttgart, Germany aut NATIONALLICENCE 773 0- Experiments in Fluids Springer Berlin Heidelberg 56/1(2015-01-01), 1-18 0723-4864 56:1<1 2015 56 348