caa a22 4500 606197753 CHVBK 20210128100930.0 cr unu---uuuuu 210128e20151201xx s 000 0 eng 10.1007/s00348-015-2069-5 doi (NATIONALLICENCE)springer-10.1007/s00348-015-2069-5 Transitional flow in the wake of a moderate to large height cylindrical roughness element [Elektronische Daten] [B. Plogmann, W. Würz, E. Krämer] The effect of an isolated, cylindrical roughness on the stability of an airfoil boundary layer has been studied based on particle image velocimetry and hot-wire anemometry. The investigated roughness elements range from a sub-critical to a super-critical behavior with regard to the critical roughness Reynolds number. For the sub-critical case, the nonlinear disturbance growth in the near wake is governed by oblique Tollmien-Schlichting (TS) type modes. Further downstream, these disturbance modes are, however, damped with the mean flow stabilization and no dominant modes persist in the far wake. By contrast, in the transitional configuration the disturbance growth is increased, but still associated with a TS-type instability in the near-wake centerline region of the low-aspect (height-to-diameter) ratio element. That is, the disturbances in the centerline region show a similar behavior as known for 2D elements, whereas in the outer spanwise domain a Kelvin-Helmholtz (KH) type, shear-layer instability is found, as previously reported for larger aspect ratio isolated elements. With increasing height and, thereby, aspect ratio of the roughness, the KH-type instability domain extends toward the centerline and, accordingly, the TS-type instability domain decreases. For high super-critical cases, transition is already triggered in the wall-normal and spanwise shear layers upstream and around the roughness. In the immediate wake, periodic shear-layer disturbances roll up into a—for isolated elements characteristic—shedding of vortices, which was not present at the lower roughness Reynolds number cases due to the decreased aspect ratio and, thereby, different instability mechanism. Springer-Verlag Berlin Heidelberg, 2015 Plogmann B. Institute of Aerodynamics and Gas Dynamics, Pfaffenwaldring 21, 70569, Stuttgart, Germany aut Würz W. Institute of Aerodynamics and Gas Dynamics, Pfaffenwaldring 21, 70569, Stuttgart, Germany aut Krämer E. Institute of Aerodynamics and Gas Dynamics, Pfaffenwaldring 21, 70569, Stuttgart, Germany aut Experiments in Fluids Springer Berlin Heidelberg 56/12(2015-12-01), 1-21 0723-4864 56:12<1 2015 56 348 https://doi.org/10.1007/s00348-015-2069-5 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/s00348-015-2069-5 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Plogmann B. Institute of Aerodynamics and Gas Dynamics, Pfaffenwaldring 21, 70569, Stuttgart, Germany aut NATIONALLICENCE 700 1- Würz W. Institute of Aerodynamics and Gas Dynamics, Pfaffenwaldring 21, 70569, Stuttgart, Germany aut NATIONALLICENCE 700 1- Krämer E. Institute of Aerodynamics and Gas Dynamics, Pfaffenwaldring 21, 70569, Stuttgart, Germany aut NATIONALLICENCE 773 0- Experiments in Fluids Springer Berlin Heidelberg 56/12(2015-12-01), 1-21 0723-4864 56:12<1 2015 56 348