Transition Delay by Oblique Roughness Elements in a Blasius Boundary-Layer Flow

Abstract

A passive device to delay the laminar-turbulent transition in a Blasius boundary-layer flow is tested at low Reynolds numbers in the laminar water channel of the Institute of Aerodynamics and Gas Dynamics (University of Stuttgart). The passive device consists of oblique surface roughness elements lined up perpendicular to the flow. Each element has the shape of a cuboid, is oriented 45° to the direction of the oncoming flow, and generates a nonsymmetric vortex system. From this vortex system only one vortex per roughness element persists and a co-rotating vortex structure remains after 50 roughness heights downstream of the elements. These vortices generate high and low speed streaks inside of the boundary layer through the lift-up effect. The streaks alter the two-dimensional (2D) boundary layer into a three-dimensional (3D) one. For correctly placed and sized roughness elements the 2D Tollmien-Schlichting wave is altered into a 3D less amplified wave and the transition occurs further downstream. The flow is investigated by means of surface visualizations with the Temperature-Sensitive Paint (TSP) method and velocity measurements with hot-film anemometry. With the TSP method the laminar-turbulent transition was measured with high spatial and temporal resolution. The changes of transition location and changes of transition mechanism are well captured.