Graduate Seminar in Fluid Mechanics 530.807
3:00 p.m. Presentation
“Transition in Three-Dimensional Boundary Layer Forced By a Free-Stream Wake Flow”
Presented by RIKHI BOSE (Advisers: Prof. Zaki)
Lifting surfaces often operate with a sweep angle relative to the free stream. In favorable pressure gradient boundary layers, this configuration is prone to the amplification of cross-flow instabilities. In the present work, transition in three-dimensional boundary layers due to an upstream wake, akin to the interaction in multi-element airfoils is studied using direct numerical simulations. A cylinder is placed above the leading edge of the flat plate, and its turbulent wake interacts with the three-dimensional, favorable-pressure-gradient boundary layer beneath it. The sweep angle is 45 degrees and the Reynolds number is 800 based on the free-stream speed, the leading-edge half thickness and the fluid viscosity. The instability and transition processes are studied starting from the initial receptivity stage all the way through the onset of turbulence. Disturbances penetrate the boundary layer in two stages: right at the leading edge through pressure perturbations and also downstream where the wake diffuses with the boundary layer. Boundary-layer disturbances are streaky prior to breakdown into turbulent spots. Counterintuitively, in such a scenario, transition may be promoted in favorable pressure gradient due to emergent crossflow vortices.