Investigation of Cross Flow in the Presence of a Laminar Separation Bubble

Cross-flow development is investigated in conditions suitable for the formation of laminar separation bubbles (LSBs) using a NACA 64₃ − 618 airfoil at a sweep angle of Γ = 45° and a chord wise Reynolds number of Re_C = 600k. The dominant wavenumber of naturally developing stationary cross-flow is found at spanwise wavenumber of K_c = 550 for α = 0°. This is consistent with hydrodynamic stability calculations using parabolized stability equations. Discrete roughness elements are added to amplify the most unstable stationary cross-flow wavelength in the presence of an LSB (α = 0°), and also for a case of cross-flow dominated transition at α = −8°. Time resolved velocity measurements using constant temperature anemometry (CTA) were performed at several planes in the favorable to adverse pressure gradient region up to the point of laminar separation (x/c = 0.50). Spanwise measurements at x/c = 0.35 confirm the presence of stationary cross-flow vortices through the periodic nature of the boundary layer in the mean flow and RMS. Indication of travelling cross-flow is observed in regions of higher disturbances, close to the wall. Near the point of separation (x/c = 0.50), the spanwise coherent structures in the boundary layer contain characteristics of both traveling and stationary cross-flow.