Abstract
A series of experimental investigations have been carried out to examine the influence of free-stream turbulence and acoustic noise on laminar-to-turbulent transition in boundary layers on an NACA0012 airfoil. The quantitative evaluation of the influence on the total growth rate N of the most amplified disturbance obtained from the eN method at the transition location is made in an empirical relation between the total growth rate N and both the residual turbulence Tu and the sound pressure level SPL. Consequently, the transition location is detected as a linear equation N=CTu×ln (Tu) +CSPL×SPL+C for not only 2-D case with zero angles of both sweep and attack but also 3-D case with non-zero angles of both sweep and attack, though each coefficient in N depends on geometrical configuration of airfoil such as cross-sectional shape and swept angle. Surface temperature measurement technique by use of infrared camera is shown to be efficient and useful for transition detection in low subsonic flow. Usefulness is also shown in visualizing cross-flow stationary vortices growing in 3-D boundary layer.
