Environmental Influences on Crossflow Instability

Abstract

The laminar-to-turbulent transition process in swept-wing boundary layers is often dominated by an inflectional instability arising from crossflow. It is now known that freestream turbulence and surface roughness are two of the key disturbance sources in the crossflow instability problem. Recent experimental findings have suggested that freestream turbulence of low intensity (less than 0.2%) may have a larger influence on crossflow instability than was previously thought. The present work involves experimental measurement of stationary and traveling crossflow mode amplitudes in freestream turbulence levels between 0.02% and 0.2%. A 1.83 m chord, 45-degree swept-wing model is used in the Klebanoff-Saric Wind Tunnel to perform these experiments. The turbulence intensity and length scales are documented. Although a significant amount of research on the role of turbulence has been completed at higher turbulence levels, comparatively little has been done at the low levels of the present experiments, which more closely reflect the flight environment. It is found that growth of the traveling crossflow mode is highly dependent on small changes to the freestream turbulence. Additionally, previously studied attenuation of saturated stationary disturbance amplitudes is observed at these low turbulence levels. The extent of laminar flow is also observed to decrease in moderate freestream turbulence.