Stability and transition of a laminar separation bubble subjected to freestream turbulence

Experiments were conducted to study the transition and flow development in a laminar separation bubble (LSB) formed on an aerofoil. The effects of a wide range of freestream turbulence intensity (0.15% < Tu < 6.26%) and streamwise integral length scale (4.6mm < Λ u < 17.2mm) are considered. The coexistence of a modal instability due to the LSB and a non-modal instability caused by streaks generated by freestream turbulence is observed. The presence of streaks in the boundary layer modifies the mean flow topology of the bubble. These changes in the mean flow field result in the modification of the convective disturbance growth, where an increase in turbulence intensity is found to dampen the growth of the modal instability. For a relatively fixed level of Tu, the variation of Λ u has modest effects, however, a slight advancement of the non-linear growth of disturbances and eventual breakdown with the decrease in Λ u is observed. The data shows that the streamwise growth of the disturbance energy is exponential for the lowest levels of freestream turbulence and gradually becomes algebraic as the level of freestream turbulence increases. Once a critical turbulence intensity is reached, there is enough energy in the boundary layer to suppress the LSB, which in turn, results in the non-modal instability taking over the transition process. Linear stability analysis is conducted in the fore position of the LSB, and accurately models unstable frequencies and eigenfunctions for configurations subjected to levels of turbulence intensity levels up to 3%.