Transition of the Boundary Layer Subjected to Freestream Turbulence

Influence of freestream turbulence (fst) on the transition to turbulence of a boundary layer under an adverse pressure gradient (APG) is investigated using a constant temperature hotwire anemometer and electronically scanned pressure (ESP). The experiments are carried out for two fst levels of 1.05 and 3.5% at a constant APG and Reynolds number (Re). A laminar separation bubble (LSB) is observed for a fst of 1.05%, and shear layer undergoes transition via the inviscid instability. The excitation of flow is evident from the beginning for an increased fst of 3.5%, while the LSB is completely suppressed and a change in the mode of flow transition is also observed. The location of onset of transition is highly affected by the variation of fst. Thus, the transition of a boundary layer under an APG is highly sensitive to freestream disturbances.