Free-stream turbulence boundary-layer transition

Abstract. Free-stream turbulence (FST) and its effect on boundary-layer transition is an intricate problem. Elongated streamwise streaks of low- and high speed are created inside the boundary layer and their amplitude and spanwise wavelength are believed to be important for the onset of transition. The streaks are unsteady, and their amplitude grows with the square root of the downstream distance until the appearance of the first turbulent spots. The spanwise wavelength is often said to adopt to the boundary-layer thickness, giving streaky structures of aspect ratio one after some adaptation length. Furthermore, the transitional Reynolds number is often simply correlated with the turbulence intensity (Tu) and the characteristic length scales of the FST are often considered to have a small influence on the transition location. Here, we present new results from a large experimental hot-wire measurement campaign performed at KTH-Royal Institute of Technology, where both the Tu and the characteristic length scales of the FST are varied. Our results show that the integral length scale (Lx) of the FST affects the transition location differently depending on Tu in a given boundary layer. On the one hand, for small Tu an increase in Lx advances transition, in agreement with established results. On the other hand, for large Tu an increase in length scale postpones transition. In the present experimental setup both trends have been observed and a hypothesis for the trend change is formulated and tested. In addition, we believe that one has overlooked the significance of Lx in the transition process. For low Tu levels a 12% increase of Lx can advance the transition point by as much as 35%, and for high Tu levels an 18% increase of Lx can delay transition by 22%. These results contradict previous results that the streaks adopt to the boundary layer thickness independent of the FST characteristics, i.e. if the streamwise streaks are important for the transition onset, they are also likely to be affected by the FST. Our measurements show that the aspect ratio of the streaky structures correlates with an FST Reynolds number and that the aspect ratio can change by a factor of two at the location of transition. Based on these results a new transition prediction model has successfully been developed and will be presented at the symposium.