Evaluation of data from direct numerical simulations of transition due to free-stream turbulence

1. Motivation and objectives The effect of free-stream turbulence (FST) on the onset of transition has only recently received detailed attention although FST is one of the important sources leading to by-pass transition. Such disturbances are of great interest in engineering, for instance for the prediction of transition on turbine blades, where the impingement of turbulence from the wake of the stator influences the boundary layers on the rotor blades. Another important aspect is the influence of FST on wind-tunnel experiments in general. It is desirable to reduce the FST-level in order to resemble free-flight conditions, but all wind-tunnels have some background disturbances with different characteristics, and knowledge about the ‘dangerous’ FSTparameters is of interest. Attempts have been made to establish empirical correlations between the free-stream turbulence and the transition Reynolds number. At FST levels (Tu) above 5-10%, transition occurs at the minimum Reynolds number where self-sustained boundary layer turbulence can exist, i.e. at Reθ ≈ 190 where Reθ denotes the Reynolds number based on momentum loss thickness (see Arnal, 1987, for a review). At lower levels of FST, however, the results are less clear.