A comparative study of experiments with numerical simulations of free-stream turbulence transition

To date, very few careful and direct comparisons between experiments and direct numerical simulations (DNS) have been published on free-stream turbulence (FST) induced boundary layer transition, whilst there exist numerous published works on the comparison of canonical turbulent boundary layers. The primary reason is that the former comparison is vastly more difficult to carry out simply because all known transition scenarios have large energy gradients and are extremely sensitive to surrounding conditions. This paper presents a detailed comparison between new experiments and available DNS data of the complex FST transition scenario in a flat plate boundary layer at turbulence intensity level about Tu = 3 % and FST Reynolds number about Re-fst = 67. The leading edge (LE) pressure gradient distribution and the full energy spectrum at the LE are identified as the two most important parameters for a satisfying comparison. Matching the LE characteristic FST parameters is not enough as previously thought, which is illustrated by setting up two experimental FST cases with about the same FST integral parameters at the LE but with different energy spectra. Finally, an FST boundary layer penetration depth (PD) measure is defined using DNS, which suggests that the PD grows with the downstream distance and stays around 20 % of the boundary layer thickness down to transition onset. With this result, one cannot rule out the significance of the continuous FST forcing along the boundary layer edge in this transition scenario, as indicated in previous studies.

Automated summary

To date, there have been very few comparisons between experiments and direct numerical simulations (DNS) on free-stream turbulence (FST) induced boundary layer transition. This is mainly due to the difficulty in carrying out such comparisons because of the large energy gradients and sensitivity to surrounding conditions in transition scenarios. This study presents a detailed comparison between new experiments and available DNS data of a complex FST transition scenario in a flat plate boundary layer. The leading edge pressure gradient distribution and the full energy spectrum at the leading edge are identified as the two most important parameters for a satisfying comparison. The study also defines a measure of FST boundary layer penetration depth using DNS, suggesting that the depth grows with downstream distance and stays around 20% of the boundary layer thickness down to transition onset.