Free-stream turbulence induced boundary-layer transition: a comparative study of EXP with DNS

To date, very few careful and direct comparisons between experiments (EXP) 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 being 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. Direct comparisons between EXP and DNS are important for two reasons. Firstly, the two approaches are frequently used for validation of computational fluid dynamics (CFD) where mathematical models describe complex fluid physics. This type of validation is redundant unless we can certify that we are able to reproduce detailed results from EXP with DNS or vice versa. Secondly, a direct comparison may pin-point the critical parameters that are important to match for a satisfying comparison, and hence can give guidance on how to develop better CFD models. In this paper we present a detailed comparison between our EXP and available DNS data of this complex transition scenario in a flat plate boundary layer at a turbulence intensity level of about Tu = 3% and an FST Reynolds number of about Refst = 67. We identify the leading edge (LE) pressure gradient distribution and the full energy spectrum at the LE as the two most important parameters for a satisfying comparison. Matching the LE characteristic FST parameters are not enough as previously thought.