Robust features of a turbulent boundary layer subjected to free-stream turbulence

We use an active grid to generate high-intensity free-stream turbulence with 7.2% ≤ u’_∞/U_∞ ≤ 13.0% and 302 ≤ Re_λ ≤ 760, and study the impact these varying conditions have on a turbulent boundary layer. In particular, several cases are produced with fixed turbulence intensities and Reynolds numbers, but up to a 65% change in the integral scale. Qualitatively the spectra at various wall-normal positions in the boundary layer look similar, however, there are quantifiable differences at the large wavelengths all the way to the wall. It is also demonstrated that longitudinal statistics up to fourth-order are well collapsed at a given u’_∞/U_∞, regardless of the other free-stream turbulence parameters. Perhaps the most interesting feature of these flows is that the outer spectral peak appears to be governed by a set of global scaling laws while the near-wall spectral peak is independent of the free-stream conditions. This allows us to build a model whereby we can determine the entire pre-multiplied spectrogram a priori, and in particular know the location and magnitude of the spectral peaks to within 6% with only knowledge of the free-stream spectrum, the boundary layer height, and the friction velocity.