SINOPSIS

An experimental study using hot-wire based measurement and novel conditional analysis techniques has revealed interesting large-scale features in the turbulent boundary layer. It is found that a hierarchy of hairpin-type events characterise the logarithmic and wake regions. To the best of the authors' knowledge, such structures have not previously been experimentally measured in their entirety. At all detection heights, we find that these resolved structures maintain a presence in the viscous sublayer (i.e they have a distinctive near-wall footprint). Statistically we find a decay in the detection frequency with increasing distance from the wall. Also interesting, is tentative evidence for the existence of a preferred spacing mode for these events. Regardless of detection height, this spacing remains approximately constant at x⁺ ~ 300. This is a surprising result, and seems to tally with the near-wall spacing implied by the time between successive VITA detected events (for near-wall quasi-streamwise structures). These results are preliminary and we would urgently seek to test the existence of these spacing modes in higher Re flows. However, for the time being, the implication seems to be that a spacing, which was originally prescribed in the near-wall region, is maintained as the near-wall structure grows I mutates into the hairpin hierarchy that we find to predominate all the way from the buffer region to the edge of the turbulent boundary layer (δ).