Analysis of Boundary-Layer Transition Induced by Surface Imperfections

This work presents an experimental study on the effects of surface irregularities, specifically bumps and gaps, on boundarylayer transition. Transition mechanisms are investigated using hot-wire anemometry and a Preston tube. The experiments were conducted in the low acoustic noise and turbulence (LANT) wind tunnel at the Universidade de São Paulo, employing a flat-plate model with a variable insert capable of generating either a bump or a gap. The results show that bumps have a significantly stronger impact on the amplification of Tollmien-Schlichting waves than gaps, with amplification levels approximately four times higher for bumps at |h/δ∗| = 0.64. Furthermore, the critical height required to trigger bypass transition is lower for bumps than for gaps. In particular, bumps promote a rapid upstream shift of the transition location already at h/δ∗ = 1.29, whereas a comparable abrupt advancement toward the irregularity is observed for gaps only at h/δ∗ = −3.22. The data indicate that bypass transition induced by gaps is associated with Rossiter-type instabilities. In contrast, bypass transition over bumps appears to be governed by a distinct mechanism related to the growth of a downstream recirculation bubble, which induces an inflectional boundary-layer profile and gives rise to a high-frequency amplification band.