Stability of the laminar boundary-layer flow behind a roughness element

摘要: Roughness elements in laminar boundary layers generate both high shear and streaky structures. Because these phenomena interact, it is difficult to precisely ascertain the dominant instability mechanisms. With goal of explicating such interactions, we study stability a layer subject single roughness element at Reynolds number subcritical bypass transition. Our work involves two parts: bi-global linear theory (LST) analysis corroborating experimental measurements. Linear flat-plate perturbed by streamwise streaks reveals presence several unstable modes. Of modes, one exhibits spanwise symmetry other antisymmetric. These modes are termed ‘varicose’ ‘sinuous,’ respectively. Corroborating experiments were conducted water channel University Stuttgart. By simultaneously traversing hot-film probes, able confirm eigenmodes predicted LST extract relevant data for each: eigenvalues, eigenfunctions, growth rates phase distributions. The main part has been performed under ‘natural’ conditions, i.e., absence external forcing. As amplitude sinuous results much smaller than varicose hence affected measurement noise, case with asymmetric forcing presented as well. Despite some deficiencies setup, possible enhance mode respect unforced its existence an eigenmode flow.