An experimental study on the effects of two-dimensional positive surface defects on the laminar–turbulent transition of a sucked boundary layer

Abstract

Laminar–turbulent transition can be effectively delayed using Laminar Flow Control (LFC) by boundary layer suction. However, major obstacles to the industrial implementation of this technique are related to practical limitations, such as proper integration of the suction system or unreliability of current design tools. The influence of surface discontinuities that can arise from installing an LFC system (and then potentially cancel or deteriorate its stabilizing effect on the boundary layer) is scarcely documented in the open literature, adding to the complexity of improving numerical models. The present investigation, therefore, focuses on experimentally characterizing the effects of surface defects on the laminar–turbulent transition of a sucked boundary layer in a two-dimensional flow, in an effort to address some of the issues mentioned above. The experimental facility and protocol for conducting this transition study are first presented, followed by a baseline characterization of the effects of wall suction alone on transition. Surface defects, in the form of cylindrical roughness elements (wires), are then introduced on the flat plate and their effects, coupled to those of wall suction, on boundary layer stability are discussed. The critical relative height (where the onset of transition coincides with the position of the surface defect) was found to be the same for cases both with and without wall suction. For the critical cases, spectral analysis of the flow immediately downstream of the defect for all suction configurations revealed a range of amplified high frequencies in addition to or in place of the natural Tollmien–Schlichting instabilities. Wall suction was, therefore, ineffective in delaying the critical relative wire height, since, in the critical cases, the transition mechanism seemed to be governed by inflection-type instabilities, rather than viscous instabilities.