Influence of Surface Waviness for Laminar Flow Nacelle Applications

Abstract

Transition onset over an engine nacelle is influenced by a large number of factors (for instance, surface roughness, steps, gaps and damage), many of which can be introduced as a result of the manufacturing process. Rivetting processes can lead to the introduction of surface deviations, which can be represented as either a bump or depression wave function. Previous work on the influence of leading edge roughness has demonstrated that such surface deviations can either promote or delay transition onset. The current work at Queen's University Belfast seeks to conduct a comprehensive investigation into the consequences of a depression-type surface irregularity on the transitional characteristics exhibited by a laminar flow nacelle, with specific attention paid to both the depression amplitude and streamwise location. Intial work has indicated that careful positioning of the streamwise location of the depression relative to the leading edge is critical in the selection of appropriate manufacturing tolerances, as even milliscale depressions, which have no appreciable influence on the transitional characteristics of the boundary layer, can have a significant affect on the downstream growth of the turbulent boundary layer. There is also some evidence to suggest that small depressions placed close to the leading edge can have a stabilising effect on the laminar boundary layer, leading to a delay in the transitional Reynolds number in a manner similar to that observed for subcritical protrusions.