An Approach to Measuring the Effects of Surface Excrescences on Transition in the Presence of Arbitrary Pressure Gradients

An experimental study was undertaken to determine the effects of step excrescences on boundary layer transition using an approach designed to allow systematic determination of pressure gradient effects. The study made use of a model with infinitely adjustable step sizes and a single measurement surface for all cases. The installation in the wind tunnel used a variable-geometry wall to impose an arbitrary pressure gradient on the model, without the need for model modifications. The approach was shown to provide good repeatability and an effective way to develop a broad experimental database on step effects that can be used to specify manufacturing tolerances for laminar flow aircraft.