Experiments on Discrete Roughness Element Technology for Swept-Wing Laminar Flow Control

Micrometer-sized spanwise-periodic discrete roughness elements (DREs) were applied to and tested on a 30 deg swept-wing model in order to study their effects on boundary-layer transition in flight in which stationary crossflow vortices were the dominant instability. Significant improvements have been made to previous flight experiments in order to more reliably determine and control the model angle of attack and unit Reynolds number in order to minimize the uncertainties that DREs have on laminar-turbulent transition on a swept wing. Two interchangeable leading-edge surface-roughness configurations were tested: highly polished and painted. The baseline transition location for the painted leading edge (increased surface roughness) was unexpectedly farther aft than the polished. Infrared thermography, coupled with a postprocessing code, was used to globally extract a quantitative boundary-layer transition location. Linear stability guided the DRE configuration test matrix. Although the results have not reliably confirmed the use of DREs as a viable laminar flow control technique in the flight environment, it has become clear that significant computational studies, specifically direct numerical simulation of these particular DRE configurations on this model geometry and flight conditions, are a necessity in order to better understand the influence that DREs have on laminar-turbulent transition