Detrimental effects of almost immeasurably small freestream nonuniformities generated by wind-tunnel screens

Measurements are presented that were obtained during the course of a series of how quality improvements to a small stand-alone wind tunnel that has been modified for boundary-layer transition studies. The objective of establishing a Blasius boundary layer with a high degree of spanwise uniformity has been frustrated by the persistence of Klebanoff modes, i.e., weak streamwise vortices within the layer. The vortices originate at the leading edge and appear to be caused by almost immeasurably small nonuniformities in the freestream introduced by the wind-tunnel screens. The vortices cause a spanwise thickening and thinning of the layer. Contours of the background unsteadiness in a spanwise plane through the layer show locally concentrated regions with elevated levels that are associated with the vortices. These contours are used as a sensitive indicator for the how quality improvements. Although far from forming a complete parametric study, the observations should act as a valuable guide for others. For example, spanwise variations in the porosity of the screens were discovered by traversing each screen between a laser and a photo detector. Significant improvement in the spanwise uniformity of the layer was obtained by sorting the screens based on these results. The quantities that are generally agreed upon to define acceptable how quality proved to be inadequate during the final stages of refinement. The background unsteadiness within the layer has been reduced by a factor of 30 compared with the initial configuration.