INTERFEROMETRIC STUDY OF ORGANIZED MOTIONS IN TURBULENT BOUNDARY-LAYERS

Double-pulse holographic interferometry has been used to visualize org anized structures in zero-pressure-gradient turbulent boundary layers over a flat plate at M = 1.8, 25, and 3.6 (Re-theta = 1.2 X 10(4) to 2 .2 X 10(4)). Differential interferograms taken with a pulse interval b etween 2 to 600 mu s at three different Mach numbers show an interesti ng array of fringe cells that appear inclined downstream in the bounda ry layers. These fringe cells must be the interferometric images of la rge-scale outer-layer structures convecting in the main flow direction . On absolute interferograms of the boundary layers, the interferometr ic images appear as horizontal fringe lines running over the test surf ace. The wavy fringe pattern frequently exhibits distinct inclined int erfaces with a positive upstream density gradient that must be; associ ated with the backs of large-scale outer-layer bulges. Quantitative in formation of these structures has been extracted from the interferogra ms that is in agreement with data from other investigators using diffe rent experimental techniques. To aid the interpretation of the experim ental data, interferograms have been generated numerically with the da tabase from a large-eddy simulation. These interferograms also reveal regular fringe patterns in the boundary layer that are found to be pro duced by the large-scale structures in the layer.