INTERFEROMETRIC STUDY OF ORGANIZED MOTIONS IN TURBULENT BOUNDARY-LAYERS

Double-pulse holographic interferometry has been used to visualize organized 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 between 2 to 600 mu s at three different Mach numbers show an interesting array of fringe cells that appear inclined downstream in the boundary layers. These fringe cells must be the interferometric images of large-scale outer-layer structures convecting in the main flow direction. On absolute interferograms of the boundary layers, the interferometric images appear as horizontal fringe lines running over the test surface. The wavy fringe pattern frequently exhibits distinct inclined interfaces with a positive upstream density gradient that must be; associated with the backs of large-scale outer-layer bulges. Quantitative information of these structures has been extracted from the interferograms that is in agreement with data from other investigators using different experimental techniques. To aid the interpretation of the experimental data, interferograms have been generated numerically with the database from a large-eddy simulation. These interferograms also reveal regular fringe patterns in the boundary layer that are found to be produced by the large-scale structures in the layer.