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.