Three-dimensional separation was studied by analyzing the flow past a large-scale model consisting of a half-prolate ellipsoid extended by a circular cylinder ending in a flat base at 45 deg with respect to the cylinder axis. The flow past the model, including boundary layer and vortical structures, was investigated in great detail using a three-component laser Doppler velocimetry system and three-hole pressure probes actuated by a displacement system installed inside the model. This last device allowed probing of the separating three-dimensional boundary layer very close to the surface. We observed how the boundary layer evolved as it gradually sheared into a vortex roll-up and then into an organized vortex. When skewing of the boundary layer increased, the difference in direction between the velocity gradient vector and the shear stress vector also increased. For this type of flow, turbulence models based on the assumption of isotropic turbulent viscosity are inadequate for numerical analysis.
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