Interaction process between a turbulent boundary layer and a pair of streamwise vortex was investigated experimentally in terms of an extra rate of strain ∂W/∂z. Two kinds of vortex pair, e.g., common-flow up and common-flow flow down, generated by a delta wing located in the free stream were introduced into a developed turbulent boundary layer. In common-flow down case, the extra rate of strain and momentum integral relation derived for symmetrical plane can quantitatively explain streamwise decrease of momentum thickness. In common-flow up case, the streamwise vortex pair embedded in boundary layer imposes both spanwise stretching and squeezing on the boundary layer and then produces inflexion point in the mean velocity profile.

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