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Abstract: The structure at a spanwise edge of spreading turbulent region is studied in a wind tunnel experiment using conditional-sampling and ensemble-averaging techniques. A turbulent region is created by locally tripping the laminar boundary layer using two bi-morph piezoelectric actuators. The turbulent region protrudes into the laminar region at the middle height of the boundary layer. The results show that when the turbulent region spreads, a pair of streamwise vortices exists near the interface on the turbulent side, and the flow induced by the vortex pair pushes the turbulent fluid into the laminar region. An experiment in which a thin flat plate is inserted into the boundary layer reveals that the spanwise spreading of the turbulent region can be effectively suppressed by destroying these vortical structures. [Copyright &y& Elsevier]