The understanding of periodic laminar-turbulent transition is crucial for flows over turbomachinery blading. Most turbomachinery blades are swept by turbulent wakes from upstream blading and recent works on transition in turbomachinery have attempted to study - and eventually predict - the nature of boundary layer transition induced by these wakes. This work reports on the characteristics of turbulent spots in strong adverse pressure gradients. In Part I the spot behavior is described. Phase-averaged velocity traces, velocity perturbation and disturbance level contours performed at two Reynolds numbers are described. In Part II (a companion paper) the spot propagation and spreading rates are reported. These experimental data are required for validating or modifying existing correlations for transition length prediction.
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