Transition from laminar flow to turbulent flow occurs very often and plays a crucial role in many practical engineering flows. There are many different kinds of transition and broadly speaking they can be classified into three categories: classical transition in attached boundary; bypass transition in attached boundary layer and separated boundary layer transition. This paper presents a comparative study of separated boundary layer transition on a flat plate with a blunt/semi-circular leading edge. Boundary layer may separate due to an adverse pressure gradient or due to flow geometry. In the current study the geometry is a flat plate with two different leading edges: a blunt one and a semi-circular one. The main purpose of the study is to identify how similar or how different the transition process is with two different leading edges. This study shows that for both cases (blunt and semi-circular leading edges) the primary two-dimensional instability originates from the free shear layer of the separation bubble via the Kelvin-Helmholtz mechanism. Three-dimensional motions develop under any small spanwise disturbances and similar coherent structures have been observed from flow visualization in both cases, strongly indicating that the transition process is very similar.
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