The surface streamlines are seen to be nearly parallel to the endwall in this regime. The laminar boundary layer starting at the leading edge undergoes a high acceleration on the suction surface. According to Hodson and Dominy, the over- acceleration in the boundary layer causes a two dimensional separation bubble near the blend point of the circular leading edge and the suction surface 15 . This separation bubble extends across most of the span, but it is not apparent in the bottom surface ﬂow visualization of figure 7. The suction surface leading edge separation bubble is shown by the ﬂow visualization in Gregory-Smith et al. 16 . Following the re-attachment behind the separation bubble, the laminar boundary layer accelerates along the suction surface and continues to grow until the separation line S3s. (ii) Turbulent regime : This regime is limited by the re-attachment line following the separation at S3s and trailing edge and between the S2s lines. The laminar boundary layer separates at the lowest suction pressure located at axial distance at S3s because of the adverse pressure gradient (see figure 3) and forms another closed separation bubble. The boundary layer undergoes transition and becomes turbulent as it re-attaches behind the separation bubble on the suction surface. The turbulent boundary layer grows along the suction surface and may separate again due to the adverse pressure gradient near the trailing edge to form the trailing edge wake. (iii) Three dimensional ﬂow regime : This regime is indicated by the region between the separation line S2s and endwall. The regime begins at the location where the suction side leg of the leading edge horse-shoe vortex and pressure side leg vortex from the adjacent blade meet on the suction surface. The pair then emerges as the passage vortex which then moves toward the mid-span as it follows the suction surface toward the passage exit. The suction surface boundary layer separates along the S1s and S2s lines near endwalls in figure 7 as the passage vortex and suction side leg vortex climbs up the suction surface. The distinct appearance of the separation line S2s indicates that the suction side leg vortex maintains its existence in the axial development of the passage vortex which will also be shown in further detail in the next section. The inclination of the surface streamlines toward the mid-span in this regime is caused by the entrainment of the boundary layer ﬂuids (both at the endwall and the suction surface) by the passage vortex. Note that the surface streamlines are symmetric about the mid-span of the blade surface in figure 7. The patterns become asymmetric in three- dimensional cascade by the inﬂuence of radial forces as will be shown in further sections. The locations of the separation bubbles and separation lines on the blade surface are strongly inﬂuenced by the inlet ﬂow angle and Reynolds number or Mach number of the incoming ﬂow. For the high speed compressible ﬂow (with the Mach number>0.70), the
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