This research collaboration aims at simulating, in detail, the role of transition in compressor aerodynamics. The flow through the passage is subjected to adverse pressure gradient, which is destabilizing, and can lead to transition to turbulence or boundary layer separation. In addition to the mean pressure gradient, the boundary layers on the compressor blade are forced by free-stream vortical disturbances. The interaction between these disturbances and the near-wall flow play a significant role in the compressor performance.
The clip below shows three planes, one near the pressure side (bottom surface of the blade), one at mid-pitch, and one near the suction surface (top surface of the blade). Contours show the tangential velocity perturbations. It is interesting to contrast the pressure and suction surface of the blade. Despite sharing the same leading edge and free-stream conditions, breakdown to turbulence on each surface follows a distinctly different route.
The pressure side of the blade undergoes bypass transition in a manner similar to that observed in DNS of simple flat plate boundary layers. While the elongated Klebanoff streaks are clearly seen in the clip, the spots are concealed by the high-amplitude of the Klebanoff disturbance. This project therefore benefits from our previous research on bypass transition in canonical flow settings such as flat plate boundary layers (Zaki & Durbin 2005, 2006).
On the suction side (upper surface), the airfoil experiences laminar separation, followed by turbulent reattachment. The clip below shows the presence of boundary layer streaks upstream of the mean separation location. The instantaneous separation surface is therefore modulated by these streamwise streaks, and the spectral characteristics of the flow are changed.