| Abstract: | In this paper, the separation-induced transition in an LPT(low-pressure turbine) cascade is investigated at low Reynolds number with DNS(direct numerical simulation). The transition process is accurately predicted giving good agreements between the DNS and experimental results. To illustrate the secondary instability of separation-induced transition in a low-disturbance environment, the results are comprehensively analyzed in both Fourier space and physical space. It is illustrated that the effect of hyperbolic instability dominates around the saddle point of hyperbolic streamlines. This instability mechanism is responsible for the emergence of the streamwise vortices in the braid region. Elongated and intensified because of the "stretching" effect of the background flow, these vortices become the most noticeable characteristic of the flow field. Fundamental modes of small spanwise wavelength are excited in the braid region, so as some low-frequency modes. The elliptical instability plays a minor role than hyperbolic instability. It is also observed that the fundamental mode with a larger spanwise wavelength is unstable in the vortex core which is associated with the deformation of the vortex core via elliptical instability. There is no convincing evidence for the existence of subharmonic instability. |
