The aerodynamic efficiency of turbomachinery blades is highly affected by the occurrence of laminar–turbulent transition in the boundary layer, since skin friction and losses rise for the turbulent state. Depending on the free-stream turbulence level different path towards a turbulent state can be identified. When the turbulent level is low a separation bubble can trigger the transition. On the other hand, high free-stream turbulence generates streaky structures in the boundary layer, which break down into turbulent spots and cause the so-called bypass transition process to occur in the boundary layer. The presented study uses direct numerical simulation (DNS) as the main tool to investigate the flow behaviour. The spectral element code used for the simulations is Nek5000. The computational setup was designed to follow the experiments by Lengani & Simoni (2015). In the simulations, only one of the blades is calculated, with periodic boundary conditions in the cross-flow directions to account for the cascade. Isotropic homogeneous free-stream turbulence is prescribed at the inlet. The free-stream turbulence is prescribed as a superposition of Fourier modes with a random phase shift. The maximum and minimum amplitudes of the wavenumber vector are prescribed quantities and are limited by the resolution of the spatial discretization and size of the domain. Various levels of the free-stream turbulence intensity were simulated (Tu =0.02% and 5.2%), with the integral length scale being 20 mm, at the leading edge. We observed that in case of low free-stream turbulence on suction side, the Kelvin–Helmholz instability dominated the transition process and full-span vortices were shed from the separation bubble. Transition on the suction side proceeded more rapidly in the high turbulence case, where streaks broke down into turbulent spot and caused bypass transition. On the pressure side we have identified the appearance of longitudinal vortical structures, where we could observe that increasing the turbulence level gives rise to more longitudinal structures. The boundary layer near the leading edge of the blade is a key receptivity site for streaks. Evaluation of the stagnation line and its correlation with streak formation is one of the goals of this study. We will present comparison of numerical results with experimental ones, in terms of average values, rms values, turbulent quantities, freestream statistics, separation bubble length.

Lengani, D. & Simoni, D. 2015 Recognition of coherent structures in the boundary layer of a low-pressure-turbine blade for different free-stream turbulence intensity levels. Int. J. Heat Fluid Flow 54, 1–13.


Free-Stream Turbulence Induced Boundary-Layer Transition in Low-Pressure Turbines

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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