An algebraic model for bypass transition in turbomachinery boundary layer flows

Abstract

A simple algebraic model is proposed for laminar to turbulent transition in boundary layers subjected to elevated free-stream turbulence. The model is combined with the newest version of the k-ω RANS turbulence model by Wilcox. The transition model takes into account, in an approximate way, two effects: filtering of high-frequency free-stream disturbances by shear and breakdown of near-wall disturbances into fine-scale turbulence. The model only uses local variables.The model has been tuned for the flat plate T3C cases of ERCOFTAC, relevant for bypass transition and tested for flow through cascades of N3-60 (Re=6·105) steam turbine stator vanes, V103 (Re=1.385·105) compressor blades and T106A (Re=1.6·105) gas turbine rotor blades. The transition model produces good results for bypass transition in attached boundary layer state and in separated boundary layer state for flows of elevated free-stream turbulence, both for 2D steady RANS and 3D time-accurate RANS simulations. Good results are also obtained for transition in separated laminar boundary layers at low free-stream turbulence by 3D time-accurate RANS simulations, thanks to resolution of the boundary layer instability and the beginning of the breakdown of the generated spanwise vortices.