Non-Equilibrium Boundary Layers on Fan Blades

Abstract This paper investigates the effect of non-equilibrium boundary layers on the aerodynamics of transonic fans. The investigation uses high fidelity simulation (DNS/LES) and low-order modelling to determine the physical mechanisms driving boundary-layer growth. The DNS shows that the presence of a shock/boundary-layer interaction rapidly shifts the equilibrium state of the boundary layer, generating a significant non-equilibrium region where the production of turbulence (and therefore turbulent shear work) rises significantly — accurately capturing this process is important to the prediction of displacement thickness and therefore blockage. The physical mechanisms are shown to be captured by an adapted form of the shear-lag model, calibrated using the DNS data. The new model demonstrates the associated impact on the development of the boundary layer. The results show that non-equilibrium effects due to shock/boundary-layer interactions are relatively insensitive to Reynolds number and therefore remain significant even for large civil engines.