Abstract
Low pressure turbines (LPT) typically operate at low Reynolds numbers, of O(105), resulting in transitional boundary layers on the suction surface. These boundary layers are prone to separation under the strong local adverse pressure gradients on the aft portion of the blade. Intermittent free-stream turbulence, periodic wakes shed by the upstream blades and surface roughness due to gradual degradation of the blades have been shown to suppress the separation bubble on the suction surface of the LPT blade. Although this generally leads to a profile loss reduction, some of the benefit is offset by a loss increase associated with a larger turbulent wetted area. In this work, we explore a strategy where the losses in both the transitional and turbulent boundary layers can be reduced. In particular, we employ surface roughness in the transitional regime to trip the boundary layer and reduce the separation bubble related losses. Subsequently, we explore the possibility of utilizing riblets in the turbulent regime to further reduce the losses due to the turbulent wetted area. To investigate the efficacy of this ‘rough-riblet blade surface’, high fidelity eddy-resolving simulations are carried out on the configuration of a flat surface subjected to streamwise varying pressure gradients. A contoured upper wall is used to mimic the pressure distribution typically encountered on the suction surface of a high lift turbine blade. The Boundary Data Immersion Method is used to represent different riblet shapes and roughness elements, while a digital filtering approach is used to impose free-stream turbulence at the inlet. The influence of different riblet geometries on skin friction drag of transitional and turbulent boundary layers under streamwise varying pressure gradients are discussed in detail. The effect of different riblet shapes in conjunction with the roughness elements on the passage losses is shown through boundary layer integral parameters and Reynolds stresses.
