Abstract
Recent research have demonstrated the effectiveness of riblets (streamwise aligned grooves) in reducing the profile loss of low-pressure turbine (LPT) blades under high-lift loading. In this research, we pursue the efficacy of riblets in reducing the blade profile loss under various design and off-design conditions. We adopt a strategy in which surface roughness is employed in the transitional regime to minimize the separation bubble-related losses and flush mounted riblets downstream to further mitigate the skin-friction drag and boundary layer losses due to an increase in the turbulent wetted area (TWA). Several high-fidelity scale resolving simulations are carried out to test the efficacy of this ‘rough-ribbed’ LPT blade for loadings ranging from low-lift (LL), high-lift (HL) and ultra high-lift (UHL) conditions. Furthermore, two exit Reynolds numbers - 83,000 and 166,000 pertaining to engine relevant design and off-design conditions, respectively, are considered. The streamwise evolution of skin-friction coefficient and boundary layer integral parameters are compared and contrasted among different test cases. The instantaneous flow features and second order statistics such as the Reynolds stress and turbulent kinetic energy are analyzed to determine the design and off-design performance of riblets. It is found that the efficacy of scallop-shaped riblets in reducing the profile loss improves with loading. Specifically, the net skin-friction reduction increases from 3.4% under LL to 8% under UHL loading at cruise Re. There is a corresponding reduction in the trailing edge momentum thickness (θTE) from 10% to 15%. A further reduction in θTE is attained from design to off-design Re under UHL loading. Thus the effect of riblets in reducing mixing losses improves with increasing Re. It is also found that the riblets reduce flow blockage due to boundary layers. Furthermore, the necessity to optimize riblet ramp to achieve skin-friction reduction under off-design conditions is highlighted.
