Effects of Ribbed Surfaces on Profile Losses of Low-Pressure Turbine Blades

In this work, streamwise oriented riblets were installed on a flat plate exposed to an adverse pressure gradient typical of low-pressure turbine (LPT) blade and, successively, on the suction side of an LPT cascade operating under unsteady flow. Different riblet dimensions and positions have been tested to quantify their effects on the boundary layer transition and on losses. The flat plate experiments allowed the detailed description of the riblet effects on the coherent structures affecting transition, thus providing a rationale for the identification of the optimal riblet geometry once scaled in wall-units. For riblet heights equal to about 20 wall-units, a maximum loss reduction of 8% was observed. Otherwise, for larger riblet dimensions, earlier transition occurs due to enhanced boundary layer instability and losses increase. Interestingly, the streamwise extension of the ribbed surfaces with respect to the transition region was found to play a minor role compared with the riblet dimension. The riblet configurations providing the highest reduction of viscous losses were then tested in the LPT blade cascade for different Reynolds numbers and with impinging upstream wakes. An overall profile loss reduction comparable to that observed in the flat plate case has been confirmed also in the unsteady operation of the turbine cascade. Low sensitivity of the profile losses to the riblet streamwise extension was also observed in the cascade application. This confirms that positive effects in terms of loss reduction can be obtained even when the exact transition position is not known a priori.