Optimal Riblets Applied to Gas Turbine Compressor Blades Studied via Direct Numerical Simulation (GT2024-122305)

Streamwise micro-groove surfaces (‘riblets’) are well-known as a passive surface treatment to reduce drag, which may assist in increasing overall gas turbine efficiency. The first direct numerical simulation of micro-scaled riblets on an axial high pressure compressor blade at engine-relevant conditions (Kozul et al., GT2023-101038) demonstrated the ability of riblets to reduce the skin-friction along most of the chord of a high pressure compressor blade, at both on- and off-design inflow incidences. However this previous study only considered riblets with fixed physical dimensions. The present work considers a ‘locally optimal’ riblet geometry applied to the high pressure compressor blade, where the riblet geometry is smoothly adjusted along the chord to target optimal riblet dimensions in viscous units, known from fundamental fluid mechanics studies. The present locally-optimal riblets do indeed reduce the skin friction on the surface of the high pressure compressor blade at both design and off-design inflow incidences, and are generally at least as effective as the previous fixed-geometry riblets. Yet the present riblet surfaces tend to increase pressure wake losses. In particular, at positive incidence, although the locally-optimal riblets generally further reduce the skin friction compared to the fixed riblets, the wake loss increases by a significant 20% with respect to the reference smooth-blade case. A complex flow interaction with the suction side separation bubble at the leading edge may be responsible for the significantly increased loss at positive inflow incidence.