CFD-based analysis and surrogate-based optimization of bio-inspired surface riblets for aerodynamic efficiency

For dynamic systems that operate in the transitional range of Reynolds number (laminar to turbulent flow), passive and active surface features can play an important role in maximizing the aerodynamic efficiency or control authority. This paper focuses on passive riblet-like features that are inspired by those naturally observed in marine animals. Unlike more conventional riblet geomtries, e.g., sawtooth and scalloped surface ridges, a smoother and parametrized riblet design is considered (defined by a bell-shaped function), studied and optimized to obtain maximum aerodynamic drag reduction. A CFD-based analysis is performed using RANS solvers to quantify the aerodynamic forces on a 3D airfoil section (NACA0012 airfoil is used), with the riblet-like features that run along the chord-wise direction on the top surface of the airfoil. To pursue optimization of the riblet geometry and spacing, surrogate modeling is performed first to alleviate the prohibitive computational cost of the CFD simulations, and a variable fidelity optimization method is used to subsequently maximize drag reduction for different angle of attack cases. Up to 6.6% reduction in drag is observed with optimal riblet design, compared to the bare 3D airfoil section. Further insights are derived into the flow physics driving the aerodynamic efficiency benefits of smooth riblets, with riblets of 0.5 height-to-spacing ratio (and that impedes upward momentum transfer) identified as particularly promising in our case studies.