Reynolds-averaged Navier–Stokes assisted direct numerical simulation of low Reynolds number flow over airfoils

The simulation of insect flight, like that of dragonflies operating at low Reynolds numbers, has numerical challenges due to the complex morphological structure. The corrugated airfoils trap vortices, and in these recirculation zones, turbulence models may be inadequate to resolve the near-wall flow features well. Hence, accurately capturing the laminar–turbulence transition and identifying the point of separation and reattachment requires resorting to direct numerical simulations (DNS) over a large domain, that is computationally expensive. We propose conducting DNS over a truncated subdomain constrained by Reynolds-averaged Navier–Stokes (RANS) solution to reduce the computational domain size and costs. A precomputed RANS simulation over a large domain is used to prescribe a velocity boundary condition (BC) at the truncated domain of the DNS; a convective BC is imposed as outflow. We validate the proposed RANS-assisted DNS (DNSR) by simulating subdomains of varying sizes and comparing the mean and fluctuating velocity fields, and aerodynamic characteristics with the DNS with free-slip BC. This technique reduces domain size and computational cost significantly (by at least half). A criterion for the ideal subdomain size is determined by satisfying the condition at the location where the non-dimensional mixing length is approximately 60. We validated our criterion by simulating flow over the NACA (National Advisory Committee for Aeronautics) 0012 airfoil at angles of attack α≤12°, corroborating with established literature. Finally, we study a three-dimensional corrugated airfoil with our approach, to capture the transition from two- to three-dimensional structures in the wake as the angle of attack increases.