On Improved Understanding of Airfoil Performance Evaluation Methods at Low Reynolds Numbers

Abstract

Airfoil performance for the Eppler 387 airfoil at low Reynolds number ([Formula: see text] to 460,000) is studied numerically and compared to existing experimental data. Two- and three-dimensional unsteady laminar Navier–Stokes simulations are performed to evaluate the large-scale dynamics of the flow. All cases show laminar separation of the boundary layer followed by the shedding of large coherent vortices resulting in reattachment only observed in the mean flow, referred to as bubble flapping. Integrated aerodynamic coefficients, pressure distributions, and separation–reattachment locations are presented and compared to experimental data. Absolute averaged section lift and drag errors in the range [Formula: see text] to 300,000 are 5 and 7%, respectively, and good agreement of the pressure distributions is obtained. Laminar separation locations are predicted within 2%-chord, and turbulent reattachment is on average predicted within 5%-chord of the experimental values, which is improved to 3%-chord when updated experimental data are used. The work illustrates that bubble flapping, and the resulting mean airfoil performance, can be analyzed using laminar unsteady Navier–Stokes simulations, and that it does not necessitate external disturbances or three-dimensional instabilities. The shear layer transition over airfoils is found to be dominated by two-dimensional vortex shedding resulting in unsteady reattachment.