NACA0012 airfoil at Reynolds numbers between 50,000 and 140,000 — Part 2: Oscillating streamwise flow

In many engineering applications, lifting surfaces operate in the low Reynolds number regime where the flow is prone to laminar separation, transition and reattachment, leading to the formation, in the time-average sense, of a so-called laminar separation bubble (LSB). Under unsteady freestream conditions (e.g. gusts), the dynamics of the laminar separation bubble that forms on low Reynolds number airfoils may be significantly affected by unsteady effects (as opposed to quasi-steady effects discussed in the previous paper, part 1), thereby affecting aerodynamic loads. In this paper, we analyze the response in lift of a NACA0012 airfoil at Reynolds number O ( 1 0 5 ) to varying freestream velocity through wind tunnel tests, high-fidelity numerical simulations and unsteady thin airfoil theory, and we correlate this response to first theoretical principles and the dynamics of the LSB. We show that analytical modeling, which does not account for viscous and streamwise pressure gradient effects, fails at predicting lift response when unsteady effects lead to separation without reattachment to separation with reattachment during the cycle. Accordingly, differences between viscous and inviscid approaches help reveal the role of viscous effects on lift response. • Lift response of a NACA0012 airfoil to oscillating freestream is investigated. • Results from analytical model, numerical simulations and experiments are compared. • Force partitioning method helps correlate lift response to changes in flow topology. • We explore regions of the parameter space that complement previous work.